The Century of Space Science

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JOHAN A.M. BLEEKER*, JOHANNES GEISS** AND MARTIN C.E. HUBER*** The century of space science

In the course of its brilliant evolution through the twentieth of our Universe have changed the perception of the world century, space science brought spectacular results and led around us, for scientists as well as for the general public. to fundamental scientific findings. In the early quest for For about a quarter of a century after the end of World higher altitude, cosmic rays were discovered by Viktor Hess War II, in what we might call the early epoch of space sci- during a balloon flight in 1912. Following the second ence, experiments in space were regarded as daring but World War, sounding rockets were used, starting in 1946, rather extravagant. But in the course of the 1970s, the meth- to study the structure of the terrestrial atmosphere – the ods and techniques of space science began to enter the threshold of space. Two landmarks of the space age stand mainstream of science, and by the end of the century, space out: the launch in 1957 of the first satellite, Sputnik 1, science had become a natural complement, and often an which sensed the near-Earth environment at orbital altitude, integral part of study in many fields of science. and in 1969 the first landing by humans on an extraterres- The present two volumes of The Century of Space trial body, the Moon. Today, sophisticated space probes Science tell the story of space science as it evolved during explore distant worlds, and space telescopes look back in the twentieth century. The origins of space research, the time towards the early Universe. enabling technology for space transportation and the ‘early The scientific exploration of space was at the origin and, epoch’ of space science mentioned above, are addressed in indeed, the motivation for our first ventures away from the Chapters 2–14. The chapters in the main body of this Earth. It is well known, however, that rockets – the enabling Reference Work then describe the development and results tools of space research – were developed with support from of the scientific topics that have benefited extensively from the military, and that the ‘space race’, later on, served as a space investigations. A chronology of the space age and a substitute battleground during the Cold War. Today, fortu- list of space science missions appear as appendices. nately, the adversities of those years have given way to We have had to restrict the number of fields covered, and peaceful global collaboration that would have been techno- have therefore concentrated on those topics with the most logically and ideologically impossible during most of the comprehensive record in space, namely the exploration of twentieth century. the Solar System, astronomy and gravitational physics. We The experiences of the astronauts on the Moon, their pic- note, however, that space science – far from being a coher- tures of the Earth from above, the visits of unmanned space- ent discipline itself – has by now become a widely used craft and robots to the distant planets and their satellites, and method that complements in an essential way the investiga- the probing of the largest distances and the earliest epochs tive tools of an impressive range of initially laboratory- and ground-based research fields. * SRON – National Institute for Space Research, Utrecht, We have also included three chapters on Earth science. The Netherlands ** ISSI – International Space Science Institute, Bern, Switzerland This field is important not only because of its scientific *** ESA – European Space Agency, Paris, France achievements, but also because, from early on in the space

Johan A.M. Bleeker, Johannes Geiss and Martin C.E. Huber, The Century of Space Science, 3–22 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 4 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER age, it started to raise humanity’s awareness of the fragility EARTH SYSTEM SCIENCE of their home planet – ‘spaceship Earth’ – which protects them from the hostile cosmic environment. It is appropriate Space-borne observational platforms gave us, for the first therefore to include here an image of ‘the blue planet in time, the means to survey the many features of our planet space’, photographed during one of the Apollo missions rapidly and effectively from a global perspective. This global (Figure 1). view of the Earth from space – together with the maturation Rather than use this chapter to summarize the contents of distinct Earth-science disciplines, such as geophysics of of the rest of the book, we instead highlight a number of the solid Earth, oceanography, and atmospheric dynamics results which demonstrate how space science has made and chemistry, as well as the recognition of the human role in major contributions to our knowledge, and sometimes has global change – has stimulated a new approach for studying even forced us to change our concepts of nature. In outlining our home world, what we might call Earth System science. these results, we start with Earth System science and then In this approach the Earth System is studied in the context of proceed – roughly following the historical development – a related set of interacting processes rather than as a collec- from the terrestrial environment and the Solar System to the tion of individual components. Interactions among oceans, Milky Way, and then to extragalactic space and cosmology. ice, land-masses, the atmosphere and biological systems are (Note, however, that the sequence chosen for the topics pre- significant but also very complex. The transport of energy sented in the main body of this work is the opposite: it and material within and among these subsystems occurs on a begins with gravitational physics and cosmology and ends global scale across a wide range of time-scales. Observations with Earth science.) from space are indispensable for present and future research,

Figure 1 The Blue Planet, with the dry lunar surface in the foreground. THE CENTURY OF SPACE SCIENCE 5 whose ultimate goal is an understanding of the processes responsible for the evolution of the Earth on all time-scales. The importance of a synoptic view of planet Earth is convincingly demonstrated by the global monitoring of stratospheric ozone (Figure 2). Ozone provides crucial protection against hard ultraviolet radiation from the Sun. The mapping and monitoring of the evolution of the Antarctic ozone hole has helped us to develop and validate representative models for ozone depletion and restoration in the upper atmosphere.

PLASMAS IN SPACE: MAGNETOSPHERE AND HELIOSPHERE

The International Geophysical Year (1957/58) was the first worldwide coordinated effort in space science. Its goal was to understand the aurorae and to map the variation of the Earth’s magnetic field. Space science has indeed clarified to a large extent the relation between the two. Sounding rockets on suborbital trajectories, satellites in various orbits together with ground-based observations of geomagnetic field variations were instrumental in revealing the processes that cause auroral phenomena. These early efforts immediately led to the discovery of a magnetically trapped, collisionless particle population, namely the radiation belts and the terrestrial magnetosphere, including its extended magnetotail (Figure 3). It was found that what is responsible for the aurorae (Figure 4) is a medium-energy particle component being injected – during magnetic substorms – from the magnetotail into the auroral zone, rather than, as had been believed, trapped high-energy particles in the radiation belts. Space in the terrestrial neighbourhood was also exploited as a natural laboratory: in situ measurements led to the discovery of discontinuities in the collisionless space plasma and uncovered the phenomena of magnetic reconnection and shock acceleration, both fundamental processes throughout the cosmos. The terrestrial magnetosphere later became the prototype for modelling other planetary magnetospheres and the magnetospheres of neutron stars, pulsars and rotating black holes. The heliosphere is formed, because the corona of the Sun is in a state of expansion, forming a continuously flowing solar wind. Because of its expansion into three dimensions, the solar wind pressure is reduced until it can no longer overcome the ambient interstellar plasma pressure. A termination shock, formed at an estimated heliocentric distance of 80–100 AU, converts the supersonic wind into a Figure 2 Assimilated total ozone maps showing the evolution subsonic flow. of the Antarctic ozone hole between September 2000 and January 2001. The charged particles in the external interstellar gas cannot penetrate the heliopause, the boundary of the heliosphere. However, interstellar grains and neutral atoms do enter, and 6 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Figure 3 The terrestrial magnetosphere, which results from the interaction of the solar wind with the magnetic field of the Earth, was completely unknown before the advent of the space age. This schematic drawing gives an impression of the complexity of the interaction. they have been found and investigated by spacecraft in the Evidence gathered from Lunar exploration by manned space between the planets and throughout the heliosphere, and unmanned spacecraft shows that water of crystalliza- allowing a direct analysis of physical, chemical and isotopic tion and hydrogen-containing minerals are absent from properties of the matter in the local interstellar cloud. Lunar material, and that other volatile elements, such as A rich variety of heliospheric phenomena have been carbon and nitrogen, are virtually absent as well (Figure 6). discovered, and the underlying processes are now generally Furthermore, neither in situ observations by astronauts nor well understood. And the heliosphere has become a para- pictures taken on the lunar surface or from lunar orbit digm for the interaction of a main sequence star with the revealed any traces of past water flows or sedimentation interstellar medium. (Figure 7). The extremely high temperature of the material ejected by a giant collision can readily explain this complete lack of volatile material. THE MOON AND THE TERRESTRIAL PLANETS It is also remarkable that the Moon has much less iron than does the Earth or meteorites. This is indeed what we The origin of the Earth–Moon system, has long been an would expect if the giant impact occurred after the Earth enigma. Moons orbit many planets in the Solar System but, had formed its iron core and if the impact ejected mainly with the exception of those of Earth and Pluto, they are tiny in material from the Earth’s crust and mantle. Fractionation- comparison with the planet they accompany. The satellite sys- corrected abundances of the three isotopes of oxygen also tems of the giant planets, Jupiter and Saturn, consisting of point to a terrestrial origin of the Moon: they are identical more than ten moons each were probably formed from in all Lunar and terrestrial samples, yet differ from those of circumplanetary disks at the time of the planets’ formation. Martian rocks and all classes of meteorites. The exceptional relative size of our Moon points to a different The aggregate evidence in favour of the giant impact the- genesis. Among the various hypotheses of the Lunar origin, ory has become so strong that, at the end of the twentieth there is only one – the giant impact theory – that explains all century, it is generally accepted as the proper explanation for the relevant observations (Figure 5). the origin of the Moon. THE CENTURY OF SPACE SCIENCE 7 The The aurora borealis as seen from the ground and from the Space Shuttle (note the silhouetted tail fin and engine shrouds), and pictures of aurorae on Jupiter and Saturn, as observed by the Hubble Space Telescope. the Hubble Space by and Saturn, as observed Jupiter Figure Figure 4 8 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Figure 5 All evidence indicates that our Moon was created by a collision between Earth and a Mars-sized object that had formed in a nearby orbit. This painting by William K. Hartmann depicts the situation five hours after the collision.

The Apollo programme and unmanned lunar orbiters and melting in its early history. Later, between about 600 and landers have given us a remarkably complete picture of the 1500 million years after the birth of the Moon, iron- and evolution of the Moon as a geological, geophysical and geo- titanium-rich lava rose from below and filled the large chemical entity. Its anorthositic crust indicates complete basins that had been excavated a few hundred million years

Figure 6 A thin section of basaltic rock collected by Neil A. Armstrong and Edwin E. Aldrin in the Lunar Mare Tranquillitatis. The Moon is virtually free of hydrogen, carbon, nitrogen and other volatile elements; accordingly, the minerals in this sample of lunar rock do not contain these elements. THE CENTURY OF SPACE SCIENCE 9

Figure 7 The dry Lunar landscape in the Mare Imbrium. Like everywhere else on the Moon, there is no trace of past water flows. The rocks are dry, devoid of hydrogen, carbon and nitrogen, although nearby there is a valley that at times in the past had been considered to be a dry riverbed. earlier by large impacts during the last stages of accretion in Solar System have important similarities: their sizes, chemical the Solar System. The lava-filled basins are the dark basaltic compositions and distances from the Sun are rather similar, planes on the Moon that were called maria by Galileo. and so comparative studies can further our understanding of The other surface features are younger and are mostly their origin and evolution. Moreover, we note that these three due to more recent impacts, because by about 3 billion years are the only objects in the Universe for which, by the end of ago the interior of the Moon had cooled to the point where the twentieth century, detailed geological surveys had been geological activity came to a standstill, and the surface of conducted and of which we possess rock samples for labora- the Moon was only occasionally changed by impacts. On tory analysis. Earth, the intrinsic geological activity goes on to this day. It should be stressed, however, that comparative plane- Impacts have only a small influence on shaping surface fea- tology is not just a matter of transferring experience and tures of our planet. However, towards the end of the twenti- information from Earth science to planetology. Quite the eth century it became more and more apparent that impacts contrary: much has been learned about the origin and the play a major role in the evolution of life. early history of the Earth from comparisons with the Moon. The space age has also completely transformed plane- The methods used to derive the relative abundances of chem- tary science. This field has evolved from a branch of astron- ical elements in the Earth and other planetary bodies were omy into a multidisciplinary field that now embraces developed by comparing results of chemical analyses of astronomy, geology, geophysics, geochronology, geochem- rocks from the Earth, Moon and meteorites, including some istry and atmospheric science. meteorites from Mars. Comparisons of the molecular and Concepts developed over centuries of Earth science can be isotopic compositions of the extremely different atmospheres extended to those objects not too dissimilar from the Earth, of Venus, Earth, the Moon and Mars have also improved our namely the terrestrial planets and the Moon. Although an knowledge of fundamental atmospheric processes such as interplanetary traveller would perceive Earth, Moon and Mars outgassing and loss by escape, and of factors that determine (Figure 8) as entirely different worlds, these members of the planetary climates, such as the greenhouse effect. And our 10 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Figure 8 Topographic map of Mars. Low levels are shown in blue, as a reminder that in the past there was probably extensive flooding on Mars. understanding of the role of impacts in surface geology and Venus, on the other hand, is so thick that surface features can in biological evolution on Earth has been greatly advanced be sensed only by radio and radar techniques (Figure 10). by the study of craters and other surface features of the Similarly on Earth, radar can ‘see through’ the clouds. terrestrial planets and the Moon. Information about the interiors of planets is obtained The technique of remote sensing from orbit permits us to from seismic data and from measurements of gravity, heat investigate not only the atmospheres but also the surfaces of flow and magnetic field. Strong magnetic fields, implying the Moon and the planets, including the Earth. Moreover, an ongoing dynamo effect inside the Earth, Jupiter and depending on the thickness of the atmosphere, the surface other large planets, have been observed. Magnetic field chemistry and mineralogy of these bodies can be probed by measurements now also indicate that Mars possessed a using parts of the electromagnetic spectrum other than the dynamo in its early history. visible, for example gamma rays for Mars and X-rays and The exploration of Mars gained momentum towards the gamma rays for the Moon (Figure 9). The atmosphere of end of the twentieth century, largely because indications of

Figure 9 A compositional map of the Lunar surface as determined by the Apollo 15 gamma-ray spectrometer. High concentrations of radioactive elements (red and yellow regions) are found in the maria and other regions on the nearside. THE CENTURY OF SPACE SCIENCE 11

Figure 10 A three-dimensional perspective view of the surface of Venus, showing the western Eistla region with the volcanic peaks of Gula Mons (left, 3 km high) and Sif Mons (right, 2 km high). The image was obtained by use of synthetic aperture radar (SAR) data combined with radar altimetry, both from the US-American Magellan mission. The hues are based on colour images recorded by the Soviet Venera 13 and 14 probes. extensive water flows at some undetermined earlier epoch then freezes upon exposure. Changes in the orientation of were discovered. These observations and findings in mete- this satellite’s magnetic axis with time are, in fact, consistent orites of Martian origin greatly increased interest in the with a conductive liquid-water layer less than 100 km below question of whether life, or at least extinct life, could be the surface. Europa’s icy shell is therefore thought to cover a found on Mars. salt-rich ocean, in which life might exist. Saturn’s largest satellite, Titan, also evoked considerable THE OUTER SOLAR SYSTEM interest (Figure 12). The temperature structure of its atmosphere and the abundance of nitrogen and hydrocarbons are Pictures and other data transmitted from probes that have reminiscent of the early Earth. Titan, however, has not visited the outer planets and their environs, and also obser- evolved because of the low temperature and the lack of liq- vations with space telescopes, have revealed a fantastic vari- uid water at 10 AU from the Sun. ety of surfaces, atmospheric phenomena, ring systems, and Io’s plasma torus. Shepherd moons, which constrain thin rings of shards circling planets, were found as well. The THE ORIGIN OF THE SOLAR SYSTEM, larger moons, in particular, showed traces of geologically METEORITES AND COMETS recent changes on their surfaces and even current activity. The most dramatic example is the ongoing activity of Io, The question of the origin of the Solar System remained the innermost of Jupiter’s Galilean moons (Figure 11). This largely a domain of physicists well into the twentieth activity not only shapes the surface of this satellite, it also century. It was concluded that the process starts with the gives rise to the famous sulphur-rich Io torus, which in gravitational collapse of a cloud of gas and dust, and is fol- turn is a major source of ions for the huge magnetosphere lowed by the formation of a disk from which the Sun and of Jupiter. Given Io’s size, which is similar to that of the planets evolved. When chemical arguments were intro- Earth’s Moon, internal energy sources could not possibly duced, meteorite research became important in studying the drive the strong sulphur volcanism that exists in the present origin and evolution of the Solar System. From isotope epoch. The answer to the puzzle is the tidal force exerted by abundance measurements in terrestrial and meteorite sam- Jupiter, which has over 300 times the mass of the Earth. ples, it was found that the age of the Earth and of the Solar Europa, though farther from Jupiter than Io, still appears System was 4.5 billion years. to be affected by this tidal force. Evidence of mobile ‘ice In 1986, at the time of the return of Comet Halley, a fleet rafts’ on the surface of Europa indicates fluid flow. Tidal of spacecraft was despatched to encounter this celestial body stress may exceed the tensile strength of the ice, lead to and its environment. The Giotto spacecraft, in particular, went cracks in the ice and expose the liquid underneath, which deep into the coma, obtaining the first detailed picture of a 12 JOHAN BLEEKER, JOHANNES GEISS AND MARTIN HUBER

Figure 11 Left: the giant planet Jupiter with two of its Galilean moons, Io (reddish) and Europa (white). While these and other moons of Jupiter have nearly the same size as our Moon, they are tiny in comparison with their parent planet. Right: close-ups of Io and Europa, both of which exhibit geological activity that stems from powerful tidal forces exerted by massive Jupiter.

Figure 12 A collage of Saturn and its moons: Dione (foreground) and (from left to right) Thea, Enceladus, Tethys, Mimas and Titan. THE CENTURY OF SPACE SCIENCE 13

presence of astronauts onboard Skylab made it possible to use photographic film and return it to Earth for developing.) With access to a halo orbit around the L1 Lagrangian point on the Earth–Sun line, it became possible to combine remote- sensing observations with in-situ solar-wind measurements. The Solar and Heliospheric Observatory, placed in such an orbit, went a step further. It also enabled space observations by the method of helioseismology, and thus provided a detailed look into the solar interior as well (Figure 14). A complete picture of the structure and dynamics of the solar interior, the processes that maintain the high temperatures in the corona and the processes of solar-wind acceleration is thus emerging from this observatory. The riddle of whether the ‘missing’ solar neutrino flux at Earth should be explained by some exotic behaviour of the energy-generating solar core or by adapting the theory of elementary particles is now resolved, since the solar core does not have any unexpected properties. It is currently thought that the electron neutrinos generated in the core change their flavour while passing through the Sun and on their way from the Sun to the Earth. Consequently, since Figure 13 The nucleus of Comet Halley imaged by the the first large neutrino detectors on Earth were only capable Halley Multicolor Camera on board Giotto. of detecting electron neutrinos, the solar neutrino flux appeared lower since some of them had become µ- or ␶- cometary nucleus and determining the composition of the gas neutrinos. and dust grains in the coma (Figure 13). This confirmed and Measurements by the Solar and Heliospheric Observatory considerably refined the so-called dirty snowball hypothesis, also confirmed that the solar ‘constant’ varies with the solar and also showed that comets have indeed preserved a wealth cycle. The solar irradiance was found to increase by about of virtually unchanged interstellar material, much like the 0.1% between solar minimum and maximum. material from which the Solar System was formed. Moreover, we now know that the Sun’s magnetic field supplies the energy that heats the corona. Waves which had previously been thought to contribute to the heating of the THE SUN corona were identified as the accelerating agent – through the ion-cyclotron mechanism – for the fast solar wind The Earth, its climate and the life it supports are all subjected (Figure 15) that emerges from coronal holes (i.e. from open to and governed by solar radiation and its subtle variations. magnetic field configurations). The astronomer, on the other hand, looks upon the Sun as a Coronal mass ejections, a phenomenon discovered by Rosetta Stone. Being the dominant object in the sky, it is Skylab, can now be followed far into interplanetary space, easily accessible to observations and has become a proving ground for advanced observing techniques. Indeed the first space experiments that went beyond investigating the terrestrial atmosphere and ionosphere were devoted to recording solar ultraviolet spectra. Among the early astronomical satellites were the series of Orbiting Solar Observatories, followed in 1973 by the pioneering Apollo Telescope Mount on Skylab. From this first, albeit short-lived, space station the outer solar atmosphere and the extension of the outer corona into the heliosphere could be investigated by a multiwavelength complement of imaging, spectroscopic and coronagraphic telescopes having focal lengths that enabled observations Figure 14 Temperature and rotation in the solar interior. down to arc-second resolution. (At a time when photo- The images show the deviations of these properties, as electric imaging detectors were not readily available, the derived from SOHO measurements, from current models. 14 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Figure 15 Dopplergram showing the (blueshifted) source Figure 16 A coronal mass ejection propagating into the regions of the fast solar wind, namely the boundaries and heliosphere. Stars are visible in this coronagraph image boundary intersections of magnetic network cells in a coronal whose field of view is thirty solar diameters wide. hole. and also as they propagate towards the Earth (Figure 16). water in circumstellar and interstellar environments. Water This provides a useful means of predicting disturbances in possesses a very large number of strong rotational transi- the Earth’s environment – a range of phenomena which tions in the far infrared, which suggest that it can act as a have been collectively termed ‘space weather’. Given the major or even dominant coolant in shocks and circumstellar increasing dependence of our civilization on communica- outflows. In particular, the Infrared Space Observatory tion, navigation and Earth-observing satellites – all vulnera- unveiled a host of water-emitting cosmic sites. Among them ble to major disturbances in the space environment – space are the asymptotic giant branch (AGB) stars, confirming weather forecasting is becoming another important service that water molecules are the dominant coolants of the stel- whose roots lie in space science. lar winds emanating from these stars (Figure 17). Moreover, the Infrared Space Observatory revealed, for the first time in a circumstellar medium, polyacetylenic THE MILKY WAY chains, including C4H2 and C6H2, and benzene (C6H6). From these observations it appears that carbon-rich protoplanetary The impact of the space age on the evolution of, and progress nebulae are capable of producing prebiotic matter in space in astronomical research has been huge, since the full breadth (Figure 18). of the electromagnetic spectrum and primary cosmic-ray par- While our picture of stellar evolution has been estab- ticle population became available as information carriers for lished with the aid of ground-based observations, progress cosmic diagnostics. Space observatories exploring the in the study of star formation has come to depend increas- infrared, ultraviolet, X-ray and gamma-ray wavebands have ingly on observations from space. Diagnosing the process revealed a dynamic and violent Universe harbouring a zoo of of star formation requires penetration into the interiors of exotic objects. Inevitably, we have to limit ourselves here to protostellar clouds, provided by measurements at infrared selecting a few scientific highlights from space-borne astron- and microwave frequencies. Observations from space have omy and obviously they reflect a subjective choice. played a crucial role in improving our understanding of star Although ground-based radio astronomy has unveiled a formation, notably in the form of spectroscopic measure- variety of molecular species, a major asset among the sci- ments from the Infrared Space Observatory, and astrometric entific harvest of space-borne spectroscopy in the infrared measurements and imaging from Hipparcos and the Hubble and far-infrared regions has been the molecular signature of Space Telescope, respectively. THE CENTURY OF SPACE SCIENCE 15

Figure 17 Water acts as a coolant in the stellar winds of AGB stars.

Figure 18 Carbon-rich protoplanetary nebulae as organic chemistry factories in space.

Observationally, it has now been confirmed that practically cate a great diversity of morphology in planetary systems: all low-mass stars like our Sun are born via the formation of a Jupiter-like planets are also found much closer to the cen- disk, which arises from the angular momentum conservation tral star than in our Solar System and they are, most likely, of the gravitationally collapsing protostellar cloud. The final formed first. Earth-like planets are believed to form later, dimensions of these disks are comparable to the size of our through the aggregation of a large number of planetesimals Solar System, and they have a mass approximately ten times a few kilometres in size. the mass of our ‘heaviest’ planet, Jupiter, which implies the Observations from space have not only helped to identify presence of sufficient material to form a planetary system. the early stages in the life of a star, but have also revealed Disks of planetary material orbiting newborn stars are the most extreme remnants of stars, namely stellar-mass called protoplanetary disks. The Hubble Space Telescope black holes. The strongest evidence for the existence of has recently obtained high-resolution images of such disks such black holes comes from observations of compact X-ray around young stars in the Orion Nebula, where they show binary sources in which the orbital velocity of the normal up as dark silhouettes in images obtained in visible light companion star can be measured from the Doppler shift of (Figure 19). Ground-based observations of exoplanets indi- its characteristic spectral lines at optical frequencies. 16 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Using Kepler’s laws, a quantity called the mass function, hole. So far about ten black hole systems (i.e. objects with f(M), of the system can be determined from measure- f(M) greater than 3M᭪) have been found in our Galaxy. The ments of the orbital period of the compact binary and the largest mass function was found for the recurrent nova V404 semi-amplitude of the radial velocity. This mass function is Cygni, a low-mass X-ray binary with a 6.5-day periodicity, the observational lower limit of the presumed black hole for which f(M) = 6.26 (Figure 20). mass. Fundamental physical arguments show that, among A remarkable achievement of space-borne astronomy very compact objects, only a black hole can have a mass in concerns astrometry, the oldest, most classical subdiscipline excess of 3 solar masses (M᭪), so if mass function values of astronomy: measuring precise positions, parallaxes and above 3M᭪ are found, the compact source has to be a black proper motions of stars to study the Galaxy’s structure and kinematics. The Hipparcos satellite has boosted this field by providing positions of nearly 120 000 stars to milli-arcsecond accuracy, and established accurate distances for tens of thousands of stars. In fact, Hipparcos pushed the effective range for parallax measurements from 30 pc out to 300 pc. Accurate distances yield accurate luminosities for all kinds of stars, and consequently theories of stellar evolution can be tested much more rigorously (Figure 21). The kinematic data provided by Hipparcos through the measurement of proper motions coupled with accurate distances allows an in-depth assessment of the dynamic evolution of the Milky Way system. Among other things, these dynamical studies will display the interplay between gravity and pressure and the role of instabilities in our Galaxy. As mentioned in the opening paragraph of this chapter, the discovery of cosmic rays may be regarded as the first result of space research. Although this discovery took place Figure 19 Hubble Space Telescope picture showing disks in 1912, the fact that the primary radiation consists of of planetary material orbiting around young stars in the Orion charged particles coming from the depths of space became Nebula. The size of the solar system is indicated by the lower generally accepted around the middle of the century. Cosmic image in the lower left corner. rays observed at the surface of the Earth are mostly secondary

Figure 20 Radial velocity curve of the black hole system V404 Cygni. THE CENTURY OF SPACE SCIENCE 17

The modulation of cosmic rays by the reversal of the solar magnetic field in the course of its 22-year cycle has proved to be an important tool for investigating heliospheric processes and dimensions. It has been suggested that the modulation of cosmic rays may also influence – via ionization and nucleation in the troposphere – the formation of low-lying clouds, and may therefore represent a coupling mechanism between solar activity and terrestrial climate variations.

THE GRAVITATING UNIVERSE AND COSMOLOGY

The notion that most of the mass of the Universe is in a form we cannot see is among the most striking discoveries of contemporary science. As early as the 1930s, Fritz Zwicky pointed out that the visible mass we can observe is not sufficient to explain the motions of galaxies in clusters. The nature of this invisible mass, known as dark matter, is under intense discussion and investigation. Despite its invisibility, it can be detected through the effects of its gravitational field, which allows us to probe its distribution in and around galaxies and in galaxy clusters. Giant clusters of galaxies form in gravitational potential wells dominated by dark matter that extends well beyond Figure 21 Hertzsprung–Russell diagram, reflecting stellar the observed galaxy population. However, the dark matter evolution, for the 16631 single stars from the Hipparcos content and distribution can be probed by observing the hot Catalogue whose distances and colour magnitudes are known X-ray emitting intracluster gas that is gravitationally bound to within 10% and 0.025 mag, respectively. The colour indi- by the total cluster mass. Space-borne observations by cates the number of stars in each pixel of the diagram. X-ray telescopes play a pivotal role in furthering this research (Figure 22). An alternative method of probing the large-scale distribu- particles. Therefore measurements had to be made from tion and concentrations of gravitating matter in the Universe space to detect the primary radiation. (In the 1930s and is provided by gravitational lenses (Figure 23). Extragalactic 1940s, cosmic rays were the main source of particles for the gravitational lensing provides us with an ‘optical bench’ emerging field of elementary particle physics. Positrons, whose length is comparable to the radius of the observable muons, pions, kaons and hyperons were discovered as sec- Universe. And here a dissimilarity with traditional astro- ondary particles produced in the atmosphere by cosmic rays, nomical observations emerges most significantly: this and their lifetimes and modes of decay were determined.) method probes all mass – not only ‘the visible 10% of the Three principal sources of cosmic rays were identified iceberg’ (i.e. the luminous content of the Universe). namely (i) high-energy particles originating in the Galaxy, Confirmation that the enigmatic gamma-ray bursts are (ii) solar particles accelerated by shocks created by solar objects in the remote Universe, rather than transient sources flares, coronal mass ejections and the co-rotating interac- lying in the close galactic vicinity, came from observations tion regions, and (iii) the so-called anomalous component made from space. The location of such events in distant of cosmic rays. The latter are accelerated pick-up ions pro- galaxies, through the optical identification of the X-ray duced from the neutral interstellar gas flowing through the afterglows detected by the BeppoSAX X-ray satellite, heliosphere. Towards the end of the twentieth century, cos- showed that gamma-ray bursts are the most powerful mic-ray research, in conjunction with gamma-ray and X-ray sources of explosive energy released in the Universe since astronomy, became more and more important for localizing its very creation in the hot big bang. and studying violent processes in the Galaxy. Supernova rem- The amount of explosive energy release is currently best nants were identified as the main source of galactic cosmic explained by the so-called fireball model. This postulates a rays. However, it is not yet clear to what extent extragalactic cataclysmic event in which the gamma-ray emission arises sources contribute to cosmic rays of the highest energies. from internal shocks generated in a relativistically expanding, 18 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Figure 22 XMM-Newton image of the hot X-ray emitting gas in the Coma Cluster of galaxies.

Figure 23 Although gravitational lenses have been discovered from the ground, the sharper pictures obtained from space permit a refined interpretation of the information provided by the deflected light. THE CENTURY OF SPACE SCIENCE 19

Figure 24 Hypernova 1998bw. optically thick plasma cloud travelling at more than 99.99% of the velocity of light. Gamma-ray burst sources would thus seem to produce the most extreme form of cosmic acceleration. Some gamma-ray bursts appear to be associated with a peculiar type of highly luminous supernova, a so-called hypernova, implying the explosive death of a very massive star in which a black hole is formed in the gravitationally collapsing core (Figure 24). The earliest picture of the Universe so far has been obtained by observing in the microwave region. The cosmic microwave background radiation (i.e. the isotropic high-frequency radio emission) was discovered in 1964 by Figure 25 The microwave temperature structure of the sky. Arno Penzias and Robert Wilson, who used a ground-based antenna. It turned out that the early Universe was very smooth. The radiation in question was last scattered from produced in the big bang, but that carbon and all the heavier the universal primordial plasma when the Universe had an elements are synthesized in the interiors of stars. Spallation age of roughly 300 000 years. by cosmic rays contributes significantly only for some of The Cosmic Background Explorer satellite mapped the the extremely rare elements, such as lithium, beryllium and temperature distribution of the microwave background over boron (Figure 26). the entire sky with an angular resolution of several degrees The theory of stellar nucleosynthesis was almost exclu- and revealed tiny spatial fluctuations of the microwave radi- sively based on element and isotope abundances measured ation field (Figure 25). This implied the existence of slight on or from the ground, namely in meteorites and in the density perturbations, the first solid observational evidence solar photosphere. But deriving the primordial abundance confirming the simplest hypothesis for the origin of large- of the isotopes of hydrogen and helium, the main products scale structure, namely that it grew out of tiny primordial of the big bang, required measurements in space as well. density perturbations. The primordial helium abundance is one of the most The first ideas about the synthesis of chemical elements important sources of information about the early Universe. Its in the early phase of the expanding Universe were devel- value derived from observation agrees, with an uncertainty of oped in the 1940s. However, the processes and locations that only a few per cent, with the theoretical prediction. This play a significant role in nucleosynthesis were identified confirms that the laws of physics derived from laboratory only in the 1950s and 1960s. It became clear that the iso- experiments can be applied without change back to the time topes of hydrogen, helium and lithium were fully or partly when the Universe was as young as one second. 20 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

had taken place, which is described by the cosmological constant ⌳ or another form of ‘dark energy’!

OUTLOOK

Space science has brought us more than new knowledge. Communication and even peaceful collaboration continued between space scientists and engineers living on both sides of the Iron Curtain, even in the darkest times of the Cold War. And space science not only helped to transcend borders between countries and political systems: it also led to intense intellectual exchanges across entrenched confines separating disciplines. As far as everyday life is concerned, we should not forget that reliability considerations and quality control for space hardware – an absolute necessity in view of the unforgiving space environment – has produced a direct payoff in the widespread use of quality assurance in industrial production. Figure 26 The three principal production sites of nuclei. However, the link between basic research and applied Carbon and all heavier elements are produced in stars. The science still needs to be strengthened because emphasizing big bang yields only the lightest species. For species with 3 7 applications without underpinning them by basic research mixed origin, such as He and Li, the relative proportions is, in the long run, a dead end. Without science, there is no change with time and location in our Galaxy – and elsewhere. science to apply. A case in point is the highly politicized discipline of cli- The average density of the Universe as derived from deu- mate research: understanding the Earth System is required terium and 3He measurements is less than 10% of the critical to predict its reaction to changes. Action on the Earth density, which is the density that would ‘close’ the Universe System must not be taken unless it is based on trustworthy and, as mentioned above, it is also less than the matter predictions. On the other hand, present uncertainties in density that is needed to account for the forces that keep some of the quantitative predictions must not be used as an galaxies and clusters of galaxies together. Thus, by the end excuse for political inaction. The large increase in the quan- of the twentieth century it was becoming clear that there tities of greenhouse gases in the atmosphere caused by exists in the Universe an exotic form of matter that con- human civilization is well documented, and it has undoubt- tributes more to the total density than does the visible, edly begun to affect the climate. Further research is urgently baryonic matter. Observations of the subtle inhomo- needed, not for recognizing the acute danger – which is geneities in the cosmic microwave background radiation obvious – but for predicting the kind of change that will currently indicate that the exotic matter consists largely of most severely influence life on Earth. weakly interacting particles that seem to be much more At the beginning of the twenty-first century, the possibil- massive than protons. ities for science in space are by no means exhausted. Perhaps the most striking example of an outcome of the Emerging propulsion technology such as solar sailing will century of space science is the evidence that the cosmologi- expand the range which our spacecraft can explore. And we cal constant, ⌳, of Einstein’s general theory of relativity is are just beginning to deploy ‘quiet platforms’ which pro- not zero. In 1914, Einstein had to give a non-vanishing vide an essentially acceleration-free environment – often at value to this integration constant because the observational cryogenic temperatures – and thus enable us to perform data available at the time pointed to a static Universe. tests of the general theory of relativity and, probably within Consequently, it was necessary to account for the non- a decade, long-baseline interferometry from free-flying collapse of the Universe by giving ⌳ a finite value. After spacecraft. Extending the technique of interferometry into Edwin Hubble had discovered the expansion of the space will widen the diagnostic potential of space science Universe in 1928, ⌳ seemed to have lost its meaning. Only tremendously. With laser interferometry we shall be able to towards the end of the twentieth century did space observa- set up giant antennas in space for detecting low-frequency tions of remote supernovae indicate that the Universe had gravitational waves, opening an altogether new window on expanded more slowly in an earlier epoch, and therefore the Universe and making possible further, more penetrating that a long-lasting acceleration of the Universe’s expansion tests of general relativity. THE CENTURY OF SPACE SCIENCE 21

The technique of “nulling” interferometry, will enable us special pleasure to thank Clare Bingham, Livia Iebba, Lidy to observe terrestrial planets that orbit stars other than the Negenborn and Silvia Wenger for their untiring assistance, Sun, and study the spectra of their atmospheres and thus and Rudolf Treumann for his comprehensive and wise look for signatures of non-equilibrium that may be caused counsel regarding the material reported in this chapter. by life. Interferometry will, of course, also benefit other fields of astronomy. Once available in the X-ray domain, for example, interferometers will allow us to expose the event horizons of black holes and thus probe the limits of FIGURE CREDITS the observable Universe in objects that are not at cosmological distances. Figure 1. Photograph by the crew of Apollo-11. Planetary exploration will be a central theme of the space Figure 2. Royal Netherlands Meteorological Institute (KNMI) and ESA. effort in the twenty-first century. How this field develops, The data were taken by the Global Ozone Monitoring Experiment and how fast, will depend on available technologies as well (GOME) on ESA’s ERS-2 satellite. as scientific findings. Evolution and breakthroughs in the Figure 3. From: Hultqvist, B., Øieroset, M., Paschmann, G. and Treumann, fields of transportation and robotics will affect the relative R., (eds.) (1999). Magnetospheric Plasma Sources and Losses. In ISSI roles of human landings and automated exploration, includ- Space Science Series Vol. 6, Kluwer, Dordrecht, and Space Science ing sample return. And any progress in the search for current Reviews, 88, Figure 7.1, p. 371. or extinct life on other planets or satellites will have an enor- Figure 4. Upper left, Jan Curtis; upper right, STScI; lower left, NASA; mous influence on the ways and means of planetary explo- lower right, STScI. ration. The search for life in the Universe, and more specifically in the Solar System, is on. And space will play Figure 5. Painting by William K. Hartmann. an important role in the new field of astrobiology, which has Figure 6. NASA/Johnson Space Center. the potential to lead to a new age of scientific understanding. Astrobiology is also dissolving many boundaries between Figure 7. Photograph by the Apollo 15 crew, NASA Photo AS-15 9212430. disciplines because it requires the integration of several dis- Figure 8. Courtesy NASA and the Mars Orbiter Laser Altimeter (MOLA) parate fields of science, as for example astronomy, biology, Team of the Mars Global Surveyor project. From Kallenbach, R. Geiss, J. chemistry, Earth sciences and physics. and Hartmann, W.K. (eds.) (2001). Chronology and Evolution of Mars. In The outlook for a new century is always uncertain. Yet – ISSI Space Science Series Vol. 12, Kluwer, Dordrecht and Space Science Reviews, 96, Figure 2, pp. 466–467. excepting a major catastrophe that affects the physical world, or a radical change in the mode of thought of humanity – Figure 9. NASA. there is now such a momentum in technology development Figure 10. Data from the Magellan mission, NASA picture ID P-38724 that the outlook for space science is good. It is advisable not (excerpt); http://www.jpl.nasa.gov/magellan/gif/p38724.gif to make any specific predictions: indeed, possibilities often seem unimaginable. As pointed out in Chapter 3, on the Figure 11. Left: Voyager photograph, 13 February 1979. Upper right: Galileo Orbiter Mission (NASA), Catalog No. PIA 025727 (excerpt); enabling technology for space transportation, an author inset: http://www.planetaryexploration.net/jupiter/io/index.html. Lower writing at the beginning of the twentieth century would prob- right:http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl? ably not have foreseen the atom bomb, worldwide air traffic, PIA01127. the Global Positioning System, the Internet or, indeed, the Figure 12. http://www.jpl.nasa.gov/saturn/gif/saturn_montage.gif supreme reign of the computer. In this sense, we might expect new concepts for access to space to provide new Figure 13. From: Keller, H.U., Curdt, W., Kramm, J.-R. and Thomas N. impetus to the overall space effort – including space science. (1994). In R. Reinhard, N. Longdon and B. Battrick (eds.) Images of the nucleus of Comet Halley. ESA SP-1127, ESTEC, Noordwijk, Vol. 1, Figure 61, p. 63.

Figure 14. D. Gough and P. Scherrer, Chapter 44, and Acknowledgements http://sohowww.estec.esa.nl/gallery

The editors deeply appreciate the enthusiastic collaboration Figure 15. From: Hassler, D.M., Dammasch, I.E., Lemaire, Ph., Brekke, P., of those who have contributed to this reference work: Curdt, W., Mason, H.E., Vial, J.-Cl. and Wilhelm, K. (1999). Solar Wind the pioneers of space research and today’s leading space Outflow and the Chromospheric Magnetic Network. Science, 283, 810–813. science investigators. We acknowledge the initiative of Figure 16. Courtesy of SOHO/LASCO consortium. Peter Binfield at Kluwer, and his safe guiding of the project through the difficulties encountered in the editing Figure 17. From: Barlow, M.J., Nguyen-Q-Rieu, Truong-Bach, Cernicharo, J., González-Alfonso, E., Liu, X.-W., Cox, P., Sylvester, R.J., phase. We commend Hermine Vloemans’ firm and diplo- Clegg, P.E., Griffin, M.J., Swinyard, B.M., Unger, S.J., Baluteau, J.-P., matic leadership during the production process. It is a Caux, E., Cohen, M., Cohen, R.J., Emery, R.J., Fischer, J., Furniss, I., 22 JOHAN A.M. BLEEKER, JOHANNES GEISS AND MARTIN C.E. HUBER

Glencross, W.M., Greenhouse, M.A., Gry, C., Joubert, M., Lim, T., Figure 21. http://astro.estec.esa.nl/Hipparcos/TOUR/hrdiagram2.html Lorenzetti, D., Nisini, B., Omont, A., Orfeil, R., Péquignot, D., Saraceno, P., Serra, G., Skinner, C.J., Smith, H.A., Walker, H.J., Armand, C., Figure 22. http://sci.esa.int/img/e5/26085.jpg Burgdorf, M., Ewart, D., Di Giorgio, A., Molinari, S., Price, M., Sidher, S., Texier, D. and Trams, N. (1996). The rich far-infrared water vapour Figure 23. Hubble Space Telescope Image STScl-PRC00-08, spectrum of W Hya. Astron. Astrophys. 315, L241–L244. http://astro.hi.is/lens/pict_th.html

Figure 18. From: Cernicharo, J., Heras, A.M., Tielens, A.G.G.M., Pardo, Figure 24. From: Baron, E. (1998). How big do stellar explosions get? J.R., Herpin, F., Guélin, M. and Waters, L.B.F.M. (2001). Infrared Space Nature, 395, 635–636. Observatory’s Discovery of C4H2,C6H2, and Benzene in CRL 618. Astrophysical Journal Letters, 546, L123–L126. Figure 25. Cosmic Background Explorer Team.

Figure 19. http://antwrp.gsfc.nasa.gov/apod/ap961207.html Figure 26. From: Geiss, J. and von Steiger, R. (1997). Production of light nuclei in the early Universe. In A. Wilson (ed.) Fundamental Physics in Figure 20. From: Casares, J., Charles, P.A., Naylor, T. (1992). A 6.5-day Space, ESA SP-420, ESTEC, Noordwijk, pp. 99–106, Figure 1. periodicity in the recurrent nova V404 Cygni implying the presence of a black hole. Nature 355, 614–617. 11

JAMES R. ARNOLD* The Moon before Apollo

When Galileo first turned his telescope toward the sky, he this name was not well chosen. ‘Deserts’ might have been made two major discoveries. One, the four large moons of better, ‘lava plains’ better still. In this period also the moon’s Jupiter, is justly famous. The other, less familiar, is perhaps libration (see below), causing somewhat more than half the more important historically. He saw the Moon with enough lunar surface to be visible, was first noticed and used. resolution to conclude that it is not a ‘heavenly body’ as The next step in quantitative precision came with Tobias that term was understood, made of perfect and everlasting Mayer (Kopal 1969, pp. 241Ð2), who in 1750 recorded rather heavenly stuff, but a rough, cratered object more like the accurate coordinates for 23 reference points on the lunar sur- Earth Ð a real world rather than a figment of the human face. Even so, actual maps using these data and new observa- imagination. Its study moved from the field of theology to tions of similar precision did not appear until almost a century that of natural philosophy, now called science. later (Beer and Mädler 1837). In Jules Verne’s deservedly We are approaching the 400th anniversary of that impor- famous science fiction novels De la Terre à la Lune (1865) tant milestone in intellectual history. I give here a brief and Auture de la Lune (1870), it was Beer and Mädler’s maps account of lunar studies before the first Apollo landing, in that were brought to lunar orbit by his three intrepid astro- 1969. For this period I owe a great debt to Kopal (1969), nauts. They remained the standard well into the twentieth to which the reader is referred for a more complete and century. The only notable advance in this period was in the authoritative history. Our knowledge of the Moon up to measurement of altitudes of various features using shadow the modern era divides itself naturally, I believe, into two lengths in early lunar ‘morning’ and late lunar ‘afternoon’. strands. The first to develop historically was a series of suc- The second area of research which was within the means cessively refined maps of its visible surface. The second of students of the Moon before the space age had to do with was the growth of understanding of the orbital motion of its large-scale structure and dynamics. These generally the Moon, and of its physical nature. require much more precise quantitative study than the map- Galileo’s first published maps (Kopal 1969, p. 226) were ping of the surface, and so were slower to develop as areas very crude. No known features can be identified on them. of research. It is worth noting, however, that Newton’s cal- The first maps showing features recognizable to the modern culation of the gravitational attraction of the Moon to the student were those of M.F. van Langren in 1649 (Kopal Earth, by his own account ‘agreeing pretty nearly’ with an 1969, pp. 228Ð9), quickly followed by others. The maps of inverse square law of attraction, was a critical early step in G.B. Riccioli and his colleagues (Kopal 1969, p. 235) show his development of his theory of gravitation. Copernicus and other big craters clearly for the first time. The study of the Moon’s orbit may be said to have begun More importantly, most of the feature names given by with G.D. Cassini in the mid-seventeenth century. Reasonably Riccioli still survive today. He seems to have been responsi- precise values of the Moon’s orbital motions, including the ble for calling the low-lying, dark, relatively smooth areas inclination of its orbital plane to the ecliptic plane (about maria or ‘seas’, names which are still attached to these huge 5.14¼), the inclination I of its axis of rotation to its orbital basaltic flows. Although they are low-lying and rather flat, plane (about 1.53¼), the semimajor axis, the eccentricity, and quite a good ephemeris of its motion were determined by * University of California San Diego, La Jolla, CA, USA Lagrange and Laplace in the late eighteenth century.

Darwin (1880) and some of his contemporaries opened surface, in fact about 59%, at one time or another. There is up a new scientific frontier by calculating the history of the also a physical libration, or actual rocking of the surface, Moon’s orbit, seeking light on the question of its origin. He but it is very much smaller. concluded that the moon has been and is receding from the We are ready, I think, to move on to the modern era. Earth, due to dissipative tidal forces exerted on the Earth by I choose to start the modern era as Harold Urey, the leading the Moon. This, he said, should cause the Earth’s rotation student of the moon ‘before Apollo’ always did, with the on its axis to slow over time as a consequence of conserva- publication of a remarkable book by Ralph Baldwin (1949). tion of angular momentum. Had the Moon ever been as In the brief span of 238 pages, Baldwin covered a lot of ter- close to the Earth as a few Earth radii, the length of our day ritory. First he reviewed the history of lunar studies to that would have been only a few hours. This analysis has proven point, with an excellent bibliography. Then he gave argu- robust. ments, clear and convincing to Urey and others able to By the early twentieth century three possible scenarios understand, for some key conclusions about the Moon. Not for the origin of the moon had been proposed and explored that he stopped the arguments! As it became apparent that to the limited degree possible with the meager data avail- they might soon be settled by direct observation, they in able. What we may call Darwin’s (1880) ‘fission hypothe- fact tended to grow more intense. sis’ held that the Moon had separated from the Earth when The most interesting of debates was over the mode of the Proto-Earth’s rapid rotation caused it to deform and origin of the abundant lunar craters, and of the circular become unstable. This idea had been embellished some- maria, such as Mare Imbrium and the ‘bull’s eye’ of Mare what, and the Pacific Ocean had been suggested as the scar Orientale. Baldwin (1949) summarized the nineteenth- of the fission process. An EarthÐMoon ‘double planet’ century literature on this topic, pointing out that the gener- scheme, analogous to the formation of double stars, had its ally accepted view, then and later, was that these features advocates. Finally, a capture model, in which the Moon was were volcanic. The other chief idea, that they were formed thought of as a small planet born elsewhere in the Solar by the impact of stray bodies, had been proposed quite System, and later captured in a close passage by the Earth, early, but was not taken seriously Ð it was even considered was seen as a possibility. While many papers on lunar a curiosity Ð by most students of the subject. origin were published over the decades, all three of these The one clear statement of the case for impacts before models continued to be advocated up to and even after Baldwin’s was given by a geologist famous in his time, the Apollo missions. They were widely considered to be G.K. Gilbert (1893). Baldwin (1949) put forward three the only three possibilities. It seems now that nature knew simple points. First, almost no lunar craters had anything better, but that is another story. resembling the conical outline familiar in terrestrial volca- While the fact that the Moon’s shape is not quite spheri- noes. There are only a few small, inconspicuous domes. cal was first deduced by Laplace, the first thorough study Second, the volume of the crater walls was always compa- was done by Jeffreys (1924). The Moon was shown to have rable, within the rather large errors associated with tele- a bulge pointed toward the Earth, accounting for its syn- scopic observations, to the volume of the central hole; chronous rotation. This was consistent at that time with averaged over many craters, the agreement was close. a model in which the bulge was caused by tides raised on Finally, while the larger craters usually show central peaks, a hot early Moon when it was much closer to the Earth. in no case does a central peak rise above the level of the However, the situation is not so simple. Moments of inertia surrounding terrain. of the Moon around three mutually perpendicular axes can The discussions of this and other questions were inter- be compared using observations made from Earth: the one rupted by a major event: the launch of Sputnik 1 on 4 called A, pointing toward the center of the Earth; C, close October 1957, followed on 3 November by the larger and to the rotation axis; and B, perpendicular to the other more ambitious Sputnik 2. On 31 January 1958 the USA two. They are all slightly different. (CA)/B is the best- responded by successfully launching the very small but determined ratio, given by Urey (1952) as 0.000629. The capable Explorer 1, which yielded James Van Allen’s dis- less certain values of CA and CB are certainly different covery of the Earth’s radiation belts. It was immediately from each other, so that to a first approximation the Moon obvious that great new possibilities for space research were is a triaxial ellipsoid. This was already known by the time about to be realized. For planetary scientists the Moon was of Urey’s book as inconsistent with a simple tidal model. the inevitable first target. The ellipticity of the lunar orbit, and the inclination Following the establishment of NASA in 1958, attention of its axis of rotation, combined with the parallax due to was given to the possibility of one or more lunar missions. the Earth’s finite size, give rise to the optical librations, Urey was particularly effective in advocating such missions. or apparent rocking, of the lunar surface in latitude and The first NASA committee to explore the subject was longitude. Thus we can see more than half of the Moon’s appointed early in 1959, with him as its most prominent THE MOON BEFORE APOLLO 273 member (and the author as a new recruit). What might be The first NASA lunar mission program was named called ‘the first Moon race’ then began. The most obvious Ranger. It began badly. Rangers 1 and 2 were engineering goal was to produce images of the then still invisible farside. test missions Ð both failed. Then missions 3Ð5, carrying The mare named Moscoviensis and the large, dark- cameras and scientific instruments and intended to floored crater named Tsiolkovsky will allow younger read- approach and impact the visible face of the moon, also ers to guess or remember who won that first race. Indeed, failed (in a different way each time.) However, the experi- the Soviet Union held the lead for a long time. After two ence gained seems to have been useful. Rangers 6Ð9, mission failures, their Luna 3 mission flew by the Moon on launched in 1964Ð65, were redesigned to carry cameras 8 October 1959 and obtained images of its hidden side, only. These four succeeded in producing the first close-up marking the first successful mission to another planetary images of the Moon, in the few seconds before impact. body. The pictures released were not very clear. This These images were clear and in focus. The final frames allowed the illusion to persist in the West for a while longer showed features smaller than one meter, sometimes much that the Soviet space probes were of poor quality, or even smaller. Some of these were craters, not very different in that the images were fraudulent. A scientist working with a appearance from the larger ones seen through Earth-based US intelligence agency undertook to evaluate the case. He telescopes. This was no surprise to those who believed they gave a very entertaining talk on the results. His conclusion were caused by impact of stray bodies, small fragments of was that the pictures were retouched ‘by experts, as good as asteroids or comets. They presented more difficulties to the ours’ before publication. He was sure that the Soviet scien- volcano party. tists actually had ‘genuine pictures better than the ones they One group of images displayed parts of bright rays origi- released.’ The better pictures obtained later in fact con- nating from one of the Moon’s youngest large craters. firmed their results. They could and did name the big fea- These were full of small pits which were elongated in a tures. The most notable difference between the nearside and common direction, appearing quite unlike the large primary the farside was the near-absence of maria on the latter. craters seen through Earth-based telescopes. There could NASA’s plans for lunar missions took some time to be little doubt that they were secondary impact features, develop. They were overtaken by President Kennedy’s formed as debris from the major crater-forming event speech to Congress in May 1961, announcing to the world moved out radially from it at subsonic speeds. It was also that he was committing the US Government to ‘placing a possible to conclude definitely what had already been man on the Moon in this decade and returning him safely to inferred from measurements of temperature versus time the Earth.’ All plans were restudied and, where appropriate, on the lunar surface (Baldwin 1949, p. 10, Figure 1 and revised. A manned flight on such a schedule required not accompanying discussion), namely that the Moon was not only the development of advanced capabilities at an accel- covered with bare rock. What the Ranger cameras saw was erated pace, but also their testing by precursor missions of fine-grained material. In general the surface in all three unprecedented scope and complexity. image sequences was remarkably smooth. Nonetheless, The new objective led to the announcement of three sets landing a spacecraft without human guidance was seen to of unmanned NASA missions to the Moon. The first, the have a finite risk of failure due to striking a rock. Ranger series, was at first largely unchanged from earlier Meanwhile the Soviet program was continuing to pro- plans. It included a series of impacts on the Moon’s visible duce important results and some more ‘firsts’. Two long face, with both scientific and reconnaissance objectives. The series of spacecraft, Luna and Zond, were launched second, the Surveyor series, was to land small instrument- throughout the 1960s and even later. Perhaps the most bearing spacecraft safely on the surface, again with multi- important was Luna 9, which was the first to soft-land on ple tasks. Both these programs were to be managed by the the Moon, on 3 February 1966, before the US Surveyor Jet Propulsion Laboratory in Pasadena, California. The program had begun. It transmitted pictures back to Earth of third, the Lunar Orbiter series, was to produce a photo- its landing site in western Oceanus Procellarum. Luna 10, graphic map of the Moon, nearside and farside, as complete which followed, was the first spacecraft to be put in orbit and detailed as possible. The goal was mainly to support around the Moon. the manned missions, but the data set would also be avail- This Surveyor series had an ambitious aim. The space- able for science. Responsibility for this was assigned to the craft were to land softly and upright on the lunar surface, Langley Research Center in Hampton, Virginia. on three extended feet. They carried not only cameras but It was a bumpy road we started down then. There were also other instruments (described briefly below). Their suc- numerous failures while scientists, engineers, and managers cess rate showed impressive progress since the Ranger learned their new roles. What is important here is not the series. Of the seven spacecraft launched, only Surveyors race, but what was learned and how that led to the remark- 2 and 4 failed to achieve their landings and scientific able scientific advances of the Apollo period. objectives. 274 JAMES R. ARNOLD

Surveyor 1 landed on 2 June 1966, at a near-equatorial Anthony Turkevich (1967). It used Rutherford scattering of western site not far from the crater Flamsteed in Oceanus alpha particles produced by radioactive decay (the same Procellarum.* This spacecraft and Surveyor 3 carried vidi- process Ernest Rutherford had used to demonstrate the exis- con TV cameras capable of producing a black-and-white tence of atomic nuclei). It could identify the nuclei, and TV image of 600 lines. The cameras could be rotated 360 hence the elements from which the particles were scattered, degrees, raised and lowered, and zoomed in and out. Over using the principle of conservation of momentum. Housed 10,000 pictures of the site (and the sky) were obtained, in a box open at the bottom, the instrument was deposited covering a full lunar day cycle in each case. on a flat spot on the surface. It accumulated a spectrum of A full report of Surveyor 1’s findings is given in Jaffe energies of backscattered particles over many hours of (1967), with an accompanying photographic section. The exposure. The concentrations of all major elements (abun- coverage of surface features was of course far more dance greater than 1%) in the top layer of the soil it sampled detailed than that available from the Ranger pictures (or the were then derived from their scattered energy spectrum. earlier Soviet ones). One fear was laid to rest by both Luna 9 The Surveyor 5 results clearly identified the material at and Surveyor 1: The two spacecraft did not sink into the this mare site as a (ground up) basaltic rock, comparable in lunar surface (confirmed to be finely divided material, now general to basalts on earth. However, it was unusual in hav- christened ‘regolith’) as one scientist had warned NASA ing a high concentration of the element titanium, 6Ð8% by that it would. Again, the surface was seen to be generally weight, which was beyond what geochemists were accus- smooth with low slopes, though there were rocks that could tomed to on the Earth. The result, though it evoked a good present serious obstacles to landing spacecraft. The princi- deal of skepticism at the time, was fully confirmed when pal scientific investigator was Eugene Shoemaker, in his the Apollo 11 soil from the same area was analyzed in the first important role in the lunar program. laboratory. This was the first chemical analysis of the com- Surveyor 3 was on the whole a repeat of Surveyor 1, position of an extraterrestrial material in situ. It was fol- enhancing the database of surface images and strengthening lowed by quite a few others, and all the data showed clearly the view that this sort of scene beheld by both craft’s cam- that the Moon is highly differentiated chemically. These eras might be typical of at least the lunar mare surfaces, and later analyses provided very useful constraints on the which because they showed fewer large craters were pre- origin and history of the Moon. ferred for the Apollo manned landings. In addition to the Meanwhile, the Soviet lunar program had not been idle. TV camera, Surveyor 3 carried a soil mechanics experi- A series of Luna missions and one mission called Zond ment, essentially a powered, fist-sized bucket with a range returned important data and images, particularly of the hid- of capabilities. In the course of about 18 hours it carried out den farside of the moon (e.g. Dolginov et al. 1967, dozens of tests of soil properties, including bearing Lebedinsky et al. 1967). strengths and response to impacts. Perhaps most important, Like the other two US mission series, the Lunar Orbiters it could excavate a trench to a depth of 17 cm and display were important both as precursors of the Apollo landings, the sampled material. Shoemaker was again chiefly respon- and for the database they provided. The method used to sible for these experiments. gather the data seems dated now, when CCD cameras and Surveyors 5, 6, and 7 landed at new and interesting spots other advances have made to job much easier. But at the on the nearside, respectively the prominent eastern Mare time it was ingenious, and above all it worked. So did the Tranquillitatis (already emerging as the most likely area for spacecraft, all five times. The first launch was in August the first manned landing), Sinus Medii near the center of the 1966, the last in August 1967. Moon’s face, and the northern rim of the crater Tycho in the Cameras with long rolls of narrow strip film were used to south. The best reference for the later Surveyor missions is take thousands of pictures of the lunar surface in a pre- Jaffe (1969). Camera and soil property instruments were planned pattern, both of whole bands of the lunar surface updated for these missions, but the most interesting new and of particular candidate landing areas at higher magnifi- development was the inclusion of an alpha-scattering instru- cation. These were developed chemically to bring up the ment for chemical analysis. This instrument was designed images, and then scanned with beams of light to produce and built at the University of Chicago by a group headed by streams of intensity data from which the black-and-white images could be reconstructed back on Earth. Sequences were designed to optimize lighting, so that shadows could help to bring out detail. Many months of exposures were * The convention used then and later reversed that used by astronomers in the preceding centuries, to one in which the direction is that seen from made and recorded. Well over 90% of the lunar surface was Earth, not as before by an observer on the Moon itself. It was adopted for eventually covered during the five missions, giving rise to a the convenience of the Apollo program. After Apollo the new convention remarkably valuable database of both the near- and farsides. was adopted by the international astronomical community. Thus, for example, Mare Orientale, which in Latin means ‘Eastern Sea’ is now at the Calculations showed that the spacecraft would remain in western edge of the visible hemisphere. orbit for a long time after the imaging had been completed, 12

HERBERT FRIEDMAN† From the ionosphere to high energy astronomy – a personal experience

In the first half of the 20th century, recognition of the storms are due to the magnetic action of the Sun; or to any existence of an electrified layer of the upper atmosphere dynamic action within the Sun, or in connection with hurri- that provided a mode for radio communication over great canes in his atmosphere, or anywhere near the Sun outside.” distances on the Earth grew with little sense of the role Furthermore he held strongly “that the connection between of invisible solar radiation in creating the electrical mirror. magnetic storms and sunspots is unreal and the seeming Solar radiation was thought to be black-body in spectral agreement between the periods has been mere shape characterized by a temperature of 6000 K with a max- coincidence.” Kelvin’s enormous prestige discouraged any imum in the yellow-green, trailing off rapidly in the infrared dispute and set back solar-terrestrial research for decades. and ultraviolet. None of this spectrum could produce signif- Toward the end of the 19th century Colonel Sabine of the icant ionization of the major constituents of the atmosphere. British army monitored a network of magnetic observatories X-rays were discovered by Wilhelm Röntgen shortly throughout the empire and noted that by “a most curious before the turn of the century and Guglielmo Marconi, a coincidence” the magnitude and frequency of magnetic dis- few years later, demonstrated trans-Atlantic radio communi- turbances was synchronized with the appearance and disap- cation via a high altitude, natural electrical mirror. Several pearance of sunspots. The direction of the compass needle ingenious physicists and electrical engineers pursued the swung in regular fashion over the diurnal cycle, but at times problems of radio reflection but had few clues to the nature the movements became more intense and rapid in the auroral of the solar radiation that keyed the phenomena of ionos- zone, giving rise to the name “magnetic storms”. It was com- pheric production and variability. The true nature of solar monly believed that interplanetary space was a vacuum and ionizing radiation remained a baffling puzzle until after that auroras were excited by direct streams of particles from WW II when the availability of rockets to carry detectors Sun to Earth unimpeded by any interplanetary medium. By directly into the ionosphere finally made the studies defini- timing the appearances of auroras and magnetic storms relatively diagnostic. Captured German V-2 (Vengeance) rock- tive to the visible outbursts of flares on the Sun, the travel ets while being studied by propulsion engineers were also speed was calculated to be about 800 km per sec, slower than turned from “weapons into plowshares”, when they were light, but consistent with concepts of particle streams. adapted to ionospheric studies. Friedrich Gauss, the great German mathematician- To preface the story of the modern era it is interesting to physicist-astronomer, as early as 1839, related fluctuations sketch the early ideas of radio propagation science. The per- in the compass needle to the passage of electric currents at ceptions of solar-terrestrial connections had developed high altitudes. In 1882, the Scotsman, Balfour Stewart, slowly over most of the 19th century. Lord Kelvin, one of defined these currents as a great dynamo of tidal move- the most influential physicists of his time, was adamant in ments of ionized air above 100 km that were driven by solar rejecting any notion (Kelvin 1892) “that terrestrial magnetic heating. The vertical movement was only 2 or 3 km, but it was sufficient to generate a great horizontal current sheet of electricity. Observations near the geomagnetic equator indi- † Naval Research Laboratory, Washington, DC, USA. Herbert Friedman died on 9 September 2000. The editors are indebted to Herbert Gursky cated circulating systems of electric currents of opposite for helping them to put the paper and its title into the final form symmetries in the northern and southern hemispheres. At

Herbert Friedman, The Century of Space Science, 277Ð286 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 278 HERBERT FRIEDMAN the equator the currents joined to form a strong flow from a critical value. At smaller angles the waves penetrated the west to east at 1100 local time, that Sidney Chapman, in the reflecting region and escaped into space. At night, skip early 20th century, named the Equatorial Electrojet. distances were greater than during the day and greater in The young Guglielmo Marconi at age 21, in 1895, built a winter than in summer in temperate latitudes. From these demonstration wireless telegraph on his father’s estate near simple observations Hulburt calculated the height of reflec- Bologna, Italy. On December 12, 1901 he transmitted a tion and the electron density (about 500,000 electrons and simple Morse code signal from England to Newfoundland, ions per cm3 at a reflection height of about 150 km). By a distance of 2900 km to the astonishment of most scientists 1926, Hulburt and Taylor were able to publish a remarkably who could not fathom how the waves, that were thought accurate account of the diurnal variation of ionospheric to travel in straight lines, could curve over the 160 km electron density. The work was almost coincident with high bulge of the surface of the earth. In 1902, Arthur E. Appleton’s and Barnett’s 1924Ð25 studies. Kennelly proposed that the radio waves were ducted around In the early years of ionospheric research, theorists the earth by an electrically conducting layer. Almost simul- speculated about particle radiation as a possible source of taneously, Oliver Heaviside reached the same conclusion ionization of the upper atmosphere. Confronted with an and the layer came to be called the Heaviside layer. With apparent 6000 K solar spectrum it was not possible to model the above background, the stage was now set for a more interactions of electromagnetic radiation with atmospheric focused scientific attack on the nature of a reflecting layer, constituents that would lead to the required ionization. now called the ionosphere, that eventually came to be asso- Within the space of a decade after the end of WW II, how- ciated with solar X-rays. In England, ionospheric research ever, the solar spectrum was revealed from its X-ray limit was lead by Edward Appleton and his student Miles Barnett throughout the ultraviolet with instruments carried on rockets [1925]; in the United States studies were conducted by to ionospheric height. Ionizing radiation was observed in E.O. Hulburt and A. Hoyt Taylor at the Naval Research X-rays and ultraviolet from the solar corona and chromos- Laboratory [1926] and by Gregory Breit and Merle A. Tuve phere, where temperatures range from hundreds of thousands at the Carnegie Institution [1926]. Successful experiments to millions of degrees K, and every spectral interval was and interpretations were achieved almost simultaneously in matched with its absorption at a particular height range. England and the United States, both groups working from Throughout this early epoch, some theorists still favored the ground. When German V-2 rockets were captured by solar particles as the source of ionizing radiation. Con- the Americans, they moved ionospheric research into space fronted with the apparent 6000 K temperature of the solar and outraced the ground-based competition. disk (Figure 1) it wasn’t possible to model atmospheric Appleton set out to determine the height of reflection interactions with solar radiation that would lead to the of a continuous wave with the cooperation of the British required ionization. At the higher chromospheric and coro- Broadcasting Co. whom he persuaded to provide him with nal temperatures shorter-wavelength extreme ultraviolet and a continuously varying signal from London at the end of X-rays would be produced but the particle concentrations the broadcast day so that he could detect the interference pattern of ground and sky waves at Oxford. He observed the elapsed time between emission and reception of the same frequency, as the broadcast frequency was oscillated back and forth. Starting with low frequencies, Appleton probed only the lower portion of the reflecting layer; working later with higher frequencies, he distinguished layered regions of reflection, that he labeled D, E, and F. For these experiments, Appleton later received the Nobel Prize. Much of the early research by NRL scientists was characterized by an admirable simplicity and economy of means. Hulburt and Taylor cooped the partnership of radio amateurs around the world who used vacuum tubes with power outputs of less than 50 watts and very short radio waves, less than 200 meters, to communicate around the world. Transmissions skipped over a “zone of silence” encircling the transmitter to a distance of 30 to 50 km and at the same time were received out to distances of hundreds Figure 1 Solar spectral energy distribution from 2000 A to of kilometers. Hulburt and Taylor showed that the waves 7000 A compared with 6000 K black body sunlight above the were reflected only when the angle of incidence exceeded atmosphere. FROM THE IONOSPHERE TO HIGH ENERGY ASTRONOMY – A PERSONAL EXPERIENCE 279 in the solar atmosphere were estimated to be too thin (emission measure too low) to provide high enough intensities to generate an E or F region. Appleton’s early measurements of vertical reflections between midnight and sunrise showed that substantial concentrations of electrons or ions persisted throughout the night, contrary to the prevailing idea that charges would disappear rapidly by attachment once the source of ionization was removed. Radio scientists were thus led to believe that an important portion of the ionization might be produced by corpuscles arriving equally by both day and night. Because a charged particle stream could not readily penetrate the earth’s magnetic field, serious thought was given to neutral particle streams. In 1928, Hulburt proposed that ultraviolet radiation shortward of 1230 Å might be the source of the ionosphere. By 1930 he was intrigued by a possible connection between solar ultraviolet and sunspots and their link to magnetic storms and radio fadeout. In 1935, J.H. Dellinger at the U.S. National Bureau of Standards (NBS), summarized observations of a series of sudden ionospheric disturbances over a period of six months and stressed the importance of understanding the connection with solar activity. He proposed a joint effort of the NBS and the solar observatory on Mt. Wilson. During the same time frame, Robert H. Goddard was developing his rocket to carry instruments to high altitudes for atmospheric research. Correspondence Figure 2 A V-2 rocket just prior to launch at the White between Hulburt and John Fleming reveals that Hulburt Sands Proving Ground in New Mexico. About 45 feet tall contemplated solar rocket astronomy to understand the and 5 feet in diameter it was fueled by 10 tons of alcohol basic physics of solar control of the ionization. He noted and liquid oxygen. The rocket is shown connected by an umbilical cable to the firing line. To service the rocket and the theoretical match between atmospheric absorption of its payload a portable ladder was brought up. Only later solar soft X-rays and the altitude of ionization and sug- was a gantry provided from which each level of the rocket gested that a good test would be to fly photographic film could be reached comfortably. A successful flight could covered with thin aluminum foil or black paper in one of reach 170 km and last for 450 sec. (NRL.) Goddard’s rockets to detect X-rays. Those early glimmerings of high altitude research with rockets were interrupted by the war but the new technolo- about 120 km (Figure 3). It appeared that a thermal corona gies of the war were soon transferred to peaceful research. at one to two million deg C made a good fit with the ioniza- In 1942, Ernst Krause in the radio division at NRL under- tion requirement of the E-region. took to develop a program of guided missiles, specifically The Lyman alpha detector and the extreme ultraviolet a new version of the German V-1 buzz bomb known as the detector showed how those radiations shaped the bottom of JB-2 and the Lark, a rocket-propelled, guided ship-to-air the ionosphere and the upper part of the reflecting E-region. missile. At the end of the war, Krause pursuaded NRL to Lyman alpha, originating in the hot solar chromosphere at commit to a substantial effort in rocket development for 10,000 K and higher, contains most of the energy in the high altitude research which led to the resurrection of V-2 extreme ultraviolet and is absorbed between 75 and 90 km rockets late in the 1940s (see Figure 2). The first generation but does not interact with any of the major constituents, of successful studies of solar X-rays and extreme ultraviolet oxygen or nitrogen, atomic or molecular. Only later on did radiation began in 1949 when my NRL group flew a set M. Nicolet, the brilliant Belgian atmospheric scientist of Geiger counters sensitive to a narrow band of X-rays point out that it could ionize nitric oxide, a trace constituent centered at about 8 A, hydrogen Lyman-alpha (1216 A), present at only 108 molecules per cm3. with almost 100% and the Schumann region, 1425 to 1600 A. As the rocket efficiency and thus have control of D-region. Radiation in climbed to an altitude of 150 km, the detectors pointing the Schumann region produced no ionization but played a normal to the spin axis swept the sky repeatedly. X-rays very important role in shaping the high ionosphere by dis- were detected above 80 km with increasing intensity to sociating molecular oxygen. By the process of dissociative 280 HERBERT FRIEDMAN

combination of a total solar eclipse and an array of rockets launched from the deck of a ship.

SOLAR FLARES

Of all the forms of solar activity, flares are the most spectacular. A solar flare creates a strong impact on the terrestrial environment, producing prompt shortwave radio blackout that may last for two to three hours. The after- effects may persist for one or two days in the form of great auroral displays, and ionospheric and magnetic storms that seriously degrade shortwave radio communications. The new arsenal of rockets that became available late in the 1950s made it possible to plan a program of solar flare studies. Although the supply of V-2 rockets was exhausted by 1952 it was replaced by smaller Aerobees and two staged Figure 3 The first measurement of the penetration of solar rockets that mated the Deacon with a Nike booster or a X-rays into the upper atmosphere made by a V-2 rocket in Skyhook balloon. The latter combinations were particularly 1949. The 8 A X-ray signal was modulated by the spin of attractive for studies that required a form of instant rock- the rocket as the Sun came into view once each roll period. etry. Launch from shipboard at sea offered range safety. By X-rays were first detected at about 90 kilometers and sailing downwind the ship could achieve nearly zero rela- reached peak intensity at about 130 km. (U.S. Naval Research tive wind conditions for inflation and release of the balloon Laboratory.) with its suspended rocket. The well deck aboard the U.S.S. Colonial measured 392 feet by 41 feet which we could recombination, molecular oxygen controls the rate of use to store three trailer-truckloads of helium while the neutralization of F-region electron density much more broad helicopter deck above served admirably for the bal- effectively than atomic oxygen. loon operations. The lumbering ship could make a speed of The 1949 measurement of harder X-rays (1Ð8 Å) led to 15 knots which was slower than the expected drift of the several years of broad band photometry of solar X-rays that balloon at altitude. To assure radio contact with the balloon extended the range of the spectrum, primarily with the aid payload we were assigned a destroyer, the U.S.S. Perkins of simple filters of beryllium, aluminum, titanium, mylar, that could track the balloon with a speed of 28 knots. formvar, etc. serving as the window materials of the photon Finally a crew of 650 sailors was tasked to man the ship for counters. It seems in retrospect that the NRL group was the naval chase. almost alone in that decade of pioneering studies of Each day as inflation began, the polyethylene balloon, the Sun. The X-ray spectral distribution from 1 Å to 44 Å most of it draped in tight folds resembling the stem of an resembled thermal emission from a thin corona at a temper- onion, rose 100 feet above the deck, crowned by a 20 foot ature of a few million degrees. Successive measurements at bulge filled with 5000 cubic feet of helium that would intervals of months to years showed flux variations of as expand further to thirty times that volume at altitude. The much as a factor of 7 for X-rays (8Ð20 Å) over the sunspot 12-feet long Deacon rocket dangled at the end of a 100-foot cycle. Such variability was consistent with ionospheric nylon line (Figure 4). At 80,000 feet, when a flare was electron density variations in the E-region, supporting a observed in visible light, a radio command would fire the direct connection between solar X-rays and E-region. But rocket and send it upward, piercing the balloon and rushing the observed variability over a solar cycle made it clear that ahead another 50 or 60 miles through the ionosphere. the concept of X-ray emission from a spherically symmetri- An NRL proposal for a naval expedition as part of the cal solar corona was very inadequate. Instead it seemed International Geophysical Year (IGY) to launch ten Rockoons that the corona was structured in condensations, formed for solar flare studies with the support of the USS Colonial, over sunspots, that produced enhanced X-ray emission. To an LSD with a large helicopter deck, was approved by resolve the question of spatial origin would require an the Office of Naval Research. Our ship was a sea-going X-ray scan of the solar disk or an X-ray photograph. Both drydock. The operational plan was to release a Rockoon methods were successfully applied at the end of the decade. each morning on ten successive days and allow it to float The X-ray photograph was obtained with primitive pinhole at 80,000 feet until the onset of a solar flare was detected, photography (Figure 6); the scan required a very special when it would be fired. One flare was successfully observed 13

BLAIR D. SAVAGE* Early ultraviolet spectroscopy from space

The period starting around 1950 and extending into the far-UV and UV bands; for a review of the beginnings of twenty-first century will probably ultimately be remembered EUV astronomy see Bowyer (1991). This delay resulted as the golden age of discovery in observational astronomy. mostly from the fear that the interstellar H I opacity in most This is because of the simultaneous occurrence of three tech- directions would be so large that only the stars closest to the nological advances: the ability to observe the entire electro- Sun would be observable. However, the highly irregular dis- magnetic spectrum, by combining observations from the tribution of H I in the local interstellar medium and the exis- ground and from above the absorbing effects of the Earth’s tence of the local hot bubble of ionized hydrogen rendered atmosphere; the creation of efficient and high-precision these fears invalid. EUV astronomy achieved its first photo- detectors of electromagnetic radiation; and the development metric detection of a stellar source during the ApolloÐSoyuz of computers for processing and manipulating large elec- mission (Lampton et al. 1976) and the first low-resolution tronic data sets. The field of ultraviolet (UV) astronomical EUV stellar spectra were obtained several years later spectroscopy at wavelengths shorter than 3100 Å has bene- (Holberg et al. 1980) as part of the Voyager 1 mission. fited from all three of these technological revolutions and In preparing this historical overview of developments in has required access to space for its very existence. the field of UV astronomy from space, I benefited substan- In this chapter I follow the development of the field of tially from reference to various papers summarizing the state UV astronomical spectroscopy from space over the period of UV astronomy over the past 40 years. These include the from approximately 1945 to 1980, with the emphasis on reviews by Friedman (1959), Wilson and Boksenberg spectroscopy of objects located beyond the Solar System. (1969), Wilson (1970), Bless and Code (1972), Code and The discussions concentrate on scientific developments in Savage (1972), Spitzer and Jenkins (1973), Boggess and the wavelength range from ~900 to ~3100 Å. The lower limit Wilson (1987), and Brosch (1999). of ~900 Å represents the wavelength where boundÐfree opacity of neutral hydrogen in the interstellar gas becomes large, and the upper limit of 3100 Å is where observations THE OBSCURING ATMOSPHERE begin to become possible from telescopes situated on the ground. The wavelength regions of 100Ð900 Å, 900Ð1200 Å, The attenuation produced by the Earth’s atmosphere in the and 1200Ð3100 Å are usually referred to as the extreme UV as a function of wavelength in angstroms is shown in ultraviolet (EUV), far-UV, and UV regions of the spectrum, Figure 1. The solid curve gives the altitude in kilometers at respectively. I shall not review the beginnings of EUV spec- which radiation normal to the atmosphere is reduced in troscopy from above the atmosphere since those beginnings intensity by a factor of 1/e. The various molecules and were delayed by a decade compared with activities in the atoms mostly responsible for the atmospheric absorption are indicated on the curve. The atmospheric absorption that *University of Wisconsin, Madison, WI, USA rapidly sets in near 3100 Å is produced by ozone (O3) while

Figure 1 This figure from Friedman (1959) shows the altitude in kilometers at which the fraction of radiation at wavelengths shorter than the visible incident on the Earth’s atmosphere is reduced by a factor of 1/e. Strong atmospheric absorption due to O3 sets in at about 3100 Å. While in the mid-UV it is possible to carry out observations from high-flying balloons, satellite altitudes are required to observe celestial sources for the UV wavelengths below ~2000 Å. (By permission of the Journal of Geophysical Research.)

absorption at shorter wavelengths is mostly from O2,O,N2, atmosphere would have several major advantages over an and N. In the wavelength range ~2000Ð3000 Å it is possible observatory on the Earth’s surface. He noted that at optical to carry out UV observations from balloons carried to wavelengths the orbiting observatory would not suffer the altitudes of 30Ð40 km (see later section on Near-UV blurring effects of the Earth’s atmosphere. Therefore the Spectroscopy from Balloons). However, rocket or satellite orbiting observatory could produce images limited only by altitudes are required to observe at UV wavelengths shorter the quality of the telescope optics and its pointing system. than ~2000 Å. Even at rocket or satellite altitudes exceeding He also pointed out that the orbiting telescope would be 300 km, the trace atomic constituents in the atmosphere can capable of observing celestial objects over the entire electro- interfere with astronomical observations. In particular, the magnetic spectrum, including spectral regions that are not hydrogen geocorona extends many Earth radii from the sur- accessible to the ground-based observer. No professional face of the Earth, and the H I scatters sunlight in the Lyman astronomers took these ideas seriously when they were first series lines producing very strong emission, particularly in discussed, and over the subsequent 20 years the only other the Lyman , , and lines at 1215.67, 1025.72, and published speculations about observatories in space that I 972.54 Å. Similarly, Earth airglow emission can be strong in am aware of appear in the science fiction literature various O I transitions. Even at the altitudes of the major (Richardson 1940). However, everything changed after modern observatories such as the Hubble Space Telescope, World War II, in part as a result of the development and there is enough residual O I in the atmosphere for terrestrial capture of the V2 rocket technology and the start of the O I absorption lines to be apparent in high-resolution spectra Cold War between the United States and the Soviet Union. of hot stellar continuum sources. The first truly serious discussions of the potential scientific importance of placing satellite observatories instrumented for UV spectroscopy above the absorbing atmosphere are found THE SCIENTIFIC IMPORTANCE OF in an internal report entitled “Advantages of an Extra- ACCESS TO SPACE Terrestrial Observatory,” written for the Rand Corporation in 1946 by Lyman Spitzer reprinted in Spitzer (1997). Spitzer’s More than 75 years ago, Oberth (1923) pointed out that report discusses the science that could be pursued with several an astronomical observatory orbiting above the Earth’s types of orbiting observatories of different sizes. In addition EARLY ULTRAVIOLET SPECTROSCOPY FROM SPACE 289 to observatories designed to observe the Sun in the UV, he spectrometer operating from 912 to 2000 Å in order to obtain discussed the scientific potential of a modest 0.25 m (10- information about the physical properties of the gas between inch) reflecting telescope designed to obtain UV spectra of the stars?” In answering the question I was intrigued by the stellar sources, and the potential value of a large reflecting possibility of using a satellite observatory to study aspects of telescope operating at optical wavelengths. the interstellar gas (the hot phase) that were difficult or The scientific programs Spitzer proposed for this instru- impossible to study from the ground. I didn’t realize at the ment included: time that this assignment was one I would continue to work on for most of my professional career. ¥ studies of the composition of planetary atmospheres ¥ measures of the structure of stellar atmospheres, including the possibility of detecting expanding atmospheres in TECHNOLOGICAL CHALLENGES the strong absorption lines of C, N, and O ¥ measures of the color temperatures of hot stars The technological challenges faced by the pioneers of UV ¥ measures of stellar bolometric magnitudes spectroscopic space astronomy were considerable. They ¥ the analysis of eclipsing binary UV light curves to included gaining reliable access to space, developing small obtain information about stellar masses and atmospheric light weight UV spectroscopic optical systems, developing properties rocket and satellite pointing and control systems, and devel- ¥ using the improved understanding of stellar atmospheric oping efficient UV sensitive detectors. The most difficult conditions from the UV studies to improve on stellar dis- challenge was to design and build complex robotic systems tance determinations that would actually operate in the space environment. ¥ measures of the composition of the interstellar gas The progression through these various technological ¥ measures of the properties of interstellar absorbing grains challenges did not always proceed smoothly. As a beginning ¥ measures of the UV spectra of supernovae in order to bet- graduate student in 1964, I witnessed the activities around ter understand the explosion processes. Princeton University involving the pioneering projects in For each of these topics Spitzer discussed how observatories UV spectroscopy through the efforts of Donald Morton and above the atmosphere could be used to make major advances Lyman Spitzer and in optical diffraction limited imagery in the given science area. For example, in the case of measur- through the Stratoscope II balloon-borne telescope program ing the composition of the interstellar gas, Spitzer pointed involving Robert Danielson and Martin Schwarzschild. I out that in interstellar space most atoms and molecules were vividly remember discussions in the halls about rockets expected to be found in their ground (lowest-energy) state. blowing up on the launch pad, pointing and control systems Therefore measures of the composition of the gas through that failed to operate, rocket parachute systems that deployed absorption line spectroscopy would require an observatory too early, and balloon flights where the 3600 kg Stratoscope operating at UV wavelengths since most absorption lines out II telescope system failed to unlatch from the vertical point- of the ground state of abundant atoms fall in the UV. Slightly ing position, or where the sealed door on the pre-cooled more than 25 years after creating this vision for the future of 0.9 m diffraction-limited primary mirror failed to open at UV astronomy in space, Spitzer pioneered UV studies of the altitude. Reflecting on all these problems and wondering interstellar medium by using the high-resolution UV spectro- how long it might take to complete an experimental thesis in graph aboard the Copernicus satellite (see the section on the space astronomy, I made an arrangement with Robert Orbiting Astronomical Observatories). Danielson and Martin Schwarzschild to pursue a theoretical Shortly after the launch of Sputnik in 1957 and the organi- PhD thesis and to briefly join the Stratoscope II team fol- zation of NASA in 1958, several papers appeared discussing lowing my graduation to continue to explore my experimen- the science that could be pursued with UV spectrometers tal interests. The lesson I learned during this period was that operating above the Earth’s atmosphere, including those of the most difficult technical challenge faced by the early pio- Spitzer and Zabriskie (1959) and Code (1960). I find the neers in space astronomy was to achieve a high level of reli- Spitzer and Zabriskie paper, entitled “Interstellar research ability for remotely controlled robotic systems operating in with a spectroscopic satellite,” particularly interesting because the space environment. Even today, it appears that this is a my professional field of interest is the interstellar medium. I difficult goal to meet on a regular basis. remember referring to that paper in 1965 when taking Professor Spitzer’s course on “Physical Processes in the Interstellar Medium.” During the course I recall a particularly SOUNDING ROCKET SPECTROMETERS interesting class assignment. Professor Spitzer asked his students, “What observations would you obtain with a 1 UV space astronomy began as an experimental field at meter satellite observatory equipped with a high-resolution about the same time as Spitzer was considering its future. 290 BLAIR D. SAVAGE

Using captured German V2 rockets which could carry a 1000 kg of equipment to 150 km altitudes, scientists working at the Naval Research Laboratory (NRL) led by Richard Tousey obtained the first UV spectrum of the Sun to wavelengths as short as 2200 Å (Baum et al. 1946). The first UV stellar photometry followed about ten years later when NRL scientists observed 59 hot stars in a 130 Å wide UV band centered on 1115 Å (Bryam et al. 1957). These early UV stellar observations involved scans of the sky with simple photometer systems from unstabilized sounding rockets. The flight of a scanning objective grating spectrometer on an unstabilized Aerobee rocket produced the first low-resolution UV stellar spectrophotometry over the 1600Ð4100 Å region at 50 Å resolution (Stecher and Milligan 1962). In this observation the dispersion direction of the spectrograph was normal to the rocket roll axis. The roll of the rocket produced the spectral scan at the slit with a photomultiplier recording the spectrum. However, to cap- Figure 2 A schematic diagram of the simple UV spectrometer ture enough photons during the short intervals for which the and fine stabilization system flown on an Aerobee rocket by stars were in the field of view, the slit needed to be wide Morton and Spitzer (1966). The fine stabilization in the spec- and the resulting resolution of 50 Å was insufficient to trometer’s dispersion direction allowed the instrument to resolve discrete stellar absorption or emission lines. obtain UV spectra of and Sco from 1260 to 1720 Å at a The first moderate-resolution spectroscopic observations resolution of 1 Å. (By permission of Donald Morton and the of stars required the development of three-axis stabilization Astrophysical Journal.) systems for pointing rocket-borne instruments at stars for long enough periods of time to record the spectra. The of the paper by Morton and Spitzer (1966) reporting the pointing control system developed by the Space General observations of first line spectra of stars in the UV, Corporation was a three-axis stabilized gas reaction system On the first two flights the attitude control system failed that could be used to orient the entire sounding rocket dur- to stabilize the rocket. On the third flight both coarse ing its free fall. This system could point the payload within and fine systems worked properly, but the parachute 3 of a desired direction and was stabilized to a limit cycle failed on re-entry so that the impact damaged the pay- jitter of 15 arc minutes. In their pioneering experiment, load beyond repair and admitted light into the film cas- Morton and Spitzer (1966) employed an additional fine sta- settes. Most of the films were totally fogged, but bilization system as part of their instrument package which underdeveloping one from the calcium fluoride camera improved the pointing in the spectrograph dispersion direc- showed wide spectra of the early B-type stars and tion to approximately 16 arc seconds. Their instrument Sco with a resolution of 1 Å. (Figure 2) consisted of two 1200 lines/mm objective plane gratings followed by f/2 Schmidt cameras with 100-mm I can recall the gloomy mood after this third flight among focal lengths and 10 fields. One Schmidt corrector was the returning Princeton scientists and engineers when they made of calcium fluoride and transmitted to 1250 Å; the discussed their totally blackened images. However, that other corrector was quartz and transmitted to 1700 Å. The mood quickly changed to one of joy when the last piece of resulting UV spectra had a dispersion of about 65 Å/mm film was underdeveloped, and revealed clear spectra of and were recorded on UV sensitive Kodak Pathe SC5 film. and Sco extending from ~1260 to 1720 Å. This small pair of UV spectrometers produced spectra with One year after the success of obtaining the first moderate- a resolution of approximately 1 Å when the fine stabiliza- resolution UV spectra of stars in Scorpius, Morton (1967) tion platform achieved its design pointing stability in the obtained high-quality spectra of six stars in Orion at 3 Å dispersion direction. resolution from 1150 to 1630 Å. Those data provided the The pioneering attempts to obtain UV spectroscopic first evidence for high-speed mass loss from hot stars in the observations of stars did not achieve instant success. form of P Cygni profiles of the Si IV and C IV stellar During these learning stages there were often problems absorption lines (Figure 3). This result must have pleased associated with making the attitude-control pointing sys- Lyman Spitzer since in his 1946 Rand Corporation study he tems work properly. For example, quoting from the abstract remarks that, “In addition the nature of unusual stellar 14

MARTIN HARWIT* The early days of infrared space astronomy

INTRODUCTION D.A. Allen’s even earlier book adds an Australian’s view- point (Allen 1975). The author hopes that contributions Infrared observations can provide a number of unique such as these may eventually be synthesized to produce perspectives on the Universe. This article concerns itself with a dispassionate, comprehensive, truly international, and the men and women who pioneered infrared space astronomy, perhaps more broadly incisive account. traces the different techniques they employed, describes the trials and tribulations they had to overcome, and lists some of the gains they achieved. In the process it also examines the The cold universe social institutions, both academic and military, that were most Much of the Universe is cold. The wavelength, , at which influential in determining the evolution of the field. stars, galaxies, and interstellar or intergalactic gases radiate While many infrared and submillimeter observations is inversely proportional to their temperature, T (Figure 1). required going into space, a large number of others were The peak emission tends to occur at 3700/T micro- carried out from the ground. The earliest successes of meters, where T is measured in kelvin (K). One micrometer infrared astronomy were largely the work of ground-based is a millionth of a meter, traditionally called a micron and astronomers. Going into space required major technological abbreviated to m (or simply in the early literature of breakthroughs that took time to materialize. A rich texture the field). Observations at infrared wavelengths from 1 of sometimes friendly, occasionally aggressive, competition to 1000 m are thus uniquely sensitive to astronomical among ground-based, balloon, airborne, rocket, and satel- sources in the temperature range from ϳ3000 K to 3 K. lite observers emerged, that persists to this day. These include the coolest stars, planets, and interplanetary As an active participant in the field, the author makes no dust, circumstellar and interstellar matter, and, at the claims to objectivity. Nor is he able, in a chapter of twenty longest wavelengths and coldest temperatures, the earliest or thirty thousand words, to give proper credit to all who known radiation emitted by the entire Universe. made significant contributions. The story told here is over- whelmingly based on personal experiences in the USA. Many of the struggles faced in the USA, however, were A dusty universe duplicated in Europe, Japan, and the Soviet Union. An excellent review written more than 20 years ago by Interstellar dust Ð microscopic particles composed of ices, J.E. Beckman and A.F.M. Moorwood (1979) may provide minerals, and common organic and inorganic materials Ð is the reader with a complementary European perspective, and abundant throughout the Universe. These particles obstruct a clear view of the cosmos at optical wavelengths. Fortunately, *Professor Emeritus of Astronomy, Cornell University, Ithaca, NY, USA; a cloud of dust that may be totally opaque in the visible or Former Director, National Air and Space Museum, Washington, DC, USA ultraviolet can be virtually transparent in the infrared. Thus,

The chemical universe The infrared band contains the spectral signatures of a variety of atoms, molecules, ions, and solids, at least some of which are found in any astrophysical environment. Infrared spectroscopy allows us to recognize large varieties of substances, ranging from cool ices in the interstellar medium to highly excited ions in active galactic nuclei. By determining the strengths of different spectral features, we obtain important, often unique insights into the chemical and physical conditions in these systems.

INFRARED ASTRONOMICAL CONSTRAINTS

The human eye ceases to respond to radiation beyond the red part of the spectrum. This limit falls at a wavelength of Figure 1 Blackbody radiation spectrum as a function of tem- ϳ0.72 m. The infrared spectral domain begins approxi- perature. As the temperature T decreases by a factor of 10, the mately at a wavelength of 0.7Ð1 m, and stretches out to wavelength of peak emission increases by the same factor and ϳ1000 m (1 mm). The wavelength range between the total power emitted over the entire wavelength range ϳ200 m and 1 mm is often called the submillimeter domain. 4 decreases by a factor of 10 . While the atmosphere tends to be opaque to much of the infrared wavelengths can probe regions deep in the core of infrared spectral band, observations can be carried out from a dusty galaxy that are inaccessible at shorter wavelengths. high mountain tops in a number of atmospheric windows. Dust grains are heated by short-wavelength, mainly visi- The lowest panel of Figure 2 shows the approximate widths ble and ultraviolet radiation. They absorb this energy, and and depths of the atmospheric windows at an observatory re-radiate it at infrared wavelengths. Because of this effi- like Mauna Kea, Hawaii, sited at an altitude of 4.2 km. cient conversion, the majority of the radiant energy emitted As seen from the upper panels, atmospheric absorption by dense, dusty regions, such as star-forming clouds and decreases and transmission increases considerably with occasionally entire galaxies, lies at infrared wavelengths. increasing altitude, and becomes quite high throughout Since the transparency of dust clouds increases at longer much of the infrared domain at aircraft and balloon alti- ϳ ϳ wavelengths, red light is transmitted more readily than tudes, respectively, at 14 and 28 km. blue. Stars lying behind a tenuous dust cloud thus appear For the astronomer, good atmospheric transmission is reddened. This reddening can be a useful measure of the not enough. Low atmospheric emission is also essential. amount of dust in the cloud. The atmosphere radiates powerfully throughout much of the infrared, and this emission is often orders of magnitude greater than the radiation from astronomical sources. The The early universe intensity of atmospheric emission also tends to fluctuate In the expanding Universe, the more distant an object the with time. In the near-infrared, at 14 m, emission greater the velocity with which it recedes from us. This cos- by OH radicals is highly variable. In the far-infrared, at mic expansion shifts starlight from distant galaxies into the around 100 m, variable atmospheric water vapor content infrared region; the more distant the source, the farther is its produces varying emissivity. Such fluctuations cause the radiation shifted into the infrared. The expansion is character- flux, or signal, from an astronomical source to be marred ized by a redshift parameter, z, where 1z gives the ratio of by superposed noise due to randomly varying atmospheric “wavelength reaching the observer” divided by “wavelength emission. actually emitted in the distant source”:1 z received/ emitted. Poor atmospheric transmission, and strong atmospheric The most distant quasars and galaxies observed to date emission are the prime reasons for undertaking infrared have redshifts z Ͼ5, so that radiation from the center of the astronomical observations from space. However, in the visual band is shifted to beyond 3 m. Because 1z also early 1960s when serious efforts to conduct such observa- represents the factor by which the Universe has expanded tions were beginning, many of these factors were poorly since the radiation was emitted, objects at z 5 are seen as understood. This led to parallel developments in infrared they were at an epoch when the Universe was only one- astronomy. Most astronomers worked with ground-based sixth of its present size. equipment taken to high mountain tops, while a few THE EARLY DAYS OF INFRARED SPACE ASTRONOMY 303

Figure 2 Atmospheric absorption in the infrared at altitudes of balloons, aircraft, and a mountain-top site like Mauna Kea (after Traub and Stier 1976). attempted to build the instrumentation required to carry out date back to those of Sir William Herschel (1738Ð1822), observations from above the atmosphere. who noted the thermal emission of the Sun beyond the visi- Since rocket payloads were relatively expensive, US ble spectral range, rapid progress did not occur until the ground-based observers feared that the National Aeronautics 1960s. Two factors limited serious advances: a lack of suffi- and Space Administration (NASA) or National Science ciently sensitive detectors and a lack of motivation. Foundation (NSF) support for rocket work would drain In 1838 William Herschel’s son John determined the total away funding from their own efforts. Uncertainties about power emitted by the Sun. His results, obtained by measur- the precise limitations of ground-based techniques often led ing the rise in temperature of water in a blackened vessel to acrimonious disputes between ground-based observers on which was focused sunlight, were remarkably accurate and those who wanted to launch sensitive infrared astro- given the simplicity of his apparatus. He found that the Sun nomical telescopes above the atmosphere on rockets. Time radiated with enormous power. This posed a problem. Julius and again, the more traditional astronomers bluntly recom- Robert Mayer’s principle of conservation of energy, enunci- mended that rocket-borne efforts be slashed or pared back ated in 1845, led to the realization that the source of this to an absolute minimum. Pioneering infrared rocket astron- energy had to be huge or the Sun very young, otherwise the omy was not a happy venture! Sun’s energy would long ago have been depleted. Though To understand the backdrop against which the first suc- many sought to resolve this puzzle, a satisfactory explana- cessful observations were obtained, one needs to understand tion did not emerge until the twentieth century and the not only the technical difficulties that had to be overcome, development of theories of relativity and nuclear physics. but also the considerable early successes of ground-based Thirty years after John Herschel’s monumental finding, observers who also had to surmount significant instrumen- William Huggins used a thermopile connected to a gal- tal limitations but were able to make headway more quickly vanometer in the far more difficult attempt to measure the in more modest efforts. heat emitted by stars (Huggins 1869). (A thermocouple is a device consisting of strips of dissimilar metals that generate a voltage when one end of the device is heated.) These mea- EARLY GROUND-BASED EFFORTS surements were not particularly convincing, but Huggins’s contemporary E.J. Stone developed the technique further While the earliest attempts to conduct observations of (Stone 1870). He used two thermopiles back to back, alter- astronomical sources beyond the red part of the spectrum nating the positioning of a star first on one thermopile and 304 MARTIN HARWIT then on the other, in a mode that we would today call If an astronomer could secure a particularly sensitive and “pushÐpull chopping” (Figure 3). He also calibrated his stable detector, it was possible to undertake novel infrared system against a radiating cube kept at the temperature of measurements. One observer who took good advantage of boiling water. With these steps he obtained more credible these detectors was Harold L. Johnson who, with various results. co-workers, pioneered the UltravioletÐBlueÐVisualÐRedÐ Struggles to perfect increasingly sensitive instrumenta- Infrared (UBVRI) photometric system still in wide use. The tion persisted. In the early decades of the twentieth cen- I band lies at 0.9 m. Johnson also made observations in tury, pioneering work on better radiometers enabled the K band at 2.2 m and at longer wavelengths. He set out W.W. Coblentz at the US National Bureau of Standards to to obtain accurate photometric data on several thousand report radiometric measurements of 110 stars, and a series bright stars and soon realized the importance of reddening of collaborations between Edison Pettit and S.B. Nicholson and extinction by interstellar dust (Johnson 1965). The gen- provided greater insights into planetary and lunar phenom- eral attitude of his time, however, remained that infrared ena (Coblentz 1914, Pettit and Nicholson 1922). astronomy would continue to be merely an adjunct to optical The history of infrared detector development, however, work. Stars, after all, were known to be visible objects; and was largely guided not by astronomers, but by military the Universe appeared to be largely an aggregate of stars. needs, such as “night vision” enabling warm objects to be Given the still-formidable technical difficulties, most discerned in the dark. World War II brought about the astronomers found little motivation for embarking on a development of lead sulfide (PbS) cells that were far more career in infrared astronomy. sensitive than thermopiles. In the post-war era, Peter Felgett This attitude changed with the work of Gerry Neugebauer in Britain was one of the first to use these devices to and Robert W. Leighton at Caltech, who decided to conduct measure the fluxes from bright stars (Felgett 1951). Later, an unbiased survey of the sky at 2.2 m, where the atmos- the Cold War further accelerated military development of phere is transparent and PbS is sensitive. Since the amount infrared sensors. One particularly successful detector, of observing time that would be required was great, and indium antimonide (InSb), came into use at wavelengths no telescope was available for the purpose, Leighton and out to 5 m. At longer wavelengths, the military was devel- Neugebauer constructed their own. Much of their inex- oping copper-doped germanium (Ge:Cu) and gallium- pensive instrumentation was war-surplus, but the 1.57 m doped germanium (Ge:Ga) photoconductors. All these (62-inch), f/1 parabola they conceived was made from alu- devices worked at their best only over a limited bandwidth, minum hollowed out on a lathe. The surface obtained in this 1 ϳ Ð roughly / 3. To obtain continuous coverage over a fashion was not sufficiently accurate, so they poured a layer broader bandwidth a succession of different detector mate- of epoxy into the paraboloid, spun it up around its vertical rials had to be employed, each cryogenically cooled to its axis and kept it spinning until the epoxy had set. Properly own optimum operating temperature. coated, this gave a reflecting surface good enough to produce images of the order of 2 arc minute resolution. This was satisfactory, since their eight-element PbS array pair- wise scanning the sky in a pushÐpull chopping mode would have to cover 30,000 square degrees. This meant that the beam employed would, of necessity, have to be sizeable. To assure themselves of the performance of their apparatus, Leighton and Neugebauer cross-calibrated against stars already known from Johnson’s work. But they also hoped to go further. They were no longer restricted to pointing at visible stars and might find sources that had no optical counterparts. The final results of their survey were not to be published until the late 1960s. But while the work was in progress the two researchers and one of their students discovered a number of truly remarkable sources. A letter to the Astrophysi- cal Journal (Neugebauer et al. 1965) broke the news. It was Figure 3 “Push–pull chopping” of a source by switching the electrifying! field of view of the telescope between an astronomical source and its surroundings on two opposite sides. By subtracting the The motivation for such a survey is to obtain an unbiased emission from the surroundings, a first attempt is made to census of objects that emit in the 2.0Ð2.5 m atmos- correct for atmospheric emission above the telescope. pheric window [which] might reveal many potentially 15

KENNETH NORDTVEDT* Verification of general relativity: tests in the Solar System

In 1900, Isaac Newton’s worldview of gravity, space, and decades. But when the post-World War II years brought time still prevailed Ð that the gravitational force was a uni- forth a stream of new technical abilities to launch space- versal, direct, and instantaneous action-at-a-distance between craft and to send ranging signals out into the Solar System, the masses of the Universe, that bodies and light rays and other supporting technologies, experiments were car- moved through an “absolute space, in its own nature, with- ried out with ever-increasing precision to confirm and out anything external” whose geometric structure was quantitatively measure the full variety of novel phenomena rigidly Euclidean without end, and that the dynamics of in the Solar System predicted by GR. physical law unfolded with respect to an “absolute, true, Although the Newtonian model of Solar System dynam- and mathematical time (flowing) equably without relation ics had been quite successful when used to discover the to anything external” (Newton 1687). Through the twentieth planet Neptune from what seemed to be unexplained per- century that edifice was overthrown and replaced by Albert turbations in the observed motions of the planet Uranus, Einstein’s general relativity (GR) perspective Ð that gravity and when it explained the Moon’s numerous orbital “irreg- is an interaction transmitted by a causal and dynamic field ularities” which result from the competition of the Sun’s whose sources are all forms of energy, including its own gravitational influence with that of Earth on that satellite, contributions, and which then acts elsewhere upon the same; the Newtonian system still faced problems, such as a robust and that the metrical relations between the clocks, rulers, anomaly found in the observed precession rate of the major and signals throughout the cosmos are dynamic, non- axis of Mercury’s orbit, and the failure of Albert Michelson Euclidean, locationally dependent, and established by the and Edward Morley to detect any change in the speed of fields of gravity. The detailed structure of metric gravita- light passing through their interferometer as the Earth tional field components in the Solar System has in all cases changed its velocity through the cosmos. Newtonian gravity been found to match the predictions of GR in a variety of and cosmology also faced theoretical challenges. This was experiments which primarily employed radar and laser rang- brought into focus in the late nineteenth century by Ernst ing between Earth and other planets or spacecraft. Mach, physicist and positivist critic of several concepts in The first half of the twentieth century was occupied physical law, who labeled the notion of absolute time “an mainly by the construction and calculational exploration of idle metaphysical conception” (Mach 1893), and stressed Einstein’s theory which he built upon the foundations of that what is observed in nature and experiment is the behav- James Clerk Maxwell’s electromagnetic field theory, his ior of clocks, not the unfolding of “time” as such. He also own special relativity theory, and then his Equivalence asserted that it was the relative feature of local motion with Principle. Further exploration and application of the theory respect to the distant “fixed stars” of the Universe that was to a variety of gedanken experiments, temporarily beyond empirically meaningful, not the notion of absolute motion the reach of experimental test, continued in subsequent relative to “space” as such. Einstein acknowledged the influence of Mach’s ideas in his formative years. Describing * Montana State University, Bozeman, MT, USA some consequences of his new theory Ð that the inertia of a

Kenneth Nordtvedt, The Century of Space Science, 335Ð352 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 336 KENNETH NORDTVEDT mass was increased by the proximity of other matter, and that gravitational field; and that clocks should tick slower the accelerated or rotating matter induced corresponding acceler- deeper they are in a gravitational potential, by a given frac- ations or rotations of nearby inertial frames, Einstein (1922) tional rate which is independent of their internal structure. pointed out that, “We must see in (these examples) a strong Measuring these predictions of the Equivalence Principle, support for Mach’s ideas as to the relativity of all inertial and checking the principle’s foundational phenomena Ð the actions.” A variety of such Machian effects indeed inspired universality of gravitational free-fall rates Ð to the highest some of the experimental tests of GR and alternative theories achievable precision continue to be at the core of the exper- which were finally carried out in the late twentieth century. imental program to test GR in the Solar System. In line The result of these tests suggested by the Machian perspec- with the field paradigm of modern physics, if the founda- tive is a more comprehensive empirical basis for theory. tion or predictions of the Equivalence Principle were found Einstein still had Mach in mind when he derived from his to be violated, this would most likely signal the existence of theory the possibility of a topologically closed Universe a previously unseen interaction field in physical law which without spatial boundaries: “If we think these (Mach’s) ideas is not “transformed away” in freely falling laboratories. For consistently through to the end we must expect the whole the purposes of discovering any such inverse-square (or inertia, that is, the whole (gravitational) g-field, to be deter- very long Yukawa range) interaction field whose strength of mined by the matter of the Universe, and not mainly by the coupling to matter is very weak compared with the gravita- boundary conditions at infinity.” tional coupling strength, the contemporary space experi- In taking on the challenge of incorporating gravity into ments designed to test the Equivalence Principle are his special-relativity structure of physical law, Einstein unsurpassed instruments. worked to replace the instantaneous, action-at-a-distance The universality of free fall also gave Einstein an impor- force of Newtonian gravity with an interaction between sep- tant clue to the type of field which could be the basis of arated matter which was carried by a dynamical field. The gravity and to the attribute of matter to which it must cou- successful electromagnetic field theory of Maxwell was his ple. From special relativity he learned that a body’s inertia guide. This view of the field-mediated interaction has equals its total energy content: E mc2. If the gravitational triumphed throughout physical law in the twentieth century. force on bodies were to be in universal proportion to the In this paradigm, two bodies interact by a staged, causal bodies’ inertial masses, then a gravitational field’s coupling process; one body is source of a dynamical field, the field strength to bodies must also be proportional to the bodies’ then spreads out in space and time from its origins in accor- energy contents. This led to consideration of a single scalar dance with its own dynamical laws, and finally another body field (tensor of rank 0) or a second-rank tensor field g of located elsewhere is acted upon by the resulting field found 10 potentials, being symmetric in its indices and which at its location. range over the four dimensions of space and time, as trans- In the search for a gravitational field, Einstein was pro- mitters of the gravitational interaction; either field could foundly influenced by the empirical fact that gravity accel- couple rather naturally to the energy content of bodies. (By erates bodies at a rate which does not depend on their contrast, the field that transmits the electromagnetic interac- chemical composition or other internal properties Ð the rate tion between charges is a first-rank tensor field A of four is apparently universal. First noted and studied by Galileo potentials, and the other interactions of physics, nuclear and and others four hundred years ago, tested by Newton to a weak, have also been found to be based on multiplets of one-part-in-a-thousand precision by comparing motions of first-rank tensor fields.) Through the decade 1905Ð15, scalar differently composed pendulums, and tested in Einstein’s and tensor gravity were both explored. For a variety of rea- time by Roland von Eötvös to precisions of a few parts in a sons Ð empirical implications and predictions, theoretical billion with torsion balances supporting different sub- uniqueness, consistency, and completeness Ð the general stances in Earth’s gravity, this strange and apparently exact theory of relativity emerged based on a pure second-rank proportionality between the inertia of bodies and the tensor gravitational field g(r, t) (Einstein 1916). strength of gravitational force they experience led Einstein, Special relativity, by itself, suggests that the motion of still short of a theory, to make a grand hypothesis. Para- bodies interacting gravitationally should deviate in detail phrasing him: since a local laboratory falling freely in grav- from Newtonian form. Since a body’s momentum was now ity “transforms gravity away” as far as the physics in that known to contain the speed-dependent modifications of laboratory is concerned, the freely falling laboratory must special relativity, application of the law of motion in the in all phenomenological respects be locally equivalent to an presence of a force inertial frame (even though it accelerates relative to the distant inertial frame). From this Equivalence Principle, p Einstein was able to predict two novel phenomena: that d d m v f dt dt 2 2 light rays should deflect downward when passing through a 1 v /c VERIFICATION OF GENERAL RELATIVITY: TESTS IN THE SOLAR SYSTEM 337 yields corrections of order v2/c2, even before considering also be found to occur in a local gravitational field of the force side of the equation or further modifications to the equivalent acceleration. (Einstein 1907) momentum of the mass. But just like electromagnetism, As one example of Einstein’s ingenious reasoning which GR’s 10 gravitational potentials produce a total force followed from this principle, consider two identically con- between bodies which includes corrections from the static structed clocks freely falling in gravity and separated by a situation (with details depending on the motion of one or small vertical distance. Light signals triggered by each tick both of the interacting objects). A further novel feature of of the higher clock are sent down to the lower clock which, GR’s interaction is its nonlinearity: the gravitational force because of its gravitational acceleration and the finite veloc- due to a sum of sources is not simply the sum of the indi- ity of light, is always receiving the signals at a Doppler- vidual forces; additional forces come into play which are shifted rate which is lower than the transmitted rate. But by proportional to the product (and higher powers) of the the Equivalence Principle, the observed phenomena should source masses. All of these modifications, fractionally char- 2 2 2 be identical to what occurs when two separated clocks at rest acterized by the factor v /c or Gm/c r, are very small in inertial space exchange signals Ð the frequency of signal throughout the Solar System, each amounting to about transmission recorded by the transmitting clock equals the 8 2.5 10 in the case of Mercury’s orbit, for example. frequency of signal as recorded by the receiving clock. This General relativity is compatible with the possible exis- will occur only for the gravitationally free-falling clocks if tence of additional, very weakly coupled long-range fields the lower clock ticks slower than the upper clock by an which have so far escaped detection. The presence of such amount needed to compensate for the Doppler shift. fields and their interactions with matter will produce phe- Unlike Newton, who had no underlying theory for the nomena which either violate the Equivalence Principle and origins and magnitudes of mass (beyond its simple additiv- metric foundations of GR, or which diverge from the pre- ity), and who therefore on empirical grounds could simply dictions of pure tensor gravity at the post-Newtonian level. adopt the equality of inertial and gravitational mass for all Testing GR can therefore also be viewed as the search for objects, Einstein had found from his special theory of rela- any “new” long-range interaction fields in physical law. tivity that a body’s inertial mass was equal to its total This chapter reflects this interpretation of the twentieth energy content, E mc2, and he was therefore led to seek a century’s achievements. theory of gravity in which the gravitational field’s coupling strength to a body was also naturally and generally proportional to the body’s total energy content. Achieving this THE EARLY FOUNDATIONAL YEARS took him over a decade. But more immediately, using only his Equivalence Principle, he predicted two novel phenom- The Equivalence Principle ena. Using the argument outlined in the previous paragraph, Soon after formulating his theory of special relativity in he predicted that the rate of any laboratory clock located in 1905, Einstein turned to the task of properly incorporating a gravitational potential U(r) will differ from the rate of an gravity, the other known force of that time, into physical otherwise identical clock located elsewhere at gravitational law. His goal was to replace Newton’s instantaneous action- potential U(r): at-a-distance with a causal field theory of gravity, Ј Ј analagous to the electromagnetic theory of interaction f f m U(r) U(r ) i , U(r) G (1) between charged particles transmitted by the Maxwell 2 f c i r ri fields. He was also profoundly influenced by the apparent fact that the gravitational forces on bodies were in universal where c is the speed of light, G is Newton’s gravitational proportion to the bodies’ inertial masses. Because of the constant, and mi are the masses responsible for the gravita- resulting identity of free-fall rates at a given location, a tional potential. Using similar forms of argument, he also localised laboratory freely falling in gravity would appear concluded that a light ray’s propagation direction cˆ will be as an inertial frame, though accelerating relative to other deflected by the transverse part of any gravitational acceler- distant inertial frames. He elevated this well-confirmed fea- ation field g through which it propagates: ture of gravity to a grand hypothesis Ð his Equivalence . ˆˆ Principle: dcˆ g g cc , g(r) U(r) (2) ds c2 All local phenomena seen in a laboratory freely falling in gravity are equivalent to phenomena in a gravity-free These effects are actually closely related. If the locally inertial frame, or conversely, phenomena present in a measured speed of light is to be a universal constant, but gravity-free but accelerated frame of reference and clock rates are universally diminished in gravitational understood from the basic kinematics of that frame must potentials, then the globally viewed speed of light must also 338 KENNETH NORDTVEDT

diminish in those gravitational potentials: from mathematician and friend Marcel Grossmann, Einstein ultimately built his theory of gravity upon a dynamical sec- 2 c(r) c(1 U(r)/c ) ond-rank tensor field of 10 potentials Ð g(r, t) Ð symmetric in its indices and , which each range over the four with c being the speed of light where the gravitational spacetime coordinates. Finding physically acceptable and potential of local bodies is negligible. As in any medium consistent field equations for these gravitational potentials, with an inhomogeneous speed of wave propagation, this including their coupling to matter, consumed several speed of light function then results in the downward deflec- years of labor. In the finished form of the theory, the gravi- tion of light wavefronts at the rate indicated by eqn (2). tational potentials fulfill a set of second-order, nonlinear Einstein soon realized that his prediction of light deflec- partial differential field equations for which the entire tion could be experimentally tested. If the path of light stressÐenergyÐmomentum tensor of laboratory matter T is between a distant star and an observing telescope on Earth the source of gravity: passes the Sun at distance of closest approach D, the integrated deflection angle will be approximately (Einstein 1911) 8 G G T c4 2GM 0.84 arcsec for a grazing ray c2D where G is a tensor constructed from the ten gravitational potentials, the square of their first partial derivatives, and The angular locations of such star images would therefore their second partial derivatives with respect to the space and move away from the Sun’s location and closer to the images time coordinates. This tensor is found to be unique by the from the less distorted regions of the starfield whose light necessity to be mathematically consistent with the property did not pass so close to the Sun. Erwin Freundlich led an that its source tensor T fulfills the traditional conservation expedition to southern Russia in the late summer of 1914 to laws of energy, momentum, and angular momentum under measure the light deflection by photographing the starfield the appropriate local conditions (Einstein 1916). The theory during an eclipse of the Sun. But World War I was breaking also specified how matter and other fields respond to out, and the expedition personnel were arrested and detained the gravitational fields; for example, an atom or object of by the Russian authorities. Such an experiment was not to negligible size will move between two spacetime locations be carried out until the occurrence of the next post-war on the trajectory r(t), which gives an “extremal” value to eclipse in 1919, a delay which permitted completion of the action integral Einstein’s theory and a change in the prediction. As early as 1907, Einstein was aware of the outstanding anomaly in Newtonian Solar System dynamics, and he S g (r, t) d x d x (3) sought to account for the discrepancy in his new theory. The accumulated astronomical observations of the previous cen- performed along that trajectory; here dx c dt,dr for tury had shown that the secular precession rate of the planet 0, 1, 2, 3. The specific mass of the atom is absent from this Mercury’s orbital major axis Ð the advance of its perihelion Ð action integral because of the identity of gravitational and amounted to 574 arcsec/century. This was a quality observa- inertial mass; a geometrical interpretation of these trajecto- tion aided by the relatively large eccentricity of Mercury’s ries as the extremal paths in a curved Riemannian space- orbit (e 0.2). But only 531 arcsec/century of this precession time geometry established by the field potentials g can could be accounted for by the perturbing gravitational accel- then straightforwardly follow. erations from the other planets in the Solar System, and no In late 1915, just weeks before arriving at the final form other explanations within Newtonian gravity emerged which of his theory, Einstein succeeded in using a slightly incom- remained plausible. It seems likely that the failure during plete version of his gravitational field equations to calculate the decade 1905Ð15 of several preliminary versions of a the static, spherically symmetric tensor gravitational field gravitational field theory to naturally account for this excess in the empty space surrounding the Sun; he then used the 43 arcsec/century precession led Einstein to the further con- equation of motion which results from the action integral siderations from which general relativity was formulated. given by eqn (3) to obtain the relativistic motion of the planet Mercury in that gravity field. Having no free theoretical parameters to adjust, his new theory nevertheless explained the anomalous 43 arcsec/century perihelion General relativity and Mercury’s anomalous advance with good accuracy. Owing to the theory’s intrinsic perihelion advance nonlinearity, the Sun’s gravitational field included an Guided by the physical insights gained from his Equivalence important correction to the Newtonian inverse-square field: Principle and special relativity theory, and with formal help it varied as the inverse third power of distance and was 16

CLIFFORD M. WILL* Verification of general relativity: strong fields and gravitational waves

At the time of the birth of general relativity (GR), experi- them. The period began with an experiment to confirm the mental confirmation was almost a side issue. Einstein did gravitational frequency shift of light (1960) and ended with calculate observable effects of general relativity, such as the the reported decrease in the orbital period of the binary pul- deflection of light, which were tested, but compared to the sar at a rate consistent with the general relativity prediction inner consistency and elegance of the theory, he regarded of gravity-wave energy loss (1979). The results all sup- such empirical questions as almost peripheral. But today, ported GR, and most alternative theories of gravity fell by experimental gravitation is a major component of the field, the wayside (for a popular review, see Will 1993a). characterized by continuing efforts to test the theory’s pre- Since 1980, the field has moved toward what might be dictions, to search for gravitational imprints of high-energy termed a Quest for Strong Gravity. The principal figure of particle interactions, and to detect gravitational waves from merit that distinguishes strong from weak gravity is the astronomical sources. quantity ϳ GM/Rc2, where G is the Newtonian gravita- The modern history of experimental relativity can be tional constant, M is the characteristic mass scale of the divided roughly into four periods: Genesis, Hibernation, a phenomenon, R is the characteristic distance scale, and c is Golden Era, and the Quest for Strong Gravity. The Genesis the speed of light. Near the event horizon of a non-rotating (1887Ð1919) comprises the period of the two great experi- black hole, or for the expanding Universe, ϳ 0.5; for neu- ments that were the foundation of relativistic physics Ð the tron stars, ϳ 0.2. These are the regimes of strong gravity. MichelsonÐMorley experiment and the Eötvös experiment Ð For the solar system, 105; this is the regime of weak and the two immediate confirmations of GR Ð the deflection gravity. Figure 1 displays these regimes and various phe- of light and the perihelion advance of Mercury. Following nomena of interest. Notice that the strong-gravity regime this was a period of Hibernation (1920Ð1960) during which (near the diagonal line corresponding to 0.5) encom- relatively few experiments were performed to test GR, and at passes Planck-scale physics where quantum gravity and the same time the field itself became sterile and stagnant, grand unification of the interactions are important, all the relegated to the backwaters of physics and astronomy. way to the observable universe. But beginning around 1960, astronomical discoveries Until the discovery of the binary pulsar, the empirical (quasars, pulsars, cosmic background radiation) and new foundation of general relativity rested on tests in the weak- experiments pushed GR to the forefront. Experimental field regime. In Chapter 15, Kenneth Nordtvedt Jr. surveys gravitation experienced a Golden Era (1960Ð1980) during experimental tests of general relativity primarily from the which a systematic, worldwide effort took place to under- point of view of Solar System and laboratory experiments. stand the observable predictions of GR, to compare and While most of these focus on weak-gravity effects, some contrast them with the predictions of alternative theories can be viewed as strong-gravity tests, primarily experi- of gravity, and to perform new experiments to test ments to search for the relics of Planck-scale physics in fundamental interactions, as might show up in tests of the * Washington University, St. Louis, MO, USA equivalence principle.

Today, much of the focus has shifted to experiments These tests mainly involve black holes, neutron stars, and which can probe the effects of strong gravitational fields cosmology, and make essential use of gravitational radia- (see Figure 1). At one extreme are the strong gravitational tion as a carrier of information about strong gravity. Even fields associated with Planck-scale physics. Will unification though the gravitational waves that bathe the Earth are of the forces, or quantization of gravity at this scale leave extraordinarily weak (and are themselves describable by observable effects accessible by experiment, even those the weak-field limit of general relativity), they often carry that themselves are performed under weak-field conditions? the imprints of strong-gravity phenomena occurring near Dramatically improved tests of the equivalence principle or black holes, neutron stars, or from the early Universe. of the “inverse square law” are being designed, to search We begin (Section 1) by defining the distinction between for or to bound the imprinted effects of Planck-scale phe- strong and weak gravity, and providing (Section 2) an intro- nomena (see Chapter 15). At the other extreme are the duction to the general relativistic description of systems strong fields associated with compact objects such as black involving compact relativistic objects and gravitational radi- holes or neutron stars or with the universe as a whole. ation. In Section 3 we focus on tests of gravitational theory Astrophysical observations and gravitational-wave detectors using stellar binary systems of compact objects, the most are being planned to explore and test GR in the strong-field, famous of these being the binary pulsar PSR 191316. highly dynamical regime associated with the formation and Section 4 focuses on the likely direct detection of gravita- dynamics of these objects. tional radiation by Earth-based antennae in the first decade In this chapter, we shall focus on tests of general relativ- of the twenty-first century, and the possible tests of gravita- ity involving strong gravity and gravitational radiation. tional theory that could emerge. In Section 5 we briefly discuss other tests of strong gravity. We use the standard “geometrized” units of general relativity textbooks (Misner et al. 1973), in which Newton’s PRESENT 1050 gravitational constant G and the speed of light c are unity. In UNIVERSE these units, mass and energy have units of length, with the MILKY WAY basic scale of length set by the solar mass: ML 1.476 km, 1040 UNIVERSE AT and velocities are dimensionless, representing fractions of HELIUM FUSION the speed of light. In the few places where indices are used, SUN Greek indices will run over the four spacetime dimensions, 1030 GPS ORBIT {0, 1, 2, 3}, with the 0 denoting time; while Roman indices NEUTRON will run only over spatial indices {1, 2, 3}. 20 STAR Rather than provide complete references to work done 10 INSIDE BLACK HOLES in this field, we will refer the reader where possible to the PRIMORDIAL BLACK appropriate review articles and monographs, specifically 10 HOLE EVAPORATING 10 TODAY to Theory and Experiment in Gravitational Physics (Will HUMAN 1993b), hereafter referred to as TEGP. References to TEGP

MASS IN GRAMS UNIVERSE AT MEASUREMENT OF 1 END OF INFLATION will be by chapter or section, e.g. “TEGP 8.9”. NEWTON’S G THE QUANTUM GRAVITY SCALE –10 1 STRONG-FIELD SYSTEMS IN GENERAL 10 STRAND OF DNA RELATIVITY PROBED BY BEST –20 ACCELERATORS 10 1.1 Defining weak and strong gravity HYDROGEN ATOM In the Solar System, gravity is weak, in the sense that the 10–30 10 20 Newtonian gravitational potential 10–30 10–20 10–10 1 10 10 DISTANCE IN METERS ϵ Ј Ј1 3 Ј U(x, t) (x , t) x x d x (1) Figure 1 Strong v. weak gravity. Representative phenomena in is much smaller than unity (recall G c 1) every- gravitational physics are indicated by filled circles; other illus- 2 trative phenomena where gravitation plays little role are shown where. So, too, are the quantities v and p/ , where v is a by filled squares and italics. Phenomena above the diagonal typical velocity of bodies in the Solar System, and where line are unobservable, because they take place inside black p and are typical pressure and density, respectively, inside holes. Phenomena close to the diagonal line are in the strong- Solar System bodies. In fact U ϳ v2 ϳ p/ ϳ . Throughout gravity regime. the Solar System, the metric of spacetime deviates only VERIFICATION OF GENERAL RELATIVITY: STRONG FIELDS AND GRAVITATIONAL WAVES 355

slightly from its flat-spacetime Minkowski form prior to a merger and collapse to a final stationary state. diag(1,1,1,1). By expanding the metric about this form Binary inspiral is one of the leading candidate sources for in powers of the small parameter , one can obtain the detection by a worldwide network of laser interferometric Newtonian limit of general relativity at lowest order, and gravitational-wave observatories nearing completion. A the first “post-Newtonian”, or “1PN”, corrections to proper description of such systems requires not only Newtonian gravity. The metric in such an expansion takes equations for the motion of the binary carried to extraor- the schematic form dinarily high PN orders (at least 3.5PN), but also requires equations for the far-zone gravitational waveform mea- 2 g00 1 2U(x, t) O( ) sured at the detector, that are equally accurate to high PN orders beyond the leading “quadrupole” approximation. 3/2 g0i O( ) (2) Of course, some systems cannot be properly described gij ij O( ) by any post-Newtonian approximation because their behavior is fundamentally controlled by strong gravity. These where the first terms beyond the Minkowski metric and the include the imploding cores of supernovae, the final merger 2 3/2 Newtonian potential at O( ) in g00, O( ) in g0i, and O( ) of two compact objects, the quasinormal-mode vibrations in gij, respectively, are the post-Newtonian terms. It turns of neutron stars and black holes, the structure of rapidly out that, in a wide range of alternative theories of gravity, rotating neutron stars, and so on. Phenomena such as these these post-Newtonian terms vary from one theory to the must be analyzed using different techniques. Chief among next only in the values of certain numerical coefficients. these is the full solution of Einstein’s equations via numeri- By inserting arbitrary dimensionless parameters in place cal methods. This field of “numerical relativity” is a rapidly of the numerical coefficients, one obtains a parametrized growing and evolving branch of gravitational physics, framework that encompasses many theories at once, and is a whose description is beyond the scope of this article (see, powerful tool for analyzing Solar System experiments. This e.g., the articles in Marck and Lasota 1997). “parametrized post-Newtonian” framework is described in more detail in Chapter 15; see also TEGP 4. In strong-field systems, this simple 1PN approximation 1.2 Compact bodies and the strong equivalence is no longer appropriate, for several reasons: principle ¥ The system may contain strongly relativistic objects, such When dealing with the motion and gravitational-wave as neutron stars or black holes, near and inside which ϳ 1, generation by orbiting bodies, one finds a remarkable sim- and the post-Newtonian approximation breaks down. plification within general relativity. As long as the bodies Nevertheless, under some circumstances, the orbital motion are sufficiently well separated that one can ignore tidal may be such that the interbody potential and orbital veloci- interactions and other effects that depend upon the finite ties still satisfy U ϳ v2 1 so that a kind of post- extent of the bodies (such as their quadrupole and higher Newtonian approximation for the orbital motion might multipole moments), then all aspects of their orbital behav- work; however, the strong-field internal gravity of the bod- ior and gravitational-wave generation can be characterized ies could (especially in alternative theories of gravity) leave by just two parameters: mass and angular momentum. imprints on the orbital motion. Whether their internal structure is highly relativistic, as in ¥ The evolution of the system may be affected by the emis- black holes or neutron stars, or non-relativistic as in the sion of gravitational radiation. The 1PN approximation Earth and Sun, only the mass and angular momentum are does not contain the effects of gravitational radiation needed. Furthermore, both quantities are measurable in back-reaction. In the expression for the metric given in principle by examining the external gravitational field of the eqns (2), radiation back-reaction effects do not occur until bodies, and make no reference whatsoever to their interiors. 7/2 3 5/2 O( ) in g00, O( ) in g0i, and O( ) in gij. Consequently, Damour (1987) calls this the “effacement” of the bodies’ in order to describe such systems, one must carry out a internal structure. It is a consequence of the strong equiva- solution of the equations substantially beyond 1PN order, lence principle (SEP), which is satisfied by general relativ- sufficient to incorporate the leading radiation damping ity, but is violated by almost all other gravitational theories, terms at 2.5PN order. including scalarÐtensor gravity (BransÐDicke theory and its ¥ The system may be highly relativistic in its orbital generalizations). SEP states that (i) all bodies fall in an motion, so that U ϳ v2 ϳ 1 even for the interbody field external gravitational field with the same acceleration and orbital velocity. Systems like this include the late (modulo tidal interactions), (ii) the outcome of any local stage of the inspiral of binary systems of neutron stars or test experiment is independent of the velocity of the (freely black holes, driven by gravitational radiation damping, falling) apparatus, and (iii) the outcome of any local test 356 CLIFFORD M. WILL

experiment is independent of where and when in the on the internal structure of each body. For compact bodies Universe it is performed. The term “bodies” here includes such as neutron stars and black holes, these internal struc- everything from laboratory-sized objects to black holes; ture effects could be large; for example, the gravitational and the term “experiments” includes everything from elec- binding energy of a neutron star can be 40% of its total tromagnetic experiments to measurements of Newton’s G. mass. General relativity satisfies SEP because it contains one, At 1PN order, the leading manifestation of this effect is and only one, gravitational field, the spacetime metric g. a violation of the equality of acceleration of massive bodies Consider the motion of a body in a binary system, whose such as the Earth and the Moon. This effect, known as the size is small compared to the binary separation. Surround Nordtvedt effect, has been tested by lunar laser ranging (see the body by a region that is large compared to the size of Chapter 15 or TEGP 8 for further discussion). the body, yet small compared to the separation. Because of This is how the study of orbiting systems containing the general covariance of the theory, one can choose a compact objects provides strong-field tests of general rela- freely falling coordinate system which co-moves with the tivity. Even though the strong-field nature of the bodies is body, whose spacetime metric takes the Minkowski form at effaced in GR, it is not in other theories, thus any result in its outer boundary (ignoring tidal effects generated by the agreement with the predictions of GR constitutes a kind of companion). There is thus no evidence of the presence of “null” test of strong-field gravity. the companion body, and the structure of the chosen body can be obtained using the field equations of GR in this coordinate system. Far from the chosen body, the metric is 2 MOTION AND GRAVITATIONAL characterized by the mass and angular momentum (assum- RADIATION IN GENERAL RELATIVITY ing that one ignores quadrupole and higher multipole moments of the body) as measured far from the body using 2.1 Introduction orbiting test particles and gyroscopes. These asymptotically measured quantities are oblivious to the body’s internal The motion of bodies and the generation of gravitational structure. A black hole of mass m and a planet of mass m radiation are long-standing problems that date back to the would produce identical spacetimes in this outer region. first years following the publication of GR, when Einstein The geometry of this region surrounding the one body calculated the gravitational radiation emitted by a labora- must be matched to the geometry provided by the compan- tory-scale object using the linearized version of GR, and de ion body. Einstein’s equations provide consistency condi- Sitter calculated N-body equations of motion for bodies in tions for this matching that yield constraints on the motion the 1PN approximation to GR. It has at times been contro- of the bodies. These are the equations of motion. As a result versial, with disputes over such issues as whether Einstein’s the motion of two planets of mass and angular momentum equations alone imply equations of motion for bodies m1, m2, J1 and J2 is identical to that of two black holes of (Einstein, Infeld, and Hoffman demonstrated explicitly that the same mass and angular momentum (again, ignoring they do, using a matching procedure similar to the one tidal effects). described in Section 1.2), whether gravitational waves are This effacement does not occur in an alternative gravi- real or are artifacts of general covariance (Einstein waffled; tional theory like scalarÐtensor gravity. There, in addition Bondi and colleagues proved their reality rigorously in the to the spacetime metric, a scalar field is generated by 1950s), or even over algebraic errors (Einstein erred by a the masses of the bodies, and controls the local value of the factor of two in his first radiation calculation; Eddington gravitational coupling constant (i.e., G is a function of ). found the mistake). Shortly after the discovery of the binary Now, in the local frame surrounding one of the bodies in pulsar PSR 1913 16 in 1974, questions were raised about our binary system, while the metric can still be made the foundations of the “quadrupole formula” for gravita- Minkowskian far away, the scalar field will take on a value tional radiation damping (and in some quarters, even about 0 determined by the companion body. This can affect the its quantitative validity). These questions were answered in value of G inside the chosen body, alter its internal struc- part by theoretical work designed to shore up the founda- ture (specifically its gravitational binding energy), and tions of the quadrupole approximation, and in part (perhaps hence alter its mass. Effectively, each mass becomes several mostly) by the agreement between the predictions of the functions mA() of the value of the scalar field at its loca- quadrupole formula and the observed rate of damping of tion, and several distinct masses come into play; inertial the pulsar’s orbit (see Section 3.1). Damour (1987) gives mass, gravitational mass, “radiation” mass, and so on. The a thorough review of this subject. precise nature of the functions will depend on the body, The problem of motion and radiation has received specifically on its gravitational binding energy, and as a renewed interest since 1990, with proposals for the construc- result, the motion and gravitational radiation may depend tion of large-scale laser interferometric gravitational-wave 17

GUSTAV A. TAMMANN* The cosmological constants

The evolution of the large scale of the Universe is but one the subsequent results in ultraviolet spectroscopy (see chapter of cosmology, yet it is fundamental to all of its Chapter 13). But optical observations above the atmosphere aspects. The understanding of the large-scale evolution has were also of fundamental interest offering almost 10 times once been described as the “Search for two numbers” smaller seeing disks and consequently the detection of (Sandage 1970). The first of these numbers is the Hubble con- roughly 100 times fainter objects than from the ground. stant H0 which measures the present expansion rate. The sec- Individual stars in crowded regions would become accessi- ond number was thought to account for the gravitational ble and much greater detail would be visible in extended deceleration of the Universe. But since evidence has been objects, as first pointed out by Hermann Oberth in 1923. found for an additional acceleration, the original deceleration However, this enormous gain was offset in part by the size parameter has been split into a decelerating term m and an limitations of a telescope in space. Plans for what was to accelerating term . m is a measure of the gravitating become the 2.4 m HST for imaging and spectroscopy in the mean matter density of the Universe, while corresponds to optical and near UV developed during the 1960s and were the energy density of Einstein’s “cosmological constant” . finally approved by the US Congress in 1977. ESA joined The three quantities H0, m, and fix the expansion the venture with a 15% share, which turned out to be of age T of the Universe and determine also whether the great benefit to astronomy in Europe. The start of HST, Universe will eventually recollapse or expand forever. planned for 1983, was delayed by the Space Shuttle acci- In principle the values of H0, m, and can be mea- dent until 1990. Since then, and particularly since 1993, sured in the present Universe as well as the age t of the oldest that is, after the repair mission for the defective mirror, objects which Ð as an essential test Ð must be smaller than T. HST has not only brought spectacular progress in astro- Yet the cosmological constants are elusive. Even the nomical research, but has also stirred an enormous public concerted effort of many of the largest optical and radio tele- interest in astronomy. scopes has not led to their satisfactory determination. As a When plans were made in 1979 (Macchetto et al. 1979; consequence observations from space were added wherever Longair and Warner 1979) for future research with HST, possible. In fact the determination of the cosmological con- the author was ambivalent about its benefits for the deter- stants was a driving force for some astronomical satellites. mination of H0, being convinced that the main uncertainties Several satellites will be mentioned in the following of the time were due to statistical problems (Malmquist which have contributed to our dawning understanding of bias; Section 1.3). He was wrong by not foreseeing that the cosmological parameters, but none has been as impor- standard supernovae of type Ia are so powerful standard tant as the Hubble Space Telescope (HST). candles that they can beat all statistical selection effects, The initial motivation for astronomers to go into space and that HST can provide Cepheid distances to their nearest were the wavelengths inaccessible from the ground. The representatives and thus calibrate their true luminosity. It reward was overwhelming with the detection of X-ray was clear, however, already in 1979 that the same super- and -ray sources in the early 1970s (see Chapter 12) and novae can be observed with HST at cosmologically significant redshifts and that this is the route to determine the * Universität Basel, Switzerland cosmological parameters q0 (or m) and .

1 THE HUBBLE CONSTANT H0 poses no problem. The crux, however, is introduced by peculiar motions which are superposed on the regular Hubble Since the Big Bang space has expanded. During the early flow. They are caused by the gravitational pull of galaxies stages the expansion was governed by complex processes, and clusters of galaxies and of any additional dark matter. For but after an inflationary period the Universe turned into a instance our Local Group of galaxies is a sufficient density constant growth rate carrying with it all matter and the enhancement to have turned its original expansion into a con- emerging galaxies. As a consequence any observer in the traction. Our nearest neighbor, the Andromeda galaxy, M31, Universe has the impression that all galaxies are being car- has thus a blueshift and approaches us at about 100 km s 1. ried away, and the faster they move the larger the distance. The largest peculiar velocity we partake of is a 630 km s 1 Yet strictly speaking the distant galaxies do not recede with velocity which manifests itself as a dipole anisotropy of the any velocity, but space between the observer and the galaxy cosmic microwave background (CMB, see below). is stretched. In spite of this the spectra of nearby galaxies are The peculiar velocities cause a dilemma for the determi- interpreted as if they had recession velocities. This is permis- nation of H0. On the one hand one wants to determine it sible here because the determination of the present expan- locally to find the present value H0; on the other hand one sion rate, that is, the Hubble constant H0, must be carried out must determine it at sufficiently large distances, where the at small distances where the look-back times are short. recession velocities are significantly larger than any possible If light travels through an expanding space its waves will peculiar motions. The best compromise is to determine H0 be stretched and when it arrives at the observer it will between recession velocities of 10 000 and 30 000 km s 1. appear redder than at the time of emission. Also the spectral But the determination of the corresponding distances poses lines of the galaxies will be shifted to the red. These lines formidable problems. can be ascribed to certain neutral or ionized elements and With this requirement in mind a new strategy has devel- each appears in the laboratory at a fixed wavelength 0. oped to determine H0. The idea is to map the Hubble flow If that particular line appears in the galaxy spectrum at out to the necessary distances by means of relative dis- wavelength one defines a redshift z of the galaxy as tances of a set of galaxies, for instance from “standard can- z ( 0)/ 0. The redshift is determined by the stretch dles” of constant luminosity, and then to determine the true factor the Universe has experienced during the light travel distances of one or a few nearby galaxies of the same set. time, that is, z (R0/Re) 1, where R0 is the present radius The latter step has been greatly facilitated by HST. The of the Universe (more exactly its radius of curvature) and route is described below. Re is the radius at the time of emission. As long as z is small, say z 0.2, the redshift can also be interpreted with 1.1 The Hubble diagram of standard candles sufficient accuracy as a Doppler effect. The recession velocity v is in that case simply given by vcz, where c is The expansion of the Universe implies that the recession the velocity of light (in km s1). velocities v increase linearly with the photometric distance. To pin down the expansion rate of the Universe one Astronomers measure, in general, photometric distances in introduces the Hubble constant H0 which is defined as terms of the distance modulus (m M), which is defined as (m M) 5log rMpc 25, where m is the apparent magni- ϵ . ϽϽ H0 R0 /R0 v/r0 (v c) (1) tude of an object and M its absolute magnitude, that is, the apparent magnitude the object would have if seen from a where v is the “recession velocity” of a galaxy and r0 is its distance of 10 pc. Objects with (nearly) constant absolute 1 1 distance (in Mpc). Thus the units of H0 are km s Mpc magnitude are called “standard candles.” The apparent (the units are not repeated in the following), but its dimen- magnitudes m of standard candles are a measure of their sion is simply (time)1. relative distances, or more exactly of their log r values. It should be noted that H0 is constant in the sense that it A plot of log v υ. the apparent magnitude m of extra- is the same anywhere in the present Universe, but it has a galactic standard candles constitutes the so-called “Hubble different value at other times. This is obvious because the diagram.” The standard candles concentrate in a linearly distance r is constantly increased by the expansion of the expanding Universe along a straight line of slope 0.2. Sys- Universe and consequently the Hubble parameter H must tematic deviations from this line beyond 30 000 km s1 decrease with time. are caused by the deceleration or acceleration of the Universe Equation (1) gives the false impression that the determi- (provided that the apparent magnitudes are corrected for the nation of the Hubble constant H0 is simple. One just deter- z-dependent photometric K-term). mines the redshift z of the spectrum of any given galaxy, and Since Hubble and Humason (1931), the Hubble diagram hence its recession velocity v, and divides the latter by the has provided the fundamental evidence for the linear expan- galaxy’s distance. Indeed the determination of the redshift sion of the Universe. The ever-increasing persuasive power THE COSMOLOGICAL CONSTANTS 375 of the Hubble diagram from Humason et al. (1956) to conclusions follow from this. (1) If the total observed Sandage (1966) and Kristian et al. (1978), who used first- scatter of the Hubble diagrams is read vertically as an ranked cluster galaxies as standard candles, has shattered effect of peculiar motions, a generous upper limit is set of all attempts to interpret redshifts as a non-cosmological v/v0.05, which holds for the range of 3500v effect. 30 000 km s1. The (all-sky) distance-dependent variation

1.1.1 The Hubble diagram of supernovae of type Ia 12 14 16 18 20 Following an early suggestion of Kowal (1968) it has 4.5 become increasingly clear that supernovae of type Ia B (SNe Ia) are the most powerful standard candles to date. This can be physically rationalized because a SN Ia out- 4.0 burst is caused each time a white dwarf is pushed over its

Chandrasekhar limit by a mass-shedding companion. log v For 35 SNe Ia in the distance range 1200v30 000 3.5 km s1 good B and V magnitudes at maximum light are known, for 29 of them also Imax magnitudes. The SNe Ia, listed by Parodi et al. (2000), are selected to have Bmax 3.0 Vmax 0.06, after correction for galactic reddening to guard against absorption within their parent galaxies and to exclude a few intrinsicly red objects with peculiar spectra. 4.5 Two blue SNe Ia with peculiar spectra at early phases V (SN 1991 T, 1995 ac) are also excluded. The selected objects of the sample all have normal spectra as far as is 4.0 known (Branch et al. 1993).

The 35 SNe Ia define Hubble diagrams in B, V, and I log v m with a very small scatter of M 0 .2. This means that SNe 3.5 Ia are standard candles to better than 20% of their luminosity. In spite of this the residuals about the Hubble line correlate with second parameters. The SNe Ia with slow 3.0 decline rates m15 are somewhat brighter, where m15 is the magnitude decline during the first 15 days after B maximum. The relation between magnitude residual and decline 4.5 I rate is roughly m 0.45 m15. Also the intrinsically bluer SNe Ia are somewhat brighter than their redder counterparts by about mp(BV), where p is 4.0 1.2Ð2.5 for I, V, and B magnitudes, respectively (for the exact relations, see Parodi et al. 2000). Corrected magni- log v corr tudes mB,V,I are obtained by reducing all SNe Ia to the 3.5 mean decline rate m15 1.2 and to the mean color BV0m.01. corr The corrected magnitudes mB,V,I define Hubble diagrams 3.0 as shown in Figure 1. Their tightness is astounding. The 12 14 16 18 20 Cerro Tololo collaboration, to whom one owes 70% of the mcorr(max) photometry of the fiducial sample, quote a mean observational error of their m values of 0m.10 and of their col- Figure 1 The Hubble diagrams in B, V (and I) for the 35 (29) max corr m SNe Ia of the fiducial sample with magnitudes m , that is, ors (B V) of 0 .05 (Hamuy et al. 1996). This alone would suffice to explain the observed scatter of 0m.12 corrected for decline rate m15 and color (B V ). Circles are m SNe Ia in spirals, squares in E/S0 galaxies. Small symbols are 0m.13. An additional error source are the corrections for SNe Ia whose observations begin eight days after B maximum galactic absorption which were adopted from Schlegel et al. or later. Solid lines are fits to the data assuming a flat universe (1998). In fact, if one excludes the nine SNe Ia with large with m 0.3 and 0.7; dashed lines are linear fits with a m galactic absorption corrections (AV 0 .2) the scatter forced slope of 0.2 (corresponding approximately to m 1.0 m decreases to 0 .11 in all three colors. Two important and 0.0). (Parodi et al. 2000.) 376 GUSTAV A. TAMMANN

of H0 must be even smaller. (2) If, however, the scatter is Cepheids are the most uncontroversial distance indica- read horizontally and if allowance is made for the observa- tors in extragalactic astronomy. Their pulsational period is a tional errors of the apparent magnitudes (and for any pecu- function of their mass which in turn correlates with the liar motions) one must conclude that the luminosity scatter luminosity. The corresponding log PÐabsolute magnitude M of blue SNe Ia, once they are homogenized in m15 and relation was discovered in 1912 by Henrietta Leavitt who color, is smaller than can be measured at present. In other studied the many Cepheids in the Large Magellanic Cloud words, they are extremely powerful standard candles. (LMC) which all appear at the same distance. The slope of Linear regressions, which correspond closely to a the log PÐM relation for different passbands is today well Universe with m 1, 0, to the data in Figure 1 determined from the LMC Cepheids. Remaining uncertain- yield ties concern the zeropoint of the relation and the effect of differences of the metallicity of the Cepheids in different corr log v 0.2m c (2) galaxies. All authors agree that the metallicity effect is small, but they do not agree on the sign of the correspond- m with cB 0.6760.004, cV 0.6730.004, and ing correction. Therefore an uncertainty of 0 .10 will be cI 0.6160.004. included into the error budget. It is easy to transform eqns (1) and (2), remembering At present it is customary to fix the zeropoint by adopt- (m M) 5log rMpc 25, into ing (m M)LMC 18.50. There is growing evidence that this value should be increased by 0m.05Ð0m.10. Based on log H0 0.2M c 5 (3) observations with the International Ultraviolet Explorer (IUE) and HST, Panagia (1998) has derived a geometrical where M is the absolute magnitude of SNe Ia in the appro- distance of (m M)LMC 18.58 0.05 from the fluorescent priate passband. Thus the problem of determining the ring of SN 1987A, a peculiar type II supernova in the LMC. large-scale value of H0 is reduced, with c now being Also the calibration of galactic Cepheids with the astromet- known, to measuring the absolute magnitude M of one (or ric satellite HIPPARCOS tends to give a somewhat larger a few) nearby SNe Ia. It must be stressed that the recession distance to the LMC (Madore and Freedman 1998; Feast velocity of the nearby calibrator(s) does not enter in any 1999; Pont 1999; Groenewegen and Oudmaijer 2000) in way. The solution for H0 is therefore independent of any agreement with RR Lyr stars and other distance indicators peculiar motions. (Walker 1999; Gratton 2000; Sakai et al. 2000; Romaniello et al. 2000). Smaller LMC moduli based on red-clump stars and RR 1.1.2 Cepheids, HST, and the luminosity calibration Lyr luminosities, calibrated through statistical parallaxes, of SNe Ia have comparatively low weight. Why two eclipsing binaries With the luminosity calibration of a SNe Ia in mind, a small in the LMC give an unacceptably small modulus of 18.3, team was formed to observe with HST the Cepheids in including HST observations (Fitzpatrick et al. 2000), is not IC 4182, home of the well observed SN 1937C. The understood at present. Cepheids would give a good distance to that inconspicuous DIVA, the planned next-generation astronomic satellite, galaxy and hence fix the absolute magnitude M in the B and is expected to measure very accurate parallaxes of 30Ð40 V band. Through eqn (3) this would lead to a high-weight galactic Cepheids. This should settle for good the question determination of H0. of the zeropoint. ESA’s very ambitious GAIA project would The team consisted of Allan Sandage, Abhijit Saha, in addition measure the parallaxes of many supergiants and Lukas Labhardt, Duccio Macchetto, Nino Panagia, and the thus make a frontal attack on the distance of the LMC. author. Their application for HST observing time was For the moment we will retain the customary zeropoint received with skepticism. Was the project not too easy for with (m M)LMC 18.50 and return to the question in HST? Could it not be executed from the ground? It was a Section 1.1.3. happy incidence for the team that the mirror of the most The HST distance of the Cepheids in IC 4182 provided complex and most expensive telescope ever build produced MB,V of SN 1937C which, inserted into eqn (3), gave a rather miserable images when it was finally launched in 1990. IC low value of H0 52 9 (Saha et al. 1994). Essentially the 4182 became an ideal target. In fact the photometry of the same IC 4182 distance had been derived before from the extended stellar images on the CCD frames was easier than ground from its brightest stars (Sandage and Tammann 1982), later, after the repair mission in 1993, when the crisp stellar but the result had been criticized. Now, with the HST result images became smaller than the pixel size of the WFPC-2 the light curve of SN 1937C from archival material came camera, but, of course, offering higher resolution essential under fire. The obvious step was to calibrate other nearby for more distant galaxies. SNe Ia, which became feasible after the repair of HST. 18

K.M. GÓRSKI* AND A.J. BANDAY** COBE, dark matter and large-scale structure in the Universe

PREAMBLE 1 LARGE-SCALE STRUCTURE OF THE UNIVERSE BEFORE COBE NASA’s first dedicated cosmological space mission, the Cosmic Background Explorer (COBE), has provided com- Following closely upon the launch of the COBE satellite prehensive full-sky observations of both microwave and far- in November 1989, the January 1990 issue of “Scientific infrared frequencies. Each of the three instruments onboard Ð American” presented an article “The Cosmic Background the Far Infrared Absolute Spectrometer (FIRAS), the Explorer”, which announced the following: “NASA’s cos- Differential Microwave Radiometers (DMR), and the Diffuse mological satellite will observe a radiative relic of the Big Infrared Background Explorer (DIRBE) Ð has contributed Bang. The resulting wealth of data will be scoured for clues significantly to our fundamental understanding of the uni- to the evolution of structure in the universe”. In this article verse. FIRAS has measured the Planckian nature of the relic we describe how the outcome of this remarkable mission blackbody radiation to unprecedented accuracy, thus estab- did indeed provide important scientific results which helped lishing stringent constraints on the thermal history of the in a very major way to improve our understanding of the universe. DMR has discovered full sky structure in the tem- universe. However, to start our story at the beginning, let us perature of the blackbody radiation over a range of angular first recall what was the status of our knowledge about the scales down to 10 degrees, thus establishing stringent lim- universe before COBE, and why COBE was needed for its on the evolution of large-scale structure in the universe subsequent exploration of the universe. and providing us with our first glimpse of the initial condi- It is very well known that our modern understanding tions for structure formation. DIRBE has provided the first of the universe as a physical system rests on three major evidence for the existence of a cosmic infrared background, pillars of observational cosmology: thus providing important constraints on the integrated cosmological history of star formation in various pregalactic ¥ Universal Expansion: On cosmological distance scales, objects, protogalaxies, and galaxies, and the subsequent astronomical objects recede from one another with veloci- conversion of starlight into infrared emission by dust. ties proportional to their separations Ð a remarkable dis- Thus COBE has become a tour de force of modern cos- covery by Hubble in the 1920s. As we now know, the mological studies; it has influenced physical cosmology universe on the largest scales is very nearly homogeneous with its results dramatically, and established a splendid and expands isotropically. legacy for the community with its unparalleled view and ¥ Primordial Nucleosynthesis: The observed universal interpretation of the multiwavelength sky. abundances of light elements proved inconsistent with the idea that they were produced in stars. This puzzle was * European Southern Observatory, Garching bei München, Germany; Warsaw University Observatory, Warszawa, Poland explained in the 1940s by Gamow, Alpher, and Bethe in ** Max-Planck-Institut für Astrophysik, Garching bei München, Germany the context of an initially hot and dense phase in the early

evolution of the expanding universe. This necessarily galaxies matured sufficiently to put strong constraints on required that a residual relic thermal radiation permeated theories of galaxy and large-scale structure formation. the universe, and its temperature was theoretically ¥ Dark matter in the universe: Ever since Zwicky’s realisa- expected to be 4Ð5K. tion in the 1930s that clusters of galaxies consisted predomi- ¥ Microwave Background Radiation: The relic thermal nantly of matter in some nonluminous form, the “missing radiation of temperature 3 K was serendipitously dis- mass” or later “dark matter” problem was one of the most covered by Penzias and Wilson (1965) and immediately serious puzzles in astronomy. The following observational recognized by Dicke et al. (1965) as the “missing link” picture was built over time: astronomical objects of increasing between the primordial fire-ball of the young Big Bang size (galaxies, groups, clusters, superclusters) appear to con- universe and its present day mature phase dominated by tain more and more mass that does not manifest itself via evolved astronomical objects. This discovery delivered a luminosity, but can be detected due to its gravitational effects. direct proof of the dense and hot early stage of evolution The amount of this hidden mass is large Ð perhaps about of the universe. 20Ð30% of what is required to render the universe spatially flat. This fraction is sufficiently large that (1) it clearly These three phenomenological ingredients laid a solid exceeds the census of baryons in the universe, i.e. the dark foundation for the Hot Big Bang model, which became matter in known astronomical objects is unlikely to be com- nearly unanimously accepted as the framework for our prised entirely of ordinary matter, and (2) it invites specula- understanding of the universe at large. The Cosmic tion that perhaps the universe indeed is spatially flat, and Microwave Background (CMB) radiation itself has subse- therefore that the required dark matter content for the whole quently become the subject of increasingly vigorous theo- universe must still be larger than for individual astronomical retical and observational studies. It was recognised very objects. This finding tied in with a rich supply of theoretical rapidly following its discovery that the CMB should con- candidates for weakly interacting massive particles, which tain “fossilized” imprints of the processes which could, or could dominate the matter content of the universe, and the even had to, occur in the early universe, and that such sig- idea that perhaps the dynamically detected matter (compris- natures of past events should be pristine in comparison with ing both luminous, baryonic objects and dark material) is the clues provided to us through the studies of nearby, well more clumped than the remaining more smoothly distributed evolved astronomical objects. Hence, both the nature of the dark matter (the so called biasing effect). An alternate specu- CMB electromagnetic spectrum, and its possible deviations lation involves the idea of a cosmological constant, or vacuum from a perfectly thermal (Planckian) form, and the CMB energy density, which was originally introduced by Einstein, anisotropy, or dependence of its temperature on the direc- and thereafter enjoyed various degrees of popularity with tion of observation on the sky, very quickly became attrac- astronomers trying to determine the global properties of the tive theoretical and observational research targets, which universe. Indeed, the currently available combination of CMB were pursued relentlessly up until the COBE mission, and, anisotropy, large scale structure, and high-redshift supernovae indeed, even more so afterward. observations can be interpreted as supportive of these cosmo- It is beyond the scope of this contribution to review logical constant dominated models of the universe. systematically the steady progress of observational and theoretical cosmology from the discovery of the CMB until ¥ CMB phenomenology: Immediately after the discovery the COBE mission, but let us try to mention the essential of the CMB radiation, it was realised that the background developments. radiation should be carefully measured to search for 1. any deviations of its electromagnetic spectrum from ¥ Large-scale structure of the galaxy distribution: It has thermal, and been assessed that galaxies not only form groups and clus- 2. the expected deviations from isotropy in the angular dis- ters, but their tendency to aggregate in space extends to tribution of its temperature on the sky. even larger scales of superclusters (separated by voids), and perhaps beyond. For a long time the extreme scales of Both effects, if found, would provide invaluable clues to our detectable inhomogeneity in the 3-D galaxy distribution understanding of physical processes occurring in the early coincided with the largest scales surveyed, which rendered universe. It would be the case regarding spectral distortions a determination of the scale of transition to the expected because only significant energy releases (e.g. bulk annihilation homogeneity of the universe at large somewhat elusive. A of exotic particles at some epoch) at very high redshift could rigorous quantitative description of the galaxy distribution measurably perturb the Planckian spectrum of the CMB. It in space has been developed, including number counts, cor- would also be the case regarding CMB anisotropy because it relation functions, and power spectra. During the 1980s, could reveal to us the early predecessors of presently observ- astronomical measurements of the spatial distribution of able structures in the universe, as outlined below. COBE, DARK MATTER AND LARGE-SCALE STRUCTURE IN THE UNIVERSE 401

It was well understood that the primary dynamical factor amplitude of 0.1% of the mean, superposed on the that must have driven the evolution of structure in the uni- isotropic background (Smoot et al. 1977). This effect, how- verse was gravitational instability. To understand how the ever, was expected and explained as being dominated by presently existing structure in the universe has arisen, it is our own motion (i.e. a combination of motions of the solar necessary to postulate the existence of perturbations in system in the Galaxy in the Local Group) with respect to the primordial spatial distribution of matter, which over the rest frame defined by the CMB radiation, and, hence, a time were amplified by their self-gravity. The logical local rather than cosmological effect. Despite this apparent appeal of the proposition to use the expected minute CMB lack of a tangible result, there was ongoing and steady anisotropies to study directly the matter distribution in the progress stimulated by the increasingly stringent limits universe as it just emerged from its embryonic stage was on theoretical predictions of expected CMB anisotropy built upon the following considerations: depending on particular scenarios of evolution of structure in the universe. In fact, the search for evidence of 1. the universe is opaque beyond redshift 1000, thus one anisotropy in the CMB by the experimental community may think of CMB photons as having been emitted from the became somewhat akin to the quest for the Holy Grail. last scattering surface at the epoch corresponding to such a redshift (typically about a few hundred thousand years after ¥ Mainstream theoretical cosmology in the 1980s: The the Big Bang); hence the CMB photons, which are influ- rapid development of theoretical and observational cosmol- enced very little by the nearly transparent universe at red- ogy after the discovery of the CMB radiation resulted in shifts 1000, bring to us precious information about the very impressive support for the Hot Big Bang model as a conditions as far away and early on in the universe as we basic paradigm for understanding the universe at large. can ever probe with electromagnetic radiation; However, there were still some nagging paradoxes left 2. gravitational instability in an expanding universe is a which could not be answered simply within the framework slow process (described by power law functions of time, of the model. Among those were the following: rather than the familiar Jeans exponentials in a non-expanding medium), hence the amplitudes of large scale inhomo- 1. overall homogeneity and isotropy of the universe Ð why geneities observed today must have then been small but not are the very distant regions in the universe, which were never negligible; in causal contact (i.e. could not have interacted at any time 3. the temperature of the CMB radiation photons when during the history of the standard Hot Big Bang universe), so they were emitted at the epoch of last scattering was physi- similar as suggested by the apparent isotropy of the CMB? cally related to the perturbations of the matter distribution 2. flatness of the observed universe Ð why is the average via both the gravitational potential of inhomogeneities (the density of the universe so close (within a factor of a few) to so called Sachs-Wolfe effect), the emitting plasma velocity the critical density, which makes the universe spatially flat, (or the Doppler effect), and the thermal effects in gravita- just right now, when we (the human race) exist? tionally compressed or rarified plasma. Thus one is led to 3. where did the initial perturbations which seeded struc- conclude that the physical conditions at the time of the last ture formation come from? scattering of the CMB photons are necessarily “imprinted” on a sky map of CMB temperature anisotropy. All these questions could be by-passed with a reference to With such understanding firmly established, all that still the initial conditions for the evolution of the universe. This, remained to be done was to measure these CMB tempera- however, satisfied practically no-one, so when the idea of ture fluctuations. inflation appeared in the early 1980s, it was rapidly Surely, the excitement related to the opening of a win- embraced as a compelling explanation for the shortfalls of dow in the background radiation through which direct the standard cosmological model. Inflation postulates that observations of the truly embryonic stages of evolution of the evolution of the early universe is driven by a scalar the large scale structure of the universe could be attempted field, called the inflaton, which dominates the energy den- was missed by neither experimentalists nor theoreticians sity. Random regions, which get trapped in a state of false in cosmology. But the required measurements proved to be vacuum (or a local minimum of the potential energy) of the an incredibly hard task, as we can only now appreciate inflaton, end up expanding very rapidly due to an effec- in hindsight. A brief, and somewhat unjust, summary of tively negative pressure (hence the term inflation). A typical the experimental CMB anisotropy efforts that were con- inflating region ends up so big that the currently observed ducted for more than two decades up until the COBE mis- astronomical universe would be just a small fraction sion is simple: there were no detections of cosmological thereof. Hence, the explanation of homogeneity and anisotropy, only upper limits. The only exception was the isotropy of the universe that we see. Near flatness is measurement of the CMB dipole temperature pattern, at an explained because, again, our astronomical universe would 402 K.M. GÓRSKI AND A.J. BANDAY

be just a small sector of an original arbitrarily curved mani- To achieve the full benefit of space observations, the goal fold stretched to enormously large size. Density perturba- of the mission and instrument design was to ensure that tions which seeded the presently observed structures were the scientific measurements conducted by COBE were generated due to zero-point quantum fluctuations of the ultimately limited by the ability to model the various inflaton. Even though inflation is not a verified theory astrophysical components and distinguish them from the its logical appeal is huge, since it is apparently solves a cosmological information sought. This goal thus drove the number of seemingly unrelated paradoxes at the price of mission strategy, spacecraft and operations design and introducing just one new puzzle Ð what is the physical choice of instruments. nature of the inflaton? 2.1 Satellite overview It should be now clear, even with this rather sketchy description of the development of experimental and theoret- The need to minimise, control and measure the impact ical cosmology after the discovery of the CMB, that there of systematic errors led to the requirements for an all-sky were important questions about the universe which could survey, a minimal survey period of 6 months, and con- be answered with accurate measurements of the attributes straints on the amount of interference from local sources of of the relic radiation. A hard-learned lesson from early radiation such as the Earth, Sun, Moon and radio emission ground-based and balloon-borne experiments was of the from the ground. The orbit, spacecraft attitude, and instru- enormous difficulties involved with attempts to perform ment enclosures were therefore carefully selected to avoid such measurements successfully, and that the better approach direct exposure to the Earth and Sun, and maintain a stable would be a well-planned experiment in space. An opportu- thermal environment for the instruments. For COBE, nity to realise such a project was provided by NASA in depicted in Figure 1, a 900 km altitude, Sun-synchronous 1976, when it selected the Cosmic Background Explorer, orbit was selected so that the orbit, with a duration of 103 COBE, for a design study. minutes, precessed by 1 degree per day. In order to meet the scientific requirements of the mission (as will be discussed below), the spacecraft must also spin. A rate of 0.8 rpm was 2 THE COBE PROJECT adopted, with the spin axis tilted back from the orbital direction by 96 degrees, so that residual atmosphere did not Perhaps the first issue to address is the simple question, affect the instruments. The attitude of the spacecraft was why a satellite? The most succinct response to this can be controlled by inertia wheels and electromagnets, and deter- found in the proceedings of a meeting held in Copenhagen mined from Sun sensors, Earth sensors and gyroscopes. from June 25Ð29, 1979. “The Universe at Large Redshifts” contained a brief paper by Ray Weiss designed to serve as 2.2 Instrumental design an introduction to a more complete review of the COBE mission (Mather and Kelsall 1980). Nevertheless, the most Three complementary experiments were flown on the relevant issues for the reader to understand are indeed COBE spacecraft: the Far-Infrared Absolute Spectrometer recorded there. Based on the integrated experiences of the (FIRAS) designed to measure the frequency spectrum of COBE team members with ground-based, balloon-borne sky radiation from 100 microns to 1 cm (see Figure 2), the and air-borne experiments to measure the background radi- Diffuse Infrared Background Explorer (DIRBE) to map the ation, and more importantly the limitations of these plat- sky from 1 to 300 microns, and the Differential Microwave forms, the arguments in favour of a space-borne experiment Radiometers (DMR) to search for anisotropies in the CMB were then, and remain today, as follows: on scales larger than 7 degrees. Due to the authors’ involvement with the analysis and 1. Freedom from atmospheric emission and fluctuations in interpretation of the COBE-DMR data, and the aim of this that emission. contribution being the assessment of the impact of COBE’s 2. Full sky coverage with a given single instrument. results on our understanding of the large-scale structure of 3. A benign and controlled thermal environment to reduce the universe, the remaining text is focused on the DMR systematic errors. instrument and the measurements of CMB anisotropy con- 4. The ability to perform absolute primary calibration in ducted with this instrument. Those readers interested specif- flight without the necessity of windows to avoid conden- ically in the results of DIRBE and FIRAS are referred to sation of the atmosphere on calibrators and instruments. NASA GSFC www page http://space.gsfc.nasa.gov/astro/ 5. Sufficient time both to peform tests for systematic errors cobe/ and references therein. and to gain the increase in sensitivity permitted by A differential radiometer is a device which outputs a extended observation time. voltage proportional to the difference in power received by 19

HUBERT REEVES* The origin of the light elements in the early Universe

Shortly after World War II, George Gamov and his corresponding steps in the chain could be achieved, so the collaborators (Alpher et al. 1948) considered the possibility scheme was largely abandoned. that all chemical elements might have been generated by a Around the same time, Fred Hoyle and his collaborators succession of nucleon capture reactions in the cooling pri- (Burbidge et al. 1957) proposed a stellar origin for the mordial Universe. The successful prediction of the exis- chemical elements. This scheme turned out to be highly suc- tence of a fossil radiation left over from the earliest times of cessful in accounting for the chemical elements from carbon the Universe (Gamov 1946) emerged from the hypothesized to uranium (12 A 238). However, it ran into problems formation of helium in the big bang. There are two steps to with the lighter elements. Because of their small electric this argument: (1) in order to have nuclear reactions, the charges, these nuclides rapidly undergo self-destroying temperature must have been in the MeV range (1010 K), and nuclear reactions at the high temperatures of stellar interiors. (2) given the fact that a free neutron decays in approximately Other formation mechanisms were clearly needed to 15 minutes, the transformation of some but not all of the pri- account for their presence and abundances in the cosmos. mordial nucleons into helium requires the mean free path for One requirement for the selection of the formation mecha- neutron capture (and hence the cosmic density) to lie in an nism of these fragile, light nuclides is that, after their forma- appropriate range. At too low a density, the Universe would tion by appropriate nuclear phenomena, they should be be of pure hydrogen; at too high a density it would be of pure rapidly evacuated into low-temperature regions. Two different helium. It is, in fact, about one-quarter helium and three- physical processes meet this requirement: (1) BBN, where quarters hydrogen (in fractional mass). By combining these the rapid cooling of the universe preserves the newly formed numbers, Gamov and his colleagues obtained the first esti- nuclei from further thermonuclear reactions; and (2) galactic mate of the ratio of nucleons to photons (of the order of cosmic-ray (GCR) bombardment of interstellar atoms 1010). From there, they predicted the existence of a fossil (mostly C, N and O) ejecting spallation residues (mostly Li, radiation in the millimetric range (CMB, cosmic microwave Be and B) into the cold interstellar medium (ISM). background) and estimated its present temperature (a few kelvin). The theory of big bang nucleosynthesis (BBN) is essentially based on these arguments. However, the hypothesis of a primordial origin of all the THE CASE FOR BIG BANG NUCLEOSYNTHESIS chemical elements soon ran into major difficulties. In the proposed chain of successive nucleon capture, extending The best evidence in favour of the BBN of the ligther from hydrogen all the way to uranium, the nuclear instabil- nuclides can be seen from Figures 1Ð3. Figure 1 shows the ity of aggregates containing five and eight nucleons cons- abundance of 4He as a function of the time-integrated stel- tituted a fatal flaw: it could not be seen how the lar activity, represented by the oxygen abundance in various galaxies. Although an increase in helium from approxi- * Centre National de Recherche Scientifique, Paris, France mately 20 to 30% (fractional mass) is clearly visible,

0.30

0.28

0.26

0.24 He Fractional mass abundance He Fractional 4 0.22

0.20

0.18 0 50 100 150 200 250 O / H x 106

Figure 1 4He fractional mass abundances as a function of 16O in chemically unevolved galaxies (after Pagel et al. 1992).

manifesting a stellar contribution, no celestial objects are Another problem is the untangling, for each nucleus, of known to have less than 20% helium (10 times less helium the relative contribution of different production mecha- than hydrogen, in numbers of atoms). nisms: BBN, GCR spallation and stellar nucleosynthesis. A similar situation is found for 7Li. As we go backward This is most important for the isotope 7Li, for which the in time Ð by observing progressively older stars, represent- three mechanisms all contribute. For this reason, the ing the state of the ISM at the moment of their formation Ð cosmic-ray spallation processes and their resulting abun- the 7Li to hydrogen ratio decreases progressively from 109 dances will be reviewed in some detail. to 1010, where it levels off for all older stars (Figure 2). In the same fashion, the deuterium to hydrogen ratio appears to increase (Figure 3) with decreasing time. These observations suggest that these light nuclides NUCLEAR PHYSICS OVERVIEW were already present in the primordial galactic gas. They point to a common origin: nuclear reactions in the hot big It is well known that many features of the universal abun- bang. Can this suggestion be made quantitative? Can the dance of chemical elements can be qualitatively understood observed abundances be accounted for in terms of detailed through a knowledge of their nuclear properties. The iron computations? This is the subject of this chapter. It will peak (at A56) corresponds to the most stable nuclear con- appear that the answer is definitely yes, although some figurations. The secondary peaks correspond to nuclei with aspects are still obscure. so-called magic numbers of neutrons (at N50, 82 and One important difficulty stems from the fact that the 128), and also to the light nuclei with an integral number of available observations (of galaxies, stars, interstellar gas, alpha particles (at A12, 16, 20, 24 and 28). In this respect, planets) cannot be readily compared with the computed pri- it is most informative to begin our study of the origin of the mordial yields. Many factors have modified the abundances light elements with a brief review of their nuclear proper- of the nuclides between the big bang and the moment ties. Since the three elements lithium, beryllium and boron at which they are observed in the history of the cosmos were potentially formed in the big bang, we shall extend (e.g. the birth of the stars in which they are measured). this review up to A 12. Extrapolation from the observed data back to BBN has Two important factors play a crucial role in the nucleo- been a difficult task for many decades. synthesis of the light nuclei: their small electric charges and THE ORIGIN OF THE LIGHT ELEMENTS IN THE EARLY UNIVERSE 425

Ð8

8 (H+3He)/H

D/H

Li)/H Ð9 7 3He/H Li +

6 6 log( Ð9 ) –5

4 Ð11 Ratio ( ϫ 10

Ð9 2

Ð10 (H)]

(B) / Ð11

n 0

log[ Ð12 02468101214 Age (Gyr) Ð13 Deuterium evolution

3 Ð14 Figure 3 D and He abundances as functions of time from the big bang (14 Gyr ago) to the present. The value of 3He at 14 Gyr is not an observation but results from BBN calcula- Ð10 tions. The apparent variation of this nuclide at later times is well within the uncertainties (adapted from Geiss and Ð11 Gloeckler 1998). (H)]

n * Ð12 * 6 7 9 11

(Be) / comes Li at 2.0 MK, Li at 2.5 MK, Be at 3.5 MK, B at

n Ð13 5.0 MK and 10B at 5.3 MK. As a result, the light elements log[ (except 4He and, to a certain extent, 3He) cannot resist the Ð14 heat of stellar interiors. However, in view of the particle instability of 4Li and 5Li, the helium isotopes cannot be destroyed in nucleon-induced reactions, but only by Ð3 Ð2 Ð10 helium-induced reactions, resulting in an appreciably (Fe/H) higher Coulomb barrier energy and higher thermal resis- Figure 2 LiH, BeH and BH stellar abundances as a func- tance (Figure 4). tion of FeH abundances. The logarithmic scale of the x-axis is The second important nuclear property is the large bind- in units of fractional solar iron abundance. For more data, see ing energy of 4He, due to the large pairing effect of nuclear Cunha and Smith (1999) (after Gilmore 1992). forces when the nucleons are paired four by four: neutronÐ proton; spin up, spin down. As a result, every nucleus in this mass range is quite unstable toward a rearrangement involving 4He nuclei. No nucleus of mass 5 manages to be stable; the large binding energy of alpha particles two relative to the lifetimes are 1021 s. The isotopes 8Li and 8B are beta- other nuclear arrangements. unstable with respect to 8Be, which quickly (1016 s) breaks At BBN or stellar temperatures, the kinetic energy fac- into two alpha particles. tors are well below the Coulomb barrier. Coulomb penetra- The nuclear stability situation at a mass of 9 is deeply tion factors, proportional to the electric charge, govern the marked by the alpha stability. 9B is unstable against two destruction rate of the light nuclei by proton capture. alphas p (1019 s), but 9Be barely escapes disintegration Because of its low Z and low nuclear stability, deuterium is (it has a very small binding energy). This weak stability of doubly fragile. It disappears at 0.5 million K (MK). Next 9Be is reflected in the fact that the endothermic Q values 426 HUBERT REEVES

corresponding to its formation by spallation reactions are 9 11 9 10 remarkably large, and the exothermic Q values correspond- –1 ing to its destruction are small. It is also reflected in the fact

6 that it has only one ‘bound’ state; all the excited states are 10 –2 unstable against particle break-up. These facts are instru- 7 mental in explaining the low natural abundance of 9Be (one 7 6 7 10 11 –3 11 of the lowest in nature). The isotopes Li, Li, B and B fare better but all remain comparatively fragile; in high- energy proton-induced reactions they all quickly break –4 p+D p+p+n down into alpha particles and other products. 6 4 10 The high binding energy of He is also responsible for ) –5 the fact that, at all but the lowest temperatures, hydrogen is

arn 4 3

(b D transformed rapidly into He. The nuclei D and He are

des –6 3 He+γ intermediate steps in this chain of reactions; very small p+D amounts of them remain at the end of the process. These og l –7 facts dominate the scenario of BBN and also of main 11 sequence stellar energy generation. –8 The influence of these nuclear properties on the formation rate of the light elements is reflected in their formation 10 cross-sections of the spallation reactions resulting from the –9 bombardment of atoms of C and O by protons (Figure 5). D The link is best illustrated by phase-space arguments. In the –10 6 high-energy region, the break-up of the excited nuclei into a 9 given configuration is proportional to the number of possi- –11 ble channels, which is itself a function of the binding 7 energy, and also of the number of bound excited states for –12 the given configuration. Above 100 MeV or so, the cross- –2 –10 sections reach a plateau which is maintained all the way up log Ep (MeV) to the highest energies. As expected, the cross-sections for the formation of 9Be are the smallest, while for Li and B Figure 4 Total destruction cross-sections of light nuclides by isotopes they have higher values. protons in barns (1024 cm2) as a function of energy. The number close to each curve refers to the mass number of the corresponding nuclide (after Reeves 1974).

100 p + 16O p + 12C 11B

10B 11B 7Li 6 7Li Li 10B

6 10 Li 9Be

9Be Cross-section (mbarn)

1 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV

Figure 5 Spallation cross-sections in millibarns (1027 cm2) for the formation of the Li, Be and B isotopes by proton-induced reactions on 12C and 16O (after Reeves 1974). 20

JEAN SURDEJ* AND JEAN-FRANÇOIS CLAESKENS* Gravitational lensing

In his theory of general relativity (GR), Einstein established 6. the size of absorbing intergalactic gas clouds, that a massive object curves spaceÐtime and that photons 7. upper limits on the density of a cosmological population move along geodesics of this curved space. Einstein’s of massive compact objects. prediction was confirmed during the solar eclipse of 1919. This not only provided a decisive, quantitative proof of the In this chapter we summarize some of the theoretical and validity of GR but also gave confirmation of the concept observational evidence supporting these claims and show that electromagnetic waves do undergo deflections in gravi- how space observations have significantly contributed in tational fields. some of the areas listed above. Atmospheric lensing effects may lead to a distortion or to The chapter is organized as follows. We first review the the formation of multiple images of a distant source, result- historical background of gravitational lensing in Section 1. ing in the perception of a ‘mirage’. They sometimes literally Unlike most other astrophysical discoveries made during deform our view of the surrounding areas. Similarly, gravita- the last century, the physics of gravitational lensing was tional lensing may perturb our view of the distant Universe understood well before the first example of a multiply- and affect our physical understanding of various classes of imaged extragalactic object was found. extragalactic objects. The great interest in gravitational lens- In Section 2 we describe the basic principles and concepts ing comes from the fact that this phenomenon can be used as of gravitational lensing. We first establish the exact form of an astrophysical and cosmological tool. Indeed, gravitational the lens equation. By making use of the Einstein deflection lensing may help in deriving: angle, we derive the expression for the angular diameter of an Einstein ring, which is also the typical angular separation 1. the distance scale of the Universe Ð via the determination between multiply-lensed images when the conditions of of the Hubble constant H0 Ð and possibly the values of perfect alignment between the observer, the lens and the other cosmological parameters (the cosmological density source are no longer fulfilled. We then set up a sufficient 0 and the cosmological constant 0), condition for an observer to see an Einstein ring, or multiple 2. the mass distribution M(r) of the lens, images, of a distant source that is located behind a deflector, 3. the extinction law in the deflector, usually located at and go on to derive, for the case of a point-mass lens model, high redshift, expressions for the expected image positions and their ampli- 4. the nature and distribution of luminous and dark matter fications. Using wavefront and ray tracing diagrams, we in the Universe, introduce the important concepts of caustics and time delays. 5. the size and structure of quasars, We then show how all the lensed image configurations observed in the Universe may be understood in terms of the relative location between the observer and the caustics * Université de Liège, Liège, Belgium associated with an asymmetric lens. Several outstanding

examples of gravitational lens systems observed with the a light ray passing near the solar limb should be deflected Hubble Space Telescope (HST) serve as illustrations. by an angle given by In Section 3, we discuss the most remarkable astrophysi- 2 cal and cosmological applications of gravitational lensing. ˆ 4GM/(c R) 1 (1) These include the independent determination of the Hubble constant H0 based upon the measurement of the time delay where G is the gravitational constant, c is the velocity of t between the observed light curves of multiply-imaged light and M and R are the mass and radius of the Sun (or of quasars. The possibility of weighing the mass of lensing any other compact lens), respectively. This deflection angle galaxies, galaxy clusters and intervening dark matter from turns out to be exactly twice the value derived by Soldner the observation of multiply-imaged distant sources, arcs and by Einstein himself in 1911; the factor of 2 merely and arclets is also reviewed. We further discuss the possibil- reflects the metric curvature. Note that in Newtonian terms ity of using gravitational lensing to determine the cosmolog- the Einstein deflection also follows if one assumes a refrac- 2 ical density 0 ( 8 G 0/3H0) and the reduced cosmological tive index n associated with the Newtonian gravitational 2 2 2 constant 0 ( c /3H0 ) of the Universe, the cosmological potential U (|U| c ) via the relation density of dark compact lenses, the size of intergalactic L 2 absorbing clouds, dust extinction laws in deflecting galaxies n 1 2U/c (2) and the structure and size of quasars. We also present the sta- The analogy between atmospheric and gravitational lensing tus of optical searches for massive astrophysical compact halo then becomes obvious. Note, however, that the bending of objects (MACHOs) based upon microlensing, the formation light rays in gravitational fields is predicted to be totally of giant luminous arcs in galaxy clusters and the observation ‘achromatic’ (see eqn (1)). of ‘weak lensing’ in the Universe. Using photographs of a stellar field taken during the The anticipated contributions of ongoing and future solar eclipse in May 1919, and six months apart, Arthur space missions such as Chandra, XMM-Newton, MAP, Eddington and his collaborators (Dyson et al. 1920) were FIRST-Herschel, Planck, NGST and GAIA are presented in able to confirm, within a 20Ð30% uncertainty, the deflec- Section 4. tion angle predicted by Einstein (Figure 1). This was the second correct prediction of GR Ð the successful interpretation of the advance of Mercury’s perihelion being the first Ð 1 HISTORICAL BACKGROUND and marked the full acceptance of the work of Einstein. It seems that Eddington (1920) was the first to propose One of the consequences of the theory of GR is that light the possible formation of multiple images of a background rays are deflected in gravitational fields. Although this pre- star by the gravitational lensing effect of a foreground one diction was made in the twentieth century, speculations that (but see the finding by Renn et al. (1997)). Note, however, light rays might be bent by gravitation had been proposed that Oliver Lodge (1919) had already characterized massive much earlier. Indeed, considering that light is composed of elementary constituents, Isaac Newton suggested as early as 1704 that the gravitational field of a massive object could possibly bend light rays, just as it would alter the trajectory (c) of material particles. A century later, Laplace independently made this same suggestion. Furthermore, the astronomer Johann von Soldner (1804) at the Munich Observatory (b) found that, in the framework of Newtonian mechanics, a light ray passing near the limb of the Sun should undergo an angular deflection of 0.875. However, because the wave description of light prevailed during the eighteenth and nineteenth centuries, neither the conjecture of Newton nor the result of Soldner were ever taken seriously. In 1911 Albert Einstein had re-derived the latter result on the basis of the equivalence principle, unaware of Soldner’s work. (a) In the elaboration of his theory of GR, Einstein predicted that a massive object curves spacetime in its vicinity O and that any free particle, massive or not (e.g. photons), Figure 1 Curved spacetime around the Sun (a) and deflection will move along geodesics of this curved space. After deriv- of light rays from two distant stars as seen by an observer (O) ing the full field equations of GR, he predicted in 1915 that during a solar eclipse (b) and six months apart (c). GRAVITATIONAL LENSING 443

Figure 2 The paths of light rays in a point-mass (PM) lens model. In accordance with eqn (1), a set of parallel rays emitted from a distant source are deflected from their original paths as they cross the deflector plane. Different observer positions are represented from where an Einstein ring (O1) or a double image (O2) may be seen. objects like the Sun as imperfect focusing lenses since they of Einstein’s notebooks. It does therefore seem that by 1936 had no real focal length; the light from a background object Einstein had totally forgotten his pioneering gravitational being mainly concentrated along a focal line of ‘infinite’ lensing work from 1912, as if he had already convinced him- length (Figure 2). In 1923, E.B. Frost, then director of the self that the idea was too speculative to have any chance of Yerkes Observatory, initiated a programme to search for empirical confirmation. multiple images of stars in the Galaxy, but it seems that Fritz Zwicky (1937a,b) was the first to realize the very these observations never really took place. Orest Chwolson high probability of identifying a gravitational lens mirage, (1924) suggested that, in the case of a perfect alignment (i.e. one composed of several distinct images of a single between an observer and two stars located at different dis- object) among extragalactic sources. He even proposed to tances, the observer should see a ring-shaped image of the use galaxies as natural cosmic telescopes to observe other- background star around the foreground one. wise too faint and distant background objects. He also Independently, at the request of Rudi W. Mandl, a Czech emphasized the possibility of determining the mass of distant electrical engineer, Einstein (1936) rediscovered the major galaxies simply by applying gravitational lens optometry characteristics (double images, the ‘Chwolson’ ring usually and, in addition, to test the theory of GR. In 1937 Zwicky referred to as the ‘Einstein’ ring, and so on) of one star lensed stated that ‘the probability that galactic nebulae which act as by another, but he was very sceptical about the possibility of gravitational lenses will be found becomes practically a cer- observing this phenomenon among stars, probably because tainty’, and he was therefore very much surprised some the expected angular separations between the lensed images twenty years later that no such lensing effects had yet been were so small. What seems remarkable is that Einstein had found with the 5 m (200-inch) Hale Telescope at Palomar first established the whole theory of the formation of multi- (Zwicky 1957). ply-lensed images (the lensing equation, formation of double After several decades of little activity, interest in the the- images by a point-like deflector, amplification of the lensed ory of gravitational lenses was revived by Klimov (1963), images, and so on) during the spring of 1912, three years Liebes (1964) and Refsdal (1964a,b, 1966a). Some of their before completing his theory of GR. This finding has recently proposed applications were particularly promising because been reported by Renn et al. (1997), who had examined some of the recent discovery of quasars by Maarten Schmidt 444 JEAN SURDEJ AND JEAN-FRANÇOIS CLAESKENS

(1963). It would indeed be much easier to prove the lensing (Section 3.5). Since 1982 a group of French astronomers origin of multiple QSO images rather than that of extended led by L. Nottale from Meudon Observatory had regularly and diffuse galaxy images, since the former ones consist of submitted observing proposals with the CanadaÐFranceÐ very distant, luminous and star-like objects. (For more Hawaii Telescope (CFHT) to search for such arcs in the centres details on quasars, see Peterson (1997).) In 1964 Sjur of rich and compact galaxy clusters, among them Abell 370. Refsdal proposed to apply geometric optics in order to esti- However, the observing programme committees were not at all mate the time delay between the arrival times of two parts of receptive to this kind of proposition. Although these predictions the same distorted wavefront at the observer; this proposal, were entirely based on the theory of GR, and had been which formed the second part of his master’s thesis, was promoted several decades earlier by Zwicky, they probably (erroneously) judged of uncertain quality by his supervisor. still looked too revolutionary to conservative observing pro- Nevertheless he succeeded in publishing his findings gramme committee members. Furthermore, during the first (Refsdal 1964a,b); the referee of these papers was Dennis international conference that was explicitly dedicated to the Sciama! Refsdal finally, obtained his Ph.D. in 1970 on the field of gravitational lensing, entitled ‘Quasars and Gra- basis of that controversial second part of his master’s thesis. vitational Lenses’ (24th Liège International Astrophysical On the basis of the strong similarity between the spectra Colloquium, 1983), some of the organizers were very sur- of quasars and of the nuclei of Class 1 Seyfert galaxies, prised by the high degree of scepticism that was still present Barnothy (1965) proposed that high-redshift quasars could among a significant number of the participants. Probably half actually be the lensed images of distant Class 1 Seyfert the audience still then considered gravitational lensing to be galactic nuclei. Sanitt (1971) criticized this view on the an unproved phenomenon, while many others thought that basis of statistical arguments. the proposed lensed monsters would remain a mere cosmic Theoretical work continued at a low level of activity curiosity, with no further scientific interest. It also seems that through the 1970s. Refsdal (1965, 1970) and Press and until the 1980s, in some institutes gravitational lensing was Gunn (1973) discussed problems on lens statistics, Bourassa not always regarded as an acceptable subject to study. and Kantowski (1975) considered extended non-symmetric Astronomers who wished to pursue the subject would not lenses (Bourassa et al. 1973) and Dyer and Roeder (1972) have received a research grant for work dedicated to gravita- derived a distanceÐredshift relation for the case of inhomo- tional lensing Ð nor would they be invited to deliver a semi- geneous universes. In spite of clear theoretical predictions, nar on the topic. We know from Refsdal, Nottale and many the interest from observers was largely absent, and no other pioneers that they had the feeling of being considered systematic search for lenses was initiated. somewhat heretic by some of their colleagues. Forty-two years after Zwicky’s prediction, the first exam- Following the pioneering detections of multiply-imaged ple of a distant quasar (Q0957 561) doubly imaged by a quasars and giant luminous arcs, the levels of observational foreground massive lensing galaxy and its attendant galaxy as well as theoretical activities have increased dramatically. cluster was serendipitously discovered by Walsh et al. (1979). More than fifty multiply-imaged quasars and an even larger This system consists of two lensed quasar images separated number of gravitational arcs have now been discovered, and by approximately 6. Good evidence that Q0957 561 A and more than 3100 scientific publications have been written B correspond to twin lensed images of a single quasar was over the past twenty years on the subject of gravitational provided by lensing (Figure 3). Space observations, and in particular 1. the similarity between their spectra, 2. a simple lens model which could naturally account for the slight morphological differences detected between the optical and radio images of the quasar (Young et al. 1981), 3. the discovery of the lensing galaxy between the twin quasar images, made possible by the use of modern CCD imaging, 4. the observation of a time delay of 1.14 years, first reported by Vanderriest et al. (1989), between the light variations of the two quasar components.

In parallel with the discovery of several other multiply- Figure 3 The number of scientific papers published annually imaged quasars, an even stronger interest in gravitational on gravitational lensing over the past forty years. (Based on the lensing studies developed with the first identification of gravitational lensing bibliography compiled by Pospieszalska- giant luminous arcs in 1986, by two independent teams Surdej et al. (2000).) 2

ARTURO RUSSO* The space age and the origin of space research

INTRODUCTION immediate post-war period. Then the Cold War was to fuel the development of more powerful missiles, capable of The exploration of space is one of the greatest achievements delivering a nuclear weapon across the oceans. of the twentieth century. In the ideal photo album of this cen- These missiles were also capable of launching a satellite tury one could hardly avoid including such pictures as our into orbit, and a new dimension was thus added to the Cold blue planet seen from space and the human footprint on the War confrontation: the demonstration of technical superior- Moon’s surface; the close-up images of a comet’s nucleus ity in a field which could greatly affect the public imagina- and the rough landscape of Mars; the floating of astronauts tion. This was seen as being particularly important in Third in the black vacuum around a space station and the rings of World countries, which were, at that epoch, emerging from Saturn photographed from an approaching spacecraft. the colonial era and in search of ideological principles, eco- Space technologies have provided us with the possibility nomic models and international patronage. Being first in of real-time global communications, as well as with unprec- space, first on the Moon, became an important political goal edented knowledge of cosmic phenomena. Traditional for both superpowers, wanting to demonstrate the superior- research fields like astronomy underwent dramatic changes ity of their cultural values, the efficiency of their political when telescopes could be flown beyond the atmosphere; institutions, the performance of their industrial system, and others emerged brand-new when physicists could put their the strength of their armed forces. sophisticated instruments onboard satellites and space-probes. Scientists on both sides of the Iron Curtain were able to Knowledge of our own planet took a big step forward when take advantage of the opportunity offered by this new dimen- it became possible to investigate, from space, the delicate sion, building instruments which could be fitted into the equilibrium of its various components. nose-cones of sounding rockets or the payloads of spacecraft, This book is devoted to the great advances in scienti- and designing space missions which could solve old and new fic knowledge that space technologies have made possible. problems in astronomy and geophysics. A new scientific com- Scientific research, however, can hardly be considered to be munity emerged from this experience, identified less by their the main driving force in space activities. Political and mili- disciplinary affiliation than by the technical means they used. tary considerations, in fact, played the major role in making Space science, in fact, is not a well-defined research the old dream of exploring space a technical reality. To the field. It includes, in principle, any scientific investigation deliberate attempt of the Nazi regime to develop the ulti- conducted by the use of rockets, Earth-orbiting satellites mate weapon to win the war we owe the V2, the prototype and deep-space-probes. In terms of established scientific of the rockets which helped the United States and the disciplines, it covers fields as different as atmospheric Soviet Union to start successful space programmes in the physics, geophysics, plasma physics, cosmic-ray physics and the various branches of astronomy (solar, stellar and * Università degli Studi di Palermo, Italy planetary; from radio wavelengths up to the gamma-ray

written in the second century, whose heroes are taken to the Moon by a terrible whirlwind that lifts their ship into space. In Lucian’s second book, entitled Icaro-Menippus, the trip to the Moon is carefully planned and eventually performed by Menippus, the hero, with the help of wings fastened to his shoulders. At the height of the Italian Rinascimento, a voyage to the Moon was conceived in 1516 by the great Italian poet Ludovico Ariosto in his epic poem Orlando Furioso. Searching for the lost mind of Orlando, Astolpho is carried to the Moon by Elijah’s chariot drawn by four red horses. He visits the Moon, where everything lost on Earth is stored in a valley, and eventually finds the paladin’s mind in a phial. A century later, in Germany, one of the fathers of modern astronomy, Johannes Kepler, told of a Moon flight in his Somnium (Dream), posthumously published in 1634. In Kepler’s dream, conceived as an allegory for astronomy which reveals the reality of celestial bodies, supernatural demons (that is, astronomical knowledge) drive the learned traveller to the Moon along the shadow of the Earth during a lunar eclipse, and then they drive him back along the Moon’s shadow during a solar eclipse. Figure 1 Astronaut’s footprint on the Moon, 1969. (Source: NASA.) region of the electromagnetic spectrum). Space sciences have also revitalized the old subdiscipline of astrometry and have helped make observational cosmology a practical and very fruitful area of research. Moreover, they have made important contributions to material sciences, biology and medicine. Each of these disciplines was dramatically reshaped by the advent of space technologies, both regarding their scientific content (such as in situ measurements of ionospheric phenomena, chemical and isotopic investigation of the Moon’s soil, astronomical observations in new spectral bands) and with respect to their increasingly inter- disciplinary nature (such as the growing importance of physicists in astronomical research). This chapter presents an historical overview of the origin of space science within the framework of the major political and military events which shaped the early phase of the space age. Given the nature of this overview, it was decided to include a comprehensive bibliography at the end, rather then to insert specific references to the vast literature on the history of space into the body of the text.

DREAMS OF SPACE FLIGHT

A journey to the Moon and beyond has for centuries been the object of popular fantasies, philosophical speculations and poetic dreams. Space travel literature can be traced Figure 2 Johannes Kepler. (Source: Istituto e Museo di Storia back to Lucian of Samosota’s Vera Historia (True History), della Scienza, Florence.) THE SPACE AGE AND THE ORIGIN OF SPACE RESEARCH 27

The same theme was dealt with in those years by two works in which space travel and encounters with alien people English bishops. Francis Godwin, Bishop of Hereford, serve as witty means for conveying the author’s opinions wrote The man in the Moon: or a discourse of a voyage about mundane matters. thither under the pseudonym Domingo Gonsales. The book But in the century which saw the birth of modern science, was published in 1638 and it became very popular all over other works tried to deal with the old dream of space flight Europe. The protagonist of the book, Gonsales himself, in the light of the new astronomical knowledge (sunspots, flies to the Moon by a chair pulled by wild geese. In the Jupiter’s satellites, Saturn’s rings, mountains on the Moon, same year, John Wilkins, Bishop of Chester, published The and so on), and in accordance with the tenets of the new discovery of the man in the Moon: or a discourse tending natural philosophy. A major event was the publication, in to prove that ‘tis probable there may be another habitable 1686, of Bernard de Fontenelle’s Entretiens sur la pluralité world in that planet. Wilkins tells nothing about the means des mondes, a popular astronomy book which was widely of propulsion but he speculates that no effort would be read throughout Europe for its fascinating speculations on the required beyond the point where the influence of terrestrial nature and habitability of Solar System bodies. Fontenelle gravity ceases, which he assumes to be some thirty kilometres claimed that each planet had its own inhabitants and dis- above the Earth’s surface. cussed their likely appearance, civilization and habits. At the court of the king of France we find the most The belief that planets are inhabited by different species remarkable author of Moon tales of the seventeenth cen- of rational beings was also expressed by the great Dutch tury. This is Savinien de Cyrano Bergerac, dramatist and scientist Christian Huygens in his Cosmotheros, or conjec- poet, philosopher and swordsman, whose enormous nose tures concerning the planetary worlds, first published in found a place in the history of French literature thanks to Latin in 1698. However, if the Moon and the planets were Edmond Rostand’s famous play of 1897. Cyrano’s L’autre no longer considered as metaphorical symbols but as real monde, ou les états et empires de la Lune, posthumously worlds subject to universal physical laws, tales of space published in 1657, tells of several attempts by himself to travels had to cope with the problem of escaping the Earth’s reach the Moon. First, he tied a string of vials filled with gravitational pull. In David Russen’s Iter lunare or voyage dew around himself, so that the rays of the morning sun to the Moon (1703) a huge spring device is proposed as a could draw him upward when evaporating the dew, but he means of launching a manned vehicle towards the Moon. only arrived in Canada on that attempt. Two years later, Daniel Defoe’s The consolidator tells us He then tried to launch himself from the top of a hill that ancient people mastered the art of space flight and used by a special spring-loaded vehicle, but the vehicle crashed to travel to and from the Moon. Defoe describes in particular down into the valley. After curing the bruises by smearing an intriguing engine based on the use of hollow bodies ox marrow on his body, the hero returned to his vehicle, but filled with ‘a certain spirit’ which produced a continuous he found that some soldiers had taken possession of it and fire. The fire acted on a spring-and-wheel mechanism were tying fireworks to it in order to make it appear as a which kept two large wings in motion. flying fire dragon. He tried to stop the soldier who was Many other tales of lunar travel can be found in eigh- setting fire to the fuse, only to find himself trapped in the teenth century literature, including Samuel Brunt’s A voyage vehicle while the latter lifted off to the clouds. to Cacklogallinia (1727), whose heroes are propelled into Tiers after tiers, all the rockets were rapidly exhausted, space by intelligent birds, and Murtagh McDermot’s A trip but instead of crashing down to Earth with the vehicle, to the Moon (1728), where a whirlwind takes the hero to Cyrano continued his ascension to the sky. The waning the Moon and gunpowder is used to return him to Earth. In Moon, he explained, used to suck up the marrow of ani- Voltaire’s Micromégas (1752) we find the first extraterres- mals, and that is why his greased body was pulled upwards. trial visitor of literary history: an intelligent giant arriving A few days later he finally landed on the Moon, where he on Earth from the star Sirius after a long journey through had many adventures and philosophical discussions with its the Milky Way and the Solar System. inhabitants. After returning to Earth, the hero decided to In the nineteenth century, science fiction became a visit the Sun, and the story is told in Cyrano’s second book, well-established literary gense, nourished by the dramatic Histoire comique des états et empires du Soleil (1662). progress of astronomy and by the growing importance of While presenting the first space travel performed by science and engineering in everyday life. Romantic tales and means of rocket propulsion, Cyrano’s works do not deal philosophical speculations were gradually replaced by scien- with science, nor even science fiction. They are instead tific and technical discussions of the various aspects of space remarkable examples of philosophical novels, a literary flight. Science fiction writers, however, still lacked the most device for the author to expound his materialistic ideas and important element: a credible means to escape the Earth’s his libertine vision of human affairs. European literature of gravitational field. An antigravity material called lunarium the seventeenth and eighteenth century counts several other was devised by the University of Virginia professor George 28 ARTURO RUSSO

Tucker in his novel A voyage to the Moon with some firing of the cannon and the story is then picked up in the account of the manners and customs, science and philoso- second book. Here we follow the heroes during their space phy of the people of Morosofia and other Lunarians, pub- travel. Owing to a large meteorite passing close to their lished in 1827 under the pseudonym of Joseph Atterley. Eight spaceship, the latter is displaced out of the planned trajec- years later, Edgar Allan Poe, a former student of Tucker’s, tory and ends by orbiting around the Moon. An attempt to sent his hero Hans Pfaall on a lunar trip in a home-made steer the vehicle towards landing by the use of the rockets balloon. In 1865, the Frenchman Achille Eyraud imagined fails and the voyagers are pushed towards the Earth, safely a spaceship powered by a reaction engine based on the descending in the Pacific Ocean where they are rescued by ejection of water for his Voyage à Venus. an American ship. The recognized father of modern science fiction is Jules Verne’s stories took advantage of the most advanced scien- Verne, whose novels De la terre à la lune (1865) and tific knowledge of his times, including calculations of escape Autour de la lune (1870) became very popular all over the velocity and flight trajectories, a sound determination of the world. The former tells of the long and careful preparation best geographic location for the launch cannon (in Florida, of a voyage to the Moon, to be accomplished by three not far from the present US launch facilities), and a realistic astronauts sitting inside an artillery shell fired into space by description of weightlessness in outer space. Inspired by the a huge cannon, called Columbiad. The shell was equipped general scientific optimism of the late nineteenth century, with a special mechanism for absorbing the shock, a chemi- Verne and his followers were less interested in writing cal system for air regeneration, and rockets for soft landing Moon tales (his heroes, in fact, do not land on the Moon), on the Moon’s surface. The book ends with the successful than in discussing in literary form the possibilities for

Figure 3 Illustration from De la terre à la lune by Jules Verne. Figure 4 Illustration from Autour de la lune by Jules Verne. (Source: AKG.) (Source: AKG.) 21

RICHARD MUSHOTZKY* Clusters of galaxies

Clusters and groups of galaxies are now recognized to be with infall velocities of ~2000 km s1 and total masses 15 65 fundamental constituents of the Universe. According to of 10 ML have a kinetic energy of 10 erg. The shocks modern theory, most of the material in the Universe that has and structures generated in the merger have an important collapsed into coherent structures lies in groups of galaxies, influence on cluster shape, luminosity and evolution and while clusters are the only entities in the Universe that may generate large fluxes of relativistic particles. These should be representative of the Universe as a whole. Thus mergers are best studied with X-ray imaging and spec- the study of groups and clusters of galaxies is of tremendous troscopy studies of the temperature, abundances and gas importance. One of the most surprising distinguishing char- density, which can reveal the entropy and pressure in these acteristics of these, the largest and most massive objects in systems. X-ray imaging is also required to distinguish the Universe, is that they are filled with hot X-ray-emitting between true mergers and the projections that one expects in gas whose mass exceeds (often by factors of 3Ð7) the mass a hierarchical universe (Henriksen and Markevitch 1996). of the galaxies in the groups and clusters. This gas is Numerical studies of large-scale structure formation enriched in heavy elements (e.g., oxygen and iron) and is show that groups are the tracers of the cosmic web, being thus the repository of most of the metals in the Universe. the connection of galaxies to clusters in a hierarchical Clusters are also exceedingly sensitive tracers of the cos- merger “tree”. Thus detailed surveys of groups will provide mological parameters of the Universe (such as its mass den- the best measure of the nature and form of large-scale 1 sity m, its age H0 , and the cosmological constant ) and structure, which in turn will provide detailed cosmological the nature of the primordial spectrum of density fluctua- information. tions. Detailed studies of clusters and their evolution as Because detailed measurements of the mass, metallicity, well as their baryonic content are among the most robust evolution, number density, and structure of groups and clus- estimators of the cosmological parameters (Bahcall et al. ters of galaxies are best performed with X-ray data, 1999) and at present are one of the strongest arguments for X-ray astronomy has become central to the study of clus- a low- universe. X-ray surveys, at present, are capable of ters. At present, X-ray data can yield the cluster mass, detecting clusters in a uniform robust way to redshifts 1.2 structure, and iron abundances out to redshifts of ~0.8 and thus provide the fundamental material for the study of (Donahue et al. 1999), measuring the abundances and dis- cluster evolution and its cosmological impact. tribution of other elements at z ~ 0.1, finding clusters at z ~ Cluster mergers are thought to be the prime mechanism 1.24 (Rosati et al. 1999), and determining the nature and of massive cluster formation in a hierarchical universe form of cluster number and mass evolution (Henry 2000) to (White and Frenk 1991) and represent the most energetic z 0.5. Because groups are much less luminous, their events in the Universe since the big bang. These mergers study so far has been limited to the local volume of space (z 0.05). Since they are extremely numerous there is * NASA Ð Goddard Space Flight Center, Greenbelt, MD, USA hope that detailed X-ray surveys optimized for the study of

Richard Mushotzky, The Century of Space Science, 473Ð498 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 474 RICHARD MUSHOTZKY groups will be able to map the cosmic web seen radiation from a collisionally dominated plasma with about in detailed numerical simulations (Cen and Ostriker 1999). a third of solar abundances (see Sarazin 1988 for a review). The hot gas in the cluster potential well will scatter While there may be significant contributions at both lower the microwave background and generate a signal whose and higher energies from synchrotron or inverse Compton strength depends on the path integral of the pressure in the radiation from relativistic particles (Sarazin 1999), the bulk cluster but is independent of distance (Sunyaev and of the radiated energy and the total energy in the cluster is Zel’dovich 1981). The ratio of the X-ray properties to the due to thermal particles. The observed range of temperatures microwave properties is distance dependent, and provides a for clusters and groups is 0.2Ð15 keV, and the luminosities direct physical means of determining the angular distance to range from 1042 to 3 1045 erg s1 (Mushotzky and Scharf the objects and thus measuring the Hubble constant and . 1997). There is a strong correlation between cluster lumi- (Frequently the Hubble constant is given in units of h, which nosity and temperature (Mushotzky 1984). The strongest means that it is normalized to a value of 100.) The emission line complex from a plasma of kT 3 keV is the Sunyaev–Zel’dovich effect provides an independent esti- He-like 6.67 triplet from Fe XXV, and the first studies of mate of the cluster gas mass, which has confirmed the X-ray cluster abundances were only able to measure the flux from estimates and reinforces the large baryonic ratios in clusters. that line complex. In more modern work, with the ASCA Knowledge of the nature of the galaxies in clusters and (Advanced Satellite for Cosmology and Astrophysics) and their evolution with cosmic time has been vastly enhanced BeppoSax satellites, the emission from the more abundant by the imaging capabilities of the Hubble Space Telescope elements heavier than oxygen has been measured. At the (HST). The detailed Hubble images from the UV to the lower temperatures representative of “poor” clusters and near-IR have enabled the age dating of elliptical galaxies, groups the spectrum is line dominated, much of the flux the most dominant population of cluster galaxies. being emitted by a veritable sea of Fe L lines. The HST optical data, when combined with the X-ray There is very little variance in cluster metal abundances measurements of cluster evolution, indicates that clusters are for massive systems with kT 3 keV, but at lower tempera- very old, even the most massive systems having formation tures (and masses) there appears to be a wide range of epochs at z 1, and most of the stars in the galaxies forming abundances and a variation in the range of Si to Fe abun- at z 2. This is contrary to the strong predictions of a 1 dances, indicative of a different formation of heavy ele- closed universe (Turner 1999). The great age of the stars in ments as a function of cluster mass. These lower-mass cluster galaxies also indicates that most of the metals, which systems also have more entropy per unit particle than is are found in the cluster gas, were formed at high z. indicated by the numerical simulations, indicating, perhaps, The exquisite Hubble images have also enabled the robust that there has been additional energy injected into the inter- measurement of gravitational lensing of background galaxies galactic medium (IGM) other than by gravity. The amount by the foreground cluster. While the lenses were discovered of energy implied is rather large and should have significant from ground-based imaging, one requires the HST images to effects on group and cluster formation. obtain accurate cluster mass profiles. These data can even Detailed analysis of the cluster temperature and gas den- place constraints on the fine-grain distribution of dark matter sity provide, via the equation of hydrostatic equilibrium, in clusters. In addition to providing measurements of the robust estimates of the total cluster mass and gas mass. It dark matter, lensing amplifies the light from the lensed became clear at the beginning of the 1990s (White et al. galaxies, allowing detailed measurements to be made of the 1993) that the dark matter in clusters accounted for “only” properties of high-redshift galaxies (Steidel et al. 1999). ~80% of the total mass. However, in the closed 1 Detailed numerical simulations of cluster formation and Universe popular at the time, dark matter should account evolution which include the effects of dark matter and gas for ~94% of the total mass. Since clusters are thought to be have reached quite high levels of sophistication (Eke et al. fairly representative samples of the Universe, this was a 1998). These calculations indicate that the gas is almost in strong indication that either the theory of cluster formation hydrostatic equilibrium and that the clusters should follow had a serious error or that 1, with a best estimate for a set of scaling laws. One of the most important of these being 0.3, for example the ratio of the baryons in the clus- laws is the relation between gas temperature and the total ters to the dark matter compared to the cosmic ratio. mass of the cluster, including gas, galaxies, and dark mat- In the centers of a large fraction of luminous clusters the ter. Theory indicates that the total mass should scale as the X-ray-emitting gas has a cooling time, in the absence of X-ray temperature to the power 1.5 and that for massive any known sources of heat, which is considerably less than systems the X-ray temperature is an accurate estimate of the Hubble time, so one anticipates that the gas should be the cluster mass. cooling (Fabian 1994). In the most spectacular examples of The 0.2Ð15 keV X-ray emission from most clusters of this phenomenon the gas should be cooling at a rate of 1 galaxies is dominated by thermal bremsstrahlung and line 1000 ML yr . X-ray spatial and spectral measurements CLUSTERS OF GALAXIES 475 provide striking confirmation of this idea, but the eventual it was not until the 1930s (Zwicky 1937, Smith 1936) that repository of the cooled material has proven to be elusive. they were recognized as very large conglomerations of The luminous galaxies that reside at the centers of cooling galaxies at great distances. The first dynamical analysis of flows have a wide variety of unique properties in the UV, clusters showed that there must exist much more gravita- optical, IR, and radio bands which indicate that the cooling tional material than was indicated by the stellar content of flow phenomenon has important consequences not only for the galaxies in the cluster. This was probably the first dis- the hot gas but also for galaxy formation and evolution. If covery of the preponderance of dark matter in the Universe. the final repository of the cooling gas is stars, as it clearly is However, the field did not advance much until the early for some of the material (O’Connell and McNamara 1988), 1960s, for what seems, in retrospect, to have been the sim- then this process could be a major factor in massive galaxy ple reason that astronomers did not want to accept that most formation. of the material in the Universe was dark. The field has undergone an enormous change, driven With ever larger ground-based telescopes, cluster primarily by a vast improvement in technical capability research developed along the lines of measuring the num- since the first discovery of X-ray cluster emission during bers, sizes, luminosity, and spatial and velocity distribution a rocket flight in 1968 (Friedman and Byram 1967). The of the galaxies. The development of large catalogs of clus- development of spacecraft allowed the first all-sky surveys ters (Abell 1958, Zwicky and Herzog 1963) based on eye with Uhuru in 1971 and Ariel 5 in 1975 (Gursky et al. estimates of the number of galaxies per unit solid angle 1971, Villa 1976), the development of low-background pro- was a major milestone in the field. The relatively strict cri- portional counters resulted in the first X-ray spectra with teria for the Abell catalog proved to be a good guide to the Ariel 5 (Mitchell et al. 1976) and OSO 8 (Serlemitsos et al. physical reality of the objects, and it is amazing that 40 1977); and the development of imaging X-ray optics years later we are still using this catalog. Extensive optical made possible the first X-ray images, with the Einstein follow-up studies of objects in these catalogs was more Observatory (Jones et al. 1979), and the large number of time-consuming, and it was not until the early 1970s (Rood images and spectra with Rosat, ASCA, and BeppoSAX in 1974) that the first large samples of estimated cluster mass the 1990s. The development of X-ray photon-counting from measurement of the velocity distribution of the galax- CCDs combined with high-throughput thin-foil optics led ies via the use of the virial theorem were obtained. to the first high-quality spectra of numerous clusters with By the early 1970s it became clear (see the pioneering ASCA. The tremendous improvement in sensitivity, spec- paper of Rood et al. (1972), and the detailed study of the tral resolution, and spatial resolution allowed by the spec- Coma Cluster by Kent and Gunn (1982)) that clusters of tacular increase in angular resolution made possible by the galaxies were dominated by dark matter, with galaxies rep- Chandra mirror and the technology breakthrough resulting resenting less than 5% of the total mass, and that there were in the very large collecting area of XMM-Newton will push definitive patterns in their galaxy content (Dressler 1980). X-ray cluster studies to new levels of sophistication and Thus the issue of the “missing mass” or “dark matter” detail in the first decade of the new century. The spectacular became the central theme of cluster research. improvement in spectral resolution that would have been The nomenclature of the field was partially defined by brought about by the calorimeter on Astro-E will have to Abell’s survey of the northern sky for clusters based on a await a future flight of this technology, due to the failure of visual search of the Palomar Observatory Sky Survey Astro-E to reach orbit. plates. He defined a set of “richnesses” and distance classes To go beyond these three new missions will require even based on the apparent magnitudes of the galaxies and the larger collecting areas and better spectral resolution, as will number of galaxies inside a fixed “metric” radius. Abell be possible with the Constellation-X mission under study in used strict criteria to define the physical reality of a cluster. the USA. While the progress in the 1990s was truly spec- “Rich” clusters (i.e., those with many galaxies inside a tacular, we anticipate even more rapid progress in the next fixed metric (Abell) radius) had a preponderance of “early” decade. type (elliptical and S0) galaxies, while “poorer” clusters had a larger fraction of spiral galaxies. It was clear that many clusters had a rather unusual central galaxy, a cD, or HISTORICAL CONTEXT centrally dominant galaxy (Morgan and Osterbrock 1969) which is very seldom, if ever, found outside clusters. There (See Bahcall (1977) for an early review.) Clusters of galax- was also an unusual type of radio source found primarily in ies are the largest virialized structures in the Universe, with clusters, a so-called WAT, or wide-angle tailed source 13 15 masses of 10 Ð310 ML and sizes of 1Ð3 Mpc. They (Owen and Rudnick 1976). There were the first indications were discovered rather early in the history of modern astron- of cluster evolution (Butcher and Oemler 1978) in which omy Ð by Herschel (as noted by Lundmark 1927) Ð but distant clusters at z ~ 0.2 tend to have “bluer” galaxies than 476 RICHARD MUSHOTZKY

low-redshift clusters (to an optical astronomer elliptical obtain accurate multi-color photometry for many hundreds galaxies have rather “red” colors while spirals tend to be has vastly improved the quality of the data. At present it is bluer), but the morphology of these galaxies was unknown. the combination of X-ray, optical, and even radio and IR However, the fundamental realization (Binney and data that yields the best constraints on the physical parame- Strimpel 1978, Heisler et al. 1985) that the mass of the ters of clusters. cluster could not be precisely determined from galaxy posi- The ability of modern instrumentation to observe clus- tion and velocity distributions severely hampered further ters at all mass scales over a wide range of redshifts has progress. The mass of clusters was parametrized by the so- made them one of the prime areas of extragalactic research. called mass-to-light ratio (in solar units, so the Sun has a Their use as cosmological probes, as large samples of value of 1). This is the ratio of the two quantities that can galaxies, and even as “fair samples” of the Universe has be well estimated by optical data: the total amount of light made the study of clusters a rather exciting area of research. in the system and a dynamical estimator of the mass (most From a theoretical point of view the rather “simple” physics often the virial estimator). For most clusters the values involved in their formation and evolution makes their prop- were ~400 h1 (the mass to light ratio depends on the dis- erties amenable to “first principles” calculation and allows tance to the cluster since the total light is proportional to a direct comparison of theory and observation. A quick flux distance2, while the virial mass estimator is propor- look at the main journals shows hundreds of papers a year tional to velocity dispersion cluster size). There were being published in this area, and the rate of increase is also technical difficulties with measuring the effective clus- rapid. ter size and the total amount of light, and with determining the velocity dispersion in the presence of sub-structure in THEORETICAL CONTEXT the cluster and foreground and background contamination (see Oegerle et al. (1989) for a status report). Physical models of clusters Since the mass-to-light ratio of “normal” galaxies is in the range of 2Ð12, this large value for clusters indicated the Modern theories of cosmology and large-scale structure are presence of large amounts of dark matter. However, mass- built on the assumption that cold dark matter (CDM) is the to-light ratio is not a basic astrophysical quantity since it main form of dark matter in the Universe and that, based on depends on the age, initial mass function, dust content, and inflation, there is an initial power density spectrum of fluc- metallicity of a stellar system as well as on the optical color tuations. This theory has a set of “free” parameters Ð the system in which it is obtained. Sensible values from 1 to 20 Hubble constant H0, the mass density in gravitating mater- can be obtained without the need for dark matter. Thus, ial m, the fraction of the closure density in cold dark mat- while these data strongly indicated the presence of dark, ter particles, the value of the cosmological constant , the non-baryonic material, its abundance and nature could not normalization of the power spectrum 8, the baryonic frac- be strongly constrained. tion (the fraction of the closure density in baryons) b, and One of the most surprising discoveries of the space age the amount of “hot” dark matter. While this is rather a large has been that most of the baryonic material in clusters is in set of free parameters, most of them are fairly well deter- the form of hot X-ray-emitting gas which is enriched in mined by observation. In this CDM theory, structure forms heavy elements to about a third of solar abundances. This in a “bottom-up” scenario in which small masses form first discovery came about in stages (see the next section), and it and larger structures such as clusters form later. However, is now realized that X-ray emission from clusters is a domi- this is a vast oversimplification, and a wide range of mass nant process and that, in some sense, clusters of galaxies scales collapse at a wide range of redshifts. For a m 1, are X-ray objects. This discovery was essentially entirely 0 universe (the paradigm before the late 1990s), clus- unpredicted and has led to fundamental change in our ters form at rather low redshifts, and Gott and Gunn (1972) understanding of the Universe. It is rather surprising to referred to the present as “the epoch of cluster formation.” realize not only that most of the material in the Universe is Because clusters are the largest virialized systems in the dark and non-baryonic, but that most of the baryons in the Universe they are at the tail end of the distribution of Universe do not shine in optical light. The anthropomorphic fluctuations, so-called 3 fluctuations. Their formation and picture that the Universe is best studied with the light visi- evolution is thus very sensitive to the cosmological parable to our own eyes is not only seriously in error, it drives meters (see Bahcall et al. 1999 for a recent review). Most science in the wrong directions. cosmological models, in fact, are normalized by the COBE Since the discovery of X-ray emission from clusters, spectrum of fluctuations and the abundance of massive the nature of the optical data has changed radically. The clusters Ð so-called cluster normalized models. ability of multi-fiber spectrographs to obtain velocities for Before the late 1970s theoretical work focused on hundreds of galaxies in the cluster and of CCD imagers to mechanisms for “relaxation” (so-called violent relaxation 22

RALPH A.M.J. WIJERS* Gamma-ray bursts

The only feature that all but one (and perhaps all) of the very Like Jansky (radio), Penzias and Wilson (microwave), and many proposed models have in common is that they will not be Giacconi et al. (X-rays), it was what they were not looking the explanation of -ray bursts. Unfortunately, limitations of for that started a new field: occasional flares of gamma rays time prevent me from telling you which model is the exception. lasting for just seconds, from extraterrestrial sources. Soon, (If I did so, I would suggest Black Hole ridden by Accretion as other satellites confirmed the discovery. the favorite in the race with Glitch as a dark horse if only A number of reviews have appeared that summarize the because so many different horses and jockeys are riding under that name.) Ð Mal Ruderman, Texas Conference, 1974 knowledge up to that point. Ruderman’s paper (1975) and its observational counterpart by Strong et al. (1975) give an When Mal so ended the first theory review on gamma-ray account of the initial explosion of discovery and thought. bursts (GRBs), I was on the other side of the world, practic- Higdon and Lingenfelter (1990) give a good overview of ing my fractions. But had he spoken these same words at the the pre-BATSE state of the field. The new discoveries made end of the fifth Huntsville GRB meeting in October 1999, his with the Burst and Transient Source Experiment (BATSE) favorite model (though not his reasoning) would have been on board the Compton Gamma Ray Observatory (CGRO), instantly recognized by all present as a very plausible and and the bewilderment following its overthrow of the galac- popular contender for GRBs. His dark horse is still easily tic disk neutron star paradigm, are described in the review identified with soft gamma repeaters Ð Plus ça change, plus by Fishman and Meegan (1995). The consequences of the que ça reste le même. What happened roughly in between the discovery of counterparts to GRBs and the state of the art two events is the topic of this tale. We shall see how the field at the end of the 1990s are described in reviews by Piran emerged out of the mutual distrust of the Cold War adver- (1999; emphasis on theory and early stages of the burst) saries in the 1960s, and was finally resolved by a determined and by van Paradijs et al. (2000; emphasis on observations effort of just about the entire membership of the United of afterglows Ð the later part of this chapter is based on Nations in the late 1990s. Its progress, of course, was in fits parts of that review). To the historically inclined, review and starts, interwoven with stagnation and frustration. papers are only part of the story because they reflect more Since rays are absorbed by less than 1% of the atmos- the final outcome of research than the struggle to reach that phere, they are quintessentially the domain of space science. outcome. To get a flavor of the latter throughout the history The initial motivation for launching satellites with -ray of the subject, I recommend a number of conference pro- capability was not, however, scientific. The purpose of the ceedings. Theorists’ struggles to come to terms with the US Air Force’s Vela satellite program was the detection of phenomenon in the early days are reflected in the contribu- nuclear explosions in the upper atmosphere or in space, tions to the 1974 Texas Symposium (Bergmann et al. which were prohibited by the first ever test ban treaty. 1975). Various stages of progress in the next decade are documented in the proceedings of meetings in La Jolla * State University of New York at Stony Brook, NY, USA; present (Lingenfelter et al. 1982) and Stanford (Liang and address: Universiteit van Amsterdam, The Netherlands Petrosian 1986). The first conference of the BATSE/CGRO

era and the accompanying wind of change is documented in At the beginning of a new century, this field is likely to the proceedings of the first Huntsville GRB symposium explode into a major branch of astrophysics. (Paciesas and Fishman 1992). After the discovery of coun- The organization of this chapter is mostly historical, terparts, a meeting was called on short notice in Elba, Italy though where dictated by logic I complete the discussion of (26Ð27 May 1997). Its atmosphere of excitement mixed some topics when they first arise, to prevent too much sub- with astonishment and confusion is firmly etched in my division of the narrative. It begins in the days when every memory; since the organizers wisely chose not to burden gamma-ray photon had a name, and it ends with the demise the attendees with writing proceedings papers, this feeling of the CGRO, halfway between the popular and the logical will forever remain the privilege of those who were there. end of the twentieth century. I dedicate this account to Jan The excitement was still very great during what I think of van Paradijs, who taught me so much. Despite his untimely as the best meeting I ever attended: the fourth Huntsville death on 2 November 1999, his science will last well into GRB symposium (Meegan et al. 1998). the twenty-first century. At this point I should describe my own path into the field, since it may have an impact on the tone and content of this review. I was in primary school when the discovery of GRBs 1 THE EARLY DAYS was announced and remained blissfully unaware of them until some time during my PhD training in Amsterdam. I did 1.1 Coming in from the cold not start active research in the field until Bohdan Paczynski« ’s unlimited enthusiasm swept me into it in 1991. Therefore As described by others in this volume, high-energy astron- my accounts of the pre-BATSE era are entirely derived omy started in the 1950s, because it required high-altitude from the literature, and from conversations with those balloon and satellite technology to get equipment above the researchers I interacted with Ð by no means an unbiased atmosphere for long times. Even 1% of the atmosphere sample of the field. Reviews are written with the sanitizing absorbs most gamma rays, so being in space is essential censorship of 20/20 hindsight and thus do not fully reflect (though some observations and new technology tests can be the bouts of inspiration and confusion that characterize done with balloon experiments). In the history of GRBs, the ongoing research: here lies a challenge for those who were other important point to note is that the early days of satel- there to see it. Having come into GRBs by way of lite astronomy coincided with the peak of the Cold War. A Princeton also means that my allegiance in the distance practical way was needed to verify compliance with the first scale debate of the early to mid-1990s was firmly extra- test ban treaty, which prohibited the detonation of nuclear galactic. While that point of view did eventually prevail, weapons in space. Since it was possible in principle to hide this outcome cannot have been so clearly predictable as I the initial explosion (by detonation behind the Moon), one thought it was then, as is nicely illustrated by the reports of type of detector was designed to catch the MeV gamma rays the “Great Debate” on the GRB distance scale (Fishman from radioactive decays in the debris cloud that would drift 1995, Lamb 1995, Paczynski« 1995). A good perspective on into view some time after the explosion. For this purpose, the merits of such debates can be obtained from Trimble’s the US Air Force in collaboration with the Los Alamos and (1995) discussion of the ShapleyÐCurtis debate. The field Sandia Laboratories operated the Vela satellites (Figure 1; of soft gamma repeaters started as part of GRBs but has the name is derived from the Spanish velar, which means slowly split off as a separate discipline; its recent develop- “to watch over” or “to hold a vigil”). They flew in pairs on ment has been similar in rapidity and magnitude to the opposite sides of a 250 000 km diameter orbit. GRB revolution, but this has been somewhat overshadowed Clandestine nuclear explosions were not found, but by the latter. I discuss them only briefly in this chapter something else was Ð short, intense bursts of gamma rays (Section 4). from random directions in the sky. The first one on record A large part of this chapter is devoted to the events fol- dates from 2 July 1967 (Figure 1), but the discovery was lowing 28 February 1997, when the first X-ray and optical not published until 1973 (Klebesadel et al. 1973). It is counterpart to a GRB was discovered. The developments sometimes stated that this was due to the classified nature since then have been fast and furious: the first two counter- of the mission, but this is false: since the objective of the parts settled the distance debate over GRBs firmly in favor satellites was to deter violations of the treaty, their exis- of the extragalactic scale, making them the most powerful tence was well advertised ( just as shops conspicuously explosions since the Big Bang. Many issues are still not advertise their closed-circuit cameras). However, since only quite settled, and thus this part of the story has a decidedly a few people worked on the data, and did so by hand, it less finished feel. Still, the discovery that they are somehow took time for them to convince themselves that spikes in the associated with young stars and can be observed out to very highly variable -ray background were real signals and had high redshifts opens up many new avenues of development. a cosmic origin. The basic method for doing this is still GAMMA-RAY BURSTS 501

1500

1000

counts/second 500

0 –4 0–2 2468 Time (seconds)

Figure 1 The Vela satellite (here Vela 5b), used by the USA to verify compliance with a ban on nuclear explosions in space (left). The light-curve of the first GRB it ever saw, on 2 July 1967, is also shown (right). (Images courtesy of “Astronomy Picture of the Day,” at http://antwrp.gsfc.nasa.gov/apod/astropix.html.) used in modern instruments: First, one requires more than contains an interesting scribbled note to the conclusion one detector to see the signal; and second, one uses the dif- section: the reader comments that such intense flashes in ference in arrival time of the burst signal between two TeV gamma rays should be visible with the TeV telescope widely separated satellites to constrain the arrival direction at Mount Hopkins. With the kind help of the people at (Figure 2). In this way, the early researchers eventually ADS, I discovered that the owner of that copy of the convinced themselves that the transient gamma-ray events Astrophysical Journal was Professor J. Grindlay, who con- they saw did not come from the Earth or the Sun. firmed to me that he had indeed considered the issue seri- It is interesting to note that the first observational paper ously. The problem, of course, was the same as with all already had a model to discuss. A few years earlier, Colgate other counterpart searches: one did not have any means of (1968) had published his idea that a flash of gamma rays getting the very early and accurate locations required to may be produced when the shock powering a Type II super- aim a telescope at the GRB locations quickly enough. nova breaks out of the surface of a red giant. Because the Klebesadel et al. (1973) commented that there have been shock conserves energy, it becomes ever hotter as it moves no detected supernovae near the GRBs they detected, and down the density gradient, and at the point where the therefore dismissed the model. Very recent developments (Thomson) optical depth becomes unity, it emits photons have put supernovae very much back into the picture with energy comparable to the electron rest mass. Since the (Section 6.2). medium is moving towards us with a Lorentz factor of The first few facts about GRBs emerged quickly. Cline about 1500, we see these blueshifted to GeV energies, and et al. (1973) used a hard X-ray detector on the IMP 6 satel- the flash time shortened to tens of microseconds. In later lite, designed for solar flare observations, to confirm the papers (Colgate 1973, 1974), he considers cooling before discovery and show that the spectra of the bursts really did shock break-out and revises his estimates to more nearly peak in gamma rays (thus excluding the possibility that the observed burst durations and photon energies. The they were just high-energy tails of some type of X-ray scanned version of the 1974 paper held in NASA’s ADS event). A telescope with some directional capability on database (http://adsabs.harvard.edu/abstract_service.html) board OSO 7 confirmed the extrasolar origin of one burst 502 RALPH A.M.J. WIJERS

observed with Apollo 16 that has enough signal above 1 MeV to show that the high-energy part is also a power

GAMMA law (Metzger et al. 1974). The time histories are usually BURST quite spiky, with overall durations of 0.1 to 100 s and sharp A fluctuations down to 16 ms, the finest time resolution of the Velas. The locations of the bursts seem to show no significant preference for any part of the sky, except that two events are consistent with the location of the well-known accreting source Cyg X-1 (a fact that contributed consider- θ ably to Ruderman’s pronouncement on theories). It is also D12 noted that the fluence distribution for bright events is S1 S2 B N (S) S3/2, the expectation for sources whose density 2–3 ANNULUS C∆T does not depend on distance (often referred to as the COS θ = S3 D12 “Euclidean” distribution). Cline (1975), in the same volume, 1–2 ANNULUS confirms the high-energy end of the spectra to be a power law, and argues that the claimed flattening of the fluence THE TRIANGULATION METHOD distribution at low fluence is not significant. With hindsight, his balloon detections must have been something other than Figure 2 When two or more satellites detect a GRB, the GRBs, since the fluence distribution is now known to be arrival time difference of the signal between each pair of flatter than Euclidean at the fluence values he reports. satellites can be used to constrain the location of the burst to a Enter Ruderman, who, also in the same volume, has the narrow circle on the sky. With three or more satellites, a true task of deciding which of the models to date survive error box – sometimes as small as a few arc minutes across – scrutiny. The overview of models later given by Nemiroff can be constructed. This principle underlies the Interplanetary (1994) lists 15 models dated 1974 or earlier; most of them Network (IPN), still in operation. (Image courtesy of K. Hurley.) are discussed in Ruderman’s (1975) paper. He recognizes the limitations of theorists: “Most theoretical astrophysicists function well in only one or two normal modes. (Wheaton et al. 1973). Incidentally, the first GRBs for Therefore, we often tend to twist rather strenuously to con- which a cosmic origin could be established were observed vince ourselves and others that observations of new phe- with the Vela 5 and 6 series satellites, which had enough nomena fit into our chosen specialties.” He finds the timing accuracy to pin down the directions to GRBs suffi- brilliant solution of inviting as many as possible to enlist: ciently well. The burst of 2 July 1967 was detected with the “For theorists who may wish to enter this broad and grow- Vela 3 and 4 series, and is classified as the first GRB only ing field, I should point out that there are a considerable on the basis of its similarity to the later ones, not because number of combinations, for example, comets of antimatter the direction to it is known to be inconsistent with a solar or falling onto white holes, not yet claimed.” terrestrial origin. Ruderman largely dismisses the extragalactic models, in The first published Soviet observation of a GRB, using part because Colgate’s supernova model is damaged by the the Kosmos 461 satellite, dates from 1974, and is by the non-detection of supernovae coincident with known GRBs. Leningrad group (Mazets et al. 1974). Igor Mitrofanov has However, he also derives a constraint on distances from a told me that they too knew earlier that they might have black-body limit to the luminosity, assuming the observed something cosmic on their hands, probably in about 1971. photon energies are comparable to the thermal energies of the emitting particles, which virtually excludes extragalactic models beyond 30 Mpc. He acknowledges this to be 1.2 The earliest observations and theories wrong for some radiation mechanisms, but nonetheless the From here on the pace of discovery picked up quickly. In constraint appears to have impressed the audience. One of the review by Strong et al. (1975), in the proceedings of his acknowledged ways around the constraint is synchro- the seventh Texas Symposium, 34 bursts are listed. Their tron radiation, since for this mechanism the emitted pho- “sizes” (now called fluence or time-integrated flux) are in tons do have much lower energies than the particles that the range 105104 erg cm2. The spectra, where avail- emit them; of course, we now know synchrotron radiation able, show a power law at low energies and then a bending to be the dominant contributor. Among the galactic models to a steeper decline at a few hundred keV. At first this he discusses a variety of “conventional” models, namely bending is fitted with a thermal bremsstrahlung function, magnetic flares on various types of object and accretion that is, an exponential cutoff, but there is already one burst events in compact objects. He then presents a number of 23

MARTIN ELVIS* Quasars

To the naked eye the night sky is filled with stars. But outside output. They also shoot out enormous jets of fast particles, of a quite narrow slice of the whole electromagnetic spectrum travelling at 99.999% of the speed of light. These jets stay a radically different sky is seen. Over two huge frequency narrow over huge distances, easily as large as the distances ranges, each about 10 decades broad, utterly different objects between whole galaxies. Such unusual objects as quasars dominate the view. Both at the long, radio wavelengths, and should surely be found in unusual places, and indeed quasars at the high X-ray and -ray energies, the brightest things in inhabit the very centers (“nuclei”) of galaxies. We believe the sky are quasars. Only in the three decades of frequency that the origin of the strange properties of quasars is a black from the ultraviolet, through the visible band and into the hole with a huge mass, 106 to 109 times that of the Sun. infrared do stars dominate, and in the infrared much of the Here I give a personal view of quasars across these starlight is seen only indirectly, having been absorbed and 20 decades of spectrum, and across four decades of dis- re-emitted by dust around the stars. Three decades is much covery, of which I have only witnessed two myself. After broader than the octave-wide band to which our human eyes telling the history of quasar discovery, I set out seven key are sensitive but, compared with the 20 decades of frequency quasar mysteries, describe how space obsevations gradually opened up to astronomy during the space age, it is small. brought the whole quasar phenomenon into view, and then Quasars1 are truly space-age astrophysics. Of the two explain each mystery, so far as we now can. I end with a wide bands where quasars dominate, the radio sky can be few speculations on where quasar research will go next. explored quite well from the ground. To explore the high- energy sky though we have no choice but to use telescopes mounted on spacecraft. 1 A QUICK QUASAR HISTORY What are these objects that dominate the sky over most of the spectrum? In complete contrast to stars, quasars are Understanding the quasar puzzle has been a long-term extremely distant objects seen back to early cosmic times, program. There were hints of strange activity in the nuclei and are the most powerful (“luminous”) continuous2 sources of some galaxies as long ago as 1917 when Slipher found of radiation in the Universe. Moreover, the light emitted by extraordinarily broad emission lines coming from the cen- quasars, even though it reaches to the highest energy -rays ter of the galaxy NGC 1068, and the next year when Curtis we can measure, is not the whole of the quasar power found a jet pointing straight out of the center of the elliptical galaxy M87 in Virgo. Quasar prehistory took another step, 25 years later, when Karl Seyfert’s 1943 PhD thesis * Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA described a few galaxies with extremely rapid gas motions 1 For simplicity and clarity I will refer to all the types of non-stellar in their nuclei (as determined from the Doppler-shifted “activity” in galactic nuclei as “quasars,” even though the research literature carefully distinguishes high-luminosity “quasars” from lower width of their peculiar spectral emission lines; Slipher had luminosity “active galactic nuclei” (or AGNs). Evidence for any basic rejected this explanation). Just a few years later, using sur- physical difference between these types of active objects has diminished, plus World War II radar equipment, radio astronomy blos- essentially to the vanishing point. 2 For a few seconds or minutes a gamma-ray burst can outshine any somed, and for the first time found a sky not dominated by quasar. stars. In 1953 some of these new radio sources turned out to

Martin Elvis, The Century of Space Science, 529Ð548 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 530 MARTIN ELVIS come in pairs, either side of a galaxy, suggesting ejection find no strong arguments against it. The results for from the galaxy, but there was little hard evidence for this. 3C 273 and 3C 48 were published six weeks later in four The true history of quasars did not begin for another consecutive articles in Nature (Hazard, Mackay, and decade, until 1963. Hard-won precise positions of radio Shimmins 1963; Schmidt 1963; Oke 1963; Greenstein sources enabled the large optical telescopes of the day to take and Matthews 1963). spectra of whatever optical object lay at that position. Some showed the starlight of normal galaxies, but often at large So these things were not stars, they simply appeared distances (as measured by the redshifts of their spectral lines, stellar on photographs, and hence they were dubbed “quasi- using the Hubble relation between redshift and distance stellar objects” which quickly became “quasars” (Hazard that describes the expansion of the Universe) (Chapter 17). 1979). Astronomers were not prompt to accept the idea that The spectra of those with compact, “stellar”-looking, optical quasars were at cosmological distances. This was not objects, though, had a baffling series of strong, broad emis- because this distance seemed too great: after all, there was sion lines. It took three years before Maarten Schmidt real- already the example of the large redshift (z 0.46) of the ized that one of these “radio stars” was actually at a large radio galaxy 3C 295. The problem was that, if the redshifts redshift, and so Ð given the Hubble relation between redshift implied distances according to the normal Hubble law and distance Ð presumably lay at a large distance from us. (Chapter 17), then it implied a second power source must This in turn implied a prodigous luminosity. be at work in the Universe, beyond the nuclear burning at Schmidt (1990) relates how his key discovery was made: the centers of stars. That is because it is one thing is to find that a steady source is very distant and powerful, and quite The puzzle was suddenly resolved in the afternoon of another thing to find that a variable source is far away, February 5, 1963, while I was writing a brief article because then it must emit its power from a region smaller about the optical spectrum of 3C 273. Cyril Hazard had than the time it takes light to cross it. In the case of quasars written up the occultation results for publication in this means the power of a whole galaxy of stars comes from Nature and suggested that the optical observations be a region smaller than the Solar System! (r(Solar System) published in an adjacent article. While writing the manu- 50r(EarthÐSun) 50 AU 7.5 1014 cm.) Any such script, I took another look at the spectra. I noticed that source has to be more efficient than nuclear burning can be four of the six lines in the photographic spectra showed (see next section). And the first quasars found, 3C 273 and a pattern of decreasing strength and decreasing spacing 3C 48, indeed varied quite strongly. It was this variability from red to blue. For some reason, I decided to construct that had fixed the idea in astronomers’ minds that these an energy-level diagram based on these lines. sources, of stellar optical appearance, must be nearby stars. I must have made an error in the process which But the high redshifts made that position untenable. In the seemed to contradict the regular spacing pattern. 40 years since the first redshift of 0.16, quasars have contin- Slightly irritated by that, I decided to check the regular ued to be found out to larger and larger redshifts (Figure 1), spacing of the lines by taking the ratio of their wave- reaching 5.8 in 2000, implying they were already around lengths to that of the nearest line of the Balmer series. when the Universe was only 5% of its current age, or less The first ratio, that of the 5630 line to H-beta, was 1.16. than 1 billion years old. The second ratio was also 1.16. When the third ratio was 1.16 again, it was clear that I was looking at a Balmer spectrum redshifted by 0.16. I was stunned by this development: stars of magnitude 13 are not supposed to show large redshift! When I saw Jesse Greenstein minutes later in the hallway and told him what had happened, he produced a list of wavelengths of emission lines from a just completed manuscript about the spectrum of 3C 48. Being prepared to look for large redshift, it took us only minutes to derive a redshift of 0.37. The interpretation of such large redshifts was an extraordinary challenge. Greenstein and I soon found that an explanation in terms of gravitational redshift was essentially impossible on the basis of spectroscopic arguments. We recognized that the alternative explanation in terms of Figure 1 The largest redshift of quasars as a function of time. cosmological redshifts, large distances, and enormous The current record is z 5.80. (From Fan et al. 2001.) For luminosities and energies was very speculative but could early data see Schneider et al. (1992) and Fan et al. (1999). QUASARS 531

2 SEVEN QUASAR MYSTERIES either long or short wavelengths is the feature that makes quasars dominate the sky over most of the spectrum. The huge research literature on quasars revolves around a 3. Quasars accelerate material to high velocities. The quite small number of central questions or mysteries. While widths of the emission lines reach up to 10% of the these are capable of infinite subdivision I believe there are speed of light (0.1c), although most are a smaller, but just seven basic mysteries. still considerable, 0.03c. Some quasars show absorp- It is remarkable that with the first quasar papers, already tion lines, and some others show X-ray emission lines, four of the seven quasar mysteries were laid out: implying velocities up to 0.2cÐ0.3c. Most extreme of all, to create the observed “superluminal” motions in blazars 1. Quasar luminosities are enormous, and arise from tiny requires 10, i.e., υ 0.999c (Chapter 24; Ghisellini regions. The power from quasars covers a wide range et al. 1993). Ways to produce the broad emission and (107) from object to object: from 1040 erg s1 to absorption lines without using mass motions are con- 1047 erg s1, and can compete with, and easily exceed, trived, but not impossible. Large velocities in astronomy that of a whole galaxy of 1011 stars (1044 erg s1). imply deep gravitational wells (to accelerate matter, or When Mathews and Sandage (1963) found that quasars to produce a general relativistic gravitational redshift), could vary their power output by 40% in just a few except for short-lived explosive events like supernovae. months, then it seemed that all this luminosity must 4. Quasars have linear symmetry. As radio maps became come from a region similar in size to our Solar System. more detailed through the exploitation of interferomety (From the simple “light travel time argument”: a source (work which led to a Nobel prize for two pioneers, Martin cannot vary coherently in less time than a light signal Ryle and Anthony Hewish) most of the radio sources were takes to cross the source. So if a source varies in a day, it found to have a “double lobe” structure (Jennison and is no bigger than a light-day across (but beware of high Das Gupta 1953) extending well beyond the galaxies they velocities; Chapter 24).) Moreover the radio galaxies straddled. With the advent of the Very Large Array (VLA) required that quasars put out enormous power over a in the 1970s these lobes could be mapped in detail at high long time period since the total energy stored in the dynamic range revealing pairs of large (1022Ð1024 cm) giant radio lobes must be 1060 ergs (Burbidge 1959). highly structured bubbles (Figure 2). In many, but by no At this point it became a good bet that an energy source means all, cases the central galaxy had a nucleus with a quite different from, and much more efficient than, spectrum like that of a quasar. Forty years later the fine nuclear burning in stars was at work. detail of Hubble Space Telescope images showed that 2. Quasar spectra are nothing like that of starlight. Stars even the quasars that are not bright radio sources have a have more or less black-body spectra with well-defined linear symmetry. Clear cone-shaped structures were seen temperatures of a few thousand degrees, so their emission stretching over a galaxy scale (1022 cm). In high-redshift in the radio band is tiny. Instead quasars can have huge radio-loud quasars (primarily in “radio galaxies”) Hubble radio luminosities with spectra that follow a simple images showed galaxy-scale optical line emission aligned straight-line form in log (flux) vs. log (frequency), i.e., a with the radio jets. A big clue to the inner structure of “power law” from 100 MHz to 100 GHz. So being quasars has to lie in this symmetry. bright in the radio does not simply mean that they are very cold objects. Their optical spectra have no obvious From these four mysteries an outline of quasars was thermal peak either, but also have a power law shape from already becoming clear: a large mass in a small region sug- 1 m to 0.1 m (Figure 3). Confusingly at first this gested that the, then outlandish seeming, concept of a black meant that quasars were both “bluer” than stars (i.e., had hole might underlie quasar physics, while the symmetry sug- more short-wavelength emission than expected based on gested a spin axis, either of the black hole or of a rotating gas their visual brightness) and “redder” than stars (that is, the disk. A short paper by Donald Lynden-Bell (1969) put this same at long wavelengths)! The whole spectrum is not together with extraordinary elegance. He pointed out that just one power law though, since the radio and optical energy release from gravitational infall is more efficient than power laws do not join up. When X-ray spectra first nuclear burning for any mass large enough to produce quasar became available, they too had a power-law shape, and luminosities. He then outlined how to release this energy as again it did not connect with the optical or radio power radiation: any material falling onto a black hole from some laws (Figure 3). In this sense quasars have no tempera- large distance will have some angular momentum around the ture, and so were called “non-thermal” sources. That does black hole and so, unless the incoming matter is perfectly not mean though that no combination of thermal spherically symmetrical, the material will fall into a flattened processes underlies the production of the quasar contin- shape to form an orbiting disk. (This is common in astron- uum (see Section 5.2). This failure to drop off in power at omy, recall Saturn’s rings, or nascent planetary systems.) If 532 MARTIN ELVIS

Figure 2 The complex two-sided radio structure of Cygnus A as imaged by the VLA. The large features, or “lobes”, are fed by a jet of relativistic particles coming out of the central “core” (white dot), which lies at the nucleus of a large galaxy. (Perley et al. 1984; courtesy NRAO.) enough matter accumulates then some (undertermined) form to pick out from normal stars in large numbers just from of viscous friction between adjacent orbits will drag on the a pair of photographs taken through different color fil- faster moving material, moving angular momentum outward, ters. This “color selection” was used to find the quasar and allowing matter to fall inward, eventuall accreting into using the, initially quite uncertain, radio source posi- the central black hole. As the matter falls inward the gravita- tions. Hence quite a large area of sky was photographed tional potential energy is released as radiation. At each radius for each radio source. Alan Sandage (1965) first saw that the radiation will be more or less a black body, but since along with the radio-emitting blue quasar, there were each radius emits a different temperature (getting hotter many more blue “interlopers” that had nothing to do inward, as the potential gradient of the black hole steepens) with the radio source. Spectra showed that these too were so the summed emission from all radii looks nothing like a quasars, but ones which were “radio quiet.” Radio-quiet black body, in fact it can mimic a power law. quasars turn out to be 10 times more common than the These “accretion disks” have since been found in our original quasars, which are now called, with impeccable galaxy around many compact objects: white dwarfs, neutron logic, “radio-loud quasars.” The difference between radio- stars, and stellar-mass black holes (Chapter 32), beginning loud and radio-quiet quasars is found only in the radio with the discoveries of the Uhuru satellite (Giacconi and spectrum (and possibly the -ray spectrum). The rest of Gursky 1974). These smaller systems in which it was possi- the spectrum, from far-IR to hard X-rays is essentially ble to establish that a black hole was almost certainly pre- the same (Elvis et al. 1994). (A slight difference of sent made the idea of black holes and accretion disks more power-law slope in the X-rays being the only exception.) familiar and better understood, and they are now a conven- Yet the difference in the radio band is huge, a factor of tional part of astrophysics (Frank et al. 1985, 1992). Nikolai 100Ð104 in the ratio of radio to optical luminosity sep- Shakura and Rashid Sunyaev (1973) expanded on accretion arates the two classes. disk theory by cleverly hiding all the unknown physics of 6. Quasars are far less common now than they were in the viscosity in a parameter, , and working out the disk behav- distant past. This was termed quasar “evolution”.* The ior as a function of . It took only a few years of searching for fainter and * I find this an unfortunate choice. All we really mean is “change with fainter optical counterparts of radio sources up to higher time.” In biology evolution demands mutation, reproduction, and selection, and higher redshifts to discover two further mysteries: and is an active, unpredictable process that will produce different outcomes even with identical starting conditions. “Evolution” in astronomy is merely the playing out of physical processes, with no reproduction or competition 5. Radio-loud quasars are just the tip of the iceberg. The and with a predictable outcome. Chaotic systems are an exception but still blue “non-thermal” spectra of quasars made them easy involve no analogs to the biological process of evolution. 24

GABRIELE GHISELLINI* Blazars

Up to the seventeenth century, the unaided eye was the only hole gravity, it is compressed, heated, and then it radiates. receiver that humanity could use to observe the Universe. This was realized quite early (Salpeter 1964; Zeldovich Evolution was able to adapt this “instrument” to be sensitive 1964; Lynden-Bell 1969; Shakura and Sunjaev 1973). to the light of the star we happen to be orbiting, the Sun. Another major advance came with the opening of the The invention of the telescope amplified the sensitivity of X-ray window, first (in the 1960s) with rocket experiments the eye and its angular resolution, letting humanity discover, pioneered by Riccardo Giacconi, Bruno Rossi, and others, less than a century ago, that other galaxies exist, far beyond and then with the first X-ray satellites in the early 1970s. The the Milky way, and that these galaxies are moving apart: the Uhuru, Ariel 5, HEAO 1 and then Einstein satellites made Universe expands. However, all we could discover using the clear that all kinds of quasars were strong X-ray emitters: at eye and its extension, the optical telescope, regarded a tiny, the same time, it started to be believed that quasars were per- very tiny, part of the entire electromagnetic spectrum. As haps the major contributors to the cosmic diffuse X-ray back- soon as technology enabled us to open new windows, we ground, already discovered in 1962 (Giacconi et al. 1962). discovered other phenomena, other objects, and could push A third qualitative “jump” was the improvements of the our knowledge farther out in space and time. interferometric technique in the radio band, again in the The opening of the radio window made the 1960s the early 1970s. Radio telescopes in different continents, look- golden decade for astronomy, with the discovery of the ing at the same source, can resolve details as close as a few microwave background and of pulsars. The third great dis- tenths of a millisecond of arc. This enabled us to discover covery made in that decade was that of quasars (Chapter 23). that some radio-emitting quasars have spots of radio emis- The term “quasar” originally stood for “quasi-stellar radio sion which are observed to move. Sometimes this motion source.” In fact when an optical telescope is pointed towards corresponds to velocities that exceed the speed of light. the direction of these radio sources, which can be as Far from challenging special relativity, this “superluminal” extended as minutes of arc in radio maps, the resulting motion, as it is now called, was even predicted before it was optical plate shows a source which looks like a star, that observed, by Martin Rees in 1966, and corresponds to real is, an unextended, or “pointlike” object. This apparently fast motion (but slower than the velocity of light!) at an innocuous point is instead a gigantic energy plant, able to angle close to our line of sight. produce much more power than an entire galaxy like our The fourth advance came with the Compton Gamma own, in a volume which is extremely small, if compared Ray Observatory (CGRO, launched in 1991) which discov- with the dimension of the Galaxy, and comparable with our ered that the subclass of quasars called blazars are very Solar System. The process that powers the stars, thermonu- strong -ray emitters, producing at energies greater than clear reactions, is not efficient enough to power quasars. We 100 MeV more power than in the rest of their electromag- believe that at the core of these sources there is a massive netic spectrum. The properties of blazars are very extreme, black hole, with a mass between a million and a billion that and we believe that this is partly due to special relativistic of the Sun. Matter around the hole is attracted by the black effects because their emitting plasma is moving with a bulk motion at large (υ 0.99c) speeds. Blazars and gamma-ray * Osservatorio Astronomico di Brera, Merate, Italy bursts are indeed the realm of special relativity: effects that

Gabriele Ghisellini, The Century of Space Science, 549Ð560 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 550 GABRIELE GHISELLINI were of academic interest up to 10 years ago suddenly quasars). At this time (the 1980s) the LPQs (low polariza- became the keys to understand their behavior. tion quasars) were left out, according to the belief that the The discovery of quasars and blazars literally opened up polarization characteristics were peculiar and important. the scale at which we can see the Universe, since they are But this belief became progressively weaker, up to the so powerful that they can be easily seen up to great dis- discovery of the (often dominant) -ray emission in both tances. For 30 years quasars were the most distant known blazars and LPQs. Then LPQs joined the class of blazars in objects in the Universe. And studying these sources we the 1990s. Linear polarization in the radio band, when study the effects of both general and special relativity, that observed, is an important tool to diagnose the absence of is, how space and time change as measured in different any cold electron component: if present, these electrons frames. could depolarize the flux due to Faraday rotation of the polarization angle (Wardle 1977). This argument does not apply in the case of electron and positron pairs (they make the polarization angle rotate in opposite directions, and the BL LACERTAE OBJECTS AND BLAZARS depolarization effect cancels out). All blazars so far can be classified as radio-loud objects, The “strange star” BL Lacertae but the level of radio emission, with respect to the optical, The star BL in the constellation of Lacertae had been varies widely. In fact objects selected through X-ray surveys known for many years from its optical variability. When the (Extended Medium Sensitivity Survey (EMSS) (Wolter radio source VRO 42.22.01 was associated with it, people et al. 1991), the Einstein SLEW survey (Perlman et al. thought they had discovered the first radio star. But unlike 1996), and the Rosat All Sky Survey (RASS)) can be con- other stars, its optical spectrum was completely featureless sidered as radio weak, since they have a ratio between the and this puzzled astronomers. Lots of other things were radio flux and the X-ray or optical flux much smaller than peculiar about BL Lacertae. The shape of its spectrum did the blazars found in radio surveys. not look thermal, but it was a power law (that is, F() To classify a source as a blazar is not an easy task. ). In addition, the optical light was highly polarized, There is, however, a useful theoretical definition: There are and a high degree of polarization was taken as the signature sources characterized by a relativistic jet producing non- of synchrotron emission, produced by ultrarelativistic elec- thermal radiation. This radiation is Doppler boosted along trons spiraling along magnetic field lines. A lot of observ- the velocity direction, coincident with the jet axis. We call ing time was invested in this source to obtain deeper blazar a source whose non-thermal radiation dominates images and spectra. In the end weak absorption lines were over the isotropic components (for example, thermal emis- observed, from which a redshift of z 0.07 was obtained sion from the accretion disk and nonthermal radio emission (Oke and Gunn 1974; Miller and Hawley 1977). This estab- from extended regions). lished that BL Lacertae was not a star after all, but a type of Observationally, we have the following properties of quasar. blazars: After the discovery of this prototype, other “stars” were found with similar characteristics, which were named “BL ¥ Rapid variability Ð The flux of all blazars varies violently, Lacertae objects,” or “Lacertids” (or “BL Lacs” for short). with large amplitudes in short times. Variations of a factor At the same time, it was realized that other radio sources, of two in hours are common at X- and -ray energies, showing the broad emission lines of typical quasars, were slightly smaller variations are seen in the optical in a also extremely variable in the optical and were named single night. Some objects have varied by more than two OVVs (optically violent variables) because of this. Their orders of magnitude over a few years. There are many optical spectrum does not generally show the flattening types of variability: there are objects whose flux varies towards the UV indicating the presence of the thermal com- continuously at all frequencies, without any quiescent ponent due to the blue bump (the thermal emission from phase. There are other objects undergoing violent outburst the accretion disk) and is therefore non-thermal in origin. phases from normally quiescent states. Therefore, if an Another class of radio-loud quasars, the HPQs (high polar- object is not continuously monitored for a long time, it ization quasars) showed polarization at a level greater than may be difficult to detect its violent variability. 3%, and it was finally realized that HPQs were also opti- ¥ Emission at all wavelengths Ð Blazars are active emitters cally variable, that is, OVVs and HPQs were the same class. all across the electromagnetic spectrum, from radio to It was then found that both BL Lacertae objects and HPQs -rays. If we plot the quantity F versus their spectrum have a flat radio spectrum, at least at high frequencies, is characterized by two very broad humps. The peak above 1 GHz. These similarities led to the definition of the energies are located in the IRÐsoft X-ray band and in the general class of blazars (contraction of BL Lacs and radio MeVÐGeV (and even TeV) bands, respectively. BLAZARS 551

¥ Non-thermal continuum Ð Most of the emission (and of the source light up at different times, giving us the (false) power) has a power-law shape in restricted energy ranges impression of motion. But observations made it clear that (a mere decade or so), with weak or absent blue bump. what was observed was always a motion from the center to ¥ Flat radio spectrum Ð All known blazars, above 1 GHz, the outer parts of the source, while the “Christmas tree” have a flat radio spectrum dominated by the core of the jet. model predicts also contraction “motions.” Furthermore, if the source is moving relativistically, then its radiation is beamed in the direction of motion, as described below, and JETS AND FAST MOTIONS we indeed have strong evidence for such beaming.

We are now used to seeing beautiful radio maps which can Superluminal motion “zoom” in a radio source from the megaparsec to the parsec scale, showing a jet which remains collimated at all scales. According to the now accepted explanation of superluminal υ Often there are lobes and hot-spots of more intense radio motion, the apparent speed app c app of the radio blobs emission at the ends of the jets. There the energy accumu- which are observed to move is due to a projection effect. lates through million of years, and only a small part of it is Once the constancy of the speed of light is accepted, super- radiated away, the rest is used to make the lobe grow and luminal motion can be explained by simple geometry, with advance in the intergalactic medium. no further involvment of special relativity. In the beginning, it was not clear at all how the energy The key point is that the source, while emitting photons, could be transported from the very small center of radio is moving towards the direction of the observer. Suppose galaxies to regions million of light years away. Then Martin we take two maps of the same radio source, separated by a Rees, in his PhD thesis (1966), made the suggestion that time interval tobs. When the source emits the photons of this job could be done by matter moving relativistically in a the first map, it is located at the position A (Figure 2). jet. The spectacular prediction he made, out of his idea, was After a time tem, the source is located at B. The segment superluminal motion: blobs of radio-emitting matter appar- AB c tem c tem, and the distance of the two light ently moving at a velocity greater than the light speed. fronts in the direction of the line of sight is HC Five years later, with the completion of the first Very c tem c tem cos c tem(1 cos ). This translates Long Baseline Interferometry (VLBI), superluminal motion to a time interval tobs tem(1 cos ). The two was indeed observed in the blazar 3C 279 (Figure 1; maps show the “blob” in two different positions, whose Whitney et al. 1971). projected distance, HB, is c tem sin . We then obtain Soon after the discovery of the superluminal motion in app sin /(1 cos ). For small viewing angles υ 3C 279, 3C 273, and other sources, alternative theories (a few degrees) and for relativistic speeds ( c), app can were challenging the idea of Rees. One of the most debated be greater than c. was the so called “Christmas tree” theory: different regions Definition and importance of beaming It is believed that in blazar jets the plasma is flowing with a bulk speed c close to the speed of light. In terms of the

Figure 2 Explanation of the observed superluminal motion. When in A, a “blob” emits a photon. After a time tem, the Figure 1 The spectrum of 3C 279 at various epochs. Note source is in B and emits a second photon. The two photons the extremely variable flux at all energies, particularly in the are separated by HC c tem(1 cos ) and arrive at CGRO band (0.1–10 GeV). (Adapted from Wehrle et al. 1998 the observer separated by a time interval tobs tem and Maraschi et al. 1998.) (1 cos ). 552 GABRIELE GHISELLINI

corresponding Lorentz factor, (1 2)1/2, we derive produce the radio emission we see. Knowing directly the values between 5 and 20. Therefore it is essential, in study- size by VLBI, one derives the particle and the photon densi- ing blazars, to take into account relativistic effects, the most ties. Therefore one can estimate the probability of interac- important of which is called the beaming effect. This is the tions between synchrotron photons and relativistic electrons, sum of three effects: predicting the flux at high energies, especially in the X-ray band. These estimates often turn out to be orders of magni- (1) Assume that in the frame co-moving with the plasma, tude greater than observed. If we now assume that instead the radiation is emitted isotropically. Then half of the source is moving towards us at relativistic speeds, the the photons are emitted in the forward directions, half fluxes we receive are enhanced, therefore less electrons are in the backward directions. An observer, however, sees needed to produce this emission, and the calculated photon the plasma moving and so will see that half of the density at the source is lower. The interaction probability photons are contained in a cone of semi-aperture angle is less than before. The estimate of the X-ray flux then equal to 1/. This is due to the aberration of light. depends on the Doppler factor: by matching the predicted (2) Since the source of radiation is moving, we must take X-ray flux with the observed flux we can put an upper limit into account the change of frequency of the observed to the amount of beaming. It is an upper limit because the photons (Doppler effect). radio region we are observing at the VLBI may not be the (3) The rate of arrival of photons is not equal to the rate of only contributor to the X-ray flux. emission, as measured in the co-moving frame (if we run towards a source, we will collect more photons per second than if we do not run). Radiation processes These three effects also depend on the angle between In tenuous plasmas interactions between particles (which our line of sight and the velocity vector of the plasma. They depend on their density) are not efficient in sharing the conspire to make the emitting source much brighter if we energy between the particles. If the injection of energy is see it at small viewing angles (photons are concentrated in due to the acceleration of few ultrarelativistic particles, then this small solid angle, they are seen blueshifted, with an the particle distribution has no chance to become thermal enhanced rate of arrival), and much dimmer at large view- (that is, described by a Maxwellian distribution). In rarefied ing angles. Therefore the (frequency integrated) intensity I relativistic plasmas the most efficient ways to produce of a moving source depends both on its velocity and the radiation are the synchrotron and the inverse Compton viewing angle: I 4I where I is the intensity seen by a processes. The first depends on the strength of the magnetic co-moving observer, and 1/[(1 cos )] is the so field, the second on the density of the seed photons to be called Doppler boosting factor. We can see that the intensity scattered. When these seed photons are produced by the can be enhanced by orders of magnitudes when is small synchrotron process the resulting mechanism is called the (a few degrees) and c. synchrotron self-Compton process. For 20 years this was Note that relativistic motion greatly affects the variabil- thought to be the main mechanism responsible for the ity timescales we measure. Suppose that in the co-moving observed radiation, from the radio to the X-ray band, until frame the source flares in a time t. According to special the high-energy -ray emission was discovered (e.g. relativity, this time would be seen, at Earth, longer by a fac- Ginzburg and Syrovatskii 1965; Rees 1967; Blumenthal and tor . But this is true only if we do not use photons to mea- Gould 1970; and Jones et al. 1974a,b). The origin of the sure this time! If instead we do, then we must take into high-energy -ray emission is still controversial, since the account that the source has emitted the first photons when it contribution of radiation produced externally to the jet is was located in a certain region, and the last photons when enhanced by the bulk motion (see below) and can overtake it was located in another position. Therefore the two the locally produced synchrotron radiation as a producer of bunches of photons traverse paths of different length to seed photons to be scattered (Maraschi et al. 1992; Dermer reach us. This is the Doppler effect, and contributes a factor and Schlickeiser 1993 Sikora et al. 1994; Blandford and 1 cos . The two effects combine to give t t Levinson 1995; Ghisellini and Madau 1996). It is also fair to (1 cos ) t/: for small viewing angles the mention that even the inverse Compton nature of the -ray observed time is shorter, not longer! emission is controversial: as Mannheim (1993) suggested, The enhancement of the observed radiation by beaming it could be due to the synchrotron process, due to an addi- can be tested through the following argument. If one assume tional population of ultrarelativistic electrons as a result of that the radio emission, as mapped by VLBI techniques, is photonÐmeson interactions. Independent of its nature, the produced by the synchrotron process in a stationary source, high-energy emission (in the X- and -ray bands) firmly then one can calculate how many electrons are needed to established the need for relativistic bulk motion. 25

GIUSEPPINA FABBIANO* AND MARTIN F. KESSLER** X-ray and infrared properties of normal galaxies

WHY GALAXIES FROM SPACE population peaks. The importance of widening the observing OBSERVATORIES? window on galaxies is made clear by ground-based radio and near- to mid-IR observations, which have provided a Galaxies are key components of the Universe and have been way of penetrating deeper into dusty star-forming regions; studied in astrophysics for several different and important have allowed the study of the cold ISM and thus the discov- reasons. Galaxies trace the distribution of matter and can be ery of extended, dark, massive halos in spiral galaxies; and used as standard candles to derive information on the expan- have revealed the properties of both galaxian magnetic fields sion and large-scale structure of the Universe. From these and highly energetic particles. results constraints can be set on both cosmology and cos- With space observatories, the observing window on galax- mogony scenarios. Galaxies evolve: they form, their stellar ies has been opened even further, and a whole new set of population evolves, they interact and merge. Understanding phenomena has come into view. The waveband coverage has the forces governing galaxy formation and evolution is para- been extended to the UV (International Ultraviolet Explorer, mount for our understanding of the properties of galaxies IUE; Hubble Space Telescope, HST; Extreme UltraViolet and their components. Galaxies are where primordial matter Explorer, EUVE), mid- and far-IR (Infrared Astronomical gives birth to stars, and where stars evolve and form Satellite, IRAS; Infrared Space Observatory, ISO), and X- elements, enriching the interstellar medium (ISM) from rays (Einstein; Roentgen Satellite, Rosat; Advanced Satellite whence the next stellar generations will be born. Therefore for Cosmology and Astrophysics, ASCA; BeppoSAX; our understanding of stellar evolution and chemical evolu- Chandra; XMM-Newton). Each of these wavebands has tion is intimately connected with our understanding of given astronomers the means to explore new aspects of galaxies and their evolution. galaxies, including states of extreme star formation and Most of the work on galaxies Ð be it related to cosmol- nuclear activity, as well as the different phases of the ISM ogy, galaxy formation and evolution, or present-day galaxy (cold, warm, hot, gaseous, dusty) and the latest stages of stel- properties Ð has been supported by ground-based, mostly lar evolution. In this chapter we concentrate on the impact of optical observations. While a lot has been learned this way, X-ray and IR space observatories. This is not a complete the optical window is only a small part of the entire emission review of all the work that has been done in these areas, spectrum of galaxies, and the information it provides is but is meant to give a general feel for these fields as they mostly limited to the normal stellar component, whose emis- are today. sion peaks in the optical wavelengths. Moreover, optical light is significantly absorbed by ambient dust, especially in the blue, where the emission of the massive younger stellar A BRIEF HISTORY OF X-RAY OBSERVATIONS OF GALAXIES

* Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA ** ESA Ð European Space Research and Technology Centre, Noordwijk, The first X-ray satellite, Uhuru, was launched from Kenya The Netherlands in December 1970. It was sensitive to photons in the

2Ð10 keV energy range. Uhuru is not an acronym; it means 39 “freedom” in Swahili, and was so named because the day of the launch was the National Holiday of Kenya. Uhuru had 38 been conceived and built by a team of physicists-turned- UHURU M31 10 sqm astronomers, led by Riccardo Giacconi at AS&E in XRB 37

Cambridge, MA. Uhuru led to the discovery of intense Chandra (0.1 sqm) x X-ray emission from compact stellar remnants (neutron 36 XMM-Newton (0.5 sqm) t~10 4 s

stars, black holes, white dwarfs) in binary systems in the SNR

Log L (ergs/s) N>100 Milky Way, as well as from active galactic nuclei (AGNs; 35 Einstein / ROSAT / ASCA (0.01-0.02 sqm) see Chapter 23), and hot plasmas in clusters of galaxies. X-ray emission from sources in the Magellanic Clouds and 34 Virgo CVO* from M31 was detected with Uhuru, but other normal galaxies escaped detection because their fluxes were below 33 2 3 the Uhuru source detection threshold (see Giacconi and 0.1 1 10 10 10 Gursky (1974) for a review of Uhuru results). Distance (Mpc) To be able to detect and study in X-rays a significant number of normal galaxies, we had to wait for the first true Figure 1 The sensitivity of different X-ray satellites to the detection of 100 counts in 10,000 s observations from typical X-ray telescope, the Einstein Observatory, also developed by galactic sources at different distances. For imaging telescopes a team led by Giacconi, including groups at the Smithsonian the collecting area is indicated. Note, however, that unless Astrophysical Observatory, Massachusetts Institute of the telescope has good enough angular resolution (Figure 3) Technology, Goddard Space Flight Center, and Columbia the sources are likely to be confused, and therefore their University. Einstein was launched by NASA in 1978 individual contributions cannot be detected. The y-axis gives (Giacconi et al. 1979). With the Einstein grazing-incidence X-ray luminosities as well as typical X-ray emitters in given mirrors, X-rays from celestial objects could be focused onto luminosity ranges, from luminous O stars to cataclysmic position-sensitive focal plane detectors. Focusing optics variables (CV), supernova remnants (SNRs) and X-ray binaries resulted in two very significant advantages: the relatively (XRBs). The x-axis is a distance scale. Two benchmark small beam (~5 to ~45, depending on the Einstein focal distances, to M31 and the Virgo Cluster of galaxies, are plane instrument) meant that the background noise in the marked by vertical stripes. detection area was much smaller than in any of the previous non-imaging X-ray satellites, resulting in the detection of Virgo z=0.5 1 thousand-fold fainter sources (Figures 1 and 2); and the imaging capability for the first time let us see clearly the 43 t~10 4 s morphology of the X-ray sources (Figure 3). These capabil- N>100 ities were crucial for opening up the X-ray window on galaxies. Normal galaxies feature in two papers in the first 42 UHURU ever set of Einstein publications. The survey of M31, the

Andromeda Galaxy, the large spiral galaxy closest to us, x 41 revealed a population of bright point-like sources and gave us for the first time a picture of what our own Milky Way Log L (ergs/s) might look like in X-rays to an external observer (Van 40 Einstein / ROSAT / ASCA Speybroeck et al. 1979). The peculiar asymmetric extended emission of M86 in the Virgo Cluster suggested the pres- ChandraXMM-Newton ence of a hot gasous halo interacting with the hot cluster 39 10 sqm gaseous medium, which had been discovered with Uhuru 10 102 10 3 10 4 (Formal et al. 1979). The foundations of what we now know about the X-ray emission of galaxies were estab- Distance (Mpc) lished with Einstein observations (Fabbiano 1989). Figure 2 The sensitivity of different X-ray satellites to the Subsequent X-ray satellites, the GermanyÐUSA Rosat, the detection of global emission from galaxies. Galaxy luminosi- JapanÐUSA ASCA, and the ItalyÐNetherlands BeppoSAX, ties are mostly in the 1039–1041 erg s1 range, but some E and have further expanded our knowledge of the X-ray proper- S0 galaxies can be as luminous as 1042–1043 erg s1 (Fabbiano ties of galaxies and widened the observable X-ray window 1989). Note that a significant advance in collecting area in several ways. Rosat was more sensitive at lower energies (to 10 m2) will be needed to begin studying normal galaxies at than Einstein and had slightly better angular resolution. cosmologically relevant distances. X-RAY AND INFRARED PROPERTIES OF NORMAL GALAXIES 563

M31 Virgo z=0.5 inferior to those of Einstein and Rosat, and as a result detailed studies of individual galaxies cannot be pursued in 104 most cases. We are now at a very exciting time for X-ray astronomy. In July 1999, NASA successfully launched Chandra, the 103 third Great Observatory, following the Hubble Space Telescope and the Compton Gamma Ray Observatory. The European Space Agency (ESA) successfully followed suit 102 with XMM-Newton in December of the same year. Chandra Chandra (0".5) and XMM-Newton represent a complementary quantum leap XMM-NewtonEinstein /(15") ROSAT (5") in sensitivity, imaging capabilities, and spectral resolution Linear size (pc) 10 ASCA (~2') compared with the previous generation of X-ray observatories, and both are going to deepen and perhaps revolutionize the way we look at galaxies in X-rays. 1 0.11 10101023 X-RAY PROPERTIES OF GALAXIES Distance (Mpc) Extrapolating from the results of Uhuru and subsequent Figure 3 Linear sizes (in parsecs) equivalent to the angular resolution of X-ray telescopes as a function of the galaxy dis- non-imaging X-ray missions (the UK Ariel 5; the tance. While Chandra’s sub-arcsecond resolution is needed to USÐNetherlands Astronomical Netherlands Satellite, ANS; resolve dense stellar regions in M31 (e.g., near the nucleus), and the US SAS 3 and HEAO 1), one could conclude that Einstein, Rosat, and XMM-Newton can resolve and study most not much excitement lay ahead in the X-ray observations of bright X-ray sources. At the distance of the Virgo Cluster, galaxies. This was indeed the feeling of a good number of Chandra achieves a linear resolution comparable to that of scientists in the Einstein team during the initial discussions Einstein and Rosat for M31. The individual source detection of time allocation to different observing programs. It was sensitivity of previous missions (Einstein and Rosat), as well as thought that normal galaxies would emit because of their that of XMM-Newton, is significantly impaired by confusion population of galactic sources, as happens in the Milky within the parent galaxy at the Virgo distance. The angular res- Way, and that the X-ray emission would be totally domi- olution of ASCA is significantly inferior to that of the other nated by the nuclear source in the case of AGNs. While this imaging X-ray missions. Note that 104 pc – ASCA’s linear resolution at Virgo – is a typical galaxy size. is true, it is only a small part of the story. Twenty years ago, when X-ray astronomers first pointed Einstein at normal galaxies, they were engaged in a very successful fishing These characteristics have led to a better understanding of expedition. The Einstein survey of normal galaxies led to the spatial morphology of the X-ray emission of galaxies, the discovery of hot ISM in E and S0 galaxies and of hot and to the detection and study of the soft X-ray-emitting gaseous outflows from starburst nuclei, and demonstrated ISM of spiral galaxies. ASCA, was the first X-ray observa- how X-ray emission is not an isolated phenomenon to be tory equipped with CCD detectors. These detectors have studied by X-ray astronomers, but is instead closely con- spectral resolution 5 to 10 times superior than previous nected to the global emission properties of galaxies (see instruments, and are able to reveal X-ray emission lines reviews by Fabbiano (1989, 1996a) and the Einstein catalog from hot plasmas. Thus for the first time a study of the and atlas of galaxies (Fabbiano et al. 1992)). metal composition of the hot ISM of galaxies could be seri- Galaxies are key objects in the study of cosmology, the life ously attempted. ASCA (and BeppoSAX) also extends the cycle of matter, and stellar evolution. X-ray observations have observing windows to energies of 10 keV, significantly given us a new key band for understanding these building higher than the 2Ð4 keV upper reaches of Einstein and blocks of the Universe, with implications ranging from the the Rosat. This allowed the study of different spectral compo- study of extreme physical situations, such as can be found in nents of the X-ray emission, peaking in different energy the proximity of black holes, or near the surface of neutron ranges. For example, a softer component from a hot ISM stars; to the interaction of galaxies and their environment; and a harder component possibly from a population of to the measure of parameters of fundamental cosmological X-ray binaries can be detected in the ASCA spectra of importance. In this chapter we discuss the unique contribu- elliptical galaxies (Matsushita et al. 1994). However, tions to our understanding of the Universe that are provided ASCA’s angular resolution (Figure 3) is ~2, significantly by X-ray observations of galaxies. 564 GIUSEPPINA FABBIANO AND MARTIN F. KESSLER

X-ray binaries and supernova remnants telescope (~10 m2) with Chandra-like resolution will be A review of the Einstein and first Rosat results on discrete needed to sample at significant depths the luminosity func- X-ray sources in external galaxies can be found in Fabbiano tion of discrete X-ray sources in galaxies at the distance of (1995a). Although the sample of detected sources has the Virgo Cluster and beyond (Figures 1 and 3): The poten- increased since, the basic conclusions are still valid. X-ray tial of these X-ray population studies is great, with implica- observations of the Milky Way and the Magellanic Clouds tions for the study of the most massive portion of the stellar demonstrate that the majority of bright, individual X-ray mass distribution, that can be detected through remnants in sources are binary systems containing one of the compact luminous X-ray binaries, and for a systematic study of the remnants of stellar evolution: white dwarfes, neutron stars, or interplay of SNRs with the ISM in different galaxies. black holes. The X-ray emission is the result of gravitational Super-Eddington sources: Black hole accretion of the atmosphere of the companion star onto this hunting grounds compact object. Thus, the discrete X-ray source population of galaxies gives us a direct view of the end stages of stellar A surprising and interesting result of imaging observations of evolution, and of the higher-mass component of the stellar nearby galaxies has been the large number of sources population. While the study of individual sources is best detected with luminosities well above the Eddington restricted to the Milky Way, because of the much higher luminosity for solar-mass (1ML) accreting objects fluxes, external galaxies present unique advantages in the (~1038 erg s1). The Eddington luminosity is the luminosity study of the population properties of X-ray sources. All at which the gravitational pressure of the accreting material sources within a given galaxy are at the same distance from is balanced by the radiation pressure, and it represents a nat- us, so the study of luminosity distributions is much less ural limit on the power that can be radiated by an accreting prone to error than with X-ray sources in our Galaxy, where object. Given their luminosities, which exceed those of the distance uncertainties are large. If fairly face-on galax- Milky Way sources and sources in M31, these very lumi- ies are targeted, the effect of differences in interstellar nous sources cannot be steadily accreting neutron star bina- extinction will also be much smaller than in the Milky Way ries. They can be divided into a few categories: nuclear sample, where directional effects due to the position of the sources; X-ray counterparts of young SNRs; and other Solar System in the galactic plane affect the measurements. unidentified objects. With the exception of the SNR counter- For the same reasons, systematic studies of properties of parts, and of sources that future higher-resolution observa- supernova remnants (SNRs), that can be used to explore their tions may reveal as extended complex emission regions, evolution, are best done with external galaxies. Most SNRs these sources may host intermediate-mass to massive black are spectacular, extended, expanding shells of hot plasmas holes (Fabbiano 1995a, 1998, Makishima et al. 2000). and energetic particles, created by the shock waves of the X-ray emission from massive nuclear black holes in supernova explosion and the interaction of the stellar debris nearby galaxies gives us a probe into the accretion mecha- with the surrounding ISM. An example of a study of a well- nisms and has implications for the evolution of AGNs defined sample of SNRs, for which the distance is known (or (see Chapter 23). Relatively low-activity AGN and LINER all at the same distance within a galaxy), is the diameterÐ (low-ionization nuclear emission line region) nuclei have luminosity relation, which can probe both the age of the been detected as luminous X-ray sources (in the range remnant and the host ISM (e.g., Long and Helfand 1979). ~10391041 erg s1) (Fabbiano 1996b). Although these lumi- Because high-quality imaging is needed for these stud- nosities would be in the super-Eddington regime for a normal ies, Einstein and Rosat have been the two observatories that X-ray binary, for nuclear sources due to accretion onto mas- have contributed most to this field. Observations of nearby sive nuclear back holes, the accretion must be very inefficient. spiral galaxies have revealed variable sources with charac- This is certainly the case for the nuclear source in M104, teristics similar to those of Milky Way sources. The lumi- the Sombrero Galaxy (Fabbiano and Juda 1997), where the 8 nosity distributions appear to vary in different galaxies. dynamics of the nuclear area point to a 10 ML black hole A notable but not unique case is that of M81, where the (Kormendy and Richstone 1995). Given this nuclear mass, X-ray sources appear brighter than in M31 and the Milky the X-ray luminosity of the nucleus of the Sombrero Galaxy Way (Fabbiano 1995a). is ~4107 that of the Eddington luminosity. While these studies open exciting new avenues, much In M33, a bright and variable nuclear source is also more sensitivity and resolving power than has been available present (e.g., Peres et al. 1989), but its nature is more mys- so far are required to extend them beyond exploratory exer- terious. It is possible that it may be a super-Eddington cises. Chandra is already making a difference, as can be accretion binary (Takano et al. 1994). seen from the images released to the public, but very long Some at least of the non-nuclear super-Eddington exposures will be needed in most cases. A large-area sources may be accretion binaries. Their luminosities 26

STEVEN L. SNOWDEN* The hot part of the interstellar medium

In the preparation of this chapter there arose issues of cooler ISM of the disk which expel hot plasma into the halo semantics. Specifically, what does the “hot part” mean in (in this chapter the “halo” of the Galaxy is defined as the reference to the interstellar medium (ISM), and just what region of space above the neutral material of the disk, and part of the hot ISM should be considered? As the original includes both ionized and neutral gas associated with the task for this chapter referred to the X-ray and EUV Galaxy and affected by its gravitational potential; this is a (extreme ultraviolet), appropriate temperatures are in the fairly loose definition which extends from a couple of hun- several 105 K to several 106 K range. In a Universe where dred parsecs to tens of kiloparsecs, or even farther, above the fundamental temperature is currently about 3 K and in a the plane). For the purposes of this chapter, a partially soci- Galaxy where most of the material in the ISM has tempera- ological definition of the hot part of the interstellar medium tures less than 104 K, any component at 106 K must have will be used: that part of the ISM which is (1) observed in been produced by fairly extreme and unusually spectacular the 0.05 to 1.0 keV band as the soft X-ray diffuse back- methods. The most common of these methods in the gen- ground (SXRB) and EUV background, (2) associated with eral galactic disk are supernovae, occasionally aided and the Milky Way disk or halo, and (3) not claimed by others abetted by the stellar winds of massive stars. However, as for studies of supernova remnants or stellar wind-blown supernova remnants (SNRs) are ably covered elsewhere in bubbles. To simplify the presentation and to acknowledge this text (see Chapter 41), the topic for this chapter required that at least the observed part originates in general from further refinement. What happens to a SNR when it slips some of the same plasmas, the EUV will be included with 1 past middle age into its dotage? All of the visual features 4 keV X-rays, despite the historical separation by NASA of that allow a SNR to stand out clearly from the background the astrophysics and astronomy programs at 0.1 keV. fade: the shock fronts (the interface region between the Furthermore, by and large this chapter will focus on the 1 expanding SNR and the undisturbed ISM) with occasion- 4 keV SXRB because it is the dominant galactic back- ally spectacular radio, optical, and X-ray emission, and the ground and, at least locally, it is the best understood. hot and relatively dense interior with bright X-ray emission. Besides, SNRs that are still emitting strongly at energies 1 As the SNR fades it also increases in size to become a more greater than 4 keV typically still stand out clearly against significant, or at least a more extensive part of the ISM. In the general background and fail consideration (3) above. essence it becomes old and boring, and fades into the Despite the observational difficulties in the study of has- woodwork; a pale ghost of its former self. been SNRs and their lack of spectacular features (e.g., the Thus the study of the hot ISM, at least in the galactic wealth of structure revealed by the initial Chandra observa- disk, is essentially a study of what happens to SNRs that tions of the more youthful Cas A and Crab Nebula SNRs), a have lost their identity, though there are exceptions. few true believers have pursued the study of the SXRB over Although the study of hot plasmas in the halo of the galaxy the last 30 years with some diligence. Like X-ray astron- may have cosmological considerations, there are ties to the omy in general, it is a field that has truly required the disk through the galactic gravitational potential and through “Century of Space Science,” as photons with energies of “chimneys,” breakouts of SNRs through the confining 0.05 to 1.0 keV travel at most millimeters in Earth’s atmosphere at sea level. The detectors must therefore be placed * NASA Ð Goddard Space Flight Center, Greenbelt, MD, USA above the atmosphere at altitudes of 200 km. The SXRB

required the advent of astronomical research sounding rockets in the late 1950s and early 1960s to provide the facility for its observation. Over three decades of study, the observations of the SXRB have progressed from limited regions of the sky provided by single sounding-rocket flights, to all-sky mosaics provided by multiple flights, to all-sky surveys provided by satellite-borne observatories. 1 The initial discovery of the 4 keV SXRB in the late 1960s (Bowyer et al. 1968, Henry et al. 1968, Bunner et al. 1969) was interpreted in the context of an extragalactic background. This interpretation was an obvious one suggested by the general negative correlation between the surface brightness of the SXRB and the column density of galactic neutral hydrogen (H I)*, as well as the expectation that there could possibly be extensive X-ray emission from intergalactic space as the low-energy extension of the extragalactic background above 2 keV first observed by Giacconi et al. (1962). The negative correlation observed at 1 keV was assumed to be due to the absorption of an Figure 1 Average mean free path of EUV and soft X-ray pho- 4 tons in the Milky Way as a function of energy, along with typi- isotropic X-ray flux of distant (extragalactic or galactic cal band-response functions (detector effective areas) for a halo) origin by the foreground material of the galactic disk. sample of soft X-ray and EUV detectors. The mean-free-path The fact that the required absorption cross-sections were 3 curve assumes a constant space density of nH 0.5 cm and smaller than the theoretical values could be explained by uses Morrison and McCammon (1983) absorption cross-sec- possible clumping of the cooler ISM. The nonzero flux tions. The Be band response function is from the Wisconsin observed in the galactic plane, where the Galaxy is com- detector (Bloch et al. 1986), and has been multiplied by a fac- pletely opaque to extragalactic photons in this energy tor of 50 for display purposes. The B band response curve is range, was attributed to additional background components, from the Wisconsin survey (McCammon et al. 1983). The 1 3 of either non-cosmic (e.g., a charged-particle background) band response functions of the 4 keV and 4 keV bands are from or cosmic (e.g., unresolved galactic point sources) origin. Rosat (Snowden et al. 1995). The band response functions Thus, the first big step toward our understanding of the show the sensitivities of the respective instruments as a function of energy (the larger the values, the more able they are to SXRB went, as will be shown, in somewhat the wrong detect X-rays of that energy). direction. However, also from the first step, the study of the SXRB was correctly linked to the ISM, in this case cooler components of the ISM that can absorb an X-ray flux of through the Galactic disk, one optical depth is reached after distant origin and modulate its apparent surface brightness. a path length of 100 pc, assuming a smoothly distributed 1 3 By its nature, the 4 keV SXRB is closely linked to the ISM with a space density of 0.5 H I cm . cooler ISM, whether or not it originates in the galactic disk or as an extragalactic background. The column density of H I (NH, the amount of material along a line of sight) required for EVOLUTION OF THE DATA unit optical depth for the absorption of X-rays by the ISM is shown in Figure 1 as a function of energy. In the This section will focus on the data collected over the last 1 20 2 4 keV energy range this is NH 10 cm . Even at its mini- three decades and how they influenced our understanding of mum high-galactic-latitude column density (NH 5 the SXRB. It will not cover all contributions to the field (with 19 2 10 cm ), the Milky Way provides roughly half an optical apologies to those left out) but will hopefully give a feel for depth of absorption as viewed from Earth for X-rays of extra- the evolution of the data. While some reference will be made galactic origin. At lower latitudes with longer path lengths to models for the origin of the SXRB, the detailed discussion of such models will be deferred to later in this chapter. *Astronomy has the quaint convention of using roman numerals to indicate the ionization state of elements. However, a single “I” indicates neutral material. (In fact, the “I” is the label for the “first spectrum” of a given element, which by convention is the spectrum of the neutral overestimate.) Initial observations and the early years Thus, H I refers to neutral hydrogen and H II refers to ionized hydrogen (otherwise known as a proton). The ion O VI referred to later in this chapter As noted above, observations of the SXRB have evolved is five-times-ionized oxygen, or O5. from the results of a limited number of sounding-rocket THE HOT PART OF THE INTERSTELLAR MEDIUM 583 flights covering limited regions of the sky to all-sky surveys Bunner et al. (1969) presented the last of the initial triad produced by orbiting observatories. The first published of SXRB observations, and reported results which were observation was the result of a sounding-rocket scan by more-or-less consistent with the previous two. In their Bowyer et al. (1968) which observed the northern Galactic analysis they considered three separate alternatives for 1 hemisphere between the center and anticenter directions (the aspects contributing to the angular structure of the 4 keV anticenter refers to the direction in the Galactic disk which SXRB: from Earth is 180 from the galactic center), scanning over 1. The small effective absorption cross-sections were due, the galactic pole. They observed a distinct negative correla- as in Bowyer and Field (1969), to clumping of the tion between the surface brightness of the SXRB in the absorbing medium into optically thick clouds. Although 1 keV band and the column density of galactic neutral 4 they did not find the evidence for the existence of such hydrogen that they attributed to the absorption of an extra- clouds compelling, clumping of the H I into clouds of galactic background. The fitted absorption cross-section was N 1021 cm2 satisfied the intensity-dependence con- significantly less than the model prediction from atomic H straint of the 1 keV data with absorption column density physics, a result which was left as having mostly unspeci- 4 but was still consistent with the 0.5Ð1.0 keV data. How- fied strong implications for the ISM after a discussion of a ever, even with the assumed clumping, an additional soft possible incorrect value for the He/H ratio.* Higher X-ray component was required to explain an excess of emission intensities were observed in the direction of the galactic observed above the extrapolation of the extragalactic center, which were attributed to an anomalous emission power law identified at higher energies. component and interpreted as possible evidence for a galac- 2. The observed flux in the galactic plane was due to an tic corona (a hot galactic halo). The Bowyer et al. results unknown solar or terrestrial background. This assump- should be considered in the context of the results of Gursky tion reduced the requirement for clumping of the ISM, et al. (1963) who had identified the possibility of an extra- but an additional, softer extragalactic emission compo- galactic background contribution in their data at energies nent was still required. above 2 keV, and the suggestion by Gould and Sciama 3. The observed flux in the galactic plane was due to the (1964) and others that the intergalactic medium might emit superposition of unresolved galactic Population II sources strongly at softer (lower) X-ray energies. (the older, more numerous, and less massive stars in the Henry et al. (1968) reported the SXRB observation of Galaxy, M dwarfs for example). The required source another sounding-rocket flight, and like Bowyer et al. density, 102 pc3, was less than the population of (1968) they observed extensive emission at 1 keV. In their 4 known objects, and so could not be ruled out. analysis, they ruled out galactic corona emission (the density of the corona would need to be unreasonably high) as While there was some disagreement about the details of well as emission from external galaxies and galactic point the observational results presented by the three groups, they sources as the origin (because of both spectral considera- were all consistent with requiring an emission component tions and the excessive magnitude of the required total in excess of the extrapolation of the extragalactic power law 1 flux), leaving the conclusion that the background arose as observed above 2 keV to explain the background at 4 keV. freeÐfree emission (radiation from interactions between The “real” diffuse background was assumed to be extra- free electrons and positive ions) from a hot, dense intergalactic or galactic halo in origin while any residual flux galactic medium. Bowyer and Field (1969), when not from the galactic plane was assumed to be either non- explaining the differences between the Bowyer et al. (1968) cosmic contamination or from unresolved point sources. and Henry et al. (1968) results, suggested that the counts Diffuse X-ray emission from the interstellar medium of the observed in the galactic plane were due to an isotropic galactic disk was not yet considered. particle background, and that the apparent absorption cross- sections were reduced from the theoretical values by clumping of the absorbing medium.** The concept of a clumpy, **When the ISM is clumped into at least partially optically thick clouds X-ray-absorbing ISM would play a role of some controversy which are not resolved by either the H I or SXRB observations, the effect is to reduce the apparent ability of the material to absorb a diffuse X-ray flux. in models of the SXRB for the following 20 years. For example, if the ISM is diffusely distributed then the absorption optical depth should just be the theoretical cross-section multiplied by the observed column density. However, if the ISM is clumped into unresolved “bricks,” an X-ray would certainly be stopped by the brick, but the bricks *While the absorption column density is measured by the column den- would cover very little of the sky so most X-rays would be unaffected. sity of neutral hydrogen, NH, derived from 21 cm measurements of the ISM Once the brick is optically thick it does not absorb more X-rays if you add (21 cm is the wavelength of the neutral hydrogen electron spin-flip transi- more material to it, but more material is removed from the ISM, allowing tion), the helium associated with the hydrogen provides roughly half of the more X-rays to pass between the bricks. The effective absorption cross- 1 physical absorption at 4 keV. Therefore if the true He/H ratio is lower than section is then reduced. Besides the effect of reduced cross-sections, the assumed, the model absorption will be overestimated. absorption becomes energy independent once the bricks are optically thick. 584 STEVEN L. SNOWDEN

The 1970s intensities in the two bands remained relatively constant (with some structure) over a factor-of-3 variation in inten- The 1970s were marked by the targeting of observations to sity and associated variation in column density (Fried et al. attempt to determine the origin of the background and the 1980). Two solutions for this discrepancy were that either expanded coverage of the soft X-ray sky. A significant role the absorbing column of galactic H I was significantly was played by the decade-long sounding-rocket campaign clumped, eliminating any differential (in energy) absorp- by the University of WisconsinÐMadison, Space Physics tion, or that the emission originated in front of the H I (e.g., Laboratory under the direction of W.L. Kraushaar. One goal Sanders et al. 1977). Hayakawa et al. (1978) came to the of the campaign was to determine the fraction, if any, of the latter conclusion as well, that a majority of the diffuse observed background that was extragalactic in origin; a sec- Galactic X-ray background originated in a local region sur- ond goal was to survey the entire sky in the 0.1Ð10.0 keV rounding the Sun. They used spectral fitting of sounding- band (see below). To address the first issue, the Small rocket data with the result that little or no interstellar Magellanic Cloud (SMC) was scanned to search for the absorption is required. signature of shadowing. The use of “shadowing” here refers Another vital aspect of the SXRB became clear in the to the simple idea that a foreground object such as an H I 1970s: the observed emission most likely had a thermal ori- cloud will absorb part or all of the X-ray emission originat- gin (emission from a hot plasma dominated by collisionally ing behind it, thus casting a shadow in the light of the back- excited emission lines). For example, Williamson et al. ground source. This is analogous to the sky on a stormy day (1974) discussed several non-thermal emission mechanisms when it appears darker in directions where the clouds are and found them lacking. Synchrotron production (radiation thicker, or more starkly, when the Moon passes in front of produced by charged particles spiraling around magnetic the Sun during an eclipse. The result of the SMC study field lines) failed because the required X-ray spectral index was that at least 75% of the observed 1 keV SXRB 4 is much larger than that at radio wavelengths, and the life- originated in front of the SMC (McCammon et al. 1971). time of the required electrons (1013 eV in a 3 G mag- Another group, Long et al. (1975), searching for detailed netic field reasonable for the Galaxy) is less than 104 years. negative correlation between the column density of galactic The number of 250 MeV electrons required to produce H I and the surface brightness of the SXRB, presented 1 the keV SXRB by Compton scattering (collisions between results for a sounding-rocket observation in the direction of 4 high-energy electrons and photons that increases the energy the Large Magellanic Cloud. They found that greater than of the photon) off the 3 K background (the fossil radiation 90% of the observed 1 keV background originated in front 4 left over from the Big Bang) is unreasonably large as well. of the galactic H I in that direction; a direction which is rel- Thermal bremsstrahlung (freeÐfree) emission from a hot atively opaque to 1 keV X-rays of distant origin if the ISM 4 plasma could produce the required spectral contribution is not clumped. but also required too high a space density for the plasmas, With increasing sky coverage it became clearer that and thus excessive pressures, if galactic in origin (e.g., while there was certainly a general negative correlation Henry et al. 1968). The inclusion of collisionally excited between the SXRB surface brightness and the column den- line emission from a thermal plasma (e.g., Cox and Tucker sity of H I (i.e., the plane-to-pole inverse variation of N H 1969) provided a much more efficient emission mechanism and X-ray intensity), the evidence for detailed negative in the 0.1Ð1.0 keV energy range, reducing the required correlation was in general lacking (i.e., shadows cast by density for that plasma to a reasonable level for a galactic distinct clouds in the ISM). However, it should be remem- origin. Thus thermal emission from extended regions of bered that the early detectors were mechanically collimated hot plasma became the most likely candidate for the source and typically had fields of view on the order of 10Ð50 of the SXRB. A galactic thermal origin became even more square degrees (the Moon has an apparent diameter of likely with the Copernicus observations of a nearly ubiqui- about half a degree). In addition, the observation of the sky tous interstellar O VI absorption line (e.g., Jenkins 1978). during a sounding-rocket flight was limited to around five The width of the ISM absorption lines in the spectra of minutes of useful data collection, so the observations were Milky Way stars indicated that the O VI could only be also photon limited. produced by collisional excitation by a thermal plasma at An advantage of the Wisconsin data was a second, softer T 105.5 K. While a plasma at this temperature is a bit too band provided by a detector with a boron filter (the B band, 1 cool to be responsible for the observed keV emission, it is the boron K absorption edge yields a high-energy cut-off 4 consistent with what could be expected from H I-cloud of 0.188 keV, compared to the usual carbon K absorption- interfaces with a hotter plasma. De Korte et al. (1976) pre- edge cut-off of 0.284 keV for the C or 1 keV band, see 4 sented an analysis of the data from two sounding-rocket Figure 1). The theoretical effective ISM absorption cross- flights from the early 1970s. Using data from two detectors sections for the two bands differ by about a factor of 2; with markedly different window thicknesses (which creates however, the hardness ratio (or X-ray color) of the observed a similar but not as strong an effect as having a separate 27

EWINE F. VAN DISHOECK* AND ALEXANDER G.G.M. TIELENS** Space-borne observations of the life cycle of interstellar gas and dust

The gas and dust in the interstellar medium (ISM) form an were detected the diffuse interstellar bands (DIBs; Heger essential part of the evolution of galaxies, the formation of 1922, Merrill 1934), whose identification is still uncertain stars and planetary systems, and the synthesis of organic mol- after more than 75 years. Initially, inspired by the identifica- ecules that may lead to the emergence of life elsewhere in the tion of simple diatomic species in the ISM, these bands Universe. Over their lifetimes, stars return much of their were attributed to molecular absorbers. However, once mass to the ISM through winds and supernova explosions, interstellar dust was established as an important interstellar leading to a slow enrichment in heavy elements and dust that component, an origin for the DIBs in absorption by dust form the building blocks from which future generations of grains was taken for granted. Nowadays the pendulum has stars and planets are made. Stars also inject energy into the swung back Ð and for good reason Ð to molecules as the ISM via ultraviolet photons, shocks, and wind-blown stellar prime candidates for the carriers of these absorption features bubbles. Cosmic rays, x-rays, and ionizing photons influence (Snow 1995). The foundation of the theoretical study of the the ionization state of the gas, whereas shielding by gas and ISM and the physical conditions that may prevail there was dust leads to the cold, neutral phases of the ISM where mole- put forward by Arthur Eddington (1926) in his famous cules can flourish. As a result, the composition and structure Bakerian Lecture. It was also he who exclaimed that “atoms of the ISM is governed by a complex interplay of micro- are physics but molecules are chemistry” with the scarcely scopic and macroscopic processes. Understanding this life hidden message that astronomers should stay away from cycle of gas and dust in the ISM is a key problem in astro- molecules. Nowadays, despite Eddington’s warning, molec- physics, for understanding not only our own Galaxy but also ular astrophysics is a thriving field driven to a large extent the much more rapid cycling between the ISM and stars in by the wealth of molecular data that has become available the earliest star-forming galaxies in the Universe (Figure 1). over the last three decades, much of it harvested from space. Research on the ISM started early in the twentieth cen- With the detection of the 21 cm H I line by Ewen and tury with the ground-based detection of interstellar Na and Purcell (1951), the study of the ISM turned to ground-based Ca optical absorption lines (Hartmann 1904, Heger 1919) radio observations, and data thus gathered still provide a and the appearance of many dark regions on photographic wealth of information on the distribution and kinematics of surveys of the Milky Way (e.g., Barnard 1919). Definite evi- the neutral, atomic ISM in our Galaxy and other galaxies dence for the presence of interstellar dust came from obser- (e.g., Hartmann and Burton 1997). The detection of the first vations by Trumpler (1930), whereas the first interstellar molecules at radio wavelengths (Weinreb et al. 1963, molecules, CH, CH, and CN, were identified between Cheung et al. 1968) paved the way for the development of 1937 and 1941 (Swings and Rosenfeld 1937, McKellar millimeter and submillimeter wave astronomy. The ubiqui- 1940, Douglas and Herzberg 1941). Around the same time tous CO molecule was detected by Wilson et al. (1970), and a surprisingly large number of other molecules have * Rijksuniversiteit Leiden, The Netherlands since been found in dense molecular clouds (see van ** Rijksuniversiteit Groningen, The Netherlands Dishoeck and Hogerheijde (1999) for a recent overview).

Ewine F. van Dishoeck and Alexander G.G.M. Tielens, The Century of Space Science, 607Ð645 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 608 EWINE F. VAN DISHOECK AND ALEXANDER G.G.M. TIELENS

Life cycle of a low mass star

Diffuse ISM Dense dust cloud molecular cloud Red Giant phase

Comets

Low mass star is born Asteroids

Planets Cloud breaks up Planetesimals form

Figure 1 Schematic diagram of the lifecycle of dust and gas from the diffuse ISM, through star formation, to the late stages of stellar evolution. (Pendleton and Cruikshank 1994.)

Space-based ultraviolet observations of the neutral ISM Sun Ð was by William Herschel in 1800, while his son John started with small spectrometers flown on rockets, which Herschel was the first to measure the total power emitted by led to the important detection of H2 by Carruthers (1970). the Sun, in 1838, and Edward Stone that of stars, in The later Copernicus satellite (Spitzer and Jenkins 1975) 1869Ð70 (Harwit 1999). In the late 1870s Thomas Edison provided extensive new information on H2 and other inter- observed Arcturus and the solar corona in the infrared stellar lines, mostly of atoms in various ionization stages. from a chicken coop in order to win a bet (Eddy 1972). Together with the ground-based H I data, they stimulated However, despite these early successes the field of infrared the development of the two- and three-phase model of the astronomy lay dormant until the late 1960s to early 1970s, ISM in which the gas is heated by the action of the photo- when progress in infrared instrumentation truly opened the electric effect on grains and by large-scale supernova explo- infrared window of the spectrum. sions, and is cooled by line radiation from atoms (Field The 1958 discussion mentioned above went on to et al. 1969, McKee and Ostriker 1977). consider the observational difficulties of detecting a low- Because the typical temperatures of the cold, neutral intensity astronomical signal against a 300 K telluric back- ISM range from 10 to a few hundred K, most of the ther- ground and emphasized the advantage of balloon or mal emission occurs at mid- and far-infrared wavelengths, a satellite observations. This was elaborated upon in an early part of the spectrum which is largely inaccessible from the discussion of infrared radiation from interstellar grains by ground. It is therefore not surprising that much of our Stein (1967), who concluded that it was hopeless even to progress over the last 30 years or so has come from space- consider observing it through the mid-infrared telluric win- based observations in the infrared. To our knowledge, the dows. Fortunately he was not much impressed with his own earliest discussion of the importance of the infrared for the pessimistic prediction, and, together with (among others) study of the ISM goes back to an exchange between Fred Gillett, he went on to pioneer the field of mid-infrared Spitzer, Kahn, and Drake at the 3rd Symposium on spectroscopy of interstellar dust using a circular variable Cosmical Gas Dynamics (Burges and Thomas 1958). Of filter-wheel. By the early 1970s it was fully appreciated that course, the first detection of infrared radiation Ð from the the best way to determine the composition of interstellar SPACE-BORNE OBSERVATIONS OF THE LIFE CYCLE OF INTERSTELLAR GAS AND DUST 609

Table 1 Selected infrared space missions and instruments relevant to ISM research

Mission Instrument Acronym Wavelength Resolving Operational range (m) power period

IRAS Photometers 12, 25, 60, 100 3Ð5 1983 Low Resolution Spectrometer LRS 7.7Ð22.6 20Ð60 COBE Far-InfraRed Absolute FIRAS 100Ð10 000 100Ð500 1989Ð1993 Spectrophotometer IRTS Far Infrared Line Mapper FILM 63, 158 400 1995 ISO Short Wavelength Spectrometer SWS 2.5Ð45 2000, 20 000 1995Ð1998 Long Wavelength Spectrometer LWS 43Ð197 200, 10 000 Camera CAM 2.5Ð17 3Ð20 Camera+Circular Variable Filter CAM-CVF 2.3Ð16.5 35Ð50 Photometer PHT 3.3Ð200 3 Photometer-Spectrometer PHT-S 2.5Ð5, 6Ð12 90 SWAS Heterodyne Instrument 545, 610 5105 1999Ð

dust was through infrared spectroscopy (Gaustad 1971). neutral ISM and the formation of stars. The various phases of That was an impressive foresight and insight, as well as a the ISM considered here are summarized in Figure 2, and are change of direction in a short time. It should be remem- discussed in order from the diffuse medium to old stars. The bered that by the late 1960s and early 1970s, rocket-borne richness of the infrared wavelength region is illustrated in ultraviolet studies had revealed the ubiquitous presence of Figure 3, which shows the complete ISO Short Wavelength 2200 Å bump in the interstellar extinction curve (Stecher Spectrometer (SWS) spectrum of Orion centered at the IRc2 1965, Bless and Savage 1972), which had been successfully source (van Dishoeck et al. 1998). Its proximity and extraor- modeled in terms of small graphite grains (Gilra 1972). dinary brightness have made this source the prime target for Yet, as has become abundantly clear since then, most of the pioneering observations at infrared wavelengths advances in our knowledge of the physics and composition (e.g., Genzel and Stutzki 1989). Much of the complexity is of the dust and gas have followed advances in infrared the result of the disruption of the star-forming environment detector and heterodyne submillimeter receiver technology, by powerful outflows from the massive young star, and its and the development of airborne and space-based platforms intense ultraviolet radiation dissociating and ionizing the gas through pioneers such as Martin Harwit, Gerard Kuiper, on the cloud surfaces. Orion is the nearest and best studied Frank J. Low, Charles H. Townes, and many others (e.g., region of massive star formation in the Galaxy, and therefore Harwit et al. 1966, Kleinmann and Low 1967). The Lear also serves as a template for more distant star-forming Jet and the Kuiper Airborne Observatory (KAO) in the regions in other galaxies. The strong continuum is due to USA, and the USÐNetherlandsÐUK Infrared Astronomical thermal emission from warm dust (50Ð300 K) and peaks Satellite (IRAS), have been pivotal in developing the field, around 70 m. A wealth of superposed lines of atoms, ions, which has culminated with ESA’s recent Infrared Space and molecules is seen, which can be used to constrain the Observatory (ISO) (Table 1). Other important missions physical parameters (see Figure 8 of Genzel 1992) and assess include NASA’s Cosmic Background Explorer (COBE), the relative importance of different processes, in particular balloons, rockets, the Japanese Infrared Telescope in shocks v. ultraviolet radiation. Space (IRTS), the US Air Force project Midcourse Space limitations preclude a comprehensive review of the Space Experiment (MSX), and NASA’s Submillimeter field, and only topics in which space-based observations Wave Astronomical Satellite (SWAS). In this respect, the have played a crucial role are covered. For example, ground- future looks very promising, with the Space Infra Red based submillimeter observations of molecular clouds are Telescope Facility (SIRTF), the ASTRO-F mission, the not mentioned, but the composition of ices in such clouds, Herschel Space Observatory (formerly known as FIRST), as deduced from mid-infrared spectroscopy is. Ultraviolet and the Next Generation Space Telescope (NGST) all on observations of the atomic component of the ISM and the horizon. space-based data on ionized H II regions and continuum This chapter presents an overview of the air- and space- observations of young stars are discussed elsewhere in this borne observations that over the last 30 years have played volume. Each section starts with a brief historical review a crucial role in our understanding of the cold and dense and an introduction of the basic physics, and goes on to 610 EWINE F. VAN DISHOECK AND ALEXANDER G.G.M. TIELENS

UV starlight [C II] 158 µ m

Stellar winds Diffuse neutral ISM PAH features Far-infrared dust continuum Supernovae

[C II] 158 µm, [C I] 610 µm [O I] 63, 145 µm, [S I] 25 µm UV starlight CO high-J Molecular Clouds PAH features Shocks H 2 pure rotational and ro-vibrational lines Hydrides: H 2 O, OH, HD Far-infrared dust continuum

Ices Luminosity Circumstellar envelope Amorphous silicates embedded YSO around young star Gas-phase absorption and emission Far-infrared dust continuum

Amorphous and crystalline silicates UV radiation PAH features young star Circumstellar disk CO ro-vibrational bands, H I recombination H 2 pure rotational lines Spectral energy distribution

Amorphous and crystalline silicates PAH features IR and UV radiation Circumstellar shell CO and HCN high-J old star around old star H 2 O, OH

Figure 2 Schematic diagram of the energy input to the various phases of the ISM discussed here, and the resulting diagnostic lines at mid- and far-infrared wavelengths. Most of these features can only be observed from air- and space-borne platforms. PAH stands for polycyclic aromatic hydrocarbons or large carbonaceous molecules in general; YSO for young stellar objects.

focus on recent ISO results to illustrate the current state of Molecular Astrophysics of Stars and Galaxies (Hartquist knowledge, in particular the spectroscopic data obtained and Williams 1998), van Dishoeck and Blake (1998), van with the ISO’s Short Wavelength Spectrometer (SWS, Dishoeck and Hogerheijde (1999), Langer et al. (2000), 2.4Ð45 m) and Long Wavelength Spectrometer (LWS, Ehrenfreund and Charnley (2000), and the proceedings of 43Ð197 m) (see Table 1 for an overview). Excellent previ- IAU Symposium 197 on Astrochemistry: From Molecular ous summaries include the proceedings of the Airborne Clouds to Planetary Systems (Minh and van Dishoeck Astronomy Symposium (Haas et al. 1995), which also con- 2000). Interstellar dust has been reviewed by, for example, tains many photographs of key people in the field, and the Tielens and Allamandola (1987a,b), Whittet (1992), and proceedings of the The Diffuse Infrared Radiation and the Cox et al. (2000). IRTS (Okuda et al. 1997). Further details of ISO results are contained in Star Formation with the ISO Satellite (Yun and Liseau 1998), ISO’s View on Stellar Evolution (Waters et al. 1 THE DIFFUSE INTERSTELLAR MEDIUM 1998), Analytical Spectroscopy with ISO (Heras et al. 1997, 2000), Solid Interstellar Matter: The ISO Revolution 1.1 Very small grains (d’Hendecourt et al. 1999a), and The Universe as seen by ISO (Cox and Kessler 1999). Reviews of various aspects The infrared emission from dust in the diffuse, neutral ISM of the neutral ISM include Spitzer (1978), Habing (1988), is faint and extended, and observations have been mostly Genzel (1992), and Hollenbach and Tielens (1999). limited to what is possible with cooled telescopes in space. Recent reviews of astrochemistry can be found in The The all-sky maps by IRAS (at 12, 25, 60 and 100 m) and 28

PRISCILLA C. FRISCH* The interstellar medium of our Galaxy

Over the past century our picture of diffuse material in space the metallicity of interstellar gas, and by inference the has grown from a simple model of isolated clouds in thermal mineralogy of interstellar grains. A primary goal of equilibrium with stellar radiation fields to one of a richly interstellar medium studies has been to determine the varied composite of materials with a wide range of physical chemical composition of interstellar clouds compared to, properties and morphologies. The Solar System interacts for instance, normal Population I stars such as the Sun. with a dynamical interstellar medium. Optical, radio, and Space observations are required to observe most astronomi- UV astronomy allow us to study the clouds which form the cally interesting elements such as C, N, O, Fe, Mg, and Si, galactic environment of the Sun. The composition and distri- since the resonant ground state transitions of these atoms bution of interstellar clouds in the disk and halo tell us about fall in the ultraviolet (UV, 912Ð3000 Å). the history of elemental formation in our galaxy, and the past Gene Parker once asked me “What is an interstellar and future environment of the Solar System. cloud?” The Rashomon-like answer depends on the con- Dark lanes of dusty clouds obscuring portions of the text. Early optical data showing velocity components in Milky Way are celestial landmarks, but the realization that interstellar absorption lines led to a definition of “clouds” interstellar gas pervades space is quite recent. The twentieth as discrete kinematical units. Alternative descriptions were century opened with the discovery of a “nebulous mass” of based on the physical properties of the clouds, for example interstellar gas in the sightline towards the binary star warm diffuse intercloud material in equilibrium (Stromgren Orionis (Hartmann 1904). A series of over 40 spectra 1948, Field et al. 1969), or hot tenuous coronal gas (Spitzer showed that the Ca II K line (3933 Å ) absorption was 1956) to confine the clouds. Ground and space data now nearly stationary in wavelength, “extraordinarily weak,” show interstellar material (ISM) with densities in the range and “almost perfectly sharp,” in contrast to broader variable 104 atoms cm3 to over 103 atoms cm3, kinetic tempera- 6 stellar absorption features. Sharp stationary Na I D1 and tures 20 K Tk 10 K, and many levels of ionization. D2 lines (5890, 5896 Å ) were discovered in Ori and Within 10 pc of the Sun, we see density contrasts of over Sco by Mary Lea Heger. An explanation offered was that a 400 and temperature contrasts over 100. The distinction stationary absorbing cloud of vapor was present in space between turbulence and “clouds” has been blurred by between these binary systems and the observer. The Ca II recent high-spectral resolution data showing that low- and Na I lines constituted the primary tracer for interstellar resolution spectral data may miss over half of the velocity gas during the first half of the century. components in a sightline (Section 3.2), and that 15% of Interstellar matter (excluding dark matter) provides the mass of cold clouds is contained in tiny (AU-sized) struc- about 30Ð40% of the galactic mass density in the solar tures (Section 3.1). Are these features “clouds” or manifesta- neighborhood. Trace elements heavier than He, which form tions of a turbulent ISM? The discovery of interstellar clouds the planets, record the chemical evolution of matter in our in the galactic halo (Münch 1957) added new questions Galaxy, and provide detailed information on physical con- about the stratification of ISM in the gravitational potential ditions in interstellar clouds, represent a small proportion of of the Milky Way Galaxy, and the origin of halo gas. the interstellar atoms (0.15%). These same elements trace Surprisingly, interstellar gas constitutes about 98% of the diffuse material inside of the heliosphere, and subparsec * University of Chicago, IL, USA spatial variations in interstellar cloud properties near the

Sun indicate the solar environment could change on time their obscuration from “fine solid grains.” Eddington noted scales 104 years. Shapley’s conjecture in 1921 that inter- that radiation with energies greater than 13.6 eV (the stellar clouds affect planetary climates no longer seems ionization potential of hydrogen) would be prevented from outlandish. entering clouds by abundant hydrogen, and concluded inter- 2 Symbols used here are N (column densities, cm ), and stellar H2 would be abundant. 3 nH (total volume density for all forms of H, cm ). Early In the early part of the century, Harlow Shapley advanced optical and 21 cm radio data were insensitive to clouds the idea that interstellar clouds were linked to terrestrial with column densities N(H) 1019 cm2. UV observations climate shifts. He noted that the diffuse luminous and dark of trace elements can detect kinematical objects with nebulae are found throughout space, and that the Sun was N(H) 1016 cm2. Galactic longitudes and latitudes are receding from the Orion region where dark nebulae are quoted in System II. (System II was adopted by the prominent (Shapley 1921). Shapley suggested that a past International Astronomical Union in 1958 in order to correct climate-altering encounter between the Orion molecular earlier errors in the location of the galactic center. Galactic clouds and the Solar System, would yield a 20% variation in coordinates published before 1958 are incorrect.) The local solar radiation, which if sustained for a period of time would standard of rest (LSR) velocity frame represents heliocentric alter Earth’s climate. While encounters with dense clouds as velocities transformed to the rest frame corresponding to the envisioned by Shapley are statistically improbable, encoun- mean motion of comparison stars near the Sun, where the ters with clouds of modest density (10 cm3) are much comparison set is selected according to some criterion. (Most more likely and would destabilize the heliosphere and mod- LSR interstellar velocities presented in the twentieth century ify the interplanetary environment (Zank and Frisch 1999). assumed, frequently without explanation, a “standard” solar motion corresponding to a velocity of 19.7 km s1 towards 1.1 Optical absorption lines the apex position lII 57, bII 22. Recent Hipparcos results give a solar motion of 13.4 km s1 towards the apex The interstellar nature of the sharp stationary absorption direction lII 28, bII 32.) Radio data are usually pre- features seen in binary systems was quickly established. sented in the LSR velocity frame, where this issue is particu- Plaskett and Pearce (1930) provided a convincing discus- larly troublesome. sion that the sharp Ca II and Na lines are formed in diffuse This chapter focuses on the diffuse gas in the space space, and labeled these features “interstellar.” Observations between the stars of our Galaxy. For an eloquent summary of 1700 stars with V 10.5 mag by Otto Struve had shown of the physical properties of the ISM and data up to the that K line strengths in general increase with increasing mid-1970s, see Spitzer (1978). magnitude (and thus distance), suggesting an interstellar origin. Plaskett and Pearce measured Ca II K line velocities for 250 OB stars (to within 1.8 km s1), and found they are 1 DISCOVERING INTERSTELLAR GAS “almost exactly twice” interstellar Ca II motions expected for the galactic rotation of interstellar clouds located at half The observational and theoretical foundations of ISM space the distance to the background star. Peculiar motions within studies were formed in the first half of the twentieth the clouds were found to be equally important for line century. In 1926 Sir Arthur Stanley Eddington laid out the broadening. principles of the ionization equilibrium of atoms in space During the late 1930s, Merrill and collaborators under the influence of dilute stellar radiation fields (the surveyed the yellow Na I D1, D2 and blue Ca II H, K lines Bakerian Lecture, Eddington 1926). He evaluated the in over 400 hot, bright stars. The D2/D1 line ratios were importance of the short wavelength stellar radiation field seen to increase as total line intensity decreased, providing ( 800 Å) for cloud heating (although extreme ultraviolet an early indication of line saturation. The formula relating radiation from space was not observable in 1926), and con- absorption line equivalent width (W) and column density cluded that diffuse clouds are illuminated by a radiation (N) was found to be faulty for “deep” lines, leading to the field at a Planck temperature of 10,000 K and have kinetic development of an empirical “curve of growth” (COG) temperatures Tk 10,000 K. He found frequent collisions using the doublet ratio (Ca II H/K, Na I D1/D2) to con- would establish Maxwellian velocity distributions for elec- strain the functional dependence of W. The COG, formal- trons and ions in space. Eddington concluded that the mate- ized later by Stromgren (Section 1.3), has been used to rial creating the stationary Ca II and Na I absorption lines is derive column densities from the equivalent widths of inter- uniformly distributed, and argued that stellar dynamics stellar absorption lines for 70 years. Cloud motions were 3 implied nH 10 cm for diffuse material. He determined shown to depend on star distance, with velocityÐlongitude that in space most interstellar Ca is Ca III and most Na is plots showing a double sine pattern with an amplitude at Na II He “reluctantly” concluded that dark nebulae derive half the expected galactic rotation value (e.g., see review of THE INTERSTELLAR MEDIUM OF OUR GALAXY 649

Münch 1968). The bulk motions implied by line velocities survey provided a consistent survey of cloud kinematics. showed a chaotic or turbulent component, with dispersion Adams estimated the relative strengths of the Ca II lines by 1 vradial 5Ð10 km s , which was interpreted as moving visually estimating line strengths, which was a common “clouds” or “currents.” The Na I line strengths increased practice then; the weakest features he detected correspond with both distance and the amount of interstellar “smoke” to equivalent widths of 3mÅ. Half of the stars in the sam- (now known as dust) in the sightline (where the dust was ple were found to have complex H or K lines, while about determined by the reddening of starlight). six stars showed weak Ca II absorption at 4226 Å. About The chemical composition of interstellar clouds was 25% of the stars had weak blue/near-UV features from explored by a series of observations at Mt. Wilson, Lick, CN I, CH I, CH II, Ca I, or Fe I. Adams found 21 Ca II line 1 and the Dominion Astrophysical Observatories. Sharp inter- components with vlsr 30 km s (the velocities quoted in stellar absorption lines from Na I (3302.4, 3303.0, Adams (1949) should be updated with modern wave- 5895.9, 5890.0), Ca I (4226.7), Fe I (3719.9, 3859.9), lengths), and concluded the velocities represented peculiar Ti II (3383.8, 3242.0, 3229.2, 3073.0), K I (7664.9, cloud motions through the LSR. 7699.0), CH II (3957.7, 4232.5), CH I (4300.3, Blaauw used Adams’ data to determine the form of the 3890.2), and CN I (3875.8, 3874.0) were discovered, velocity distribution of the chaotic component of interstel- and upper limits were placed on Al I (3944.0), Sr II lar cloud velocities (e.g., see review by Spitzer 1968). He (4077.7, 4215.5), and Ba II (4554.0, 4934.1) lines compared Ca II component velocities with two possible (e.g., see review of Münch 1968). The sightlines towards distributions Ð a Gaussian and an exponential distribution 2 Ori and 55 Cyg were shown to be excellent for identify- of component velocities. Blaauw concluded that an expo- ing new interstellar lines (Dunham 1939). Dunham used the nential form fitted the observed velocity distributions better weak 3302 Å and strong D1, D2 Na I doublets to construct than a Gaussian form. Munch extended this analysis to halo an empirical COG for the ISM towards 2 Ori, avoiding sat- stars, with observations of Ca II and/or Na I towards uration problems with the D lines. Ionization equilibrium 112 stars in the northern hemisphere sky (l 50Ð160, calculated for N(Ca I)/N(Ca II) provided the interstellar elec- |z| 1Ð2 kpc; Münch 1957). He confirmed earlier studies 3 tron density, ne 0.7Ð1.4 cm . The anomalous properties of showing that bulk cloud motions indicate clouds are interstellar Ca II lines were apparent in early data. These aligned with the Orion and Perseus spiral arms, and found observations yielded N(Ca II)/N(Na I) 1.5, which was the blue-shifted absorption components from expanding inter- value expected from stellar atmosphere data at that time. stellar gas around O-star associations. Based on the Blaauw The peculiar behavior of interstellar Ca seen in these results, and reasoning that turbulence is not Gaussian so early data was a harbinger of the fundamentally different that the increased energy dissipation by supersonic turbu- properties of volatiles and refractory elements in the ISM; lence would flatten a cloud velocity distribution in compar- however, an incomplete understanding of cloud ionization ison to a Gaussian, Münch fitted the observed disk and halo prevented this recognition. When Olin Wilson showed con- cloud velocities with the form: vincingly that Ca II has a velocity distribution which is 1 peculiar in comparison to Na I, he described the results as (V) exp( V V ) 2 o “unexpected … and disappointing.” The average internal velocity distribution for Ca II was about three times larger where is the mean radial velocity, found to be 5 than for Na I (22 km s1 v. 7.5 km s1; Wilson 1939). 1kms1 by Blaauw. With the exponential distribution, a Although later high-resolution spectroscopy showed that constant value for the velocity dispersion for all Na I D2 1 the “lines” observed by Wilson were blends of several com- line strengths is obtained (2 4.6 km s ), providing a ponents (Section 3.2), the velocity distribution of Ca II was better fit to velocity distributions than a Gaussian form. This still peculiar. Merrill and Wilson showed that the ratio has created what I view as a conundrum, since the internal N(Na I)/N(Ca II) varied strongly from one cloud to another, velocity distributions of clouds are found to be Gaussian in with values 0.3Ð10 (e.g., see review of Münch 1968). cold clouds, while the bulk cloud velocities measured at Walter S. Adams published an influential survey of higher resolution show an exponential distribution. the strengths and velocities of Ca II H, K, lines and weak A study of the abundance variations between individual features from CN I, CH I, CH II, Fe I, and Ca I in 300 clouds was presented in a classic paper by Routly and disk (|z| 500 pc) OB stars visible from the northern hemi- Spitzer, in 1952, which confirmed earlier indications that the sphere (Adams 1949). The higher resolution of these distribution of Ca II cloud velocities is intrinsically larger data (6kms1), compared to earlier photographic data, than the distribution of Na I velocities. They found both that demonstrated that interstellar gas is concentrated in clouds b(Ca II) 1.5 b(Na I) and that Ca II component velocities with velocity separations larger than the atomic velocity are larger than for Na I. The systematic decrease of dispersion within individual clouds. The velocities in this N(Na I)/N(Ca II) with increasing cloud velocity is known as 650 PRISCILLA C. FRISCH

the “RoutlyÐSpitzer (RS) effect”, manifested as N(Na I)/ curve of growth for the main component (heliocentric 1 1 N(Ca II) 1. RS proposed that the process accelerating velocity, vhc 15 km s ) gives bDop 2.4 km s and the Ca II also decreased N(Na I)/N(Ca II), either through colli- column densities for these elements. The photoionization sional ionization of Na I during acceleration, or from rela- equilibrium of Na I, which equilibrates photoionization tive differences between Ca and Na depletions onto dust with the temperature-dependent recombination rate of Na grains in low- v. high-velocity clouds. The first discovery of and an electron (N(Na II)/N(Na I) n(Na II)/n(Na I); 3 1 ISM within 20 pc of the Sun was possible because of the Section 1.3) gives ne 0.36Ð0.54 cm if the 15 km s RS effect. Ca II was detected towards Oph (14 pc), cloud is 50 pc from Oph. If photoionization of heavy but not Na I (Munch and Unsold 1962). Recent data give elements (e.g., C) supplies electrons, densities nH 1 3 N(Na I)/N(Ca II) 0.1 in the Vlsr 8kms cloud 500Ð900 cm are found, indicating the cloud is a thin towards Oph (Welty et al. 1996). Small-scale (1) vari- sheet of thickness 0.15 pc. Later studies show Na is ations in Ca II line strengths suggested the presence of small depleted by factors of 4Ð5, raising densities and reducing clouds (1 pc). These data provided the first evidence of cloud thickness by the same factors. Enhanced abundances small-scale structure and shocked ISM close to the Sun. of Ca II and Ti II in the 29 km s1 cloud identify this As frequently happens, new techniques proven in first- material as “intercloud,” which Herbig suggested is local generation instruments provide limited data, but change the (since it is seen in front of many Scorpius stars). course of science. The first high-spectral-resolution (R Lewis Hobbs initiated the high-resolution era of optical 200,000) observations of optical absorption lines did not spectroscopy with a survey of the Na I D lines at resolu- achieve the initial goal of observing the Na I D line hyper- tion 1kms1 using a Pepsios interferometer at Lick fine components (separated by 1.0 km s1), but instead Observatory (Hobbs 1969). Turbulence was found to domi- demonstrated cloud velocity structure that is unresolved in nate observed line widths, since cloud temperatures of 1 photographic plates. High-spectral-resolution (R 3Ð5 100 K yielded bDop 0.3 km s , v. observed Doppler 5 1 10 ) spectrometers were developed to observe the hyperfine widths of bDop 1.5 km s . The photoionization balance of Na I lines towards Cyg using the long-focal-length Na applied to N(Na I)/N(H I) yielded typical electron densi- 3 3 McNath solar telescope (Livingston and Lynds 1964), and ties ne 0.008 cm for solar abundances, or nH 20 cm using a Pepsios spectrometer at Lick Observatory (Hobbs if electrons are supplied by the photoionization of metals. 1965). Much later, after the launch of the Copernicus satel- High-resolution optical data acquired by Hobbs, his stu- lite and after high-resolution optical data were routinely dents, and others proved to be a crucial supplement to UV acquired by Hobbs and others (Section 3.2), the Na I D-line data (Section 3.2). hyperfine splitting was discovered towards Cygnus, using a Michelson interferometer (effective resolution R 1.2 Radio astronomy – the first multispectral data 500,000; Wayte et al. 1978). Turbulence in cold interstellar clouds was found to be subsonic. The 1kms1 cloud has Radio waves provided the first multispectral window on the 1 bDop 0.38 km s and a likely temperature of 70Ð124 K, ISM. Radio emission from the Galaxy was discovered by 1 indicating turbulent velocities vt 0.26Ð0.3 km s in com- Karl Jansky in 1932 during an investigation of radio static parison to the isothermal sound speed of 0.8 km s1. in a Bell Labs shortwave (15 m) radio antenna. Jansky A thorough and now classic study of the ISM in front of recognized the interstellar origin of the hiss. Jansky’s Oph (HD 149757) was performed by George Herbig, using papers inspired Grote Reber, who, working with a private the Lick Observatory 120-inch (3 m) Coudé feed spectra radio telescope located in his backyard in Wheaton, Illinois, with photographic plates (Herbig 1968). The star Oph in 1938, detected cosmic static at 2 m and confirmed has a relatively featureless bright continuum (O9.5 Vn, V Jansky’s discovery. Over the next several years, Reber used 2.6 mag, 140 pc, vsini 380 km s1), and a rich interstellar his backyard telescope to map the northern hemisphere spectrum (E(B V) 0.32 mag) lending itself to searches radio sky at 160 MHz and 480 MHz. The advent of World for new interstellar species. It is a runaway star from the War II moved radio astronomy to the forefront of technical ScorpiusÐCentaurus association moving supersonically interest, both in the USA and Europe, and during the fol- through the ISM (space velocity 40 km s1), and with a lowing decade technical advances from the wartime use of circumstellar H II region (5 radius). Oph also has a ram- radar supported the development of the new radio sciences. pressure confined stellar wind observed around the star Shklovsky (1960) reviews the scientific basis for the devel- through 60 m radiation from heated, swept-up interstellar oping field of radio astronomy. grains. Herbig measured absorption lines from Na I, Ca II, The spectral index of the cosmic “radio static” was K I, Ti II, Fe I, CH I, CH II, and CN I, and set limits on measured and found to increase towards lower energies, in some 25 additional species at the W 1Ð2mÅ level. The contrast to the expected increase towards higher energies ISM is contained in two dominant clouds at 15 km s1 predicted for a blackbody (or “thermal”) source. This and 29 km s1 (heliocentric velocities). An empirical puzzle was explained when V.L. Ginzburg calculated the 29

FRANK B. MCDONALD* AND VLADIMIR S. PTUSKIN** Galactic cosmic rays

Our Galaxy is filled with a relativistic gas of high-energy energies of the order of 10 TeV (1013 eV). At high energies protons, electrons, and heavy nuclei. The interstellar energy (100 TeV), the cosmic-ray intensity is so small that density of these cosmic rays is ~1 eV cm 3 Ð comparable to experiments such as large, ground-based air-shower arrays the energy density of the galactic magnetic field and of the must be used. Throughout the modern era of cosmic rays Ð thermal energy of the interstellar medium Ð with an energy defined somewhat arbitrarily as 1946 onward Ð a vital stim- spectra that extends to a maximum energy above 1020 eV. ulus has been the guidance and insight from a number of High-energy particles are also an important and distinguish- excellent theoretical groups. ing feature of radio galaxies, quasars, and active galactic Cosmic-ray studies now span an epoch of almost exactly nuclei. The direct measurement by space and balloon exper- 100 years. In this brief overview we sketch the initial gesta- iments of their charge and mass composition and energy tion of cosmic-ray astrophysics. A more complete history spectra provide information on the source regions within our of the early development of cosmic rays by J. A. Simpson Galaxy, on injection and acceleration processes, and offer a can be found in Chapter 4. At the close of the nineteenth steadily increasing understanding of cosmic-ray transport century, scientists using gold-leaf electroscopes to study the through interstellar space. Observations from radio, gamma- conductivity of gases discovered that no matter how care- ray, and X-ray astronomy define the distribution of energetic fully they isolated their electroscopes from possible sources particles throughout our Galaxy and establish their presence of radiation they still discharged at a slow rate. In 1901 in extragalactic sources. The interpretation of these observa- two groups investigated this phenomenon, J. Elster and tions by allied scientific disciplines is significantly aided H. Geitel (1901) in Germany, and C. T. R. Wilson (1901) in by the detailed study of cosmic rays near Earth while our England. Both groups concluded that some unknown understanding of the sources and of the distribution of galac- source of ionizing radiation existed. Wilson even suggested tic cosmic rays is strongly dependent on the data from these that the ionization might be “due to radiation from sources other fields. outside our atmosphere, possibly radiation like Röntgen Space experiments have played a vital role in the devel- rays or like cathode rays, but of enormously greater opment of cosmic ray astrophysics. These experiments have penetrating power.” A year later two groups in Canada, determined the detailed charge and mass composition of Ernst Rutherford and H. Lester Cooke (1903) at McGill cosmic rays for the elements from hydrogen to nickel University, and J. C. McLennan and E. F. Burton (1902), at (Z 1Ð28) up to energies of ~1 GeV, have mapped the the University of Toronto showed that 5 cm of lead reduced charge distribution of many of the even Z elements to ura- this mysterious radiation by 30%. An additional 5 tonnes of nium (Z 92) and extended the direct observation of the pig lead failed to further reduce the radiation further. energy spectra of the more abundant elements up through Nothing significant happened until 1907 when Father Theodore Wulf (1907) of the Institute of Physics of Ignatus College in Valkenburg, Holland, invented a new electro- * University of Maryland, College Park, MD, USA ** IZMIRAN Ð Institute of Terrestrial Magnetism, Ionosphere and Radio scope. Wulf’s electroscope enabled scientists to carry the Waves Propagation, Troitsk, Russia search for the origin of the mysterious radiation out of the

Frank B. McDonald and Vladimir S. Ptuskin, The Century of Space Science, 677Ð697 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 678 FRANK B. MCDONALD AND VLADIMIR S. PTUSKIN laboratory, into the mountains, atop the Eiffel Tower and, radiation Ð indicating the presence of highly relativistic ultimately, aloft in balloons. Assuming that the radiation electrons throughout our Galaxy including some discrete came from the Earth, they expected to find a rapid decrease sources as well as extragalactic sources (Pacholczyk 1970). in the radiation as they moved away from the surface. They However, because of their small abundance (~1% of the did not find the decrease they expected and in some cases intensity of cosmic ray nuclei) electrons were not directly there seemed to be evidence that the radiation actually detected in the primary cosmic radiation until 1962 (Earl increased. Intrigued by the conflicting results obtained by 1962, Vogt and Meyer 1962). These discoveries made it Wulf and his colleagues, a young Austrian nuclear phy- possible to begin constructing realistic models of the origin sicist, Viktor Hess, obtained support from the Austrian and interstellar transport of galactic cosmic rays. Imperial Academy of Sciences and the Royal Austrian Aero As early as 1934, Baade and Zwicky linked the appear- Club to conduct a series of balloon flights to study the radiance of supernovae with neutron star formation and cosmic- ation. Hess obtained a license to pilot balloons in order to ray generation. Fermi (1949) regarded cosmic rays as a gas reduce the size of the crew and thereby increase the altitude of relativistic charged particles moving in interstellar mag- to which he could carry his electroscopes. On 12 August netic fields. His paper laid the groundwork for the modern 1912, using the hydrogen-filled Böhmen, Hess reached theory of cosmic ray acceleration and transport. The close an altitude of 5,350 m. Carrying two hermetically sealed link between radioastronomy and cosmic rays was conclu- ion chambers, he found that the ionization rate initially sively established at the time of the Paris Symposium on decreased, but then at about 1500 m it began to rise, until at Radioastronomy in 1958 (Paris Symposium 1959). This 5,000 m it was over twice the surface rate. Hess (1912) marked the birth of cosmic-ray astrophysics. The basic concluded that the results of these observations can best be model of the origin of galactic cosmic rays was developed explained by the assumption that radiation of a very high by Ginzburg and Syrovatskii (1964). penetrating power from above enters into the atmosphere Over the last three decades the continual evolution of and partially causes, even at the lower atmospheric layers, cosmic-ray detector systems for space missions have given ionization in the enclosed instruments. us a detailed description of galactic cosmic rays and a On a voyage from Amsterdam to Java, Clay (1927) growing understanding of their source region, acceleration, observed a variation in cosmic-ray intensity with latitude and transport. with a lower intensity near the equator, thus establishing that before entering the Earth’s magnetic field, the bulk of the primary cosmic rays were charged particles. In 1930 Bruno 1. COSMIC-RAY OBSERVATIONS Rossi, using Störmer theory, showed that if the cosmic rays were predominantly of one charge or the other there should The elemental composition of cosmic rays provides infor- be an eastÐwest effect. In the spring of 1933 two American mation on chemical fractionation in the source region as groups, Thomas H. Johnson (1933) of the Bartol Research well as some insight into the nature of this region and of the Foundation and Luis Alvarez and Arthur H. Compton (1933) propagation of cosmic rays in interstellar space. Cosmic- of the University of Chicago, simultaneously and indepen- ray isotopes probe more deeply the nature of the source dently measured the eastÐwest effect. It showed the cosmic region and the timescales of the injection and initial accel- radiation to be predominantly positively charged. In a series eration. Radioactive isotopes such as 10Be, 26Al, and 36Cl of balloon flights in the late 1930s, M. Schein and his co- reveal the temporal history of cosmic rays in the disk and workers (1941) used Geiger counter telescopes interspersed halo regions. The variation of the charge and mass compo- with lead absorbers to determine that most of the primary sition with energy Ð their energy spectra Ð can be related particles were not electrons, and hence protons were most to the acceleration process and to particle transport in the plausibly the dominant constituent. Galaxy. When improved measurements are available at ultra- In 1948 research groups from the University of high energies it should be possible to determine whether Minnesota and the University of Rochester flew nuclear these particles are of galactic or extragalactic origin. At the emulsions and cloud chambers on the same high-altitude highest energies the cosmic-ray arrival direction may also Skyhook balloon flight (Freier et al. 1948) and discovered indicate the approximate direction of the most powerful the presence of heavy nuclei in the primary cosmic radia- sources. tion. Further studies by many other groups soon established Over the past four decades of the space age there has that essentially all of the elements between H and Fe were been a remarkable improvement in the ability of cosmic present in the cosmic radiation near the top of the atmos- ray experiments to measure the elemental and isotopic phere Ð including an overabundance of the light elements composition over a steadily increasing range of charges and Li, Be, and B. Then in 1950 it was found that a significant energy. Space limitations permit discussion of only the fraction of the cosmic radio emission was synchrotron more recent results in a given area. GALACTIC COSMIC RAYS 679

The most remarkable feature of cosmic rays is their E2.7J(E) energy spectra (Figure 1). From ~10 GeV to 1020 eV these spectra, over some 10 orders of magnitude variation in intensity show a relatively featureless power-law distribution. At Energy losses + Solar modulation energies below a few giga-electronvolts the influence of solar Bump? modulation becomes important with significant temporal J(E) ∝ E–2.7 variations at 1 AU related to the 11- and 22-year solar and J(E) ∝ E–3.0 heliomagnetic cycles. At energies of less than 40 MeV the Dip? oxygen spectra in Figure 1 show the presence of so-called Knee J(E) ∝ E–2.7 anomalous cosmic rays, discussed in Chapter 4. Those are cut-off? partially ionized interstellar atoms accelerated at the solar Anisotropy <0.1% Composition ‘‘Normal’’ Anisotropy small* Anisotropy? wind termination shock. Near 10 MeV there is a highly vari- with FIP Bias All species present* Composition? able turn-up in the ion spectrum produced by particles of 1010 1012 1014 1016 1018 10 20 solar/interplanetary origin although the acceleration up to Total Energy, E (eV) energies more than tens of giga-electronvolts was registered in some solar flare events. Figure 2 Schematic representation of cosmic ray spectrum If the differential intensity in Figure 1 is multiplied by (Axford 1994). E2.7, where E is the particle energy, then the structure in the spectrum becomes more apparent at energies 10 Gev (Figure 2). The spectral slope steepens and the energy is Above 10 TeV the composition is not well known; this is the proportional to E3 above 1014 eV. It decreases back to the region where acceleration by supernova shocks becomes form ~E2.7 above 1018 eV, and extends to ~3 1020 eV. difficult. It is generally assumed that the cosmic-ray “knee” The experimental limitation imposed by the current size at ~1015 eV may reflect a gradual transition in the composi- of air-shower arrays does not allow measurements of the tion to particles of increasingly higher charge. At energies cosmic-ray intensity at energies greater than 3 1020 eV. 1019 eV questions of galactic magnetic confinement lead to the assumption that these particles are of extragalactic origin. At energies above 4 1019 eV even protons should experience significant deceleration by the 3 K black-body radiation (the GreisenÐZatsepinÐKuzmin effect). At energies of 1012Ð1014 eV there are small anisotropies of ~0.1% which are thought to be due to local effects. At this time there are no meaningful anisotropies observed at higher energies except the ultra-high energies ~1018 eV (see Section 6).

1.1 The elemental composition Over the charge region Z 1Ð28 (HÐNi), cosmic-ray experiments in space can resolve the individual elements over an extended energy range. A summary of these data (Figure 3) shows the relative abundance of cosmic rays at ~1 AU along with the Solar System abundance (Simpson 1983) for two different energy regimes, 70Ð280 MeV/nucleon and 1Ð2 GeV/ nucleon. It can be seen that H and He are the dominant elements, constituting some 98% of the cosmic-ray ions, but are still under-abundant in the cosmic rays relative to the Solar System abundance. There is reasonably good agreement between the cosmic-ray and Solar System abundance data for most of the even elements particularly for C, O, Mg, and Fe. The light elements Li, Be and B, and Sc and V in Figure 1 Energy spectrum of cosmic rays measured at the the sub-iron region are greatly over-abundant when com- Earth (Jokipii 1989). pared to the Solar System abundance Ð a result of nuclear 680 FRANK B. MCDONALD AND VLADIMIR S. PTUSKIN

106 dances and plotted against the elements’ first ionization potential (FIP), a well-defined fractionation effect is observed He (Figure 4). Very similar effects are also present in the relative 105 abundance of the solar coronae, solar wind, and gradual solar energetic particle events. However, particles with low FIP tend to be chemically 104 active and form stable compounds and structures such as dust grains. Meyer et al. (1997) have developed in detail the sce- O C nario in which galactic cosmic-ray ions originate predomi- 103 nantly from the gas and dust of the interstellar medium. This Ne model gives a natural explanation of the low abundance of Mg Si Fe H and He. With the presently available data, it is not possible 102 N S to choose between FIP and volatility. The maximum energy A Ca attainable by the shock acceleration of dust grains is of Ti Cr the order of 0.1 MeV/nucleon, so the “FIP v. volatility” argu- Ni 10 ment is really about the injection and initial “energization” Na Al processes. 1 Mn P Cl K 1.2 The ultraheavy nuclei (Z 30) RELATIVE ABUNDANCE –1 10 Co F These elements arise from a combination of s-processes (produced in an environment where it is more probable that V 10 –2 they undergo beta decay before adding another neutron) and r-processes (produced in a neutron-rich environment where Sc beta decay is less probable). It is expected that r-process ele- –3 Li B 10 ments will dominate in explosive nucleosynthesis such as occurs in supernova explosions. Binns et al. (1989) found over the charge range 33 z 60 that the FIP-corrected –4 10 Be observed abundance is similar to that of the Solar System abundance; above Z 60, they found an enhancement of r-process produced nuclei in the source region. 10–5 246810121416182022242628 Of particular importance is the actinide region (Z 88), NUCLEAR CHARGE NUMBER where the elements can only be produced via the r-process. Figure 3 The relative abundances of H to Ni in cosmic rays, These have been studied by an experiment flown on NASA’s solid line and in the Solar System, dashed line. All abun- Long Duration Exposure Facility (LDEF). The experiment dances normalized at Silicam-100 (Simpson 1983). consisted of an array of thick, inert, solid-state nuclear track detector stacks Ð sheets of polycarbonate with a collecting area of 10 m2. They were exposed in Earth orbit spallation in interstellar space by nuclei of higher charge. aboard the LDEF for some 69 months. From this exposure The secondary nuclei generated by these reactions with the Donnelly et al. (1999) measured an actinide/subactinide ratio interstellar gas will have essentially the same velocity as the given by incident primary nuclei and hence the same energy per nucleon. Their energy spectra tend to be steeper than those Z 88 of the primaries due to energy-dependent escape of the 0.0147 0.003 74 Z 87 higher-energy primaries from the Galaxy. It is possible to correct the 1 AU observations for the changes produced by nuclear interactions, ionization energy which is consistent within statistical errors with the abun- losses, and escape from the Galaxy in the journey of the ener- dance ratio 0.013 of propagated primordial Solar System getic nuclei through interstellar space and thus obtain an esti- material. Notice that the propagated present-day Solar mate of the source abundance of these elements. When these System material would give 0.0077 for this ratio. Donnelly source abundances are normalized by their Solar System abun- et al. (1999) found that the measured abundance is consistent 30

GERARD F. GILMORE* Stellar populations and dynamics in the Milky Way galaxy

Our Galaxy offers a unique opportunity to deduce the closest approach to such a view that is possible. An ideal important physics involved in galaxy formation from obser- astrophysicist would have astrometric eyes! vations of those old stars that were formed at the time of the Available information strongly suggests that the Galactic formation of the Milky Way, and whose present properties extreme Population II subdwarf system formed early, though contain some fossil record of the Galaxy’s history. Only in the duration of its aggregation into the proto-Galaxy remains the Milky Way galaxy and its immediate neighbour satellite unclear. This subdwarf system now forms a flattened, pres- galaxies can one obtain the true three-dimensional stellar sure-supported distribution, with axial ratio ~2:1. The thick spatial density distributions, stellar kinematics and stellar disk formed close in time to the subdwarf system, with at chemical abundances. Knowledge of how stars move and least the metal-poor tail of the thick disk being comparable in how they are distributed in space measures the Galactic age to the globular cluster system. The thick disk is probably gravitational potential, including its dark matter content, chemically and kinematically discrete from the Galactic old while knowledge of stellar kinematics, ages and chemistry disk Ð by which we mean those stars of the thin disk with age constrains the star formation and gas accretion history. greater than a few billion years Ð implying a discontinuous The scientific goal is to build on the powerful concept of Galactic history. The inner Galaxy is mostly old, almost cer- stellar populations, which basically distinguishes old, metal- tainly barred, but as yet remains to be studied in detail, espe- poor stars (Population II) from younger, more metal-rich cially in the innermost regions. Importantly, recent dynamical stars (Population I), to include the rich complexity of the analyses lead to the conclusion that there is no statistically real Universe. significant amount of non-luminous mass in the solar neigh- No single section of the electromagnetic spectrum pro- bourhood, and hence no evidence for dissipative dark matter. vides the ‘best’ view of the Galaxy. Rather, all views are We consider in turn the general questions of galaxy for- complementary. However, some views are certainly more mation which the science of stellar population studies aims representative than others. The most fundamental must be a to address, followed by discussion of star count and infrared view of the entire contents of the Galaxy. Such a view studies of Galactic structure, and astrometric and kinematic would require access to a universal property of matter that studies of the Galactic mass distribution. The primary space is independent of the state of that matter. This is provided missions which have contributed to these subjects are the by gravity, since all matter, by definition, has mass. Mass Hubble Space Telescope (HST), whose excellent spatial res- generates the gravitational potential, which in turns defines olution allows study in crowded regions, whose dark sky the size and the shape of the Galaxy. While the most reli- background allows the study of very faint objects and, most able and comprehensive, such a view is also the hardest importantly here, allows reliable image distinction between to derive. Kinematics and distance data are, however, the stars and resolved objects (galaxies); the sequence of infrared observatories, which have allowed observations through the * University of Cambridge, United Kingdom optically thick dust obscuring the inner Galaxy, Infrared

Astronomical Satellite (IRAS), Diffuse Infrared Background where k and mp are the Boltzmann constant and the mass of 6 1 Experiment (DIRBE) on the Cosmic Background Explorer the proton, respectively. Numerically, Tvirial ~10 R50 M12 K (COBE) and the Infrared Space Observatory (ISO); and the for gravitational (half-mass) radius, R, in units of 50 kpc, 12 High Precision Parallax Collecting Satellite (Hipparcos), and mass M in units of 10 ML. Since the disks of spiral arguably ESA’s most substantial contribution to astrophysics galaxies are cold, with T Tvirial, energy must have been in the twentieth century. lost. Since this lost energy was in random motions of individual particles, the only possible loss mechanism is through an inelastic collision, leading to the internal excita- THE FORMATION OF DISK GALAXIES tion of the particles, and subsequent energy loss through radiative de-excitation. Clearly, particles with small cross- Current understanding of the formation and early evolution of section per unit mass for collisions, such as stars, will not disk galaxies allows a description of the important physical dissipate their random kinetic energy efficiently, so that dis- processes at various levels of complexity and generality, and sipation must occur prior to star formation, while the galaxy illustrates what we have learned, what we are attempting still is still gaseous. The virial temperature of a typical galaxy- to learn, and the relative importances of surveys, targeted 6 sized potential well is Tgalaxy ~ 10 K, with a corresponding research projects and newer space-based studies. At one one-dimensional velocity dispersion of ~100 km s1. extreme, one simply considers the global evolution of a gas The physical conditions in the Universe at the epoch of cloud, and assumes that mean values of relevant parameters galaxy formation (redshift ~a few), as deduced from obser- suffice for an adequate description of generic properties. vations of quasar absorption lines (the GunnÐPeterson test Alternatively, one gives up general applicability and instead for neutral hydrogen), are such that hydrogen is ionized, adopts specific numerical values for those parameters that and correspond to temperatures of the proto-galactic gas quantify the important physics, attempting a detailed con- of ~104 K, with a sound speed of only ~10 km s1. The frontation of model predictions with observed stellar popu- collapse of this gas in Galactic potential wells will thus lations. The relation of any model prediction to detailed induce supersonic motions, and lead to both thermalization observations at a single radius in a specific galaxy clearly of energy through radiative shocks and subsequent loss of needs to be considered with some care. Mindful of this caveat, energy by cooling. It is this conversion of potential energy, we outline here the most important timescales and physical first to random kinetic energy as described by the virial the- processes that are likely to play a role in the determination orem, and then to radiation via atomic processes, the net of the observable properties of galaxies like the Milky Way. result of which is an increase in the binding energy of the system, that is termed dissipation. General models of dissipational disk galaxy formation The rate at which excited atoms can cool obviously places a fundamental limit on the amount and rate of dissi- The existence of cold, thin Galactic disks has strong pational energy loss, and hence on the maximum rate at implications for galaxy formation. To see this, consider a which a gas cloud can radiate its pressure support and col- standard picture whereby galaxies form from growing lapse. A convenient measure of this timescale is the cooling primordial density perturbations, which expand with the time of a gas cloud, which is the time for radiative processes background Universe until their self-gravity becomes domi- to remove the internal energy of the cloud. Defining the nant and they collapse upon themselves. Were there to be cooling rate per unit volume to be n2(T ), where n is the no loss of energy in the collapse, and neglecting angular particle number density, and where the functional form of momentum, the transformation of potential energy into is determined by the relative importances of freeÐfree, thermal (kinetic) energy would lead to an equilibrium boundÐfree and boundÐbound transitions and is thus an system with final radius equal to half its size at maximum implicit function of the chemical abundance, gives expansion, supported by random motions of the constituent particles. Thus an equilibrium, purely gaseous proto-galaxy 3nkT T should have temperature t (2) cool 2 n (T ) n GMm p T T (1a) It is usually of most interest to compare this timescale with virial kR the global gravitational free-fall collapse time of a system, and a stellar proto-galaxy should equivalently have velocity which is the time it would take to collapse upon itself if dispersion there were no pressure support. This timescale depends only on the mean density of the system, and is given by

2 k 7 1/2 Tvirial (1b) tff 2 10 n yr (3) mp STELLAR POPULATIONS AND DYNAMICS IN THE MILKY WAY GALAXY 701

The term ‘rapid’ is often used to describe evolution that chemical and dynamical evolution that appeal to continual occurs on about a free-fall time. infall must also satisfy angular momentum constraints. The important mass scale of any condensations is set by gravitational instability, quantified by the Jeans mass. The Jeans mass is that minimum mass at which gravity over- Specific models of Milky Way galaxy formation whelms pressure so that density perturbations of mass 8 3/2 1/2 M MJ ~10 T4 n ML are unstable and collapse upon The most widely referenced model of the formation of our themselves, where the numerical factor is for temperature T Galaxy is that of Eggen, Lynden-Bell and Sandage (1962; in units of 104 K and number density n in units of particles henceforth ELS), which was developed primarily to under- per cubic centimetre. stand their observations of the kinematics and chemical The discussion above is based on an extremely idealized abundances of stars near the Sun. Certainly the most impor- model of a proto-galaxy, in that only the global cooling and tant effect of the ELS analysis was to emphasize that quan- collapse timescales of a uniform gas cloud are considered. titative conclusions about the epoch of galaxy formation No analytic descriptions of more plausible models exist. could be derived from observations of stellar abundances The above discussion can say nothing about when or and kinematics near the Sun today Ð a task in which we are how local Jeans-mass condensations actually form stars; the still engaged. inherent assumption is that cooling is necessary and suffi- The ELS model requires the stellar spheroid to have cient for efficient star formation, though the critical distinc- formed during a period of rapid collapse of the entire proto- tion between global and local timescales is rarely made galaxy, after which the remaining gas quickly dissipated into explicit. However, it is clear that the existence of gaseous a metal-enriched cold disk, in which star formation has disks requires the star-formation efficiency to be low during continued until the present. The ELS model was designed to the early stages of disk formation. A realistic discussion of provide conditions under which the oldest stars populated galaxy formation must consider the hydrodynamics of the radially anisotropic orbits, while stars that formed later had gas in a proto-galaxy. The general conclusion from avail- increasingly circular orbits, in accordance with data which able studies is that, while it is possible to build models implied that the most metal-poor stars, assumed to be the old- which are somewhat like observations, it is necessary to est stars, were on more eccentric, lower angular momentum specify the most sensitive parameters (viscosity and, in orbits than the more metal-rich stars. This model is based on effect, the star formation rate) in an ad hoc way. Hence the two crucial assumptions: first, that a pressure-supported, pri- need to be guided by observations. marily gaseous galaxy (where T Tvirial) is stable against star The angular momenta of galaxies that formed in envi- formation, in which case the global cooling time is the short- ronments of different density might also be expected to dif- est timescale of interest, and thermal instabilities must be sup- fer. During the build-up of structure, initially strongly pressed; and second, that stellar orbits cannot be modified to bound particles lose both energy and angular momentum, become more radial after the formation of the star. while the weakly bound particles gain energy (become If the first assumption were valid, the observed high- more weakly bound) and also gain angular momentum. velocity stars must have formed from gas clouds that were There is overall alignment of the angular momentum vector not in equilibrium in a pressure-supported system. If the of different shells in binding energy. Disks of spiral galax- second assumption above were valid, these clouds formed ies would then form without significant angular momentum stars while on radial orbits at large distances from the transport, provided the baryons remain gaseous until the Galactic centre. Thus, in this picture, these clouds must virialization of the dark halo, and shock-heating, as have turned around from the background universal expan- described earlier, would homogenize the gas. The predic- sion, and be collapsing towards the centre of the potential tions of these models could be tested in detail if we knew well. Hence, the oldest stars of the Galactic spheroid must the angular momentum distribution of the outer spheroid of have formed as the proto-galaxy coalesced. To determine our Galaxy; all we know at present is that the kinematically the rate of the collapse, ELS analysed the evolution of the selected subdwarfs in the solar neighbourhood have a lower radial anisotropy of a stellar (or gas cloud) orbit as the specific angular momentum than the disk stars, by roughly Galactic potential changed, and showed it to be approxi- a factor of five, and that the metal-poor globular cluster sys- mately conserved during a slow collapse but to become tem is consistent with zero net rotation to galactocentric more radially anisotropic in a fast collapse. They argued distances of ~30 kpc. against a slow collapse on the grounds that such a collapse The angular momentum distribution of the material des- requires tangentially biased velocities (remember that pres- tined to form the disk controls the range of galactocentric sure support has been excluded by assumption), and this radii over which infall occurs at a given epoch, and the dura- tangential bias will be unaffected by the resulting slow tion of the infall at a given location. Thus models of disk changes of the gravitational potential. The observed radial 702 GERARD F. GILMORE

anisotropy of the stellar orbits then implies an initially radi- where later (rapid) collapse of either the disk or the dark ally biased velocity ellipsoid, while the calculations of ELS halo, or the merger of a few large substructures, could lead to show that such a velocity ellipsoid will have become more the rapid dynamical evolution of a central spheroidal compo- radially anisotropic during collapse. Hence, ELS deduced nent which had previously formed on a longer timescale. that the gas clouds were in free-fall radial orbits, and that Models of this type have yet to be studied in detail. the consequent collapse must have been rapid, with ‘rapid’ It is the continuing attempt to quantify the metallicityÐ in this sense meaning that the timescale for collapse is com- kinematics distribution function of stars, with sufficient parable to an orbital or a dynamical timescale, which is of accuracy and large numbers to address these questions, that the order of 108 years. It should be noted that Isobe (1974) motivates projects such as Hipparcos, and the use of HST, came to the opposite conclusion from his analysis of the COBE, IRAS and ISO to quantify the true distribution of ELS data, and favoured a slow collapse, while Yoshii and stars in space. We now consider that specific continuing Saio (1982) augmented the ELS sample and also concluded challenge. that the halo collapsed over many dynamical times. Clearly if either of ELS’s assumptions were violated, there need be no correlation between the time of a star’s formation Ð which they infer from a star’s metallicity Ð and THE SPATIAL STRUCTURE OF THE its present orbital properties. In a non-rotating pressure- MILKY WAY GALAXY supported system, all stars formed would be on highly radial orbits, as a star has too small a surface area to be pressure Counting stars is one of the few truly classical scientific supported by the gas. As mentioned above, assumptions techniques used to study high-latitude (and therefore low- about star formation in pressure-supported systems must be obscuration) Galactic structure. Early work in this subject is treated as ad hoc until we understand better the physics of well reviewed by Paul (1993). The extensive data set and star formation, so conclusions based on such assumptions understanding available at that time is reviewed by Blaauw are at best uncertain. If their second assumption were vio- and Schmidt (1965). Relatively little further progress was lated, then the stars that are now the high-velocity stars near achieved until the new deep, high-quality data of Ivan King the Sun could have originated from more circular orbits and collaborators at Berkeley became available in the late inside that of the Sun, and have present orbital properties 1970s. The application of computer modelling to these data that depend only on dynamical processes subsequent to their by van den Bergh (1979) led to a considerable resurgence of formation. The realization that a forming galaxy undergoes interest, continuing to the present. An accessible overview changes in its gravitational potential which are of order the of the recent advances in the subject is provided by Croswell potential itself (violent relaxation) means that stellar orbits (1995), while the continuing level of research activity gener- can be modified considerably. ates several conferences per year. An important specific Later N-body models (e.g. May and van Albada 1984) for example, which includes both historical and research arti- systems in which dissipation does not play a major role cles, is the proceedings of IAU Symposium 164, ‘Stellar show that the final state of the collapsed system depends Populations’, held on the 50th anniversary of Walter Baade’s both on the degree of homogeneity and on the temperature publication of the original concept (van der Kruit and of the initial state: clumps cause angular momentum and Gilmore 1994). energy transport. Violent relaxation never goes to completion, so that final and initial orbital binding energies and The fundamentals of star-count analyses angular momenta are correlated, the interior regions becoming more centrally concentrated and the outer regions being The number of stars, N, countable in a given solid angle to puffed up. The typical final steady-state velocity distribution a given magnitude limit, m, is given by a simple linear inte- is highly anisotropic outside (roughly) the half-light radius, gral equation often known as ‘the fundamental equation of and more isotropic within that radius. If violent relaxation stellar statistics’: were completely efficient, all systems would reach the same final state with isotropic velocity distribution. In the Galaxy, 3 N(m) (M , x) D(M , x)d x (4) the spheroidal half-light radius is ~3 kpc, well inside the v v Sun’s orbit. Thus the expected velocity distribution of old stars near the Sun after virialization of the spheroid is where (Mv, x) is the distribution function over absolute anisotropic, as observed by ELS, even though the dynamical magnitude and position, D(Mv, x) is the stellar space density evolution of the system is not as they envisage, and a corre- distribution, and d3x is a volume element. This (Fredholm) lation between kinematics and age is no longer an inevitable equation is rarely invertible, being ill-conditioned. In gen- conclusion. One might, for example, imagine a situation eral, the luminosity function is too broad to allow any 3

ERNST STUHLINGER* Enabling technology for space transportation

INTRODUCTION astronomers, among them K.O. Kiepenheuer, were greatly interested. The capsule was ready for launch late in 1944, but Although rockets have been in use for a thousand years, it by that time the Nazi government did not permit any activi- was only 60 years ago when concrete plans were made to ties not directly supportive of the war effort. Then, in January transport a package of scientific instruments to high alti- 1945, von Braun received orders to leave Peenemünde and tudes with a rocket. In 1942, Professor Erich Regener in move south; that terminated the Regener Tonne project. Stuttgart, Germany, a specialist in atmospheric research, Only one year later, Peenemünde rockets, shipped from ozone physics, ultraviolet spectroscopy, and cosmic rays, Germany to White Sands in New Mexico, were launched and Wernher von Braun, director of the Peenemünde Rocket with scientific payloads instead of warheads, prepared by Research and Development Center, jointly began to design a scientists from numerous universities and research insti- large scientific payload for the empty warhead of an A-4 test tutes all over the country. A very active period of high alti- rocket to be launched into a near-vertical trajectory from the tude research began which lasted for seven years. Besides “Greifswalder Oie,” a small island near Peenemünde. its immediate success as a source of scientific results and Regener and von Braun had worked together before on a discoveries, this project proved that rockets represent mar- project to try out radio communication between the ground velous tools for scientific research in far-away regions that and a transponder carried aloft with a high-altitude balloon. would not be accessible in any other way. Instruments for the capsule were built and installed in a Stimulated by the successful research projects with the large aluminum container, called a “Regener Tonne” old Peenemünde rockets, several new rockets were devel- (Regener barrel). It contained pressure and density gauges, a oped during the 1940s, among them Van Allen’s Aerobee UV spectrograph with quartz optics to determine the ozone and Aerobee High, and Milton Rosen’s Viking, which con- content of the atmosphere, thermal sensors, containers for tinued a successful program of scientific space research. atmospheric samples to be taken during the parachute They were members of a very small number of rockets descent of the capsule at various altitudes, instruments to developed exclusively for scientific purposes. A large num- measure ion densities in the atmosphere, drum recorders, ber of rocket projects evolved from about 1950 on in the radio receivers and transmitters, and a parachute that United States, in Soviet Russia, and in other countries, but unfolded under the stiffening force of pressurized tubes. most of them were developed and built for military pur- While the quartz spectrograph, built at the Regener Institute, poses. Some of them, such as the European Ariane and the had the primary objective of determining the ozone content American Delta II, and also the Space Shuttle, served of the high atmosphere by measuring the intensity of solar exclusively, or at least primarily, commercial and other util- UV radiation, it also promised to furnish the first solar spec- itarian uses; the big Saturn-Apollo rocket project arose trum deep into the ultraviolet, an objective in which solar under the pressure of political circumstances. On the other hand, all the rockets and rocket technologies * NASA Ð Marshall Space Flight Center, Huntsville, AL, USA developed and built during the past fifty years for various

Ernst Stuhlinger, The Century of Space Science, 59Ð114 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 60 ERNST STUHLINGER purposes have also found applications in purely scientific This chapter will not present a comprehensive history of endeavors. All those spacecraft which were built during the rocketry and space projects. Rather, it will highlight those past five decades specifically for astronomical and astro- specific events which gave origin to the fabulous evolution physical research, for lunar and planetary studies, and for of space sciences by providing the technical means to send solar and deep space investigations owed their launchings to instruments, and even humans, away from Earth with its rockets that had been built primarily for nonscientific pur- many disturbing influences, enabling them to make obser- poses. This intimate relationship between technologies cre- vations that would never be possible from the surface of the ated for practical purposes, and the need of scientists who Earth. Also, some thoughts will be offered about the likely are driven primarily by their quest for knowledge, has been development of space flight in the foreseeable future, as a characteristic feature of rocket and space activities during seen by those who were actively involved in the develop- the past sixty years. ment of rockets and spacecraft during past decades. In this essay on enabling technologies for space The first decisive step from dreams to realistic studies was transportation, the author tries to convey an impression of made by a Russian schoolteacher, Konstantin Eduardovitch the multifaceted challenges, problems, failures, and suc- Tsiolkovskii (1857Ð1935). Based on Newton’s laws of cesses the rocket people have encountered on their long mechanics, he derived in 1895 a simple, but very elegant way from the dreams of the 1930s to the satellites and the equation that allows one to calculate the terminal velocity, lunar, planetary and space probes that have come to life Vterm, a rocket will reach in the vacuum of empty space, far during the twentieth century, enriching our awareness and away from any noticeable gravity field, when its total initial our knowledge of the Universe to an extent unforeseen mass, Min, its mass at burning cutoff, Mco, and the velocity when the century began. of its exhaust gases, Vex, are known. He found:

Vterm Vex ln [Min/Mco] 1. EARLY THOUGHTS ABOUT SPACE TRAVEL Tsiolkovskii’s work, remaining largely unknown for many Transportation through space, without a firm substrate or a years, did present the proof that rocket behavior is surrounding atmosphere, must rely on principles of propul- amenable to exacting mathematical analysis. sion different from those which allow people, and animals, Around the end of the nineteenth century, a young to move from one place to another in our immediate earthly American physics teacher, Robert Hutchings Goddard environment. Several ways to produce a propulsive force (1882Ð1945), engaged in an extremely active pursuit of for travel through space beyond the atmosphere can be rocket and space flight ideas. In 1902, he wrote an essay The envisioned, such as gravitational forces generated by celes- Navigation of Space, and offered it to Popular Science,but tial bodies; light pressure; and the rocket principle. his paper was not accepted for publication. Convinced that Rockets did not originate from the desire to travel into the key to space travel is a suitable propulsion system, and and through space. History tells us that about one thousand that space propulsion should be based on rocket principles, years ago rockets appeared as weapons of war, first in Asia, he devoted all his further work primarily to the study and then in Europe, and later on the American continent. Today, development of rocket systems. In 1906, he wrote about military rockets play major roles in the armaments of all “electrified jets” for planetary voyages, but from about 1909 countries that must be prepared for occasional involvement on he concentrated his efforts predominantly on the theoreti- in military conflicts. cal and practical development of liquid-propellant rockets. Independent of that situation, there have been humans, Beginning in 1914, Goddard applied for and received probably for thousands of years, whose minds dreamed patents Ð 214 during the following thirty years Ð for an about voyages to the Moon, to planets, and even to stars. astounding variety of subjects: rockets with solid and liquid Toward the end of the nineteenth century, some of these propellants, multistage rockets (a principle that was men- dreamers began to study whether it would really be possi- tioned for the first time by Konrad Haas in Romania in ble, on the basis of well-established laws of physics, to 1529!), combustion chambers, nozzles, cooling systems, travel to the Moon, and perhaps even to one of the nearest valves, tanks, propellant feed systems, gyroscopic guidance planets, with spacecraft built on Earth and propelled by a and control systems, air and jet vanes, and recovery by suitable rocket motor. Today, as the twenty-first century parachute. Like Tsiolkovskii ten years before him, and dawns, we realize that those early studies have led to a vast independently Hermann Oberth ten years after him, he field of activities involving rockets. They gave the twentieth found that liquid oxygen and liquid hydrogen would make century its most characteristic feature: Mankind’s outreach the best propellant combination for high-altitude rockets. into the unfathomable expanses of space that surround the Goddard (1919) described some of his ideas in a paper A planet Earth. Method of Reaching Extreme Altitudes which was published ENABLING TECHNOLOGY FOR SPACE TRANSPORTATION 61 by the Smithsonian Institution in 1919. This essay, one of For his book, Oberth had independently derived many of very few Goddard papers published during his lifetime, led the basic formulae of rocketry, including the famous to some support from the Navy, and to the permission from Tsiolkovskii equation. In spite of its small size Ð 92 pages Ð Clark University at Worcester, Massachusetts, where he Oberth’s book is an impressively rich compendium of facts was a professor of physics, to carry out his experiments and properties of rockets, most of them presented for the under the auspices of the university. Goddard’s paper, very fist time, and also of ideas referring to the use of rockets. unfortunately, also evoked some negative comments by the Oberth described two types of rockets in great technical press which ridiculed his contention that humans may one detail, the “alcohol rocket,” and the “hydrogen rocket.” He day be able to travel to the Moon. This criticism hurt mentioned the possibility of space stations that orbit the him deeply. He discontinued publishing his articles, and he Earth indefinitely “like a little moon” without requiring any put his patents, reports, and diaries in safes where they sustaining propulsion, providing opportunities to make remained unseen until after his death in 1945. He refused to large-scale weather observations, to communicate through cooperate, and even to communicate with other rocket light signals with inaccessible regions on Earth, to investi- scholars, limiting his entire “team” to five persons, includ- gate the ultraviolet light from stars, to make experiments ing his wife Esther. and study processes under weightlessness, and to carry out Homer E. Newell, NASA Associate Administrator and observations of strategic interest. “Telescopes of any size Chief Scientist during the 1950s, wrote in 1980 (Newell could be used in space, since the images of stars do not 1980): “By his secrecy, Goddard not only dissipated most twinkle …; visiting other celestial bodies would certainly of his influence he might have had, he also deprived him- be of utmost scientific value …” And he added very cau- self of the engineering expertise that he sorely needed to tiously: “Under certain economic conditions, construction achieve his dream. The future of rocketry belonged to the of such machines”– space rockets and orbiting stations Ð team approach. It was, indeed, inextricably tied to the mas- “can be profitable. Such conditions may arise in some sive funding sources and particular purposes of the national decades.” How right he was! security state … His reluctance to work openly with others deprived Goddard … of the kind of funding support from the military …” It deprived him also of the recognition of 2. SYSTEMATIC ROCKET TECHNOLOGY peers he would have deserved so abundantly. On March 16, WORK IN GERMANY, 1923–45 1926, a Goddard rocket, fueled with kerosene and liquid oxygen, rose to an altitude of 13 meters and covered a dis- Oberth’s book of 1923 became the most important and tance of 55 meters. It was history’s first liquid-propellant influential book about rocketry ever. Many of the later rocket. Eleven years and numerous rockets later, Goddard rocket pioneers quoted it as their initiation into their own and his co-workers launched a rocket in Roswell, New rocket career. At the time when it was published, young Mexico, which reached an altitude of 2700 meters Wernher von Braun, eleven years old, lived in Berlin. As (Goddard 1948). his mother later wrote, “He involved himself in numerous It was several years after Goddard’s death that his many projects with his friends … they built all kinds of rockets, prolific notes, diary entries, manuscripts, and patents and they collected pieces of old automobiles from junk- became available to the public (Goddard et al. 1968). There yards and built ‘new’ cars, with and without rocket propul- is hardly any area of rocket technology that he did not dis- sion …” (Stuhlinger et al. 1994). At 14, he wrote an essay cuss, and for which he did not propose designs and ideas Journey to the Moon: Its Astronomical and Technical for further studies. By the time, during the late 1940s and Aspects, published in the Journal for the German Youth. early 1950s, when details of his theoretical and experimen- One year later, the teenager wrote a letter to Oberth: tal work became known, others had independently arrived “I know you believe in the future of rockets. So do I. at similar ideas, and had even developed them into modern, Hence, I take the liberty of sending you a brief paper on powerful precision rockets that transformed Goddard’s rockets that I wrote recently.” This was the beginning of dreams into reality. a very close and warm friendship between the two men, Hermann Oberth (1894Ð1989) learned of Goddard’s exis- based on great and genuine mutual admiration. It lasted tence and work in 1922 through a brief note in a newspaper. until von Braun’s death, fifty years later. He wrote Goddard a letter, asking for “a copy of your Shortly after he had decided to become a space pioneer books.” Goddard sent Oberth a copy of his Smithsonian in 1925, von Braun made another decision, typical of his paper. In return, Oberth wrote an appendix to his own book clarity of thought. If we want to travel to the Moon and to Die Rakete zu den Planetenräumen (The Rocket to Mars, he said, the most important requirement is a vehicle Planetary Spaces) which was in press at that time (Oberth that can provide proper transportation over the vast dis- 1923), with the expression of great admiration and esteem. tances in space, and that also offers accommodation first for 62 ERNST STUHLINGER

instruments to make scientific observations, and later for 2500 meters. In 1935, the Army, in a joint enterprise with human travelers who will live and work on their spacecraft the Air Force, decided to build a larger rocket development during the voyage, and then on the surface of their celestial facility at Peenemünde on the Baltic island of Usedom. target. I will devote my efforts first of all to the develop- Two years later, von Braun and his co-workers moved in ment of a powerful precision rocket that is capable of pro- and began work on a rocket for which the Army had speci- viding an adequate and safe means of transportation fied a range of about 250 kilometers and a payload of about through space. 1 ton. Von Braun, 25 years old, became the Technical Years later, recalling his early decision, he mentioned to Director of the Peenemünde Rocket Development Station; some friends: “Look at the great events of discovery and Colonel (later General) Walter Dornberger was named exploration in our history. Each of them happened when Commanding Officer. Under the Army’s auspices and pro- proper means of transportation had become available, good tection, work at Peenemünde proceeded quickly and suc- ships for Columbus, horses for the early American pio- cessfully, relatively undisturbed by the Nazi government neers, chuck wagons when the American West was opened and the Party. for European immigrants, railroads when people began In retrospect, the Peenemünde rocket project stands out to settle there in numbers, automobiles and airplanes when as the first step in the realization of von Braun’s youthful the continent became a homestead for millions. So, have dream, dreamed ten years earlier, to build a powerful preci- rocket, will travel through space!” sion rocket whose future descendants one day would enable Von Braun’s serious and systematic rocket development human voyagers to travel into and through space. The basic work began in 1930 when, as a student at the Technical features of that first rocket had taken shape in his mind University of Berlin, he assisted Professor Oberth in his before Peenemünde began: a rocket motor burning alcohol efforts to develop a proper combustion chamber and exhaust and liquid oxygen, the propellants being fed by centrifugal nozzle for a gasoline-liquid oxygen rocket motor. This was pumps; the walls of the combustion chamber and nozzle to von Braun’s first encounter with a technical enterprise based be cooled by fuel; the attitude of the rocket to be sensed by on careful planning, and on exacting theoretical studies, an gyroscopes and controlled by a combination of air and jet approach that was to become a trademark of von Braun’s vanes; the shape of the rocket to be determined by careful own work in all of his ensuing projects. wind tunnel experiments; and its velocity to be measured Around the same time, von Braun joined a small group by a set of accelerometers and integrators. of young rocket enthusiasts in Berlin. They called them- Only rudimentary knowledge and experience existed in selves “Verein für Raumschiffahrt” (Society for Space all these areas when work on the Peenemünde rocket Travel), and they built and tested their liquid-fueled rockets started. From the beginning, von Braun organized his fast- at their “Raketenflugplatz” (Rocket Proving Ground) in growing work force in such a way that intense development nearby Reinickendorf. Although it was a very small team work could be conducted simultaneously in all areas. with almost no financial means, they made noticeable Systems were continuously refined, tested, and further progress with their rocket engines. But Rolf Engel, one of refined. Actually, the Peenemünde rocket grew and the members who later became a prominent rocket expert, approached its definitive form in five distinct steps that remembered: “One day, Wernher told me: ‘Look, Rolf, we had already started in Kummersdorf. Each of the steps want to push this thing. But we have no money. The only utilized the experience gained in its simpler and smaller way how we can get the money, the assistance, and all the forerunners; the models were the A-1, A-2, A-3, A-5, and means, is the Army’.” (Reisig 1997). finally the A-4 which was built with the size and capability It so happened that during 1931Ð32, the German Army specified by the Army. Gradually, the A-4 rocket took (Deutsche Reichswehr) also had a small rocket develop- shape, and after much testing of the instruments in the labo- ment program underway. By some mixture of fate and ratories, and of the rocket engine with its jet vanes on test circumstamces (details are described by Reisig 1997), the stands, flight testing of the A-4 began in the summer of Army group made contact with the Society for Space 1942. After the first three launch attempts had failed, a test Travel and offered to von Braun a Civil Service contract to flight on October 3, 1942 was a full success. develop a rocket that could be used as a weapon replacing Some improvements were planned and started in heavy artillery that at that time was forbidden to Germany. Peenemünde, but they were completed only years later In 1932, von Braun and most of his few co-workers settled when von Braun and part of his team continued their rocket in a modest rocket testing facility in Kummersdorf near development work in the United States. Among these Berlin and developed a rocket with alcohol and liquid oxy- systems were integral propellant tanks instead of the gen as propellants. Their work was successful; in December tank-within-a-shell design, an improvement that became 1934, they launched Max and Moritz, rocket twins of possible after better aluminum alloys and welding techniques the A-2 type. Each of them reached a planned altitude of had become available; air bearings for the gyroscopes 31

WERNER BECKER* AND GEORGE PAVLOV** Pulsars and isolated neutron stars

HISTORICAL OVERVIEW Oppenheimer and George Volkoff, who calculated an upper limit for the neutron star mass. Using general relativistic The idea of neutron stars can be traced back to the early equilibrium equations and assuming that the star is entirely 1930s, when Subrahmanyan Chandrasekhar, while investi- described by an ideal (i.e., non-interacting) Fermi gas of gating the physics of stellar evolution, discovered that there neutrons, they found that any star more massive than 3ML is no way for a collapsed stellar core with a mass more (the OppenheimerÐVolkoff limit) will suffer runaway gravi- than 1.4 times the solar mass, ML, to hold itself up against tational collapse to form a black hole (Oppenheimer and gravity once its nuclear fuel is exhausted (Chandrasekhar Volkoff 1939). Unfortunately, their pioneering work did not 1931). This implies that a star left with M 1.4 ML predict anything astronomers could actually observe, and (the Chandrasekhar limit) would keep collapsing and eventu- the idea of neutron stars was not taken seriously by the ally disappear from view. After the discovery of the neutron astronomical community. Neutron stars therefore remained by James Chadwick in 1932, Lev Landau was the first to in the realm of imagination for nearly a quarter of a cen- speculate on the possible existence of a star composed tury, until in the 1960s a series of epochal discoveries were entirely of neutrons (Landau 1932, Rosenfeld 1974). Using made in high-energy and radio astronomy. the newly established FermiÐDirac statistics and basic quan- X-rays and gamma rays can be observed only from above tum mechanics, he was able to estimate that such a star, con- the Earth’s atmosphere (X-rays are absorbed at altitudes of sisting of 1057 neutrons, would form a giant nucleus with a 20Ð100 km), which requires detectors to operate from high- 2 1/2 5 radius of the order of R ( /mnc)( c/Gmn) 3 10 cm flying balloons, rockets, or satellites. One of the first X-ray in which , c, G and mn are the rationalized Planck constant, detectors brought to space was launched by Herbert the speed of light, the gravitational constant, and the mass of Friedman and his team at the Naval Research Laboratory in the neutron. In view of the peculiar stellar parameters, order to investigate the influence of solar activity on the prop- Landau called these objects unheimliche Sterne (“weird agation of radio signals in the Earth’s atmosphere (Chapter stars”), expecting that they would never be observed because 12). Using simple proportional counters put on old V2 (cap- of their small size and expected low optical luminosity. tured in Germany after World War II) and Aerobee rockets, Walter Baade and Fritz Zwicky were the first to propose they became the first to detect X-rays from the very hot gas the idea that neutron stars could be formed in supernovae in the solar corona. However, the intensity of this radiation (Baade and Zwicky 1934). The first models for the struc- was found to be a factor 106 lower than that measured at opti- ture of neutron stars were worked out in 1939 by Robert cal wavelengths. In the late 1950s it was therefore widely believed that all other stars, much more distant than the Sun, should be so faint in X-rays that further observations at that * Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany energy range would be hopeless. However, results from high- ** Penn State University, University Park, PA, USA energy cosmic ray experiments suggested that there exist

celestial objects (e.g., supernova remnants) that produce pulsating radio sources, which were called pulsars, and high-energy cosmic rays in processes which, in turn, may rapidly spinning neutron stars was provided by Franco also produce X-rays and gamma rays (Morrison et al. 1954, Pacini (1967, 1968) and Thomas Gold (1968, 1969). Pacini, Morrison 1958). These predictions were confirmed in 1962, then a young postdoctoral worker at Cornell University, had when the team led by Bruno Rossi and Riccardo Giacconi published a paper a few months before the discovery by Bell accidentally detected X-rays from Sco X-1. With the aim of and Hewish in which he proposed that the rapid rotation of a searching for fluorescent X-ray photons from the Moon*, highly magnetized neutron star could be the source of they launched an Aerobee rocket on 12 June 1962 from energy in the Crab Nebula. This prediction was based on the White Sands, New Mexico with three Geiger counters as the pioneering work of Hoyle et al. (1964), who had proposed payload, each having a 100 field of view and an effective that a magnetic field of 1010 G might exist on a neutron star collecting area of about 10 cm2 (Giacconi 1974). The experi- at the center of the Crab Nebula. The most fundamental ment detected X-rays not from the Moon but from a source ideas on the nature of the pulsating radio sources were pub- located in the constellation Scorpius, dubbed Sco X-1, which lished by Gold (1968, 1969) in two seminal Nature papers. is now known to be the brightest extrasolar X-ray source in In these papers Gold introduced the concept of the rotation- the sky. Evidence for a weaker source in the Cygnus region powered pulsar which radiates at the expense of its rota- and the first evidence for the existence of a diffuse isotropic tional energy Ð the pulsar spins down as rotational energy is X-ray background were also reported from that experiment radiated away Ð and recognized that the rotational energy is (Giacconi et al. 1962). Subsequent flights launched to con- lost via electromagnetic radiation from the rotating magnetic firm these first results detected Tau X-1, a source in the con- dipole and emission of relativistic particles. The particles are stellation Taurus that coincided with the Crab Supernova accelerated in the pulsar’s magnetosphere along the curved Remnant (Bowyer et al. 1964). Among the various processes magnetic field lines and emit the observed intense curvature proposed for the generation of the detected X-rays was ther- and synchrotron radiation. (When a charged relativistic par- mal radiation from the surface of a hot neutron star (Chiu and ticle moves along a curved magnetic field line, it is acceler- Salpeter 1964), and searching for this radiation became a ated tranversely and radiates. This curvature radiation is strong motivation for the further development of X-ray closely related to the synchrotron radiation caused by gyra- astronomy. However, the X-ray emission from the Crab tion of particles around the magnetic field lines.) Supernova Remnant was found to be of a finite angular size Since those early days of pulsar astronomy more than a (1 arcmin), whereas a neutron star was expected to appear thousand radio pulsars have been discovered (see, e.g., the as a point source. Thus, the early X-ray observations were not catalog by Taylor et al. (1993), which lists about half of sensitive enough to prove the existence of neutron stars. This them). The discovery of the first radio pulsar was very soon was done a few years later by radio astronomers. followed by the discovery of the two most famous pulsars, In 1967, Jocelyn Bell, a graduate student under the super- the fast 33 ms pulsar in the Crab Nebula (Staelin and vision of Antony Hewish at Cambridge University, England, Reifenstein 1968) and the 89 ms pulsar in the Vela super- came across a series of pulsating radio signals while using a nova remnant (Large et al. 1968). The fact that these pul- radio telescope specially constructed to look for rapid varia- sars are located within supernova remnants provided tions in the radio emission of quasars. These radio pulses, striking confirmation that neutron stars are born in core col- 1.32 seconds apart and with remarkable clock-like regularity, lapse supernovae from massive main sequence stars. These were emitted from an unknown source in the sky at RA exciting radio discoveries triggered subsequent pulsar 19h 20m, and dec. 23. Further observations refined the searches at nearly all wavelengths. pulsating period to 1.33730113 seconds. The extreme preci- Cocke et al. (1969) discovered optical pulses from the sion of the period suggested at first that these signals might Crab Pulsar, whereas its X-ray pulsations in the 1.5Ð10 keV be generated by extraterrestrial intelligence. They were range were discovered by Friedman’s group at the Naval subsequently dubbed LGM1, an abbreviation of “Little Research Laboratory (Fritz et al. 1969) and by the team at Green Man 1” (Bell 1977). However, when other similar the Massachusetts Institute of Technology (Bradt et al. 1969) sources were detected, it became clear that a new kind of three months later. Using a plastic scintillator platform, celestial object had been discovered. The link between these Hillier et al. (1970) flew a balloon-borne experiment over southern England and detected its pulsed gamma rays at a 3.5 level at energies greater than 0.6 MeV. These early * The Moon was selected as a target because it was expected that a state- of-the-art detector available at that time would not be sensitive enough to multi-wavelength observations showed that the pulses are all detect X-rays from extrasolar sources. “We felt … that it would be very phase aligned, with a pulse profile which was very nearly the desirable to consider some intermediate target which could yield concrete same at all wavelengths, suggesting a common emission site results while providing a focus for the development of more advanced instrumentation which ultimately would allow us to detect cosmic X-ray for the radiation. Moreover, the power observed at the high sources” (Giacconi 1974). photon energies exceeded that in the optical band by more PULSARS AND ISOLATED NEUTRON STARS 723 than two orders of magnitude, justifying the need for more describing data analysis techniques optimized for “sparse sensitive satellite-based X-ray and gamma-ray observatories data,” particularly the timing analysis aimed at pulsation to perform more detailed investigations of the emission searching, were therefore always valued highly. mechanism of pulsars and to survey the sky for other X-ray The first complete and detailed gamma-ray map of the and gamma-ray sources. Galaxy was provided by the ESA mission COS-B, launched The first Earth-orbiting mission dedicated entirely to celes- in August 1975. Developed by a group of European research tial X-ray astronomy, SAS 1 (Small Astronomy Satellite 1), laboratories known as the Caravane Collaboration (formed was launched by NASA in December 1970 from a launch of members from MPE-Garching, CEN-Saclay, SRON- site in Kenya. The observatory, later named Uhuru (which Leiden (today Utrecht), IFCAI-Palermo, CNR-Milano, and means “freedom” in Swahili), was sensitive in the range SSD-ESTEC), the satellite carried two scientific payloads: a 2Ð20 keV and equipped with two sets of proportional coun- digital spark chamber, sensitive in the range 0.03Ð5 GeV, ters having a collecting area of 840 cm2 (Giacconi et al. and a 2Ð12 keV collimated proportional counter that was 1971). It was designed to operate in survey mode, allowing used as a pulsar synchronizer. The on-board clock calibra- for the first time a scan of the whole sky with a sensitivity of tion was not very accurate, so the latter instrument was used 1.5 1011 erg s1 cm2. In over two years of very success- to ensure the synchronization of the X-ray and gamma-ray ful operation, 339 new X-ray sources were detected pulses from isolated pulsars, like the Crab and Vela Pulsars, (Forman et al. 1978): accreting binaries, supernova rem- and accreting pulsars in X-ray binaries. It was further used nants, Seyfert galaxies, and clusters of galaxies. By far the to determine pulsar ephemerides from the temporal analysis largest sample of objects was found to belong to the group of X-ray data, independently of the availability of exact of accretion-powered pulsars Ð neutron stars in binary sys- radio ephemerides. The high sensitivity of the gamma-ray tems accreting matter from a companion star. As the matter detector allowed Kanbach et al. (1980) to conduct the first spirals onto the neutron star’s surface or heats up in an detailed temporal and spectral study of the Vela Pulsar in the accretion disk, strong X-radiation is emitted (Chapter 32). range 0.05Ð3 GeV. The pulsar’s spectrum was found to be The next major step in high-energy astronomy was the represented by a power law, dN/dE E (with a photon launch in November 1972 of SAS 2, the first satellite dedi- index of 1.89 0.06 for the phase-averaged spectrum), cated exclusively to gamma-ray astronomy (Fichtel et al. but appreciable differences of the photon index were 1975). The detector, a spark chamber, was sensitive in the detected for different pulsar phases (e.g., the inter-pulse energy range 35Ð1000 MeV. Although the mission lasted emission, first detected in the COS-B data, was found to only seven months and was ended by a failure of the low- have the hardest spectrum). The COS-B observations of the voltage power supply, its measurements confirmed the exis- Crab Pulsar provided much improved photon statistics tence of the gamma-ray pulses from the Crab (Kniffen et al. which resulted in a more accurate pulse profile (Wills et al. 1974) and discovered the gamma-ray pulses from the Vela 1982) and detailed spectral studies (Clear et al. 1987). Pulsar (Thompson et al. 1975), which was found to be the Many radio pulsars had been observed by the mid-1970s, strongest gamma-ray source in the sky. The Vela light-curve and two of them, the Crab and Vela Pulsars, had been was characterized by two relatively sharp peaks, separated detected at high photon energies. Although the interpretation by 0.4 in phase (as observed for the Crab) but not phase of both isolated and accreting pulsars as neutron stars with aligned with the radio and optical pulses. enormous magnetic fields, 1012 G, had been generally In addition, a few unidentified gamma-ray sources were accepted, there was no direct evidence for the existence of detected, among them Geminga, a faint source in the Gemini such huge fields. The evidence came from a remarkable region from which 100 gamma-ray photons had been spectral observation of Her X-1, an accreting binary pulsar recorded, but which was not finally identified for another 20 discovered with Uhuru by Tananbaum et al. (1972). On years.* Gamma-ray astronomy, from its beginning, was often 3 May 1976, a team of the Max-Planck Institut für extrater- hampered by the relatively small number of detected photons restrische Physik in Garching and the Astronomische Institut and large position error boxes, typically 0.51. This posi- of the University of Tübingen, led by Joachim Trümper, tion uncertainty strongly complicated follow-up observations launched from Palestine, Texas a balloon equipped with a for optical and X-ray counterparts. Scientific publications collimated NaI scintillation counter and a NaIÐCsIÐphoswich detector, sensitive in the range 15Ð160 keV. They easily detected the 1.24 s pulsations up to 80 keV (Kendziorra et al. * The source was dubbed Geminga, a pun in Milanese dialect in which 1977). However, when Bruno Sacco and Wolfgang Pietsch gh’é minga means “it is not there” or “it does not exist,” by Giovanni attempted to fit the observed spectrum with usual continuum Bignami Ð See Bignami and Caraveo (1996) for a comprehensive descrip- spectral models, they found that a one-component continuum tion of the Geminga story, from the first discovery to the final identification. It is amusing to note that the name inspired Eric Cohez to choose the model could not represent the data Ð all fits gave unaccept- title of his science fiction book Geminga: la civilisation perdue. ably large residuals at 40Ð60 keV. Further data analysis 724 WERNER BECKER AND GEORGE PAVLOV

confirmed that the spectral feature was not an artifact (e.g., X-ray sources, were obtained with HEAO 2, widely known due to incomplete shielding of the in-flight calibration source, as the Einstein X-ray observatory (Giacconi et al. 1979), 241Am, which emitted a spectral line at E 59.5 keV). It was which carried the first imaging X-ray telescope on a satel- Joachim Trümper who first recognized that the excess emis- lite. Among four focal plane detectors of Einstein, two sion at 58 keV (or an absorption feature at 42 keV, depending proved to be particularly useful for detecting and studying on interpretation; Figure 1) could be associated with the res- isolated neutron stars. The High Resolution Imager (HRI), a onant electron cyclotron emission or absorption in the hot micro-channel plate detector, sensitive in the 0.15Ð4keV polar plasma of the rotating neutron star. The corresponding energy band, with about 5 arc seconds angular resolution, 12 12 magnetic field strength would then be 6 10 or 4 10 G was designed to use the imaging capability of the X-ray (Trümper et al. 1978). This observation provided the first telescope. However, it had no energy resolution and its field direct measurement of a neutron star’s magnetic field and of view was small, 25 arcmin. The Imaging Proportional confirmed the basic theoretical predictions that neutron stars Counter (IPC), the workhorse of the observatory, could are highly magnetized, rapidly spinning, compact objects. detect weaker sources than the HRI and had a wider field of Beginning in 1977, NASA launched a series of large sci- view, 1, but its imaging resolution was about 1 arcmin. It entific payloads called High Energy Astrophysical Obser- was capable of studying spectra with modest energy resolu- vatories (the dramatic history of the HEAO project and the tion in the range 0.2Ð4keV. experiments aboard HEAO satellites are described in lively Einstein investigated the soft X-radiation from the previ- fashion by Wallace Tucker (1984)): HEAO 1 (August ously known Crab and Vela Pulsars and resolved the com- 1977ÐJanuary 1979), HEAO 2 (November 1978ÐApril 1981), pact nebula around the Crab Pulsar (Harnden and Seward and HEAO 3 (September 1979ÐMay 1981). Particularly 1984). It discovered pulsed X-ray emission from two important results on isolated neutron stars, among many other other very young pulsars, PSR B0540 Ð 69 in the Large Magellanic Cloud (Seward et al. 1984) and PSR B1509 Ð 58 (Seward and Harnden 1982), with periods of 50 ms and 150 ms respectively. Interestingly, these pulsars were the first ones discovered in the X-ray band and only subsequently detected at radio frequencies. No pulsations from the Vela Pulsar were found in the soft X-ray band. Einstein also detected three middle-aged radio pulsars, PSR B065614 (Córdova et al. 1989), B1055 52 (Cheng and Helfand 1983), and B195132 (Wang and Seward 1984). Also, X-ray counterparts of two nearby old radio pulsars, PSR B095008 and B192910, were identified, based on positional coincidence (Seward and Wang 1988). In addition, many supernova remnants were mapped Ð 47 in our Galaxy (Seward 1990) and 10 in the Magellanic Clouds (Long and Helfand 1979) Ð and several neutron star candidates were detected as faint, soft point sources close to the centers of the supernova remnants such as RCW 103 (Tuohy and Garmire 1980), PKS 1209 51/52 (Helfand and Becker 1984), Puppis A (Petre et al. 1982), and Kes 73 (Kriss et al. 1985). Some additional information on isolated neutron stars was obtained by Exosat (European X-ray Observatory Satellite; Taylor et al. 1981), which was equipped with a low-energy detector with imaging capability and grating (0.04Ð2 keV) and a medium-energy proportional counter (1.5Ð50 keV). In particular, it measured the soft X-ray spectra of the middle- aged pulsar PSR B1055 52 (Brinkmann and Ögelman 1987) and of a few neutron star candidates in supernova remnants (e.g., PKS 1209 51/52; Kellett et al. 1987). Figure 1 Unfolded X-ray spectrum from Her X-1, showing In spite of the major advance in the field of high-energy the first measurement of a cyclotron line in a pulsed spectrum astronomy provided by the space observatories (particularly of an accreting neutron star. (From Trümper et al. 1978.) by Einstein) in the 1970s and 1980s, the results obtained 32

ED P.J. VAN DEN HEUVEL* Evolutionary concepts of binaries with compact objects

In 1962 the first extrasolar X-ray source, Sco X-1, was dis- a supernova event (Gold 1969; Monaghan 1969) just as had covered (Giacconi et al. 1962) and by the end of the 1960s been predicted 34 years earlier by Baade and Zwicky (1934). several dozen such strong point X-ray sources had been dis- Before 1968 neutron stars and black holes had been purely covered with rocket and balloon experiments. theoretical concepts, based on theoretical insights developed Their marked concentration in the direction of the galac- in the pioneering studies by Oppenheimer and Volkoff (1938) tic center made clear that the distances of a number of them and Oppenheimer and Snijder (1939), respectively, and sub- must be of the order of 8 kpc, implying a very large energy sequently studied by various groups, notably those of J.A. output in the form of X-rays, of the order of 1037 to Wheeler in the USA and Ya.B. Zel’Dovitch in the USSR. 1038 ergs s1 (some 104 times the total energy output of the The first to search for black holes in binary systems Sun). What could be the mechanism generating these enor- were Zel’Dovitch and Guseinov (Guseinov and Zel’Dovitch mous X-ray luminosities? Largely thanks to the Russian 1966; Zel’Dovitch and Guseinov 1966), who searched for team of Zel’Dovitch and co-workers, since the mid-1960s spectroscopic binaries with massive unseen secondary stars. the idea arose that in a binary system the process of accre- They did not yet, however, mention the possibility that such tion of matter flowing over from a normal companion star binaries might emit X-rays, but Novikov and Zel’Dovitch to a neutron star or a black hole might power these strong (1966) did so slightly later. Following the discovery of the galactic X-ray sources. Indeed, the simple process of accre- faint blue optical counterpart of Sco X-1 (Sandage et al. tion of an amount of mass m onto a neutron star (black 1966) this early work culminated in Shklovskii’s (1967) neu- hole) releases some 0.15 mc2 (0.06 to 0.42 mc2) of gravita- tron star binary model for Sco X-1. This author showed that tional binding energy which, converted into heat, is avail- the optical light in the system could not arise from the same able for emission in the form of X-rays. (The process of source as the X-rays, and that the X-ray energy distribution is mass accretion onto a supermassive black hole had already consistent with thermal bremsstrahlung from an optically thin been suggested as the energy source for quasars and active plasma accreting onto a neutron star. Since the optical spec- galaxy nuclei by Salpeter (1964), Zel’Dovitch (1964), and trum of the source resembles that of a cataclysmic variable Zel’Dovitch and Novikov (1964).) (CV: these are binary systems consisting of an accreting It should be kept in mind, however, that the existence white dwarf and a low-mass ordinary star; Chapter 33) and of neutron stars in nature was not known before the end of since no stellar spectrum is seen (implying that the compan- 1968 Ð the year in which the discovery of radio pulsars was ion star is faint) it was postulated that the neutron star is in a announced (Hewish et al. 1968). The discovery of the Crab binary and accretes matter from a low-mass companion star. Nebula pulsar in November 1968 (Staelin and Reifenstein It took another nine years before this “low-mass X-ray 1968) and the detection of its large spindown rate made clear binary” model for Sco X-1 was confirmed by the detection of that pulsars are neutron stars and that neutron stars are born in its 0.86-day orbital period by Gottlieb et al. (1975). Before that, however, the first X-ray satellite Uhuru * Universiteit van Amsterdam, The Netherlands (USA, 1970) had, in 1971, discovered the existence of the

pulsating and eclipsing binary X-ray source Cen X-3, which supergiant star HD 226868. Thus, by 1972 the existence of left no doubt about the existence of neutron stars moving in both neutron stars and black holes in close orbits around orbits around massive ordinary stars (Schreier et al. 1972). massive companion stars appeared well established. In the Shortly before this, earlier in 1971, the Uhuru group had dis- same year, Uhuru discovered the second pulsating and covered rapid X-ray variations in Cyg X-1, with timescales eclipsing binary X-ray source Her X-1, in which the 1.2 s down to 50 ms, with no obvious periodicity (Oda et al. period X-ray pulsar orbits in 1.7 days a star of rather low 1971). This rapid variability indicated that the X-ray source mass, about 2.0M (Tananbaum et al. 1972). In the 1970s in the system cannot be larger than about 104 km. Webster and 1980s more and more X-ray binaries were discovered. and Murdin (1972) and Bolton (1972) independently that They were found to fall into two broad categories: the same year identified the bright O9.7 supergiant star HD high-mass X-ray binaries (HMXBs) like Cen X-3 and the 226868 as the optical counterpart to this source. This identifi- low-mass X-ray binaries (LMXBs) like Her X-1 and Sco cation was an indirect one, through the accurate arc second X-1 (Chapters 34Ð36). position of a radio source that had been discovered indepen- The recognition that neutron stars and black holes can dently by Braes and Miley (1971) and Hjellming and Wade exist in close binary systems came at first as a surprise, as it (1971) in the X-ray error box of Cyg X-1. Webster and did not seem to fit with the then current ideas about the evo- Murdin and Bolton found the blue supergiant to be a 5.6-day lution of binary systems. It was known from stellar evolu- single-lined spectroscopic binary with a large velocity ampli- tion that the more massive a star is, the shorter its lifetime. tude (72 km s1) indicating, if the O 9.7 supergiant has a Thus, the more massive component of a binary will be the normal mass (15M) for its spectral type, the presence of a first one to undergo a supernova explosion. If more than companion of mass 3M. As this is above the upper mass half the mass of a circular-orbit binary system is explosively limit of a neutron star (Nauenberg and Chapline 1973; ejected, the orbit of the system becomes hyperbolic and the Rhoades and Ruffini 1974; Kalogera and Baym 1996) they system is disrupted (Blaauw 1961). This is a simple conse- suggested Cyg X-1 to be a black hole Ð a suggestion that quence of the virial theorem (Van den Heuvel 1994a). In a nowadays has been fully accepted. However, at the time this massive binary the mass of the neutron star remnant was, like that for Sco X-1, a rather indirect indication for the (1.4M) is negligible with respect to the masses of the existence of X-ray binaries, and not yet completely convinc- two components (and to the amount of mass ejected in the ing. For example, Kristian et al. (1971) dismissed the blue supernova), so at first sight one would always expect these supergiant as the optical counterpart of the X-ray source. On systems to be disrupted by the first supernova explosion. the other hand, there could be little doubt that the X-ray Still more puzzling were the almost perfectly circular orbits source and the radio source were connected, as the radio of Cen X-3 and Her X-1, apparently showing no trace of the source appeared just at the time when a dramatic change effects of the supernova (although Cyg X-1 still has a in the spectrum of the X-ray source occurred (Tananbaum slightly eccentric orbit). For the HMXBs like Cen X-3, it 1973). Since the radio source coincided within a few arc was soon realized (Van den Heuvel and Heise 1972) that the seconds with the blue supergiant star, it seemed quite likely survival of the systems was due to the effects of large-scale that the star, the radio source, and the X-ray source were con- mass transfer that must have occured prior to the supernova nected. However, the X-ray source did not eclipse and nei- explosion. This caused the initially more massive star in the ther the X-ray source nor the radio source showed a trace of system to have become much less massive than its compan- a 5.6-day period variability that might connect them with ion at the time of the explosion, and prevented the system the 5.6-day blue supergiant binary (only recently such peri- from being disrupted. Tidal effects subsequently circular- odic variability has been established at radio wavelengths ized the orbits during several millions of years, before the (Pooley et al. 1999)). reverse mass transfer began and the system became an X-ray For these reasons it was only with the discovery of the source. (Several other authors somewhat later independently eclipsing pulsating binary X-ray source Cen X-3 by came to similar conclusions: Börner et al. (1972); Tutukov Schreier et al. (1972) that this collection of observations and Yungelson (1973)). The model of Van den Heuvel and found a definitive explanation. Here one observed for the Heise (1972) built on earlier work on close binary evolution first time an X-ray pulsar (neutron star) that shows a beauti- developed primarily in the 1960s. Morton (1960) had been ful 2.087-day sinusoidal Doppler modulation of its 4.84 s the first one to show that large-scale mass transfer may pulse period and is eclipsed every orbit for about 0.5 days. explain why the more evolved (sub-giant) component of From the Doppler effect a projected orbital velocity of a close binary system like Algol ( Persei) may be less 415.1 0.4 km s1 was derived leading to a minimum com- massive than its brighter and less evolved companion star. panion mass of about 17 solar masses. The presence of this The reason is that in the course of its evolution the envelope massive companion made it immediately clear that Cyg of a star gradually expands, and after the exhausion of X-1 might form a similar system with the massive blue the hydrogen fuel in the stellar core, swells up to giant EVOLUTIONARY CONCEPTS OF BINARIES WITH COMPACT OBJECTS 761 dimensions. However, in a close binary system there is no matter is lost from the systems, together with its orbital room for a giant star: the sizes of the stars are limited by the angular momentum, and only the helium core of the more dimensions of their so-called Roche lobes, pear-shaped criti- evolved star is left, together with the practically unchanged cal equipotential surfaces, surrounding the stars (Section 2). low-mass companion. Still, in order to not disrupt the sys- As soon as a star becomes larger than its Roche lobe, the tem when this helium star explodes, the initial conditions matter outside the Roche lobe will flow over to its compan- must have been very fine-tuned. Therefore, the formation of ion. The more massive star in a binary will be the first one a LMXB is a much rarer event than the formation of to overflow its Roche lobe. Evolutionary calculations HMXBs (Van den Heuvel 1983, 1994a; Webbink and by Kippenhahn and Weigert (1967), Plavec (1967), and Kalogera 1994; Kalogera 1998a,b), and the same holds for Paczynski (1966, 1971a) in the 1960s showed that this leads double neutron stars. to the transfer of most of the star’s hydrogen-rich envelope An important new ingredient that has been introduced in (70% of its mass) to its companion, reversing the mass these binary evolution models since 1975 (Flannery and ratio of the system. Van den Heuvel 1975) is the occurrence of velocity “kicks” This type of “conservative” evolution (in which mass of the order a few hundred kilometers per second that and orbital angular momentum of the system are con- are imparted to the neutron stars in their birth events. served) appears to be able to explain the formation of the Ample evidence for the occurrence of such kicks has been HMXBs. inferred in the last decade from the space and velocity dis- However, for the LMXBs it was much harder to under- tribution of radio pulsars and from a variety of other obser- stand how the system survived the supernova explosion. The vational facts (Dewey and Cordes 1987; Tauris et al. 2000; same holds for the close double neutron stars, such as the Hartmann 1996, 1997; Van den Heuvel and Van Paradijs HulseÐTaylor binary pulsar PSR 191316 (Hulse and 1997). Without the occurrence of these kicks a variety of Taylor 1975), which has an orbital period of only 7.75 hours properties of the X-ray binaries and the binary radio pul- and an orbital eccentricity of 0.615. These systems must sars, including their birth rate in the galaxy, would be diffi- have survived two supernova explosions! In the LMXBs cult to understand (Verbunt and Van den Heuvel 1995; and their close relatives the cataclysmic variables the com- Kalogera 1998a,b; Kalogera and Webbink 1998). panion of the compact object is a low-mass ordinary star As mentioned above, in the mid-1970s a link was sugges- like our Sun, and the orbital periods are in general very ted between the massive X-ray binaries and the HulseÐTaylor short: mostly between 11 minutes and about half a day binary pulsar. The magnetic field of this pulsar is relatively (Chapter 34). All these systems must in the course of their weak (1010 G, some two orders of magnitude lower than lives (and, in the case of LMXBs and binary pulsars, before the average of pulsar magnetic fields) and its spin period is the last supernova in the system) have lost a very large very short (0.059 s). As it was believed at the time that neu- amount of mass and orbital angular momentum. The evolu- tron star magnetic fields decay spontaneously on a relation of these systems was therefore much more complicated tively short timescale (107 yr) the weak field suggested than that of the HMXBs. The first models for the evolution that this neutron star is old. Since in X-ray binaries with of binaries with a large loss of mass and orbital angular accretion disks angular momentum is fed to the neutron momentum were made by Van den Heuvel and De Loore star, one observes the X-ray pulsars in these systems to (1973) who showed that a HMXB may later in life turn into show a gradual decrease of their pulse periods in the course a very close binary system consisting of a helium star (the of time (Chapter 35; Section 1.4), so-called “spin-up.” helium core of the massive star) and a compact star. They Bisnovatyi-Kogan and Komberg (1975) suggested that if suggested that the 4.8 hour X-ray binary Cyg X-3 is such a the binary system is disrupted in the second supernova, this helium-star system, which was confirmed almost 20 years spun-up neutron star may again become observable as a later (Van Kerkwijk et al. 1992). The HulseÐTaylor binary radio pulsar. It was suggested by Smarr and Blandford pulsar is a logical later evolutionary product of such a sys- (1976) that PSR 191316 is the old “spun-up” neutron star tem (Flannery and Van den Heuvel 1975; De Loore et al. in the system (due to its weak magnetic field it now spins 1975). The first model for explaining the origin of an down only very slowly, on a timescale of the order of LMXB was that of Sutantyo (1975a) for the origin of Her 108 yr). The other neutron star in the system was then pro- X-1. He showed that in order to obtain such a system one duced by the second supernova explosion, and must be a should start out with a binary with components that differ young, strong-field neutron star. It is not observable, either very much in mass. In this case, due to the large difference because it has already rapidly spun down (Srinivasan between the thermal timescales of the envelopes of the stars, and Van den Heuvel 1982), or because the Earth is outside the low-mass component can hardly accept any mass from of the pulsar beam. Old neutron stars spun-up by accretion its more massive companion, once that star begins to over- in X-ray binaries, which are now observable as radio flow its Roche lobe. As a result most of the overflowing pulsars, are called “recycled pulsars” (Radhakrishnan and 762 ED P.J. VAN DEN HEUVEL

Srinivasan 1982, 1984). In order to become observable as a the different sub-types are also given. A few binaries with radio pulsar, there should no longer be gas in the system, so compact objects do not fit into the above four categories, accretion should have terminated. The companions of recy- notably the so-called “ante-deluvian” binary radio pulsars, cled pulsars should therefore either be white dwarfs or which are young pulsars in an eccentric orbit around a mas- neutron stars. The system may also have been disrup- sive star, which presumably are the progenitors of HMXBs. ted, resulting in a single recycled pulsar. The discovery of Four such systems are presently known, also indicated in the the first millisecond radio pulsar in 1982 gave the recy- table: PSR 1259Ð63 (Porb 3.4 yr), PSR 1820-11 (Porb cling idea an enormous boost: Alpar et al. (1982) and 357.8 d), PSR J1740-3052 (Porb 231 d), and PSR J0045- Radhakrishnan and Srinivasan (1982) suggested that mil- 7319 (Porb 51 d) in the Large Magellanic Cloud. In the lisecond pulsars are very old neutron stars which were first system and the last-mentioned two systems the com- spun-up by accretion in LMXBs. Because of the very long panion of the young pulsar is a B-type star, in PSR 1820-11 duration of the accretion phase in LMXBs (108 yr), a lot the companion is not known. Furthermore, there are the of angular momentum can be fed to these neutron stars, two peculiar X-ray binaries with relativistic jets, SS433 and leading to spin-up to millisecond periods. This recycling Cyg X-3. In SS433 (Porb 13 d) the companion of the jet- model has recently been beautifully confirmed by the dis- producing compact object is probably an early-type hydro- covery of the first millisecond X-ray pulsar in the LMXB gen-rich star (Margon 1983). In Cyg X-3 (Porb 4.8 h) it is a system SAX 1808.4-3658 (Wijnands and Van der Klis WolfÐRayet star (helium star; Van Kerkwijk et al. 1992). 1998). In order to spin-up a neutron star to a millisecond I now briefly describe the main characteristics of each of period, its magnetic field should have decayed to below the various types of neutron star and black hole binary sys- 109 G (Section 6). The general idea now is that the decay of tems listed in the table. In this review I will concentrate the magnetic field in accreting neutron stars is somehow mainly on these systems as these are the ones that have pri- related to the accretion process (Taam and Van den Heuvel marily been discovered thanks to observations from space. 1986), although the precise mechanisms for field decay are Only where this is appropriate will I also mention the CV still being debated (Konar and Bhattacharya 1999). systems and double white dwarfs. This chapter is organized as follows. In Section 1 an overview is given of the various observed types of binaries with one or two compact components. In Section 2 the 1.2 X-Ray Binaries orbital dynamics of binary systems is described, including High- and low-mass X-ray binaries the main effects of mass exchange and mass loss on the orbits. In Section 3 the reasons for the existence of the two The X-ray binaries can, broadly speaking, be divided into main classes of X-ray binaries, high- and low-mass sys- two main groups, the HMXBs and LMXBs, which differ in tems, are discussed. In Section 4 an overview is given of a number of important characteristics, listed in Table 2, and the evolution of single stars, and of close binaries leading to graphically depicted in Figure 1. (The characteristics listed the formation of X-ray binaries and CVs. In this section the and depicted are for systems containing neutron stars but further evolution of X-ray binaries until their final stages is most of them also hold for the X-ray binaries that contain also discussed. Section 5 deals with the globular cluster black holes; also these can be divided into HMXBs and X-ray sources and their formation and Section 6 with the LMXBs, see Figure 3.) For further details we refer here to formation of binary radio pulsars as the final products of the reviews of these two types of systems in Chapters 34 to the evolution of X-ray binaries. 36. In HMXBs the companion of the X-ray source is a luminous early-type star of spectral type O or B, like in the Cen X-3 system, with a mass typically between 10 and 40M. In LMXBs it is a faint star of mass M; in most 1 TYPES OF BINARIES WITH cases the stellar spectrum is not even visible, as the light of COMPACT OBJECTS the systems is dominated by that of the accretion disk around the compact star. The orbital periods of the LMXBs 1.1 Introduction are generally short (mostly 0.5 d), though on average somewhat longer than those of the CVs. The binaries with compact objects can be divided into the categories and types listed in Table 1. Basically they fall into the categories “compact star plus ordinary star” and “two Intermediate-mass X-ray binaries compact stars.” The first category consists of the X-ray binaries and the CVs, the second of the binary radio pulsars and Only recently it was realized that apart from these two main the double white dwarfs. Each of these categories can be groups of X-ray binaries, which each contain some 102 divided into a few main types, that can be further divided known systems in our Galaxy (Van Paradijs and McClintock into sub-types, as indicated in the table, where examples of 1995), there are a few X-ray binaries in which the companion 33

PHIL A. CHARLES* White dwarf binaries

The white dwarf binaries considered in this chapter are the cool, mass-losing companion is less massive than the Sun better known as cataclysmic variables (CVs) and are inter- and hence very faint. The white dwarf may be hot but it is acting binaries in that the white dwarf is accreting material very small (comparable to the Earth in extent) and hence from its (usually) cool, late-type companion star in a short unable to contribute significantly to the light output except at (of the order of hours) orbital period. They are one of the the shortest wavelengths. CVs are also extremely common few classes of object considered in this book that were throughout our Galaxy and hence there are nearby objects for actually known and observed prior to the twentieth century. convenient study (of the several hundred CVs catalogued, the The novae become naked-eye objects (e.g. Nova Cyg 1975 majority are estimated to be within 200Ð300 pc; Ritter and which, at a peak of 2nd magnitude, completely transformed Kolb 1998, Downes et al. 1997). A valuable consequence of the appearance of Cygnus for a few weeks in the summer of this is the low interstellar absorption for most CVs which 1975) and hence have been observed throughout human his- allows their study over a wide range of wavelengths. This is tory, and dwarf novae were first reported in the middle of the particularly important for the inner part of the accretion disk nineteenth century (Hind 1856). It was the non-periodic but and the white dwarf surface which can reach temperatures continuous eruptions displayed by dwarf novae (such as exceeding 20,000 K and hence the bulk of their radiation is U Gem and SS Cyg) that led to their cataclysmic designa- in the far and extreme UV which can only be accessed tion, a term now widely applied to all interacting binaries from space. Furthermore the CV geometry (Figure 1) is well where white dwarfs are accreting. However, their physical defined by orbital mechanics and Roche surfaces, thereby nature was not understood until the pioneering spectroscopic providing an outstanding astronomical ‘laboratory’ with studies of Kraft in the late 1950s which revealed their binary which to test and improve our understanding of the physics signature. CVs are of considerable importance for astronomy of accretion onto compact objects, in particular the nature of in general because of the significance of accretion processes the viscosity which acts within the disk. CVs thereby provide on virtually all scales, from star and planetary formation (the constraints on the basic physical properties of accretion disks proto-star accretes material from its surrounding molecular which can then be applied in many other fields. cloud) to accretion onto supermassive black holes in the cen- The typical luminosity and range of temperatures pro- tres of active galactic nuclei. Unfortunately, in most of those duced in CVs can be estimated in the following way. If circumstances, the geometry and mass accretion are very material is being accreted by the white dwarf at a rate Mú poorly constrained, there are usually multiple emitting then the release of gravitational potential energy will pro- ú sources that are complex to unravel and the timescales can be duce a luminosity L GMWDM/RWD. For a white dwarf of 4 uncomfortably long relative to human observing lifetimes. mass MWD 1M and radius RWD 10 km accreting at 10 1 In CVs the visible light we see is normally dominated by 10 M yr (a rate expected from calculations of the the accretion process itself (the transferred material usually evolution of close interacting binaries) then total luminosi- forms an accretion disk around the white dwarf and this disk ties of 1033 erg s1 will result. And if this is assumed to be is heated to 5000Ð10,000 K by internal viscous processes) as radiated uniformly from the white dwarf surface as blackbody radiation (L 4 R2T 4) then T 30,000 K and we * University of Southampton, United Kingdom would expect CVs to be copious sources of ultraviolet (UV)

Figure 1 Basic geometry of a white dwarf interacting binary, in which a low-mass normal star transfers material into an accretion disk surrounding the white dwarf. Note that the mass-transfer stream produces a bright spot on the accretion disk, which is seen as the large hump in the light curve. The binary and resulting light curve is shown at different orbital phases using quiescent (low-state) observations of the eclipsing dwarf nova Z Cha as an example. (Adapted from Marsh 2001).

radiation. Indeed, most CVs show very blue colours rising at survey of CVs that definitively established their binary the UV end of the optical to imply that the bulk of the emis- nature. The presence of a white dwarf in these systems as sion will lie at shorter wavelengths. However, if the accreting the primary was first demonstrated observationally by material is in free-fall onto the white dwarf surface then it Crawford and Kraft (1956). These events and others are should reach velocities of 5000 km s1 which correspond given chronologically in Table 1, and the properties of a to temperatures of 109 K and we would therefore expect selection of dwarf novae are summarised in Table 2. hard X-ray emission to be produced. With these expectations Both spectroscopy and photometry demonstrated that both UV and X-ray satellite instruments were used to study the continuum emission from CVs almost always exhibited these objects in the early days of space astronomy. a hot, blue, non-stellar component. With orbital periods of hours and stellar companions of usually G-M spectral type, it became clear that the cool star (referred to as the HISTORICAL SUMMARY ‘secondary star’ or ‘companion’) must be filling its Roche (equipotential) surface and hence transferring material into Dwarf novae are CVs which exhibit major optical outbursts the hot accretion disk (Figure 1) that gives rise to the emis- (increasing in brightness by several magnitudes or more) at sion lines and blue continuum. For details (and equations) irregular intervals (usually of the order of weeksÐmonths). of the Roche geometry in interacting binaries see Warner The brightest are accessible with binoculars (e.g. SS Cyg (1995), the primary reference source for CV properties. and U Gem, both of which reach V 8Ð9 at peak of outburst) and amateur groups have made fundamental contri- The accretion disk: importance of multi-wavelength butions to their long-term light curves for more than a studies hundred years (Warner 1995). Walker’s discovery (Walker 1954) that DQ Her is an eclipsing binary, combined with The material leaving the secondary’s inner Lagrangian spectroscopic observations revealing occasional absorption point carries its parent star’s orbital angular momentum and and emission components, culminated in Kraft’s (1962) hence cannot (in the absence of other forces) accrete WHITE DWARF BINARIES 793

Table 1 Chronology of white dwarf binaries

Year Discovery

Pre-history Naked-eye observations of nova outbursts 1855 U Gem at maximum light (Hind 1856) 1896 SS Cyg found from Harvard plates (Wells 1896) 1938 Term ‘dwarf novae’ coined by Payne-Gaposchkin and Gaposchkin (1938) 1949 Flickering found in UX UMa (Linnell 1949, 1950) 1956 71 s pulsations in DQ Her (Walker 1956) 1956 Identification of WD as primary in CVs AE Aqr secondary filling Roche lobe (Crawford and Kraft 1956) 1962 All CVs are binaries undergoing mass transfer onto WD (Kraft 1962) 1971 Accretion disk responsible for outburst light (Warner and Nather 1971) 1972 Hard X-rays detected from EX Hya (Warner 1972) 1974 Novae at maximum seen in UV by OAO 2 (Gallagher and Code 1974) 1977 Large, variable polarization in soft X-ray source AM Her (Tapia 1977) 1977 Term ‘polars’ coined by Krzeminski and Serkowski (1977) for AM Her systems 1978 Soft X-rays from SS Cyg in outburst and quiescence (Heise et al. 1978) 1980 14.3 min X-ray pulsations in H2252-035 (Patterson and Price 1980) 1981 ‘Supersoft sources’ (SSS) found by Einstein in LMC (Long et al. 1981) 1981 Term ‘intermediate polars’ coined by Krzeminski for asynchronous WD rotation 1983 Einstein X-ray survey of CVs (Córdova and Mason 1983) 1991 IUE spectra reveal cooling of U Gem WD between outbursts (Kiplinger et al.) 1992 Steady nuclear burning WD model for SSS (van den Heuvel et al. 1992) 1996 AE Aqr ‘magnetic propellor’ model (Eracleous and Horne 1996)

WD white dwarf.

Table 2 Properties of selected dwarf novae CV type Porb V rec decay sup Spectral (h) (d) (d) (yr) type min. inter. max.

U Gem IP Peg 3.8 15.8 12.3 95 1.5 M4V U Gem 4.2 14.6 9.4 101 1.2 M4.5V SS Cyg 6.6 11.7 8.2 40 2.4 K5V RU Peg 9.0 12.7 9.0 65 3.4 K2V BV Cen 14.6 12.6 10.7 149 7 G5-8IV-V Z Cam KT Per 3.9 16.0 12.3 11.7 26 1.3 M3 RX And 5.0 13.6 11.8 10.9 13 1.7 Z Cam 7.0 13.6 11.7 10.4 23 2.6 K7V EM Cyg 7.2 14.2 12.9 12.0 22 3.6 K5V SY Cnc 9.1 13.7 12.2 11.1 27 2.3 G8V SU UMa WZ Sge 1.36 14.9 — 7.1 30 OY Car 1.51 15.6 12.4 11.5 50 — 1 IR Gem 1.64 15.5 11.7 11.2 26 0.8 0.5 VW Hyi 1.78 13.3 9.5 8.5 27 0.7 0.5 Z Cha 1.79 16.0 12.4 11.9 82 — 0.6 M5.5V SU UMa 1.83 14.8 12.2 11.2 19 1.4 0.4 YZ Cnc 2.09 14.5 11.9 10.5 12 0.9 0.4

(i) in the V magnitude range, “inter” corresponds to the standstill brightness of Z Cam stars, and the normal maximum of SU UMa stars. (ii) rec is the mean recurrence time between outbursts. decay is the decay time from outburst. sup is the mean recurrence time between super outbursts. 794 PHIL A. CHARLES

directly onto the white dwarf. It therefore forms an and Tmax is the disk’s maximum temperature given by extended, rotating accretion disk which has the compact object at its centre, and some physical process acting within ú 3 0.25 T MM R max 41,000( WD / WD) K (2) the disk creates an internal viscosity which allows angular momentum to be transported outward in the disk, and mate- where the mass transfer rate Mú is in units of 1016 gs1, the rial to accrete inwards. The nature of this viscosity has been white dwarf mass MWD is in solar masses and the radius 4 one of the major unsolved problems in modern astro- RWD is in units of 10 km. The total radiated emission from physics, and is being actively investigated today (e.g. the disk is then the sum of all the contributions from these Balbus and Hawley 1998). Nevertheless, by wrapping up annuli, as demonstrated in Figure 2 (la Dous 1989), the our lack of understanding of viscosity into the parameter result of which is the continuum rising into the blue and /csH ( is the effective turbulent viscosity, cs the sound UV regions. This produces the characteristic UV excess by speed and H the half-thickness of the disk), Shakura and which many CVs have been discovered. At this range of Sunyaev (1973) produced the so-called -disks which temperatures it is necessary to observe from the near-IR formed the starting point for observers and theorists alike. into the far-UV to fully characterise the spectrum of In the steady state, the radiated spectrum of the disk does the disk. not depend on , but not surprisingly the disk’s response By application of the virial theorem it can be shown that ú time to changes in accretion rate does. Consequently, it is half of the total accretion luminosity (GMWDM/RWD) is the outbursts of dwarf novae and the intervals between deposited in the disk, the other half in the boundary layer them which can constrain and hence the physical with the white dwarf. An optically thick boundary layer has processes that must be occurring (e.g. Lasota 2001). temperatures of 105Ð6 K and thus radiates predominantly The original Shakura and Sunyaev formulation is sum- in the EUV and soft X-ray regions. marized in the review by Pringle (1981), and can be used to demonstrate the importance of multi-wavelength observa- Early UV and X-ray Observations tions of CVs. The circular, thin disk is divided radially into annuli of radius r, each of which is assumed to radiate as a This basic model of a CV is very similar to that of a black body at temperature T(r) where low-mass X-ray binary (Chapter 34), apart from the difference in radius of the compact object (by a factor of 103)

when the neutron star is replaced by a white dwarf. Early 4 4 3 1/2 T(r) T (r/R ) [1 (R /r) ] max WD WD (1) (non-imaging) X-ray satellites of the 1970s (Uhuru, Ariel-5,

Figure 2 Simulated spectrum (dashed line) of an accretion disk computed by adding the contributions of individual rings or annuli in the disk, each of which is assumed to radiate as a black body. The contribution of each ring is labelled with its radius and temperature (coolest on the outer edge). (Adapted from La Dous 1989). 34

JAN VAN PARADIJS† AND MICHIEL VAN DER KLIS* Low-mass X-ray binaries

The connection between cosmic X-ray sources and compact time showed that this binary star contained an accreting stars is an old one: soon after the discovery of the first such object that is likely to be a black hole. source, Sco X-1 (Giacconi et al. 1962), it was proposed that The idea that all bright galactic X-ray sources are mass- these objects are young, hot neutron stars, formed in recent exchanging binary stars with a compact accretor became supernovae, which cool through thermal radiation from their widely accepted with the observation, with the Uhuru satel- surfaces (Chiu 1964, Chiu and Salpeter 1964, Finzi 1964). lite, of regular eclipses of the pulsating X-ray source Cen However, the finite extent of the X-ray source associated X-3 (Giacconi et al. 1971, Schreier et al. 1972). The vari- with the Crab Nebula (Bowyer et al. 1964), and the non- able delays of the pulse arrival times, in phase with the Planckian shape of the X-ray spectra of this source (Clark periodic (2.1 days) eclipses of the X-ray source, showed 1965) and of Sco X-1 (Giacconi et al. 1965) showed that persuasively that in Cen X-3 the X-rays are generated by this was not, in general, a good model for X-ray sources. accretion onto a strongly magnetized neutron star, rotating Accretion onto a compact star had meanwhile been sug- at a 4.8 s pulse period, in orbit around a massive (10M) gested as a possible source of energy for quasars and X-ray companion star. The discovery of the binary nature of Cen sources (Salpeter 1964, Zel’dovich 1964, Zel’dovich and X-3 was soon followed by more observations of eclipsing Guseinov 1966), and together with the peculiarities of the X-ray sources, some of them pulsating, and by the identifi- optical spectra of the counterparts of Sco X-1 (Sandage cation of these X-ray sources with early-type stars. In addi- et al. 1966) and Cyg X-2 (Giacconi et al. 1967) this led to tion, a general framework for the origin and evolution of a the idea that these sources are mass-exchanging binaries with massive X-ray binary, as a rather normal episode in the life a compact component (Shklovsky 1967; see also Burbidge of a massive close binary star with successive stages of 1972, Ginzburg 1990). The spectrum of Sco X-1 was similar mass transfer between the two components, was readily to those of old novae and U Gem-type stars, which were by accepted (Van den Heuvel and Heise 1972). Thus, within a then known to be binary stars, in particular through the work few years the existence of a galactic population of high- of Crawford and Kraft in the 1950s and 1960s (Crawford mass X-ray binaries (HMXBs), with accreting neutron stars and Kraft 1956; Kraft 1962, 1964). However, the single-most (or occasionally a black hole) was well established. important characteristic of a binary star, that is, an orbital Already in the 1960s (e.g. Dolan 1970) it had become clear periodicity, was not found until many years later, in spite of that there is a clustering of bright X-ray sources within 30 substantial observational effort (see Hiltner and Mook (1970) of the direction of the galactic center. This concentration was and Kraft (1973) for discussions of early optical observations not accompanied by a strong background of unresolved of X-ray sources, and references). sources, which showed that these sources are located in the The discovery of the first X-ray binary, Cyg X-1, by central regions of the Galaxy (Ryter 1970, Setti and Woltjer Webster and Murdin (1972) and Bolton (1972) at the same 1970). It was, therefore, suspected that apart from the above- described HMXBs there is a class of low-mass X-ray binaries (LMXBs) with donor star masses of the order of a solar mass † Universiteit van Amsterdam, The Netherlands. Jan Van Paradijs died on 2 November 1999 or less (e.g. Salpeter 1973), but proof for this idea was hard to * Universiteit van Amsterdam, The Netherlands obtain. Apart from the difficulty of finding orbital periods, the

apparent heterogeneity of the properties of LMXBs may have played a role. Compared to the HMXBs the first handful of known systems now classified as LMXBs (Her X-1, Sco X-1, Cir X-1) show rather more diversity than similarity in their properties. As a result, only at the end of the 1970s did it become clear that there are such objects as LMXBs, which form a group with “family traits,” distinct from the HMXBs with respect to their sky distributions, X-ray spectral characteristics, optical properties, and types of X-ray variability (e.g. Lewin and Clark 1980). The LMXBs comprise the globular- cluster X-ray sources, X-ray bursters, soft X-ray transients, and the bright galactic-bulge X-ray sources. Roughly speaking, the reason for the bi-modal distribution of the masses of donor stars in X-ray binaries is that for stars less massive than 10M the stellar wind is too weak to power a strong X-ray source; however, Roche lobe overflow is unstable for stars more massive than a neutron Figure 1 Artist’s impression of an X-ray binary. (Credit: Rob star, and proceeds on a very short timescale, as a conse- Hynes). quence of which the accreting neutron star is completely engulfed and X-rays cannot escape. is not a neutron star (or black hole) but a white dwarf, bear The differences between the X-ray properties of LMXBs a general resemblance to those of LMXBs, showing emis- and HMXBs (with accreting neutron stars) may be linked to sion lines superposed on a continuum (see Warner 1995 for a difference in the strength of the magnetic fields of the neu- a comprehensive review of CVs). However, the equivalent tron stars they harbor. The natural assumption that the dif- widths of these lines in LMXB spectra, in particular that ference in donor star masses corresponds to a difference in of H, tend to be much smaller than those in CV spectra the ages of LMXBs and HMXBs has led to the idea that the (Van Paradijs and Verbunt 1984, Shahbaz et al. 1996). magnetic fields of neutron stars decay with time. In LMXBs the donor star transfers mass by Roche lobe Within the limits of this review it would be meaningless overflow (Figure 1). This mass arrives at the compact star via to strive for completeness. For detailed reviews on a variety a relatively flat rotating configuration, the accretion disk, in of topics related to X-ray binaries we refer the interested which it slowly spirals inward. Gravitational potential energy reader to Lewin et al. 1995). Further background information is converted into heat in this process. In the inner few tens of can be found in Shapiro and Teukolsky (1983), Frank et al. kilometers of this disk temperatures of 107 K are attained, (1992), Ögelman and Van den Heuvel (1989), Ventura and which leads to the emission of thermal X-rays. Part of these Pines (1991), Van den Heuvel and Rappaport (1992), Alpar X-rays irradiate the disk further out. The optical emission of et al. (1995), and Buccheri et al. (1998). References on indi- LMXBs originates primarily from these irradiated outer vidual sources can be found in Bradt and McClintock (1983) regions of the accretion disk, which produce their radiation and Van Paradijs (1995). An extensive summary of X-ray mainly through reprocessing of incident X-rays into optical satellite missions has been given by Bradt et al. (1992). and UV photons. This reprocessing dominates the internal energy generation of the disk due to conversion of gravitational potential energy into heat, generally by a large factor 1 OPTICAL COUNTERPARTS (Van Paradijs and McClintock 1995). For internal energy generation alone, as occurs in accretion disks in CVs, the Whereas the optical properties of HMXBs are dominated by (local) effective temperature, T, in the disk varies with radial the emission of their very luminous mass-donor stars, the distance, r, from the central compact star approximately as optical spectra of LMXBs (e.g. Shahbaz et al. 1996) show T(r) r3/4. In LMXB disks, where reprocessing of X-rays only a few emission lines, particularly H,H, He II 4686, dominates the energy budget, one would expect T(r) r1/2. and C IIIÐN III 4630Ð50, superposed on a rather flat (in Thus, the farther out in the disk, the more reprocessing domi- frequency) continuum. In a few cases the signature of a com- nates internal energy generation. According to the simplified, panion star can be discerned. According to Motch and Pakull but self-consistent calculations of Vrtilek et al. (1990), in (1989) the relative strength of the C IIIÐN III emission com- which also the effect of X-ray irradiation on the height of the plex relative to the 4686 emission provides a good measure disk is taken into account, T(r) r3/7. of the heavy-element abundances in the accreted matter. Many LMXBs show a regular orbital variation of their Spectra of cataclysmic variables (CVs), in which the accretor optical brightness, with one maximum and minimum per LOW-MASS X-RAY BINARIES 813 orbital cycle. Minimum light occurs at the superior con- (White et al. 1995), and optical brightness variations (Van junction of the X-ray source. These variations are caused Paradijs and McClintock 1995). In several cases periodic by the changing visibility of the accretion disk and the radial velocity variations have been found for emission X-ray heated side of the secondary, insofar as the latter is lines (originating from an accretion disk), or for absorption not in the X-ray shadow of the disk (see Van Paradijs 1991 lines (secondary star of soft X-ray transients in quiescence). and Van Paradijs and McClintock 1995 for reviews of the The known orbital periods of LMXBs range between 685 optical light curves of LMXBs). seconds and 16.6 days (Van Paradijs 1995, Liu et al. 2001). For transient LMXBs (“soft X-ray transients”) in quies- This large range indicates that LMXBs have a variety of cence, X-ray heating of the accretion disk and the compan- mass donors (white dwarfs, main-sequence stars, giant ion star is not very important, and the optical emission of stars). The orbital-period distributions of CVs and LMXBs the system is dominated by the Roche lobe-filling secondary (Figure 2) are different. Compared to the CVs a much star. These systems then show ellipsoidal light curves, larger fraction of LMXBs have periods above about half a reflecting the tidal and rotational distortion of the secondary. day. A possible explanation for this difference is that for The (reddening-corrected) color indices B V and such long periods the companion star masses are expected U B of LMXBs have average values of 0.09 0.14 and to substantially exceed the mass of a typical white dwarf, 0.97 0.17, respectively (errors are 1 standard devia- but not that of a neutron star. This will make the mass trans- tions), close to those expected for a flat continuum (F fer at long periods unstable for CVs, but not for LMXBs. constant). The distribution of the ratio of X-ray to optical The LMXB period distribution does not show the well- fluxes is rather sharply peaked. Expressed in terms of an known period gap in the distribution for the CVs (e.g. “optical/X-ray color index” B0 2.5 log FX( Jy), the peak Verbunt 1984, Spruit and Ritter 1983 for studies of the occurs near 21.5, corresponding to a ratio of fluxes emitted period gap); in fact, there are no LMXBs in the period in X-rays (2Ð11 keV) and in optical light (3000Ð7000 Å) of range between 80 minutes and 2 hours (i.e., below the 500 (Van Paradijs and McClintock 1995). period gap), which is well populated by the CV. This may The optical luminosities of LMXBs are, in general, be the result of the high probability that LMXBs form at much higher than those of CVs (e.g. Warner 1987, 1995); periods of about half a day or longer, whereas a large frac- this is because the gravitational potential well of a neutron tion of CVs form at periods below 2 hours (King and Kolb star is much deeper than that of a white dwarf, and X-ray 1997). Perhaps evaporation of the LMXB secondaries plays heating of the accretion disk is not important in CVs. a role after the LMXB has reached the upper edge of the Absolute visual magnitudes MV have been estimated for period gap; mass transfer then stops, and the rapidly rotat- LMXBs with distance estimates: (i) LMXBs in stellar sys- ing neutron star (spun up by accretion torques) then tems at a known distance; (ii) X-ray burst sources (Section 5) becomes active as a millisecond radio pulsar (Ruderman showing bursts with Eddington-limited photospheric radius expansion; (iii) Z sources (Section 6); when these are in the “normal-branch” state their X-ray luminosity is very close to the Eddington limit (e.g. Van der Klis 1995); (iv) soft X-ray transients, whose distance can be determined in quiescence from the spectral properties of the companion star. The absolute magnitudes of active LMXBs range between 5 and 5 (Van Paradijs and McClintock 1994). This large range is the consequence of the large range in X-ray luminosity, LX, of the central source, and in the size of the accretion disk. For a simple model of reprocessing of X-rays the optical luminosity, LV, of the disk is expected to 1/2 2/3 scale with LX and orbital period P as LV Lx P , in agreement with the values of MV for LMXBs with known orbital periods. This is confirmed by numerical calculations of X-ray-heated accretion disks (De Jong et al. 1996).

2 ORBITAL PERIODS Figure 2 Distributions of orbital periods for LMXBs and CVs. Orbital periods are known for some three dozen LMXBs, Data have been taken from Van Paradijs (1995) and Ritter and mainly from regular X-ray eclipses and X-ray “dips” Kolb (1998). 814 JAN VAN PARADIJS AND MICHIEL VAN DER KLIS

et al. 1989, Van den Heuvel and Van Paradijs 1988); how- and Van Paradijs 1996). The wide z dispersion of LMXBs ever, whether or not complete evaporation of the secondary with neutron stars requires that the neutron stars in these star occurs is a matter of debate. systems formed in an asymmetric supernova explosion which gave them an extra kick velocity (Brandt and Podsiadlowski 1995, Van Paradijs and White 1995, Ramachandran and Bhattacharya 1997); the kick velocity 3 GALACTIC DISTRIBUTION distribution is consistent with that of single radio pulsars (Lyne and Lorimer 1994, Hansen 1996, Hansen and The sky distributions of the HMXBs and LMXBs are quite Phinney 1997, Hartman et al. 1997). different, as shown in Figure 3. The galactic HMXBs are 2 36 There are 10 persistent luminous LMXBs (LX 10 distributed along the galactic plane, without an obvious erg s1) in the Galaxy (Van Paradijs 1995). During the last concentration to the galactic center. They have an average decade it has become clear that in a large fraction of the II latitude b 0.8 6.9 ; if we leave out X Per and a transient LMXBs the compact star is a black hole; the num- few other nearby high-latitude Be/X-ray systems identified ber of such transient black-hole binaries in the Galaxy is II by Tuohy et al. (1988) we find b 0.4 1.9 . This estimated to be between a few hundred and a few thousand fits the idea that HMXBs are young population I objects. (Tanaka and Lewin 1995, White and Van Paradijs 1996). The galactic LMXBs (excluding the globular-cluster The kinematic properties of LMXBs have been studied II sources) have a wider latitude distribution ( b by Cowley et al. (1988) and Johnston (1992). Based on the 0.4 9.1 ), and are also more concentrated to the direction large velocity dispersion and low galactic rotation velocity, of the galactic center. The scale height of the (assumed Cowley et al. (1988) concluded that LMXBs are among the exponential) distribution of distances from the galactic oldest objects in the Galaxy. However, since LMXBs get a plane, for LMXBs with neutron stars at known distances is kick velocity at the formation of the neutron star, such a 900 pc (Van Paradijs and White 1995). The z distribution of direct interpretation of the kinematic properties of LMXBs LMXBs with a black hole is substantially narrower (White in terms of their ages cannot be made.

4 X-RAY PULSATIONS

Almost all HMXBs show X-ray pulsations, which indicates that the accreting compact stars in these systems are strongly magnetized neutron stars (for reviews of various aspects of X-ray pulsars see, e.g. Joss and Rappaport 1984, Nagase 1989, White et al. 1995, Bildsten et al. 1997). Pulse arrival time measurements for pulsating HMXBs, in combination with radial velocity observations of their massive companions, have provided information on the masses of accreting neutron stars. X-ray pulsations in LMXBs are very rare. In spite of very sensitive searches (e.g. Vaughan et al. 1994) none of the bright LMXBs have shown the predicted millisecond pulsations, with upper limits to their amplitudes well below the 1% level. One case of a millisecond pulsar powered by accretion has recently been found. This pulsar, with a spin period of 2.5 ms, was discovered with the Rossi X-ray Timing Exporer (RXTE) in the faint transient LMXB SAX J1808.4-3658 (Wijnands and Van der Klis 1998). It has an orbital period of approximately two hours (Chakrabarty and Figure 3 Sky maps (in galactic coordinates) of the HMXBs (top) and LMXBs (bottom); the latter also includes the globular- Morgan 1998). In the last several years evidence for millisec- cluster sources (indicated by open circles). The 27 LMXBs ond spin periods of the neutron stars in LMXBs has also within 2 of the galactic center have not been included to been obtained from the “burst oscillations” seen in some avoid congestion of the map. These maps are based on the X-ray bursts (Section 5); indirect evidence from aperiodic catalog of Van Paradijs (1995). variability for millisecond spin periods also exists (Section 6). 35

NICHOLAS E. WHITE* High-mass X-ray binaries

High-mass X-ray binary (HMXB) systems were first identi- Cyg X-1 has strengthened and it is now widely accepted. fied in the early 1970s amongst the handful of mysterious While this was one of the first HMXBs to be found, it is a bright X-ray sources found in the 1960s from brief sound- relatively unique system in containing a black hole. ing rocket and balloon flights. The launch of NASA’s Uhuru Following these initial discoveries several more HMXBs (SAS 1) satellite, which made the first all-sky X-ray survey, similar to Cen X-3 were found, primarily from the identifi- established the class. One of the first results from this mis- cation of other Uhuru sources. These included Vela X-1 sion was the discovery by Giacconi et al. (1971) of 4.8 s (Ulmer et al. 1972; Forman et al. 1973), SMC X-1 X-ray pulsation from Cen X-3 (Figure 1). Precise pulse tim- (Webster and Murdin 1972; Schreier et al. 1972), 4U1700- ing and the observation of an eclipse of the pulsar showed 37 (Jones et al. 1973), and GX 301-2 (Vidal 1973; Jones that Cen X-3 is in a 2.1-day orbit around a massive early- et al. 1974).These discoveries stimulated a flood of theo- type star (Schreier et al. 1972). Within the Uhuru error retical papers. Pringle and Rees (1972), Davidson and box of Cen X-3 Krzeminski (1974) identified the optical Ostriker (1973), Lamb et al. (1973), and Shakura and counterpart as a 13th magnitude highly reddened early OB- Sunyaev (1973) all proposed and discussed variants on the type star. It is ironic to note that if the radio astronomers basic model that successfully explained the broad observa- had not discovered radio pulsars in 1967, then X-ray astron- tional properties of Cyg X-1 and Cen X-3. The X-rays are omy would surely have found them a few years later. driven by material captured from a relatively normal star, Around the time of the launch of Uhuru, Bolton (1972) and falling into the deep gravitational potential well of a neu- Webster and Murdin (1972) identified the optical counter- tron star in the case of Cen X-3 and a black hole in Cyg part to the then anonymous X-ray source Cyg X-1 (discov- X-1 (Figure 2). The strong wind of the OB star provides a ered by Bowyer et al. 1965) with an OB supergiant natural source of material to be captured by the pulsar or HD226868. For a brief time it was thought that Cyg X-1 black hole. The wind may be enhanced due to the fact that was also a pulsar (Oda et al. 1971), but it turned out the the OB star is close to filling its critical Roche potential pulses were not coherent but rather a manifestation of “shot lobe, so that material spills through the inner Lagrangian noise” (Terrell 1972), caused by chaotic energy release in point onto the compact object. the accretion disk around the black hole. The mass function These early results provided the first direct confirmation of the HD226868/Cyg X-1 system suggested that the X-ray of the earlier suggestion by Shklovsky (1967) and source was too heavy to be a neutron star, and might be a Pendergast and Burbidge (1968) that the mysterious bright black hole with a mass of 10 or more solar masses. This X-ray sources were binary systems containing a compact caused headline news at the time, as it provided the first object. These earlier models were based on the identifica- direct evidence that these enigmatic objects might exist in tion of Sco X-1 and Cyg X-2 with nova-like objects. These nature. Over the course of time the black hole candidacy of eventually became the class of low-mass X-ray binaries (LMXBs) where the mass donor is a late-type star. But it * NASA Ð Goddard Space Flight Center, Greenbelt, MD, USA was the HMXB systems that provided the first direct proof

Figure 1 The discovery of X-ray pulsations from Cen X-3 made by Giacconi et al. (1971). The 4.8 s pulsations are seen as the Uhuru satellite scanned across the source. away over several tens of day (Ives et al. 1975). Several were identified with Be star optical counterparts. Maraschi et al. (1976) suggested they formed a distinct class of HMXB object, with the X-ray outburst driven by mass-loss episodes from the Be star. This idea was subsequently confirmed with the determination by Rappaport et al. (1978) using the SAS 3 satellite of a 24-day orbital period for one of these transients, 4U011563. The Be X-ray binaries are much more widely separated than the supergiant systems. Their typical X-ray luminosity is many orders of magnitudes lower, both because the wind is less strong and the neutron star is further out. The Be stars are well known as a class of star that is rotating close to breakup velocity. They are thought to have a disk in the equatorial plane and to undergo mass-ejection episodes. Most are only seen as X-ray sources when the Be star undergoes such a mass- Figure 2 An artist’s impression of the HMXB Cyg X-1, which ejection episode, so that the X-ray luminosity increases dra- has an orbital period of 5.6 days. This system contains a black matically (Stella et al. 1986). hole accreting from the wind of a massive OB star. Not all the Be systems are transients. The Uhuru error box of the persistent, relatively faint source 4U035252 contained X Per, a bright Be star that up until then had been for both the presence of accretion onto a neutron star and assumed to be a single star. White et al. (1976a) using the for the binary orbit. small X-ray telescope on the Copernicus observatory dis- The OB star companion has a substantial stellar wind, covered a coherent 13.9-minute pulsation from the X-ray removing between 106 and 1010 solar masses per year source associated with X Per. The luminosity of this system with a terminal velocity up to 2000 km s1. Roche lobe is 1033 erg s1, 1000 to 1 million times fainter than the other overflow has also been discussed as a supplement to the members of the HMXB class. The Be star systems are by mass transfer rate in some of the supergiant HMXBs. But the far the most populous members of the HMXB class. full Roche lobe overflow is not likely to be the case They tend to be nearer by (by virtue of their higher space because when the mass ratio of the compact object to its density) and have a lower luminosity. companion is greater than unity, then mass transfer via The term “high-mass X-ray binary” did not enter the ver- Roche lobe will become unstable 105 yr after it starts nacular until the early 1980s. By then the systematic studies (Savonije 1983). As the supergiant approaches its Roche using the X-ray observatories of the 1970s (Uhuru, Ariel V, lobe, the reduced gravity and/or X-ray illumination can SAS 3 and HEAO 1) had resulted in the identification of cause a focusing of the wind towards the compact object 25 systems associated with OB stars (Bradt and (Friend and Castor 1982; Day and Stevens 1993). McClintock 1983). By that time it was clear that the overall The newly launched Ariel V observatory detected tran- properties of an X-ray binary were dictated by the nature of sient X-ray pulsars that brightened quickly, and then faded the mass donor. The spectral type of the mass donor in an HIGH-MASS X-RAY BINARIES 825

LMXB is K or later and does not have a strong wind Ð Roche Tidal effects will tend to circularize the orbit, and will be lobe overflow drives the X-ray emission. In an HMXB the more important in the shorter orbital period systems. All wind of the OB star provides a ready source of fuel for the but one of the supergiant systems have orbital periods less X-ray source, and also attenuates the X-ray source. This nat- than 15 d, and dominate this part of the period distribution. ural division also means that HMXBs are younger systems, Most Be star systems have longer periods of several tens or where the neutron star has been relatively recently formed. hundreds of days. The orbital periods of the Be X-ray binaries are not so well known, except that they must be long to avoid any tidal modulation of the optical star or variations of the pulsar arrival times. For these systems it can take PULSE AND ORBITAL PERIODS many years of observations to reveal the orbital period. For example the orbital period of X Per remained elusive for Orbital periods many years until very recently the Rossi XTE, by monitor- The orbital period and eccentricity of a representative ing the pulse timing over several years, revealed a 250-day selection of HMXB systems are listed in Table 1. The period (Delgado-Martí et al. 2001). orbital periods range between 4.8 h and 250 d. The super- As the baseline of the pulsar timing measurements giant systems typically are eclipsing and show extreme increased it became possible to search for changes in intensity and absorption variability on all timescales. The HMXB orbital periods. Kelley et al. (1983) determined that longer orbital period systems are more eccentric. This the orbital period of Cen X-3 is decreasing on a timescale eccentricity is to be expected from the “kick” imparted dur- of 500,000 yr. This decrease is the result of tidal ing the supernova explosion that created the neutron star. torque between the neutron star and the outer layers of the

Table 1 The parameters of some well-known HMXBs

Name Alternative Orbital period Pulse period* Eccentricity Type† name (day) (s)

Gamma Cas 4U 0053604 ? ? Be A 1118-61 He3-640 ? 405 Be transient Cyg X-3 V1521 Cyg 0.20 — WR LMC X-4 Sk-Ph 1.4 13.5 SG LMC X-3 — 1.7 BHC Be Cen X-3 Kra 2.1 4.8 0.0008 SG 4U 1700-37 HD 153919 3.41 — SG 4U 1538-522 QV Nor 3.73 529 SG SMC X-1 Sk 160 3.89 0.71 0.0007 SG Cyg X-1 HD226868 5.60 BHC SG 4U 1907097 8.38 438 0.22 SG Vela X-1 HD77581 8.96 283 0.092 SG OAO 1657-415 10.4 38 SG 4U 011563 LS I65 010 11.6 850 0.34 Be SS433 4U 1909048 13.1 — SG 1E 1145.1-6141 V830 Cen ? 298 SG A 0535-668 16.7 0.069 Be transient 4U 0115634 V662 Cas 24.3 3.6 Be transient 4U 1553-542 30.6 9.3 Be? V 033253 BQ Cam 34.25 4.4 0.31 Be transient GX 301-2 Wra 977 41.5 696 SG EXO 46.0 41.8 Be 2030375 A 0535262 HD 245770 111 104 Be transient GX 304-1 V850 Cen 133 272 0.47 Be 4U 1145-619 Hen 715 187.5 292 Be X Per 4U 035230 250 835 Be

*BHC, black hole candidate. †SG, supergiant; WR, Wolf Rayet. 826 NICHOLAS E. WHITE supergiant. A similar decay in the orbit of SMC X-1 has periods of the order of minutes, the slowest rotating neutron been measured by Levine et al. (1993). There are several stars known. During the mid-1970s many of these long- other HMXBs where marginal evidence for a change in period X-ray pulsars were found using a combination of period has been reported, including LMC X-4, Vela X-1, Ariel V, SAS 3, and Copernicus. These missions provided and X0115634 (Nagase 1992, and references therein). the first pointed capability where observations lasting many hours to days were possible (as opposed to Uhuru which scanned a narrow field of view across the sky). Many of Neutron star orbit and mass determination these long-period pulsars were discovered in quick succession (e.g. McClintock et al. 1976, 1977; White et al. 1976b). The X-ray pulsar can be used as a precise clock to deter- A striking example of how common these long-period mine the orbital parameters of the neutron star and to obtain pulsars are was the discovery of the “twin pulsars” by White a mass function (Figure 3). A mass function can also be et al. (1978) using Ariel V. The field of view of the instru- obtained for the OB star using optical spectroscopy. These ment being used for the search was 3.75. In one field con- can be combined with other constraints to determine the taining the Uhuru source 4U1145-61 a power spectrum of orbital inclination, such as the duration of the eclipse, to the time series revealed two peaks at 292 and 298 s. This yield the mass of the neutron star. These measurements also suggested there were two pulsars within 1 of each other, give the overall system parameters. with very similar pulse periods. This result was confirmed a The neutron star mass has now been determined in seven year later when the Einstein Observatory took the first X-ray binary systems and three radio pulsar binaries. The images of the region to reveal two separate sources, pulsing uncertainties for the X-ray pulsars are dominated by those at the two periods found by Ariel V (Lamb et al. 1980). The in the optical mass function (which can be difficult to mea- original 4U1145-61 source is in a widely separated Be sys- sure) and the system inclination. Within the uncertainties, tem with an orbital period of 292 days (Watson et al. 1982). the neutron star masses obtained are consistent with 1.4 The longer pulse periods were an unexpected discovery. solar masses, the canonical Chandrasekhar mass. While there was no reason to exclude such a population, the radio pulsars cuts off at about 10 s because they cross a “death line” where the dipole radiation is no longer efficient. Pulse periods These newly found long-period X-ray pulsars were dubbed HMXB pulse periods are also given in Table 1. They range “slow rotators” (e.g. Fabian 1975; Lea 1976) and provided a from 60 ms to 850 s. Many of the HMXBs have long pulse new view on the evolution of neutron star spin periods.

Figure 3 The orbit of SMC X-1 determined from an arrival time analysis of the 0.7 s pulsations. (Taken from Primini et al. 1977). 36

YASUO TANAKA* Black-hole binaries

1 INTRODUCTION 1. to demonstrate the presence of an event horizon in a compact (gravitationally collapsed) object. The black hole is the most exotic object in the universe predicted as a direct consequence of general relativity. However, this demonstration is extremely difficult. The Research of stellar mass black holes dates back to 1939, next best thing is when Oppenheimer and Snyder (1939) predicted the possi- 2. to find the general relativistic effects (particle motion near ble presence of black holes. They discovered that, based on the speed of light, a large gravitational redshift, etc.) that general relativity, a sufficiently massive star would collapse are events unique to close vicinities of event horizons. indefinitely when all thermonuclear energy was exhausted and disappear inside a sphere of a limiting radius from which For the reasons discussed later, attempts along this line even photons could not come out (the “event horizon”). This have not been as yet successful (although successful for limiting radius, called the Schwartzschild radius, equals some Seyfert galaxies). So far, the most reliable evidence 2 twice the gravitational radius, rg ( MG/c , where M, G and for a black hole has come from mass determination: c are the mass, the gravitational constant and the light velocity, respectively). Such an object is what is later called 3. showing the mass of a compact object exceeds 3M. a black hole. However, black holes remained merely theoretical objects for a long time. To the best of our current knowledge, any compact object The actual existence of stellar-mass black holes came to more massive than 3M, the mass upper limit of a stable light for the first time in the early seventies, as described neutron star, is believed to have no other fate than to collapse below. At present, the presence of stellar mass black holes in into a black hole. a certain type of X-ray binaries has become beyond doubt. As of the end of 1999, the compact objects in twelve The “discovery” of black holes was a great victory for gen- X-ray binaries have been shown to have a mass greater than eral relativity theory, and was certainly one of the highlights 3M (see Table 1, Section 4.1). On this theoretical basis, of astronomy in the twentieth century. However, the presence they are considered to be “reliable” black holes. (It is to be of stellar mass black holes in our Galaxy has not been estab- noted that black holes can in principle have any mass, hence lished by a single indisputable discovery (unlike the discov- black holes of 3M, if they exist, are not recognized using ery of neutron stars with radio pulsars). In actuality, their real this criterion.) Yet, genuine general relativistic tests such as existence has become convincing as a result of steadily the above-mentioned (1) and/or (2) are still lacking. In this growing evidence accumulated over a length of time. sense, one can say that the presence of stellar mass black It is not straightforward to identify black holes observa- holes in some X-ray binaries is virtually certain, but rigor- tionally. The most direct proof for a black hole would be ously speaking the observational proof is not yet perfect. A wealth of observational results on X-ray binaries has become available in the last few decades. Accordingly, * Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany and Institute of Space and Astronautical Science, studies of the X-ray properties of these sources have made Sagamihara Kanagawa-ken, Japan great progress, and we have acquired a fair understanding

Yasuo Tanaka, The Century of Space Science, 839Ð856 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 840 YASUO TANAKA of the nature of X-ray emission. In particular, we find that in the sky) must, on observational grounds, be a neutron star, in certain circumstances, X-ray binaries containing a black which later turned out to be the case. This speculation hole (hereafter referred to as “black-hole X-ray binaries”) occurred even before the discovery of neutron stars. Neutron show X-ray properties distinctly different from those stars were discovered in 1967 by Hewish and Bell (Hewish containing a neutron star (“neutron-star X-ray binaries”), et al. 1968) as radio pulsars. Soon, a rapidly spinning neu- a point to which we will return in a later section. Based on tron star was found in the center of the Crab Nebula. This these properties, one can identify the most probable black discovery provided direct evidence for the birth of a gravita- holes with the X-ray observations alone. As a result, it has tionally collapsed object as a consequence of supernova become certain that many more black-hole binaries exist in explosions, a phenomenon predicted earlier by Baade and our Galaxy. Interestingly enough, most of the black-hole Zwicky in 1934. These discoveries re-excited interest in binaries are not bright in X-rays all the time, but they are black holes, another class of gravitationally collapsed object, transient sources. The current observational facts on these that might exist in reality. X-ray sources were considered to transients make us suspect that there exist as many as several be the best locations to look for black holes because the deep hundred or even more black-hole binaries (though most of gravitational potential of the latter make them strong X-ray them are X-ray quiet) in our Galaxy. emitters. However, in early days of X-ray astronomy there It is worth emphasizing that X-ray observations, which are was no observational clue to identify black holes. only possible from space, have played a unique role in the An epoch-making development concerning black holes investigation of black holes. The currently known stellar- took place in 1972. This was with Cyg X-1. It is worth a mass black holes have all been discovered among bright brief account here. Cygnus X-1 was one of the bright X-ray X-ray binaries. There are good reasons for this, since, sources known from the earliest X-ray observations in explained later, X-ray observation is practically the only sixties. From the early days, this source drew much atten- means to discover stellar-mass black holes. Thus, one can say tion because of its distinct characteristics among then that without the development of observations from space, this known X-ray sources. It showed a hard X-ray spectrum fundamentally important field could not have been opened. much harder than others, and also irregular time variabili- The above summarizes the present status of observa- ties. (For those who are not familiar with the expression tional research into black-hole binaries as of the end of “hard” or “soft”, see explanation in Section 4.3.) The first 1999. We shall discuss these topics in detail in the rest of X-ray astronomy satellite, UHURU launched in 1969, this chapter. For previous reviews, see e.g. Tanaka and found that Cyg X-1 had a bimodal behavior switching Lewin (1995), Tanaka and Shibazaki (1996), and references between a low-intensity hard state and a high-intensity soft therein. The Research in this field has been long and is state (see Section 4.3). This added another peculiarity to the expanding rapidly. A great many investigators have been source. Various efforts had been made to localize the posi- involved in the course of development. However, it is tion of the source, but they were not yet precise enough to beyond the capability of the author to cover all the impor- allow optical identification. In 1971, a new radio source tant contributions and giving them their credit due. Readers was detected within this error region by Braes and Miley must be aware that this article is not intended to be a com- (1971) and Hjellming and Wade (1971). It became convinc- plete review of the subject and that the coverage of topics ing that this radio source was Cyg X-1 itself, since the radio and the assessment of the results and interpretations may source had emerged in coincidence with the epoch when well be subject to the author’s personal bias. Cyg X-1 had changed from a high/soft state into a low/hard Following this introduction, we begin with a brief his- state. The accurate position of the radio source immediately tory of the early developments that led to the discovery of led to an identification of the optical counterpart of Cyg X-1 stellar-mass black holes. with the O-type supergiant (O9.7Iab) HDE226868. A big surprise soon followed, when Webster and Murdin (1972) and Bolton (1972) discovered a sinusoidal variation 2 EARLY DEVELOPMENTS with a 5.6-day period in the optical Doppler curve of HD 226868 (Figure 1), clearly indicating that it is a binary Soon after the discovery of X-ray stars in 1962 (Giacconi system. This was the first discovery of the binary nature et al. 1962), the concept of an accreting compact object of Galactic X-ray sources, and was prior to the discoveries emerged in order to account for the large X-ray luminosity. of binary X-ray pulsars. The invisible companion must be If a compact object is in a binary system and accretes mat- a compact object accreting matter from the supergiant and ter from a companion star, the matter falling into a deep emitting strong X-rays. Not only that, but the mass function gravitational potential well of the compact object would (see equation below) obtained by them indicated that the be heated to a very high temperature and efficiently emit mass of the compact object probably exceeded 3M (more X-rays. Shklovsky (1967) argued that the compact object of recent results listed in Table 1). They independently consid- Sco X-1 (the first discovered and the brightest X-ray source ered that this binary system contained a black hole, Webster BLACK-HOLE BINARIES 841 and Murdin wrote, “it is inevitable that we should also spec- It is to be cautioned, however, that the above-mentioned ulate that it might be a black hole”. Bolton wrote, “this raises X-ray properties of Cyg X-1 are no longer unique, nor are the distinct possibility that the secondary is a black hole”. they signatures of black holes. Similar properties are Because of the great impact on the astronomy commu- observed from many other X-ray binaries regardless of nity, this discovery excited various critical discussions. whether the compact object is a black hole or a neutron star. However, none of the suggested alternatives to exclude the black hole hypothesis (e.g. invoking an equation of state with strange matter, or a three-body system, etc.) survived. 3 BLACK HOLES IDENTIFIED FROM Thus, Cyg X-1 remained as a strong candidate for a black- MASS FUNCTIONS hole binary. Details of these accounts on Cyg X-1 are found in a review by Oda (1977). During the following ten years, Cyg X-1 was the only black hole candidate until two bright X-ray sources in the Large Magellanic Cloud, LMC X-3 and LMC X-1, were discovered. Both of them were optically identified as early-type stars (Cowley et al. 1983 for LMC X-3, and Hutchings et al. 1983 for LMC X-1). The mass functions obtained indicated that the masses of the compact objects in these sources were also larger than 3M. (The result for LMC X-1 in Table 1 was taken from the follow-up work by Hutchings et al. 1987). The mass function f(M) is given by the following equation:

3 3 3 M i x sin PK f(M) 2 (Mx Mc) 2 G

where Mx and Mc are the masses of the X-ray emitting compact object and of the companion star, respectively, and i is the inclination angle of the binary orbit. P is the orbital period, Figure 1 The radial velocity curve of HD 226868 plotted and K denotes the amplitude of the Doppler curve (giving the against the phase of 5.6-day orbital period. The vertical scale line-of-sight component of the radial velocity) of the compan- is in units of km s1. (From Webster and Murdin 1972.) ion, which are both optically measurable quantities.

Table 1 Black-hole binaries established from the mass functions

Source Spectruma Companion F(M) BH mass Ref.b name (M)(M)

Cyg X-1 S PL O9.7Iab 0.241 0.013 16 ( 7) 1 LMC X-3 SPL B 3 V 2.3 0.3 72 LMC X-1 SPL O 7Ð9 III 0.14 0.05 63 J042232 XNova Per ’92 PL M 2 V 1.21 0.06 3.2 4 0620003 XNova Mon ’75 SPL K 5 V 3.18 0.16 7.3 5 100945 XNova Vel ’93 SPL K 7Ð8 3.17 0.12 4.4 6 1124684 XNova Mus ’91 SPL K 0Ð4 V 3.1 0.4 67 1543475 XNova ’71, ’83, ’92 SPL A 2 V 0.22 0.02 2.7Ð7.5 8 J165540 XNova Sco ’94 SPL F 3Ð6 3.24 0.09 7.020.22 9 1705250 XNova Oph ’77 SPL K 3 V 4.0 0.8 610 2000251 XNova Vul ’88 SPL early K 4.97 0.10 6Ð7.5 11 2023338 XNova Cyg ’89 PL K 0 IV 6.26 0.31 8Ð15.5 12

a X-ray spectrum at high luminosities, SPL: soft power-law, PL: power law. b References: 1. Gies and Bolton 1982 7. McClintock, Bailyn and Remillard 1992 2. Cowley et al. 1983 8. Orosz et al. 1998 3. Hutchings et al. 1987 9. Orosz and Bailyn 1997 4. Filippenko et al. 1995a 10. Remillard et al. 1996 5. McClintock and Remillard 1986 11. Filippenko et al. 1995b 6. Filippenko et al. 1999 12. Casares et al. 1992 842 YASUO TANAKA

It is clear from the equation that f(M) gives an absolute might not be correct for the stars in close binary systems lower limit of the mass of the compact object. Once f(M) is such as X-ray binaries that transferred a large amount of obtained, the actual mass of the compact object can be esti- mass and might have experienced an unusual evolution- mated if i and Mc are known by some means. These quanti- ary process. According to the critical discussions, e.g. ties are however subject to a fair amount of uncertainty. In McClintock et al. (1992), Mx 3M is secure for Cyg X-1 particular, the possible systematic effects in the early type and LMC X-3, but less secure for LMC X-1. (Yet, we shall systems (large Mc), where the f(M) values are usually much see later that the X-ray spectrum of LMC X-1 supports the smaller than Mx (see Table 1), complicates the setting of a proposition that it is a black-hole binary.) firm mass lower limit for the compact object. For instance, Up until 1986, these three were the only known black the companion mass Mc is usually estimated from the opti- hole binaries, and they were all high-mass (Mc M) cal spectral type and our knowledge of the masses of those systems. Since 1986, research on black hole binaries stars of the same spectral type. However, such an estimate entered into a new era of rapid development. It began with

Figure 2 X-ray light curves of the transient outbursts of four black-hole binaries. The observed fluxes are shown in units of the Crab Nebula flux in an energy band indicated for each source. (Tanaka and Shibazaki 1996.) 37

CHRISTOFFEL WAELKENS* The formation of stars and protoplanetary disks

The discovery that stars and galaxies evolve, and with them infrared and submillimeter telescopes in space are crucial the Universe, is probably the major legacy of twentieth- for unravelling the star formation process observationally century astronomy. On the threshold of the new millen- and for detecting YSOs which can be observed in adjacent nium, questions about the origins of the Universe and its wavelength intervals from the ground. A second window constituent galaxies, stars and planets are the main drivers suitable for the detection of YSOs from space is the X-ray of astronomical research. Answers to these big, funda- window: low-mass YSOs develop convective envelopes, mental questions will require a timely interplay between the activity of which generates X-rays that pass relatively ground-based and space-borne observations. unaffected through the molecular cloud material. Stars are the main energy sources in the Universe, and Observations from space have, moreover, revealed the their study has always been a central theme in astronomy. existence of circumstellar disks not only of YSOs, but also In the quest for our origins, stars will continue to play a of stars which already have significantly evolved on the crucial role. ‘Galaxy formation cannot be understood with- main sequence. Such disks were first discovered from out incorporating a detailed theory of star formation’ (Silk their thermal infrared radiation, and subsequently directly 1997). Such a theory also is a prerequisite for understand- observed in scattered light, in particular by the Hubble ing the formation of planetary systems in the circumstellar Space Telescope. The study of such disks, and the realiza- disks with which most stars appear to be born. tion that their occurrence is fairly widespread, have trig- While the basic insights into the gravitational instability gered very active research on the evolution of such disks of molecular clouds date back to Jeans’s work (Jeans 1902), towards planetary systems. Only from space can the spectra the theory of star formation is still far from complete. of these disks and their observational links to Solar System The formation of structure in the Universe turns out to be objects be studied in detail. The Infrared Space Observatory a complex process which needs sustained observational (ISO) has highlighted the rich chemistry and mineralogy of efforts to be unravelled. Our present database on star for- the dusty environments of YSOs. Observations from above mation is still restricted, since many relevant phenomena the Earth’s atmosphere can also provide the high angular occur in regions that are shielded from our view at optical resolution that is necessary for probing the structure of such wavelengths. Ground-based and space-borne facilities that disks, with possible links to the presence of planets. are able to probe newly formed stars have been developed only recently. While optical pictures of star-forming regions are often spectacular (Figure 1), one must realize that STAR FORMATION: BASIC SCENARIOS AND the spectral energy distribution (SED) of young stellar TERMINOLOGY objects (YSOs) peaks at far-infrared wavelengths, where the Earth’s atmosphere is a strong emitter. Consequently, Following Shu et al. (1987), star formation can be described as consisting of four stages. The first stage is * Katholieke Universiteit Leuven, Belgium the formation of dense cores in molecular clouds; these

a) b) t=0

1 pc 10 000 AU Dark cloud cores Gravitational collapse

5 6 c) t ~104 Ð 105 yr d) t ~10 Ð 10 yr

Figure 1 Hubble Space Telescope image of the star-forming region in the Eagle Nebula. (Courtesy of NASA.) Protostar, embedded in 100 AU 8000 AU envelope; cores are supported by turbulent and magnetic pressure, disk; outflow T Tauri star, disk, outflow which gradually decrease due to ambipolar diffusion 7 (Mouschovias 1991). Magnetic pressure is not able to sup- e) t ~106Ð107 yr f) t >10 yr port the more massive cores, which are thought to collapse more rapidly. In the second stage, a central protostar already occurs: it collapses rapidly in its centre, and less so in its outer layers. Matter originating far from the rotation axis has too large an angular momentum to fall onto the protostar and settles in a circumstellar disk; the mass of the latter itself, may, in fact, be higher than that of the central 100 AU 50 AU object. The third stage is characterized by a bipolar outflow aligned with the rotation axis. The confinement of the out- Pre-main-sequence star, Main-sequence star, flow to a narrow cone has led to the suggestion that it is remnant disk planetary system (?) related to magnetic activity, but the occurrence of powerful outflows in massive objects and in the accretion-powered Figure 2 Schematic representation of stellar formation and early stellar evolution (Hogerheijde 1997). With respect to the UX Orionis stars is consistent with the hypothesis that original scheme, the third (outflow) phase has been split up accretion energy feeds the outflows. During the fourth stage in two subphases before and after the evaporation of the enve- both infall and outflow are terminated, and a newly formed lope, and a sixth phase, where planets have formed in the star with a circumstellar disk emerges. A schematic view of disk, has been added. (After Shu et al. 1987.) this evolution process was pictured by Hogerheijde (1997) and is shown in Figure 2. are characterized by a more or less reddened stellar compo- From an observational point of view, YSOs emerge as nent and an infrared excess. This classification scheme has optically visible, but still heavily obscured, objects during been extended towards ‘class III’YSOs, the infrared excess the third stage in the Shu et al. (1987) scenario. Younger, of which has essentially disappeared, but for which circum- still completely embedded, objects have SEDs with a posi- stellar gas causes atomic emission lines, and also towards tive spectral index in the far-infrared and are called ‘class I’ ‘class 0’ sources, which emit the bulk of their energy in the sources in the observational classification scheme proposed sub millimetre and millimetre domains (André et al. 1993), by Lada and Wilking (1984; see also Lada 1987, Lada and and even towards ‘class-I’ sources (Boss and Yorke 1995), Shu 1990). This scheme is illustrated in Figure 3. YSOs in where the collapse has just been initiated and which still stages 3 and 4 are also called ‘class II’ sources; their SEDs await detection. THE FORMATION OF STARS AND PROTOPLANETARY DISKS 859

Mayor 1991). In high-mass SFRs an excess of binaries is still observed, but it is less marked. Prosser et al. (1994) derive a ‘normal’ binary frequency for the Orion Trapezium cluster, but current data are felt to be insufficient for firm conclusions. The two main scenarios for binary-star formation are fragmentation (where the cloud cores fragment before rapid collapse is initiated), and gravitational instability in the massive circumstellar disk. It is possible that the inventories of binaries in SFRs teach us that binary-star formation is the natural outcome, and that single stars are either disrupted binaries, following dynamical interactions which indeed are more common in the denser high-mass SFRs, or objects whose disks have been rapidly photo- evaporated. Dynamical interaction in SFRs clearly also influences the evolution of the circumstellar disks, as does the interaction with the ionized winds of already formed stars. In various clusters it has been noted that for stars which occur close to each other in the HertzsprungÐRussell diagram, and thus are of about the same mass and age, the infrared excesses Ð which directly probe the mass of the disk Ð widely differ (for example, NGC2244; Pérez et al., 1987). The highest fraction of infrared excesses are found for low- mass SFRs (e.g. Hillenbrand et al. 1995), where dynamical interactions are less frequent and the ionizing flux is small. The dynamical evolution of the disks and their ablation by the ionizing radiation may thus indeed contribute to the lower binary fraction in high-mass SFRs. In addition, infrared excesses as well as X-ray surveys, as tools for detecting YSOs, have revealed a surprisingly high number of putative YSOs that occur outside known SFRs, suggesting the possibility of almost isolated star formation, with relatively long-lived disks. In the HertzsprungÐRussell diagram, the ‘stellar birth- line’ (Stahler 1983) is defined as the locus of points at which a protostar no longer accretes mass, and thus initiates its pre-main-sequence phase. For galactic stars more massive than about eight solar masses, the circumstellar envi- Figure 3 Observational classification scheme of YSOs. The ronment is still optically thick when the central object ‘black bodies’ in the plots incorporate a correction factor to initiates core hydrogen burning and thus arrives on the account for circumstellar reddening. (After Lada and Shu 1990.) main sequence; in more metal-deficient and therefore less dusty galaxies, this mass limit is higher. High-mass pre- main-sequence stars are thus not observable at optical Recent results have provided evidence for the fact that wavelengths. YSOs with masses between two and eight many stars, if not all, are formed as binaries. Inventories solar masses, which emerge before having reached the main of binary objects in star-forming regions (SFRs) indicate a sequence, are called Herbig-Ae/Be stars, hereafter called larger fraction of binaries than for main-sequence stars in Haebe stars. During their quasi-static contraction towards the field and in evolved clusters (Mathieu 1994). Some the main sequence, they have radiative envelopes. YSOs studies suggest that in low-mass SFRs nearly all stars are of lower mass are called T Tauri stars and have convective born as binaries: for the Taurus SFR, Leinert et al. (1993) envelopes, with associated activity which causes intense and Ghez et al. (1993) find that some 85% of the objects emission of X-rays. Class-II T Tauri stars are also called are binaries, that is, much more than the 53% found among ‘classical T Tauri stars’ (CTTs), and class-III T Tauri stars G dwarfs in the solar neighbourhood (Duquennoy and correspond to ‘weak-lined T Tauri stars’ (wTTs) or ‘naked 860 CHRISTOFFEL WAELKENS

T Tauri stars’ for which the circumstellar material has fact that the isolated nature of an object may imply a less almost disappeared. disturbed and therefore longer evolution for its circumstellar structures. The so-called Bok globules, which are small, dark clouds and are easily detected from the obscuration DETECTING THE YOUNGEST STELLAR they cause to the background, appear to be the natural sites POPULATIONS for isolated star formation (Bok and Reilly 1947). Some 10 or even 25% of the known giant molecular The cool and embedded nature of YSOs implies that clouds appear devoid of current star formation (Blitz 1993). infrared, sub millimetre, millimetre and radio techniques, Though for distant clouds this apparent absence of star for- from the ground and from space, are needed to detect and mation may be due to the limited sensitivity of current sur- study these objects. All techniques are plagued with their veys, Mooney and Solomon (1998) argue that the case for own selection effects, involving sensitivity as well as angu- quiescent molecular clouds is genuine. Detailed mapping of lar resolution, so that a suitable combination is required in these clouds with future instrumentation should reveal order to obtain a complete picture. The all-sky survey in the whether they represent an early stage in molecular cloud 12 to 100 m region by the Infrared Astronomical Satellite evolution or whether star formation is inhibited in them. It (IRAS) led to the detection of a large number of YSOs is clear that we are currently still lacking a definitive theory of classes I and II, but was hampered by both relatively for understanding the processes of fragmentation and core low sensitivity and low spatial resolution. A significant formation in molecular clouds (André 1997). improvement has been obtained with ESA’s Infrared Space Observatory (ISO), especially from the ISOCAM surveys. Nevertheless, all space surveys so far are confusion limited CIRCUMSTELLAR DISKS REVEALED except for the few nearest SFRs, so that an unbiased picture will probably not emerge before the surveys foreseen by Disks appear to be a natural byproduct of star formation, but the Herschel satellite, to be launched in 2007, are available. direct confirmation of the disk-like structure of the matter Overcoming the confusion limit in surveys is essential for a close to young stars has had to await the recent advances in proper understanding of the initial mass function (IMF) in high-angular-resolution observing techniques. With the Wide various environments. Field and Planetary Camera on the Hubble Space Telescope Large-scale, ground-based continuum surveys are sensi- (HST), Burrows et al. (1996) for the first time were able to tive to the small amounts of ionizing gas occurring in YSOs resolve the vertical structure of a YSO disk, for the Herbig- of several classes, but often miss the low-mass YSOs for Haro object HH30 (Figure 4). The object, viewed nearly which such radiation is faint. However, X-ray surveys have edge-on, appears as a circumstellar disk, the equatorial part proved rather efficient in detecting the population of young, of which is optically thick, and which shows a vertical often naked, objects in which vigorous stellar activity takes decrease in density so that scattered light from the central place. After initial successes in this field with the Einstein object can emerge. Perpendicular to the disk, highly colli- observatory (Vaiana et al. 1981), substantial progress has mated bipolar jets are observed. ISOCAM and ISOPHOT been made with the ROSAT all-sky survey. Optical follow- observations of this object (Stapelfeldt and Moneti 1999) up of the ROSAT survey in various SFRs has impressively reveal a double-peaked SED, where the scattered light peaks increased the amount of Ð mostly weak-lined Ð known T around 2 m and the thermal emission from the disk peaks in Tauri stars (for a review, see Neuhauser 1997). In the the far-infrared. Taurus region, Neuhauser et al. (1995) detected with In the youngest stellar objects, circumstellar disks are ROSAT 43 out of 65wTTs, but only 9 out of 79 CTTs; they strongly affected in their appearance by their surroundings. found that the larger detection rate in weak-lined objects The so-called ‘proplyds’ (Figure 5) are flattened circumstel- was intrinsic, being related to, among other parameters, a lar clouds of gas and dust which are rendered visible as a higher rotation rate. Interestingly, the X-ray technique has result of the ionizing radiation of hot stars in a H II region also proved useful for detecting objects in the brown dwarf to which they belong, or close to which they are located. In mass range (Neuhauser and Comeron 1998). some cases (Figure 5) the optically thick dusty inner disk A somewhat surprising result of the IRAS and ROSAT all- can be seen in absorption against the background, but the sky surveys Ð that is, surveys not limited to known SFRs Ð optical appearance of proplyds is most often dominated by and their follow-up, has been the detection of several YSOs recombination radiation from their outer parts. Such objects which appear isolated in the sky or as small associations were detected in the central parts of the Orion Nebula (Gregorio-Hetem et al. 1992, Kastner et al. 1997). The rela- as emission-line sources (Laques and Vidal, 1979) and as tive importance of such a mode of small-scale star forma- compact radio continuum sources (Churchwell et al. 1987, tion is currently not very clear, one possible bias being the Garay et al. 1987), and were resolved spatially with 38

ROBERTO PALLAVICINI* High-energy radiation from outer stellar atmospheres

Although the Sun had been known to be a source of X-ray possess subphotospheric convective zones Ð both the high and ultraviolet radiation since the late 1940s, it was only in coronal temperatures and the various manifestations of stel- the late 1970s that normal stars of nearly all spectral types lar activity appear to be related to the generation of mag- and luminosity classes were recognized to be sources of high- netic fields through a dynamo process that involves rotation energy radiation detectable from space (see Mewe 1996 for a and convection. Thus, observations of stellar chromospheres, historical perspective). The origin of this high-energy emis- transition regions and coronae, albeit limited to the outer sion from normal (i.e. non-accreting) stars constitutes a fun- atmospheric layers, allows us to put constraints also on the damental and yet unsolved problem in stellar astrophysics, physical processes that occur deep in the interior of stars, which space observations have helped to elucidate over the with important implications for our understanding of stellar past 20 years. It was quite obvious from the very first obser- structure and evolution. Moreover, observations of stars in vations that the generation of high-energy photons by ther- different evolutionary stages, from the early phases of con- mal processes, such as the ones usually observed in normal traction to the main sequence to the late stages of evolution stars, requires temperatures far in excess to those responsible out of it, allow for the investigation of the effects of mag- for the stellar optical radiation. In particular, it requires that netic braking and angular momentum loss on the generation the temperature profile in stellar atmospheres, after reaching of magnetic fields and on the heating processes. a minimum in the upper photosphere, rises again, attaining The traditional view, first developed in the case of the Sun, values of one to several million degrees in the corona. These attributed the temperature inversion that occurs in the outer outer atmospheric layers, where the temperature increases stellar atmospheres to shock dissipation of acoustic waves outwards through the chromosphere, transition region and generated in the subphotospheric convective zone. Although corona, had been known and widely studied for a long time this theory is probably still valid for the Sun’s lower chro- in the case of the Sun, but it was only much later with the mospheric layers and for non-magnetic regions, it fails badly advent of grazing incidence X-ray telescopes and sensitive for solar magnetic regions, for active stars and in general for X-ray detectors that their existence for the vast majority of stars, both hotter and cooler than the Sun, with an internal stars was definitely proved. structure significantly different from the solar one. For cool The study of high-energy radiation from normal stars is a stars, magneto-acoustic and Alfvén waves and/or resistive powerful diagnostic tool for the physical conditions in outer dissipation of electric currents are more likely to be responsi- stellar atmospheres and for the processes in the stellar interi- ble for the observed high-energy radiation, thus stressing the ors that are at the origin of coronal heating. X-ray and UV importance of magnetic fields in the outer atmospheres of radiation in fact provides information on non-radiative heat- these stars. Hot stars, which do not have outer convective ing mechanisms as well as on the processes that lead to wind zones, are also observed to be sources of X-rays, but high- acceleration and mass loss. In the case of cool stars Ð which energy emission in this case is more likely due to shock dissipation in their massive radiatively driven winds rather than to * Osservatorio Astronomico di Palermo, Italy the presence of a high-temperature corona.

In this chapter, I will review X-ray and UV emission from 10,000 times more than the average X-ray luminosity of outer stellar atmospheres with emphasis on X-ray emission the Sun or of quiet solar-type stars like Cen. At any rate, from the coronae of late-type stars, that is stars that possess X-ray emission at appreciable levels was thought at that subphotospheric convective zones. According to the usual time to be limited to somewhat peculiar objects and not to classification based on their optical spectra, these stars be a general characteristic of all types of stars. cover the spectral types from F to M, and all luminosity Neither was there much expectation from a theoretical classes from dwarfs to supergiants. Along the main point of view of detecting stellar coronal emission (Mewe sequence, the effective temperature decreases from about 1979). The mechanism of coronal heating generally accepted 7000 K to less than 3000 K, the mass decreases from about at that time was shock dissipation of acoustic waves gener- 1.7 solar masses to one-tenth of a solar mass and the radius ated in subphotospheric convective zones. In spite of large decreases from about 1.3 to a few tenths of the solar radius. uncertainties present in available estimates of the power On the contrary, the convection zone depth increases from a dissipated by acoustic waves, the theory invariably pre- negligible fraction of the stellar radius in early F-type stars dicted that maximum dissipation should occur around spec- to the whole stellar radius in late M dwarfs. The wide range tral types F and G, decreasing rapidly towards later spectral of physical parameters covered by F to M stars along the types along the main sequence. It was not expected there- main sequence, not to mention the wider range provided by fore that X-ray emission could be detected in K and M the inclusion of giants, offers a variety of different condi- dwarfs, nor in early-type stars that do not possess outer tions for the generation of magnetic fields and for the heat- convective zones. Even more importantly, the acoustic heating of the outer atmospheric layers. In addition, the broad ing theory predicted that stars at a given position in the HR range of ages and evolutionary stages covered by cool stars diagram should all have the same level of chromospheric implies also different values of rotation rates and hence of and coronal emission, which in turn should depend only on the efficiency of magnetic field generation by the dynamo the properties of their convective zone and on the efficiency process. All this offers to our physical speculations a much of generation of acoustic noise. Spatially resolved observa- more complex picture that the one made possible only by tions of the solar chromosphere and corona (which showed observations of the Sun. The X-ray and UV observations the presence of localized regions of enhanced emission carried out from space during the past 20 years have largely associated with regions of stronger magnetic fields) were unravelled this complex picture taking us to the point where clearly at variance with this simplified picture; however, the we can finally start to understand the physical processes at effect of magnetic fields was considered only as a second- the origin of coronal activity in different types of stars. order perturbation which could hardly change the overall picture. As for early-type stars, acoustic waves could not be driven by convection but it was suggested that waves could be amplified by radiation pressure leading some authors EARLY RESULTS ON STELLAR X-RAY EMISSION to believe that early-type stars might eventually be detected in X-rays. The beginning of stellar X-ray astronomy The early observations by IUE and Einstein showed that Before the launch of the International Ultraviolet Explorer these expectations were fundamentally wrong. In particular, (IUE) and of the Einstein Observatory (formerly HEAO 2) the Einstein Observatory showed that virtually all stars, in 1978, there was only a handful of stellar coronal sources, with the exception of A-type dwarfs and of late K and M besides the Sun, known to emit X-ray and UV radiation giants, were X-ray emitters (Vaiana et al. 1981). For late- (Mewe 1979). They were either very close objects (e.g. type stars, there was little or no dependence of the X-ray Centauri) or very active stars with coronal emission many emission on spectral type, in contrast with the predictions orders of magnitude higher than that of the Sun. Most of the of the acoustic theory. Only for early-type stars was there latter objects were close binary systems of the RS Canum a clear dependence of the X-ray emission on the stellar Venaticorum (RS CVn) type, a class of binary stars known bolometric luminosity. For late-type dwarfs of spectral for their extreme activity at optical and radio wavelengths. types F to M a broad range (more than three orders of mag- One of these objects, Capella, was indeed the first non-solar nitude) of coronal emission levels existed at each effective coronal X-ray source to be detected in 1974 during a rocket temperature, indicating that parameters other than the posi- flight and is still the brightest X-ray source among normal tion of a star in the HR diagram (i.e. effective temperature stars, except for the Sun. This early discovery was soon fol- and luminosity) were responsible for the activity level of lowed by the detection with the first High Energy Astrono- stellar outer atmospheres. These early observations clearly mical Observatory (HEAO 1) of a number of other RS CVn demonstrated that the acoustic theory was basically unten- stars with typical X-ray luminosities in the range of able as far as coronal heating is concerned and that other 1030Ð1031 erg s1 in the soft X-ray band. This is 1,000 to mechanisms, possibly involving magnetic fields, were to be HIGH-ENERGY RADIATION FROM OUTER STELLAR ATMOSPHERES 877

to fluctuations in the heating rate. All this suggested that magnetic fields not only confine the coronal plasma (since the magnetic pressure exceeds the gas pressure in the corona), but are also responsible for the heating of the plasma to temperatures far in excess of a star’s effective temperature.

Coronal X-ray emission in late-type stars The broad range of X-ray emission levels observed for late- type stars of similar effective temperatures was soon found to be related to stellar rotation and age, thus enforcing the notion that dynamo-generated magnetic fields are responsible for coronal heating in stars of late spectral types (Pallavicini et al. 1981). Although a fully consistent dynamo theory does not yet exist, not to mention a generally accepted coronal heating theory, a qualitative expectation is that the efficiency of dynamo action should increase with stellar rotation rate in stars that possess subphotospheric convective zones. We may expect, therefore, that the activity level of a star, as it results from the cumulative effects of the multitude of magnetically confined coronal structures Figure 1 Soft X-ray image of the solar corona obtained on that form its outer atmosphere, should depend on the rota- 27 January 1992 with the Soft X-ray Telescope (SXT) on the tion rate, or better on a combination of rotation and convec- Japanese satellite Yohkoh. (Courtesy of the Institute of Space tion zone properties. This combination of dynamo factors is and Astronomical Sciences.) often parameterized by means of a Rossby number (Noyes et al. 1984) defined as the ratio of the star rotation period explored. These results for stars were in line with increas- Prot to a colour-dependent convective turnover time conv, ing evidence from solar observations that X-ray-emitting computed at the base of the convective zone (where most of coronae are dominated by magnetic field effects (Rosner the dynamo action is believed to occur). The observed et al. 1985), and that surface magnetic fields are perhaps dependence of stellar chromospheric and coronal emission the dominant factor in determining the structure and vari- on either rotation rate or Rossby number is a strong, albeit ability of late-type coronae and in providing for their heating only qualitative, support of this interpretation. The details (Figure 1). should depend on the still poorly understood properties of It is interesting to note that this interplay of solar and the dynamo process and of the conversion of magnetic stellar results, all pointing in the same direction, is itself a fields into plasma heating. consequence of space observations at X-ray and UV wave- Since coronal emission among late-type stars appears lengths, being carried out at about the same time, but with to depend on both rotation and age, a relevant question is different emphasis and different objectives, for the Sun and whether these parameters are related to each other and, if stars. In the case of the Sun, sensitivity is not a problem, so, which of the two is the primary one for coronal heating. since the Sun, in spite of being an intrinsically weak X-ray The existence of a dependence on age was demonstrated by and UV source, appears to us very strong because of its observations of open clusters, that is by homogeneous sam- proximity. The Sun is the only star which appears to us as ples of stars with approximately the same age and chemical an extended source; it is therefore not surprising that the composition. It was evident from early IUE and Einstein emphasis of solar research was, and still is, in obtaining observations of the Hyades and the Pleiades, and from the high-spatial-resolution observations which allow for the comparison of solar-type stars in these clusters with the study of fine details in the solar atmosphere. The X-ray and much older Sun, that there was a dependence of chromos- UV observations obtained in 1973Ð74 by Skylab and in pheric, transition region and coronal emission upon age 1980 by SMM, as well as the solar images obtained more (Stern et al. 1981, Caillault and Helfand 1985, Micela et al. recently by Yohkoh, SOHO and TRACE, revealed a highly 1990). On the other hand, it is well known that late-type structured outer atmosphere, where the coronal plasma stars with outer convective zones suffer angular momentum appears confined by the magnetic field in loop-like struc- loss during their main sequence lifetime, owing to the braking tures tracing the field lines. The same observations also action of magnetized stellar winds. As stars get older, they revealed that the solar corona is highly dynamic in response rotate more slowly, and their chromospheric and coronal 878 ROBERTO PALLAVICINI emissions (which have been found by Einstein and IUE to coronae and low mass losses (to the left of the DL) from be related one to the other, e.g. Ayres et al. 1981) both stars with high mass losses and the absence of high-temper- decline with age. Rotation, rather than age, appears there- ature plasma (to the right of the DL; note that the cool, low- fore to be the primary factor, at least for stars that have speed winds of late-type giants are fundamentally different already reached the main sequence (the situation may be from the hot, high-temperature winds of early-type stars). more complex for pre-main sequence stars, owing to the A class of stars was also found (the so called ‘hybrid’ stars) interaction of the young star with the surrounding disk). which have both large mass losses and plasma in excess That rotation is indeed the primary factor is confirmed by of 105 K in their outer atmospheres, suggesting that the the high activity and strong coronal emission of RS CVn transition between the two classes of evolved stars (with binaries which are relatively old stars (a few Gyr) that still and without coronae) may not be as abrupt as originally rotate rapidly because of the tidal interaction of the two thought. For some late giants (for instance Arcturus) components in a close binary system. extremely low upper limits to their X-ray luminosity (as The above picture was developed in the early 1980s at the low as 1025 erg s1, Ayres et al. 1991) have now been estab- Center for Astrophysics in Cambridge, MA, by the group lished, which indicates that the X-ray surface fluxes in responsible for the early analysis of the stellar data from the these evolved low-gravity stars must be many orders of Einstein Observatory. This group, led by Giuseppe S. Vaiana magnitude lower than in solar coronal holes. The onset of (an Italian scientist who, not surprisingly, had worked previ- strong mass losses nearly coincident with the disappearance ously on X-ray observations of the solar corona, and who of hot coronae strongly suggests a causal relationship had been responsible for one of the main instruments on between the two phenomena. By analogy with the Sun, the ATM-Skylab), greatly benefited from the interaction with a presence of a high-temperature corona in a late-type star number of Guest Investigators from all parts of the world implies the existence of magnetically confined structures (Jeff Linsky, Tom Ayres, Fred Walter, Rick Harnden, Joe which are denser and hotter than the surrounding regions. Cassinelli, Rolf Mewe, Bob Stern and Thierry Montmerle, On the contrary, areas of open field lines in the Sun (i.e. just to mention a few) and included at that time a number coronal holes) are somewhat cooler and much less dense of young scientists (Bob Rosner, Leon Golub, Roberto than the emitting loops: apparently the energy deposited in Pallavicini, Jürgen Schmitt) who continued for many years these regions (by either magneto-acoustic waves or electric to work in the field of stellar X-ray astronomy. Later, it currents) goes into accelerating the wind and in enhancing also included a number of young researchers from the mass losses rather than into plasma heating. By analogy, Astronomical Observatory of Palermo, where Vaiana, upon the transition across the DL could be interpreted as a his return to Italy, established an active centre of stellar change of magnetic topology in the outer atmospheres of corona research. The interpretation of stellar X-ray activity cool stars, from a corona dominated by closed magnetic in terms of dynamo-generated magnetic fields and solar structures (similar to solar coronal loops) to an essentially analogues was largely suggested by previous work on chro- open corona, with most field lines being open to the inter- mospheric Ca II H and K emission and on its dependence planetary space. Whether this picture is correct, and how on rotation and age, as well as by the lesson learned from the supposed change of magnetic topology relates to the spatially resolved observations of the solar chromosphere, dynamo process and to the internal changes experienced by transition region and corona. It received further support a star during its evolution out of the main sequence, remain from the wealth of ultraviolet data provided at about the questions to be solved. same time by IUE on stellar chromospheres and transition regions up to temperatures of about 105 K (see Jordan and X-rays from stellar winds in early-type stars Linsky 1987 for a review). The situation with early-type (O-B) stars is completely different. Their X-ray luminosity, ranging from 1029 to 1034 erg s1, Late-type giants is virtually independent of rotation, while it depends on A further surprise in these early days came from the obser- bolometric luminosity (albeit with some scatter). This vations of late-type giants and supergiants. Whereas yellow suggested that the X-ray emission might be related to the giants are typically X-ray emitters, sometimes at very strong strong, high-speed radiatively driven winds known to exist levels, this was not the case for red giants later than spectral in these massive stars. Early models of X-ray emission from type K2 for luminosity III giants and of spectral type G0 for hot stars assumed the presence of a thin, high-temperature supergiants. On the basis of UV and X-ray observations corona at the base of the wind, possibly confined by fossil from IUE and Einstein, the presence of a dividing line (DL) magnetic fields (Cassinelli and Olson 1979). This model, in the HR diagram was inferred (Linsky and Haisch 1979, however, was not confirmed by spectral observations with Maggio et al. 1990), separating stars with high-temperature the Einstein Solid State Spectrometer (SSS) which did not 39

JOSEPH P. CASSINELLI* Mass loss from stars

Stars return mass to the interstellar medium throughout finding outflow properties from line profiles. He also intro- their lives. A low-luminosity star such as the Sun is losing duced ways for interpreting the flattened continua of WR mass by a stellar wind at a rate of 1014 solar masses stars in terms of an extended atmosphere. Kosirev (1934) 1 per year (M yr ). Very luminous stars, whether they used these diagnostics to derive a rate of mass loss and out- 5 1 1 are early-type hot stars or cool giants, lose mass at a rate flow velocity (10 M yr and 1000 km s ) of a WR star, 5 1 of up to about 10 M yr , and such rates have major and his values are within a factor of three of current esti- consequences. The evolutionary track of the star on the mates. Most of the initial work on profiles had assumed that HertzsprungÐRussell diagram is modified, the terminal the outflow was transparent to line radiation. Sobolev (1947) state can be changed, and the winds affect the surrounding developed an escape probability method for treating optically interstellar medium by depositing momentum, energy, and thick lines, and his method served as the basis for essentially chemically enriched material. all of the subsequent work on line profile analysis. Underhill This chapter reviews selected highlights of the origin and (1949) computed continuum model atmospheres for hot stars development of our understanding of stellar winds and mass and arrived at the important conclusion that luminous stars loss. Much of what we currently know about stellar winds with Teff 20,000 cannot have stable hydrostatic atmos- and the effects of mass loss comes from space observations. pheres. In spite of this early activity, the understanding of The space era in this field started with the rocket UV mass loss from hot stars progressed very slowly in the sev- research by Donald Morton and his colleagues at Princeton eral decades before Morton’s rocket observations. in the mid-1960s (Morton 1967, Morton et al. 1968). The During that interval, the attention of stellar astronomers results were unexpected and led to a strong increase in activ- was on mass loss from cool stars. This began with the dis- ity regarding the subject of mass loss from hot stars. Figure 1 covery by Adams and McCormack (1935) of spectral evi- (top) shows the spectrum of one of the belt stars of Orion. dence for slow (5kms1), outward, mass motion. Lyman On it were discovered strong and broad features at several Spitzer (1939), who would later lead the development of UV resonance lines. Some of the line profiles were found to space astronomy, carried out a PhD thesis on a study of the have longward shifted emission and shortward shifted supergiants 1 Her (M5 II) and Ori (M2 Iab). In his thesis absorption, and these lines are called P-Cygni profiles after he developed the idea of “fountains” to explain the fact that the B supergiant which shows many such lines in the visible the optical spectra of cool stars show evidence for expan- part of the spectrum. An early study of P-Cygni profiles and sion, but only at a speed well below the escape speed. He their significance as indicators of outflows was carried out in postulated that there would occur a change in the ionization the 1930s by Beals (1929, 1931). He compiled catalogs to an unobservable stage, followed by a fountain-like infall. showing spectra of Wolf-Rayet (WR) stars and Carinae, Many decades later this concept of fountain flow was found and some of the novae and central stars of planetary nebulae useful for modeling the infalling gas trajectories in our which show the profiles. Chandrasekhar (1934a,b) intro- Galaxy. The first strong evidence that cool giants involve duced the now often used iso-velocity contour diagnostic for matter actually leaving the stars was developed by Deutsch (1956) from his observations of Her (M5 II G0 III). His * University of Wisconsin, Madison, WI, USA analysis showed that the expansion of the primary star’s

— Si IV— — C IV—

Figure 1 The upper panel shows Morton’s (1967) rocket observation of the ultraviolet spectrum of Ori (O 9.5 Ia). The wavelength increases to the right from 1140 to 1630 Å. Note the P-Cygni profiles at C IV and Si IV, and the shortward displaced absorption lines of C III, N V, and Si III. The lower panels show the Copernicus satellite spectra of the doublets of N V and O VI in the O4 f star Pup and the B0 V star Sco. The arrows indicate the location of the rest wavelength of the lines. The sharp lines in the O VI spectrum are interstellar lines and the strong line at 1900 km s1 is Ly-. (Adapted from Lamers and Cassinelli 1999.)

outer atmosphere extended beyond the orbit of its distant The following reviews the development of the subject of (360 R*) G0 companion. At such a large distance the flow mass loss by stellar winds in the space age. As a unifying had reached local escape speed leading Deutsch to infer that theme, we give special attention to the various dividing 7 1 there is an outflow with a mass loss rate of 10 M yr . lines and boundaries on the HertzsprungÐRussell (HR) dia- Ludwig Biermann (1951) recognized from the directions gram that mark sharp changes in outflow properties associ- of the ion tails of comets that there must be a fast and con- ated with slight changes in stellar parameters. These tinuous “corpuscular radiation” from the Sun. Eugene boundaries were discovered mostly through space satellite Parker (1958) developed an explanation of this mass out- surveys. Their locations on the HR diagram led to insight flow, which he called solar wind theory. This theory formed regarding the driving mechanisms, and the dependence the basis for all the subsequent work on stellar winds and of the winds on basic stellar properties such as effective stellar mass loss (Chapters 9 and 47). The high speed or temperature, surface gravity, and stellar rotation. “wind” outflow that he predicted was verified by the Mariner II interplanetary space mission (Neugebauer and Snyder 1962). Weymann (1960, 1963) pioneered the appli- MASS LOSS FROM EARLY-TYPE STARS cation of wind theory to other cool stars, initially using extensions of coronal or thermally driven wind theory, and In the late 1960s research was being carried out on then considering other forces such as radiation pressure on stellar atmospheres theory regarding stars with a high resonance line and dust grain opacities. luminosity-to-mass ratio, and hence near the stars’ Eddington MASS LOSS FROM STARS 897 limit (Böhm and Deinzer 1965, Cassinelli 1970). Proximity to this “superionization” in the winds, and two superionization the Eddington limit was seen as a requirement for producing lines are shown in Figure 1 (bottom). Doublets of O VI near stars with extended density distributions which have scale 1040 Å and N V near 1240 Å were seen in the spectra of heights comparable to the stellar radius. The rocket observa- stars across the entire O star spectral sequence, and reso- tions had begun to make it clear that the outer atmospheres of nance lines of C IV and Si IV extended into the B spectral hot luminous stars are not hydrostatic, but are expanding at range (Snow and Jenkins 1977). The extent of these superi- high velocities. onization zones with their well defined dividing lines on the Lucy and Solomon (1970) wrote a ground-breaking HR diagram are shown in Figure 2. These ions are anom- paper that explained the high speed of the winds from hot alous because they were not expected to be present in a gas stars. They showed that the same P-Cygni lines that had photoionized by the stellar radiation field. Their presence been used to recognize the outflow could be responsible for indicated that that there was a problem regarding the tem- 1 accelerating the winds to speeds of v 3000 km s , and perature structure assumed in line-driven wind theory, and that each strong line could drive a mass loss rate of that an extra source of heating was present and a source ú 2 M L/c , which corresponds to a total mass-loss rate of for that was needed. ú 8 1 OB supergiants of about M 10 M yr . Furthermore, In the Copernicus observation of Pup (O4 f) shown in they showed that coronal wind theory could not explain Figure 1 (bottom), the profiles of the superionization stages both the high speeds and the low-ion stages observed are fully developed as saturated P-Cygni profiles. The through P-Cygni profiles. The basic theory of outflows dri- nature of the profiles could be taken as good evidence that ven by selective absorption of the continuum radiation in the ions O5 and N4 are present at all levels in the wind, the shortward wing of the line opacity in an expanding not just near the base nor just far from the star where high atmosphere was originally developed in a sequence of speeds are reached. Initially the superionization was inter- papers by Saha (1919), Milne (1924, 1926), and Johnson preted by Lamers and Morton (1976) by the “warm wind (1925). This early interest in the acceleration of atoms was model.” In this model superionization would be produced in part to find a mechanism to explain the heating of the collisionally in a gas with a temperature above 105 K. solar chromosphere and corona. Collisional ionization required that stars with O VI should Castor, Abbott, and Klein (CAK) (Castor et al. 1975, have winds with temperatures of about 200,000 K. Lamers 1976) developed a powerful method to treat the effects of and Snow (1978) extended the warm wind idea to B super- driving of an outflow by very large numbers of lines, which giants which showed a lower stage of superionization (N V have a statistical distribution in line strengths. CAK theory and C IV and Si IV), by using progressively lower wind led to predictions of mass-loss rates and terminal velocities temperatures such that Twind 80,000 K for the later B as a function of stellar properties and the line statistics supergiants. parameters. With the modifications by Friend and Abbott Producing the warm wind temperatures required some (1986), Pauldrach et al. (1986), and Kudritzki et al. (1989), sort of mechanical heating or wave-energy deposition. The CAK multi-line theory gives good agreement with observa- amount needed was troubling, however. In the case of tionally derived values of Mú and v. In particular, strong Pup, the required mechanical luminosity was estimated to winds could be driven by radiation alone, with no extra dri- be about 10% of the radiative luminosity of the star. Such a ving associated with mechanical energy deposition. Thus it large proportion of the luminosity is required because gases was the expectation of the CAK theory that the wind tem- with temperatures near 2 105 K are near maximal effi- peratures would be cool, that is, about 80 to 90% of the ciency for radiating away thermal energy. The mechanical star’s effective temperature, as would be the case for an flux required for Pup was considered to be unacceptably extended atmosphere that is in radiative equilibrium with large given that hot stars have no obvious source of wave the photospheric radiation field. Space observations showed energy such as an outer convection zone, so alternative evidence for a quite different temperature structure. solutions were sought. At a workshop at JILA, Cassinelli, Castor and Lamers (1978) critically analyzed three models for explaining the Superionization of hot star winds superionization: Lamers’ warm wind model, Cassinelli’s The Copernicus satellite, launched by NASA in 1973, pro- “corona plus cool wind” model, and Castor’s non-LTE vided the first extensive set of data regarding winds from model with a wind temperature of 60,000 K, which was bright, hot stars. The spectra covered the far-UV region in dubbed the “tepid wind” model. In the corona plus cool the 1000 to 1450 Å band with very high resolution (/ wind idea, a geometrically thin corona with a temperature 104). Among the most surprising discoveries was the pres- of a few million kelvins, at the base of the wind was postu- ence of anomalously high ionization stages with broad, lated. The idea that a hot star could have a thin corona had strong P-Cygni profiles. Lamers and Morton (1976) called been proposed by Hearn (1975). Some mechanical flux 898 JOSEPH P. CASSINELLI

O3 6 9.5 Bo 2 4 6 Ao 5 Fo 5 Ia O3 Bo 2 4 6 Ao 5 Fo 5 III O3 Bo 2 4 6 Ao 5 V UPPER MASS LIMIT Ð13 7 250 HUMPHREYS-DAVIDSON LIMIT Ð12

50 Ð11 6 100 WOLF-RAYET HYPER GIANTS Ð10 STARS 60 Ð9 10 SUPER-IONIZATION O VI LIMITS N V 5 He 30 Ia Ð8 C IV Si IV ZAMS Ð7 L 15 log 5 NUCLEI MBOL L o PLANETARY Ð6 4 NEBULAE STRONG WIND Ð5 10 LIMIT B2p X-RAY LIMIT Ð4

Ð3 3 Be STARS 5 Ð2 Ð1 2 0 III 1

2 1 V

5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8

log Teff

Figure 2 Regions and boundaries for hot stars in the HR diagram. The locations of various classes of stars with interesting mass-loss properties are shown. Of particular interest are the “superionization boundaries” of O VI, N V, C IV, and Si IV, which demarcate the range in effective temperatures of stars that show these high-ion stages. (From Cassinelli and Lamers 1987.) from the star was invoked to produce the thin coronal argue that the superionization is produced by the Auger region. However, the amount of mechanical flux would be effect, following X-ray photoionization by the star. If an orders of magnitude below that required by the warm wind X-ray is absorbed by an ion with fewer than 10 electrons, model, because gas at million-degree coronal temperatures then two electrons are essentially always ejected from the has a much lower emissivity than does warm gas. A corona ion (Daltabuit and Cox 1972). The X-ray photon ejects a would produce X-rays at energies beyond the K-shell edges K-shell electron, and then in the transition of an electron of carbon, nitrogen, and oxygen, and the absorption of these from the L shell to the K shell, a second electron is ejected. X-rays in the cool wind above the corona could produce As a simple numerical consequence, if a wind has stage superionization. Castor showed that the photospheric radia- “n” as its dominant ion, Auger ionization will lead to an tion field of the O4f star Pup could produce some O VI overabundance of the “n 2” stage. Cassinelli and Olson ions. However, the O VI line persists through the O spectral (1979) argued that the O VI line should be present in winds range to Sco at B0.5 V. This star has a much lower lumi- for which O3 is the dominant ion. The ion O3 persists nosity and effective temperature than Pup (30,000 K v. as the dominant stage for all of the O stars and extending 42,000 K). The non-LTE effect could not explain the O VI as late as B0.5. Similarly, the UV line of N V should persist superionization at this low stellar temperature. out to spectral type B2 I for which N2 is the dominant ion Cassinelli and Olson (1979) made use of the boundaries stage, and C IV and Si IV can be produced to B8 I (Odegard of the superionization on the HR diagram (Figure 2) to and Cassinelli 1982). The mechanism involving X-rays thus 40

STUART R. POTTASCH* Planetary nebulae

HISTORY stars. He attempted to enlarge Messier’s list: Herschel discovered 2000 new nebulae in seven years. Before 1917 In a paper published in 1785, Herschel distinguished a class of nebulae that he considered to be distinct from the Several hundred years ago it became apparent to astronomers rest. He called them ‘planetary nebulae’ because they that other objects were present in the sky besides stars, plan- vaguely resembled the greenish disk of a planet. He found ets and an occasional comet. These objects had a hazy or these objects intriguing. In a paper in 1791, he reported on nebulous appearance, and it was for this reason that they an observation made the previous year: ‘A most singular were called nebulae. The nature of these objects was not phenomenon! A star of about the 8th magnitude with a faint clear and it took many years to discover that these objects luminous atmosphere, of circular form. The star is perfectly do not form a homogeneous group, but that several very in the centre, and the atmosphere is so diluted, faint and different classes of objects were being grouped together. equal throughout that there can be no surmise of its consist- In the eighteenth century telescopes were small and ing of stars; nor can there be a doubt of the evident connec- imperfect; the images were not sharp and photography had tion between the atmosphere and the star.’ Several examples not yet been discovered. This made it difficult to study are shown in Figure 1. the properties of nebulae. A controversy arose as to their Herschel’s argument that the nebulae did not consist of nature: did the nebulae consist of many faint stars that were stars was simple. He was certain that the star at the centre close together or did they consist of a luminous fluid? The and the nebula were associated because a chance coinci- regular appearance of the outer regions of globular clusters dence of such a bright star so perfectly centred on the neb- provided strong evidence for the former point of view, but ula was highly improbable. Thus, the star and the nebula the wide variety of shapes which were seen implied that are at the same distance. If the nebulosity is composed of some of the nebulae consisted of fluid or gas. stars they must either be very faint (assuming the central The first attempt to catalogue these objects was made by star to be ordinary) or, if they are normal stars, the cen- Charles Messier (1730Ð1817). He was a comet seeker and tral star must be of ‘enormous size’. Herschel rejected both his motivation was to avoid confusion between the nebu- of these possibilities from which it followed that the nebula lous objects and comets. His catalogue, published in 1784, was not composed of stars. contained 103 entries. Four of these are now known to be Figure 1 shows photographs of a sample of planetary planetary nebulae. A copy of this catalogue was given to the nebulae obtained by the Hubble Space Telescope. While all German-born English astronomer William Herschel (1738Ð nebulae are different in detail, all have a generally similar 1822), who immediately set to work to observe all these morphology. They are usually symmetric, at least about one objects with his 30 cm and 48 cm telescopes. He concluded axis, and there is always a star at a centrally located position, that most of the nebulae could be resolved into stars, and clearly indicating a physical connection. Occasionally this that the Milky Way could also be resolved into individual star is so faint that it cannot be seen above the nebular background and an observer may conclude that it is absent. But * Rijksuniversiteit Groningen, The Netherlands careful observations, which suppress background light, will

Figure 1 (a) The planetary nebula NGC 3242. The colours indicate the strength of certain lines as the image is a composite of three images of the nebula in different spectral lines, Note the strong red (N II) ansae or fliers at each end of the nebula. The central star is clearly visible. The matter appears to be dispersed in the form of a shell. This type of morphology is quite common.

almost always render the star visible. Thus Herschel’s conclu- Figure 1 (b) The planetary nebula MyCnl 8. The morphology sion that nebulae do not consist of stars (other than the central of this nebula is unusual. In addition, the central star appears to be offset from the centre. The reasons for this behaviour are star) applies to the whole class of nebulae, and implies that unknown. all are in a similar stage of development or evolution. By the middle of the nineteenth century further evidence gaseous light. The Great Nebula in Andromeda (as an became available which confirmed these nebulae as a sepa- example of ‘spiral nebulae’) was observed and showed a rate class. The spectroscope had become available and was continuous spectrum indicating starlight. It was clearly a being used with telescopes to observe the Sun and stars. very different object than a planetary nebula. It was thus Joseph Fraunhofer (1787Ð1826) had discovered that the possible to distinguish gaseous nebulae from nebulae con- Sun emitted a continuous spectrum interspersed with sharp sisting of stars by using a spectroscope. absorption lines. The planets showed many of the same fea- In 1865 Huggins used a spectroscope with higher resolv- tures as the solar spectrum. The stars also showed a contin- ing power, and was able to resolve the ‘single’ bright line into uous spectrum, but each had its own set of absorption lines. three individual lines. One line could be identified with a In 1859 Gustav Kirchhoff (1824Ð1887), working in the Balmer line of hydrogen (H), while the other two stronger laboratory of Bunsen in Heidelberg, discovered that certain lines to the red remained unidentified. When it became elements in gaseous form emit lines at just the wavelengths clear that no element known in the laboratory would produce of the solar absorption lines. In this way over 25 elements these lines, they were ascribed to a new element, nebulium. were identified in the atmosphere of the Sun. This was not the first new element named in this way. An William Huggins (1824Ð1910) was the first to examine a unidentified line observed in the solar chromosphere during planetary nebula with a spectroscope. In 1864 he observed the eclipse of 1859 was ascribed to the then unknown element the bright nebula in Draco, NGC 6543. Huggins had been helium, while a line found in the solar corona during an observing star spectra for over a year at that time, so that eclipse 10 years later was ascribed to the element coronium. the spectrum he observed was completely unexpected. Helium was identified in the laboratory in 1895. The other He found ‘a single bright line only’. This bright line pro- two ‘new elements’ were identified only much later: nebulium vided a means of distinguishing between starlight and in 1927 ([O III]) and coronium in 1939 ([Fe XIV]). PLANETARY NEBULAE 915

Figure 1 (c) The so-called Stingray nebula. This is a very young nebula in which the central star has been found to increase in temperature over a period of several years. Note the companion star, which is probably physically related.

Figure 1 (e) The planetary nebula M2-9. The morphology of this nebula suggests that it is an extreme form of the ‘bipolar’ type, but the normal nitrogen and helium nebular abundance suggests that this is not true.

Figure 1 (d) Hubble 5. This is an example of a ‘bipolar’ planetary nebula. About 15% of all nebulae have this form. It is thought that the central stars of these nebulae are of higher mass than the average nebula, and that the nebulae have a Figure 1 (f) The planetary nebula NGC 5307. An example of higher nitrogen and helium abundance. a nebula that does not show axial symmetry. 916 STUART R. POTTASCH

terms of stellar evolution. The paper begins with a thorough summary of all the observational data available, including photographs and drawings of all 78 planetary nebulae then known and observable from the Lick Observatory. Curtis plotted the position of diffuse and planetary nebulae in the plane of the sky and showed that both were to be found close to the galactic plane. However, the ‘spiral nebulae’ were distributed uniformly except for a ‘zone of avoidance’ toward the galactic plane. When coupled with the spectroscopic information it was clear to Curtis that both diffuse and planetary nebulae were ‘an integral part of our own galactic system’ while spiral nebulae were ‘very clearly a class apart Ð not only unconnected with our Galaxy but perhaps individual galaxies’. On the basis of this information, it was possible to speculate on the place of planetary nebulae in stellar evolution. Planetary nebulae were rare objects: ‘fewer than 150 are known in the entire sky. The relative proportion of plane- Figure 1 (g) He3-1475 is another very young nebula whose central star temperature has increased by a significant amount taries to the total number of stars must be of the order of 5 in the past 10 years. The origin of its unusual shape is not 10 or less. This minute percentage would seem to stamp known. the planetary nebula as an exceptional case, a sporadic manifestation of a path which has been rarely followed in stellar evolution.’ The only alternative to this conclusion was to regard ‘the planetary stage of existence as one of relatively very brief duration, through which the great majority of stars have long since passed.’ Adopting the latter hypothesis, the lifetime in the planetary stage was cal- culable; less than 10,000 years. ‘The very short life which must be presumed for the planetary stage of existence … does not seem inherently probable; it is as yet unsupported by any direct evidence.’ It is somewhat ironic that the ‘direct evidence’ actually existed. In the same volume in which the work of Curtis appeared was an article by Campbell and Moore which presented observations which are now used to show that planetary nebulae are actually about 10,000 years old. These astronomers obtained spectra at a higher resolution than was previously possible. They found evidence for important line broadening in 23 nebulae and in four cases the broadening was so large that the line was split. It is now known that this splitting is due to an expansion of the nebula with a velocity of about 20 to 30 km s1. Coupled with the size of the nebula this leads to an age of 10,000 years. Figure 1 (h) The nebula 1C 4663. The elliptical morphology But Campbell and Moore did not recognize the correct is quite common, as is the small-scale structure seen in the reason for the line splitting; they attributed it to a rotation nebula. Only the central star is related to the nebula; the of the nebula. The splitting was thought to be caused by other stars in the field are foreground or background stars. matter on the outside of the nebula, which was supposed to be rotating more slowly than the emitting matter and absorbing the central part of the line. The work of Curtis and beyond Ten years later, in 1928, this interpretation was shown In 1918 Heber Curtis published an important paper in the to incorrect when the nebulium lines were identified with Lick Observatory Publications. This is the first publication forbidden line radiation. The absorption coefficient for that attempts to define the status of planetary nebulae in these transitions is very low so that absorption as a cause 5

JAMES A. VAN ALLEN* Magnetospheric physics**

AUTHOR’S PREFACE had been flown for a wide variety of atmospheric, solar, astronomical and astrophysical investigations. How did a Ph.D. in experimental nuclear physics become a In January 1951, I returned to my Ph.D. alma mater as a planetary astronomer? Well, it wasn’t simple! Here is my professor of physics and head of the Department of Physics story. (later Physics and Astronomy) at the University of Iowa During the early days of World War II, I helped develop and, with the support of the Research Corporation and the the radio proximity fuze for gun-fired projectiles, working at U.S. Office of Naval Research (ONR), initiated and led a the Applied Physics Laboratory of Johns Hopkins student-centered program of similar research using balloons University. In November 1942, I was commissioned as a and balloon launched rockets (rockoons) as vehicles. The line officer in the U.S. Naval Reserve and was dispatched rockoon technique provided a low cost method of transport- immediately to the South Pacific Fleet to foster the adoption ing our instruments up to altitudes of 130 km. During the of this innovative, “smart” anti-aircraft fuze. During the next period 1952Ð1957, we conducted 109 rockoon flights, all three and a half years, I served a total of 17 months in two from shipboard, over a large range of latitudes from Baffin tours of duty on combatant ships and at ammunition depots Bay in the Arctic to the Ross Sea in Antarctica, under the in the South Pacific. In March 1946, now at the exalted rank sponsorship of the ONR and, during 1957, by the National of lieutenant commander, I was placed on inactive duty and Science Foundation as part of the 1957Ð1958 International returned as a civilian employee of the APL/JHU. Geophysical Year (IGY). Our principal results were a lati- Then during the period 1946Ð1950, I developed and over- tude survey of the intensity of the primary galactic cosmic saw a program of high altitude research in cosmic rays, rays, including the heavy nuclei therein, the discovery of atmospheric ozone, solar UV spectroscopy, ionospheric cur- X-rays from auroral electrons, and the measurement of rents, and photography of large regions of the Earth’s sur- ionospheric currents near the equator and at high latitudes. face. The work was supported by the U.S. Navy Bureau of Beginning in 1956, I participated as a member of several Naval Ordnance. Our instruments were transported up to alti- of the committees and panels planning U.S. participation in tudes of 160 km by the U.S. Army Ordnance Department’s the IGY and, most importantly, the one on scientific uses of post-World War II test flights of refurbished German V-2 artificial satellites of the Earth. The realistic expectations rockets and by American Aerobee rockets. The latter were for investigations with satellite-borne instruments were developed specifically for our investigations of physical heavily dependent on the experience and aspirations of a phenomena in and above the Earth’s atmosphere. Our origi- small cadre of us veterans of research with high altitude nal Aerobee and its successive upgrades were later adopted rockets during the preceding decade. by many others. By 1988, over a thousand such vehicles Following the successful launch of Sputnik I by the U.S.S.R. on 4 October 1957, there was a fast-breaking effort in the United States to speed up our efforts to place * University of Iowa, Iowa City, IA, USA ** This article, except the Author’s Preface, was first published in Icarus, artificial satellites in orbit, all within the context of the 122, 209Ð232 (1996) International Geophysical Year. As part of that response, the

U.S. Army’s proven four-stage Juno II vehicle was adapted so great that the GM tube no longer yielded resolvable indi- for the first attempt rather than the previously planned vidual pulses but only a kind of noise level of pulses too Naval Research Laboratory’s new Vanguard vehicle, still small to trigger the counting circuit. plagued by developmental problems. We then returned to the analysis of the real-time A cosmic ray instrument developed by George Ludwig Explorer I records, which had much greater dynamic range, and me at the University of Iowa had been selected previ- and we used a laboratory calibration of a similar system for ously by the relevant IGY panel as one of four payloads apparent vs. true (i.e., assuming zero dead-time) counting slated for early flights on a Vanguard vehicle. Our instru- rates to find true counting rates. The altitude dependence of ment was essentially ready for flight in late autumn 1957. counting rate was far too rapid and at far too high an alti- During the preceding several years, I had followed in detail tude to be attributed to any form of electromagnetic radia- the relative status of the Juno II and Vanguard vehicles and tion (e.g., X-rays or -rays) or to any form of corpuscular had decided that it would be wise to design our instrument radiation on direct trajectories from a distant source. so that it would be suitable for either. By virtue of this fact I concluded that the observed “effect” must be attributed and our previously established place on the short list, the to energetic, electrically charged particles trapped mechani- Iowa instrument was selected for the first Juno II attempt. cally in the Earth’s external magnetic field. This was the We then worked with the Jet Propulsion Laboratory to inte- essence of my “discovery” announcement of the existence grate it into the overall payload for the flight. of enormous intensities of geomagnetically trapped corpus- The purpose of the investigation was to greatly extend cular radiation at a joint National Academy of Sciences/ our rocket measurements of cosmic ray intensity above the American Physical Society meeting in Washington, D.C. on appreciable atmosphere as a function of latitude, longitude 1 May 1958. and altitude. Our adopted sensor for this purpose was a sin- We were immediately given a go-ahead for conducting gle Geiger-Mueller tube. follow up investigations with detector systems of far greater The Juno II, also known as Jupiter C, delivered the pay- dynamic range than that of Explorers I, II and III. We load (then named Explorer I) into a durable, moderately worked with the Army Ballistic Missile Agency in eccentric orbit on 31 January 1958 (1 February GMT). Huntsville, Alabama in building payloads for two further Telemetered data during and immediately after the launch launches of upgraded Jupiter C vehicles. The first of these sequence showed that our instrument was operating properly two launches on 26 July 1958 placed Explorer IV in a in free flight. Short segments of real-time data (typically of 2 durable orbit at an inclination of 50 (up from the 33 of the or 3 minutes duration) gradually flowed into our laboratory orbits of Explorers I and III). The second attempt on from the wide geographical distribution of receiving stations. 24 August 1958 was a vehicular failure. Some segments showed about the expected cosmic ray Our multi-fold detector system on Explorer IV yielded counting rates. In others the apparent rate was zero, a physi- immediate and massive confirmation of our earlier results cal impossibility for cosmic rays. For several weeks we puz- and a substantial advance in particle identification. In addi- zled over conceivable failure modes, but were much tion, it provided the principal observations of artificial radi- preoccupied by intense preparations for further launches. An ation belts composed of energetic electrons injected by the upgraded instrument was carried on Explorer II, a Jupiter C radiative decay of the fission products from a series of three launch failure on 5 March. Our third instrument also high altitude nuclear bomb bursts, called Argus I, II and III. included Ludwig’s magnetic tape recorder for recording data The latter body of observations was classified as secret during a complete orbit and then, upon command from a until public release by the federal government in early ground station, playing back the record within about 1959. During the following six months, we provided radia- 6 seconds. Data were also to be transmitted in real-time. This tion detectors for “moon” shots by Pioneer II, Pioneer III payload was launched successfully by a Jupiter C on 26 and Pioneer IV. Pioneer III did not achieve escape velocity March and called Explorer III. The very first full orbit play- but reached an apogee of 17 Earth radii (radial) and yielded back that we received confirmed the existence of expected data during both outbound and inbound legs of its trajec- cosmic ray rates at low altitudes, but then showed rapidly tory. These data, combined with the lower altitude data by increasing rates as the satellite moved to higher altitudes in similar detectors on Explorer IV, clearly established the its elliptical orbit, and then apparent rates of zero at the high- existence of two major radiation belts, distinguished by the est altitudes. This sequence was repeated in reverse as the markedly different absorbtivity of the trapped particle pop- satellite descended to lower altitudes. A further series of ulation. The very penetrating radiation in the inner belt had Explorer III tape playbacks gave repeated examples of simi- been well characterized by Explorers I, III and IV. lar results and established a coherent pattern. Meanwhile, Meanwhile Sputnik III (May 1958) had provided informa- Carl McIlwain demonstrated by a laboratory test of a similar tion on an outer belt. But Pioneer III gave the first complete detector system that the zero rates in Explorer I and Explorer survey of the radial distribution of trapped particles and III data were, almost certainly, caused by radiation intensity established the outer boundary of the trapping region. MAGNETOSPHERIC PHYSICS 155

Pioneer IV did achieve escape velocity and made a rather was undertaken by NASA’s Ames Research Center. Our remote pass by the Moon as it continued into interplanetary Iowa package of radiation detectors was one of several space. Our radiation data from this one-way flight through selected for these flights. the Earth’s external magnetic field confirmed and extended The missions of the two spacecraft, later named Pioneer the basic findings from Pioneer III and gave a generally 10 and Pioneer 11, were brilliantly successful. Pioneer 10 concurrent determination of the outer boundary of trapping. passed through the asteroid belt unscathed and made the Our laboratory then conducted subsequent investigations first in situ investigation of Jupiter’s huge and intense radia- on the “heavy” IGY satellite Explorer VII of the Army tion belt and magnetosphere in NovemberÐDecember 1973. Ballistic Missile Agency and on a series of complete satel- It continued outward in a solar-system escape trajectory. lites designed, built and instrumented in our small univer- Pioneer 11 made the second and somewhat different passage sity laboratory. During the period 1961Ð1974, our six Injun through Jupiter’s magnetosphere a year later. Following the and Hawkeye satellites were placed in a variety of high success of Pioneer 10, and at the urging of the scientific inclination orbits and provided a wealth of pioneering data investigators, Pioneer 11’s encounter trajectory for Jupiter on auroral radiation, solar X-rays, VLF radio waves above was chosen so that it led to a subsequent encounter with the ionosphere, galactic cosmic rays, solar energetic parti- Saturn. Its close flight by Saturn occurred in AugustÐ cles, and the structure of both radiation belts. September 1979. We discovered that Saturn is also a highly Also during the 1960s and 1970s, we participated in a magnetized planet and has a large radiation belt and magne- series of NASA missions, including the Orbiting Geophysical tosphere, though considerably smaller in magnitude and Observatories 1, 2, 3, 4 and 5, Explorers XII and XIV and intensity of trapped particles than that of Jupiter. Explorers 33, 34 and 35. There was an intense debate among the scientific investi- At one point in time, the University of Iowa had more gators and officers of NASA on the best choice of the flight space flights to its credit than the entire European Space trajectory for Saturn. One ballistic option was a very close Agency, including individual member states. passage within the inner edge of Saturn’s ring system. During the 1960s, the primary emphasis of the evolving Another was an encounter that would lead Pioneer 11 to a U.S. national space program was on manned missions in subsequent encounter with Uranus. In the end, NASA low Earth orbit, in preparation of the Apollo manned mis- selected a trajectory that would cross Saturn’s ring plane at sions to the Moon. A secondary emphasis was on about the radial distance that was being considered for future exploratory (unmanned) missions to the inner planets Venus Voyager 1 and Voyager 2 encounters, thereby calibrating the and Mars. There was a special interest in exploring Mars to survival probability for those missions. After its Saturn establish whether or not the physical conditions on Mars encounter, Pioneer 11 also had solar system escape velocity. were favorable for the development of some sort of biologi- Both Pioneers 10 and 11 continued to provide uniquely cal activity there, as had been frequently speculated. valuable data as they flew through the outer heliosphere, Our radiation instruments were carried on Mariner II, most notably on the properties of the solar wind and on the the first spacecraft to fly by Venus (1962); on Mariner IV, cosmic ray intensity over a long time span and over previ- the first spacecraft to fly by Mars (1964); and Mariner V, the ously unexplored distances from the Sun. Because of a second spacecraft to fly by Venus (1965). We found that combination of technical limitations, the flow of useful data neither Mars nor Venus has radiation belts, and we con- from Pioneer 11 terminated in January 1995 at a heliocen- tributed observations establishing significantly small upper tric radial distance of 42 AU. limits on the magnitude of their magnetic moments. As of mid-2000, after over 28 years of flight, Pioneer 10 As a member of both the Space Science Board of the continues to operate well. Valuable, though rather sparse, data National Academy of Sciences and NASA’s Lunar and on cosmic ray intensity are still being telemetered reliably Planetary Mission Board, I led special advocacy commit- from my radiation instrument on Pioneer 10, now at a helio- tees for missions to Jupiter and the other giant gaseous centric radial distance of over 75 AU (11,220,000,000 km), a planets Saturn, Uranus and Neptune. It was already known truly heroic achievement for NASA’s Deep Space Network of from radio-astronomical evidence that Jupiter had a large receiving stations. (The radiated power of the S/C transmitter radiation belt of relativistic electrons, but there was no is only 8 watts.) We are seeking the boundary of the helio- credible radio-astronomical evidence of radiation belts sphere but our most recent data show that the cosmic ray around Saturn, Uranus or Neptune. intensity at 75 AU is still influenced by solar activity. Hence, Our advocacy group provided the scientific rationale for that boundary lies beyond, possibly far beyond, 75 AU. missions to the outer planets. In 1968, NASA adopted two Meanwhile, my Iowa colleagues are serving as the prin- missions designed to pass through the asteroid belt between cipal investigators on the currently active missions of the orbits of Mars and Jupiter and to pass by Jupiter at a Dynamics Explorer I, Geotail, Cluster II, Voyagers 1 and 2, heliocentric distance of 5 AU, beyond any previous mis- Galileo and Cassini, and they are developing instruments sions. Management of the development of the spacecraft for future missions. 156 JAMES A. VAN ALLEN

INTRODUCTION floor of the ocean. Also the permanent magnetization of the cool outer crust is far too weak and fragmentary to be Space science is an eclectic mixture of the traditional disci- important on a global scale. plines of astronomy, physics, chemistry, geology and biol- It is now almost universally accepted that the general mag- ogy. The commonalty of its observational component is netic field of the Earth must be attributed to electromagnetism (a) the use of rocket-propelled vehicles for the delivery of or the flow of electrical currents in patterns resembling those instruments into orbit about the Earth, to the Moon, and of laboratory solenoids (Rikitaki, 1966). The consequent through the interplanetary medium to and beyond distant magnetic field may be complex because of the likely com- planets, comets and astroids; and (b) the use of radio teleme- plexity of the causative current system. However, at distances try as the primary method for transmitting data to terrestrial large compared to the dimensions of the current system, such stations. In addition, physical samples of the solar wind, complexity disappears and the general magnetic field is that lunar surface material and meteoric dust have been collected of a small current loop or equivalent point magnetic dipole of and returned for laboratory study. The return of samples of vector moment M. In general, M of a planetary body may be cometary dust and Martian surface material is planned. offset with respect to the geometric center of the body and The term space physics designates the sub-field of space tilted with respect to the body’s rotational axis. science that deals with certain classes of electromagnetic A first-order approximation to the Earth’s external mag- radiations of solar system origin, as well as with electric and netic field is that of a point dipole of moment M 7.90 25 3 3 magnetic fields, ionized gases (plasma), energetic electrons 10 gauss cm (0.304 gauss R , where the equatorial and energetic ions. The heritage of space physics is well rep- radius of the Earth, 1.0 R 6,378 km) located at the geo- resented by the great monographs of Chapman and Bartels metric center of the Earth and tilted by 11.5 to its rota- (1940), Störmer (1955), Mitra (1952), and Alfvén (1950). tional axis. The corresponding geomagnetic poles are at The modern epoch of space physics began in the late 78.5N, 69.1W; and 78.5S, 110.9E (epoch 1965). The 1940s with the use of high altitude sounding rockets and now vector moment M points southward. encompasses the research efforts of thousands of investigators An improved approximation is obtained by displacing throughout the civilized world. Sophisticated instruments of the same vector moment M, as in the centered dipole superb quality are being flown on all manner of automated, model, by 450 km from the Earth’s geometric center toward commandable spacecraft. The original data are, for the most latitude 17N and longitude 149E. This is called the eccen- part, descriptive of natural physical phenomena, whose inter- tric dipole model. The need for such an improved represen- pretation engages a growing cadre of theorists and modelers. tation is evident in global charts of the scalar magnitude B In addition, numerous artificial experiments are being con- as a function of latitude and longitude at zero altitude. ducted in space (Hultqvist and Fälthammar, 1990). Contemporary measurements establish the secular varia- This chapter is a tutorial review of magnetospheric tion of the Earth’s magnetic field and paleomagnetic evi- physics, with primary emphasis on the magnetic fields of dence records reversals of polarity at irregular intervals on a planets and the energetic particles therein. Aside from this time scale of the order of 105Ð106 years (Akasofu and fresh introduction, it is a reproduction of the author’s Chapman, 1972), thus testifying to the long-term instability “Kuiper Prize Lecture: Electrons, Protons, and Planets,” of the internal current system. which was sponsored by the American Astronomical Maintenance of the necessary system of electrical currents Society, published in the journal Icarus (122, 209Ð232, is attributed to a self-excited dynamo, according to the theory 1996) and reprinted in this volume with the copyright per- pioneered by Bullard and Elsasser. In this theory, it is visual- mission of the Academic Press. The relatively short bibliog- ized that any initial magnetic field, however small, is ampli- raphy comprises classical and recent monographs, reviews, fied by the convective flow of electrically conducting major compilations of papers, and a few original papers, material (e.g., molten lava) through that field to induce elec- but does not cite scores of other original papers. trical currents of such strength as to achieve a quasi-steady state between energy input and ohmic and viscous losses. Detailed theories of this effect are complex and have not MAGNETISM OF PLANETARY BODIES reached the stage at which quantitative predictions can be made, even given the interior conditions of a planet. But it The interior of the Earth has an estimated temperature of does appear that two properties of the planet are necessary: several thousand degrees Kelvin, far above the Curie points (a) an interior that is sufficiently hot to produce a fluid, elec- of all known ferromagnetic substances, i.e., the tempera- trically conducting fluid, and to drive convective flow of that tures above which they lose their ferromagnetic properties. fluid and (b) a rotational rate that is sufficiently rapid to Common evidence for such high temperatures is the flow of guide the pattern of convective flow. Plausible values of elec- molten lava from volcanoes on land surfaces and on the trical conductivity are derived from laboratory experiments 41

HIROSHI TSUNEMI* Supernovae and supernova remnants

1 GENERAL INTRODUCTION TO that go nearly completely undetected, the remaining 1% still SUPERNOVAE AND SUPERNOVA REMNANTS represents an enormous amount of energy released over a very short time. The maximum absolute magnitude of the M A supernova explosion is among the most dramatic events supernova is about 19 . This means that if a supernova 18 that can be seen. The term ‘supernova’ is somewhat mis- occurred at a distance of 10 parsecs (pc: 1 pc 3.110 cm) leading, as such an event represents not a new star (that is, a from the Earth, it would appear to be about 1,000 times as ‘nova’), but instead the end of a star’s life. Nuclear fusion, bright as a full moon. If it were to appear at the outer edge the energy source of the stars, creates heavier elements of the Galaxy, it would be as bright as Venus, if it did not from lighter elements. In this way, almost all of the ele- suffer from extinction due to interstellar matter. The Galaxy, ments that make up the universe, with the exception of however, does contain a great deal of diffuse matter so that hydrogen and helium, are created inside a star through this there is a great deal of extinction, particularly at optical process. These elements accumulate inside a star over its wavelengths. This is especially true in the Galactic plane. lifetime and are dispersed into space through a supernova Since most of the stars of the Galaxy are in the Galactic explosion. We may therefore say that all of the material of plane, only those supernovae that occur nearby are visible. the Earth (excepting hydrogen and helium) were probably On the contrary, supernovae appearing in external galaxies created inside some star many eons ago, and that they were lying outside of the Galactic plane can be easily seen. We ejected by a supernova, soon to become the primordial can therefore study the supernova rates in these galaxies. material of the Earth. Nuclear fusion inside a star can gen- The appearance of several supernovae has been docu- erate heavy elements ranging from helium up to the most mented over the past 2,000 years. Most of these were stable nucleus, iron. It may therefore be said that the iron recorded in the Eastern as opposed to the Western world, that plays such a vital role in the hemoglobin of our blood for reasons that appear to be socio-political. In the Western must have been generated inside a massive star that soon worldview, God created the universe perfect and forever went supernova. This must have happened at least five billion fixed. It is thus likely that the appearance of a ‘new star’,a years ago, since this is the age of our solar system. phenomenon at variance with a perfect, unchanging uni- Based on the statistical study of the occurrence rate of verse, would go unrecorded. Such a ‘mistake’ could not supernovae in other galaxies, we believe that a supernova be accepted. On the contrary, in the Eastern worldview, should occur every few tens of years in our Galaxy. Further, changes in the celestial sphere could result from God’s the energy released by a supernova is of the order of 1053 erg, ‘mistake’, a forecast for an impending disaster. In China, in which is two orders of magnitude larger than the energy fact, it became an important concern for the emperor to be radiated by our Sun over its entire 10-billion-year lifetime. able to predict a drought, a flood, or other such disasters. It Although 99% of this energy is carried away by neutrinos was thus a regular job for astronomers to search the sky for any omens of such events. Furthermore, a nearby supernova * Osaka University, Japan would be bright enough to be easily visible even during the

Hiroshi Tsunemi, The Century of Space Science, 937Ð960 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 938 HIROSHI TSUNEMI day, thus not only professional astronomers, but even common people would have been able to observe such events. During a supernova explosion, the heavy elements synthesized inside the star expand at very high velocity, up to 10,000 km sec1. Afterwards the supernova slowly fades through adiabatic expansion. The major heat source during this phase comes from the following series of radioactive decays: 56Ni (7 days) → 56Co (77 days) → 56Fe. This decay series will keep the supernova bright for several months. The ejecta, rich in heavier elements, will eventually form another generation of stars and planets. The ejecta first expand at high velocity with no deceleration. Eventually, it will sweep up enough interstellar material so that deceleration becomes effective. The deceleration will heat up both the ejecta and the interstellar material and the temperatures will become high enough so that the majority of the radiation will be not in the visible but in the X-ray regime. The swept-up material will form a shell-like structure, leaving a low density, high temperature interior. The expanding shell contains a mixture of interstellar matter and the heavy elements from the supernova progeni- Figure 1 Close up of the Veil Nebula (the Cygnus Loop) tor and the supernova itself. Once the temperature becomes obtained by the HST (see http://oposite.stsci.edu/pubinfo/gif/ high enough to ionize most of the elements, the shell barely CygnusLoop.gif) (courtesy of J. Hester, Arizona State Univ. and cools down. The dominant emission mechanism is thermal NASA). A high density region forms a filamentary structure. bremsstrahlung (also referred to as free-free emission), which results from free electrons colliding with the nuclei within the shell, losing energy by emitting radiation. The account the occurrence rate of supernovae, we find that the result is an X-ray continuum spectrum. When the tempera- hot gas, with temperatures of a few million degrees and ture has cooled enough so that some heavier atoms, such only visible in X-rays, potentially occupies a large volume as iron, capture electrons, much more efficient cooling of our Galaxy. processes take over. When an iron nucleus, for example, The fraction of heavy elements in our Galaxy is gradually captures a free electron, free-bound radiation is emitted. increasing. Moreover, apart from this in-flux of material, When a free electron collides with a bound electron, the there is also a flow of thermal and kinetic energy. The heavy bound electron is excited to a higher energy level. Almost elements are generated inside the stars as a by-product immediately, this electron returns to its ground state, emit- of the nucleosynthesis in the stars. The energy released by ting a photon in a bound-bound transition (line emission). this nucleosynthesis is continuously radiated away by the Once the matter cools down to the line-emitting temperature, stars as visible and ultraviolet light, while the heavy the cooling process accelerates. The lower the temperature, elements continue to accumulate within the star. When the the more efficient the cooling process. The tempera- star becomes a supernova, a large amount of the star’s grav- ture range for which the iron family (the most abundant itational energy is released along with the heavy elements. species among the heavy elements) captures electrons is These elements then disperse through the Galaxy and form thermally unstable. When the temperature reduces further, the building blocks for the next generation of stars. The the more abundant light elements such as oxygen begin to supernovae and supernova remnants (SNRs) provide the capture electrons that again accelerate the cooling process. most dramatic examples of this cycle of life-and-death in The cooling region is compressed by its surroundings and the Galaxy. further increases in density, further accelerating the cooling A star, such as the Sun, is a large sphere of hot plasma process. Sheets that are visible in the optical are formed with nucleosynthesis taking place only in its innermost as a result of the thermal instability. Figure 1 shows a por- regions. The energy is transported away from the center via tion of the Veil nebula in the Cygnus Loop, that results emission and, in the outer layers, by convection. The pho- from the radiative instability of the hot gas. tons emitted from the center (excluding the neutrinos) are The supernova leaves behind a high-temperature low- scattered, absorbed and re-emitted by the surrounding mate- density cavity a few tens of pc in diameter. It can survive rial. Therefore, photons that are created in the center of the for a few million years due to its low density. Taking into star cannot be directly detected. The photons that we detect SUPERNOVAE AND SUPERNOVA REMNANTS 939 from stars come from only a very shallow layer near to the star’s surface. This region is known as the photosphere. The supernova remnant (SNR) is a blow-up of the star, where the matter is at very low density. Because of this, we may directly detect emission from any portion of the plasma. That is, we do not need to worry so much about intervening material in the remnant absorbing photons. Such a plasma is referred to as being optically thin. In this circumstance, we can measure directly the abundances of the heavy elements in the remnant. Because of the remnant’s great size and low density, self-absorption and scattering do not play an important role in the radiation Figure 2 Atomic mass per nucleon as a function of mass process. We can therefore directly see emission from the number. Hydrogen has the highest mass per nucleon while heavy elements irrespective of their position in the remnant. the nucleons in 56Fe have the lowest ones. When heavy elements such as O, Ne, Mg, Si, S, A, Ca, and Fe emit photons at their characteristic energies (line emission) the photons will reach us undisturbed (thermal broad- Z is the atomic number) existed. This energy gap hindered ening of the lines is very small, in practice). Thus we are the creation of elements beyond helium in the first 3-minute provided with a unique opportunity to directly see these after the Big Bang. In contrast, the universe today is abun- heavy elements before they redistribute themselves through dant in high-Z elements. In the first epoch of the universe, the Galaxy. Further, the morphology of the remnants is also matter was partly clustered and formed into stars. Once the intricate, making them interesting in their own right. stars began to form, the nuclear fusion of hydrogen into Because so many remnants have already been observed, helium began in their cores. This phase of a star’s life is it is impossible to review all of them. In this chapter, called the ‘main sequence phase’, and it is the most stable we will discuss a sample of SNRs, ranging from young phase of a star’s life. The Sun, for example, has been in the remnants to old remnants. This will help us to understand main sequence phase for the last five billion years and will how the heavy elements and the energy of the supernova remain in this phase for another five billion years. are distributed throughout the Galaxy. Once the star has exhausted the hydrogen in its core, the main sequence phase ends, and the star intermittently expands and contracts. During this phase, a new class of 1.1 Nuclear fusion inside a star fusion processes proceeds, creating heavier elements such Atomic nuclei consist primarily of baryons (protons and as C, N, O, Ne, etc. Each element is created by a process neutrons) which determine the mass of the nucleus. Figure 2 that occurs at its own particular temperature. In general, the shows the average baryon mass inside the atomic nucleus higher the temperature, the heavier the element created. as a function of mass number, A. The figure gives the Because the temperature increases as one goes deeper into energy level of each nucleus: the lower the energy level, the the star and because the temperature in the core of a star is more stable the nucleus. The isolated proton, hydrogen (1H, slowly increasing during this phase, an ‘onion-skin struc- A1) has the highest energy level among the various ture’ results. That is, the elements are created in layers so nuclei. The baryons inside the helium atom (4He, A4) that, as one proceeds from the outer layers to the core of form a local minimum in Figure 2, being lower than lithium the star, different layers of elements are passed through. (6Li, 7Li), beryllium (9Be) and boron (10B, 11B) that follow it. Elements with higher Z are found deeper in the star. 12C becomes lower than 4He. Beyond this, the data points Elements with a lower energy level are more stable, so that fluctuate up and down. Generally, however, the energy level the fusion of two or more lighter nuclei into a heavier one gradually decreases with increasing mass number until 56Fe creates a nucleus with a lower energy level, that is, a more is reached. This nucleus (A56) has the lowest energy stable nucleus. The most stable nucleus is 56Fe. It therefore level among all nuclei and is therefore the most stable represents a barrier beyond which nuclei cannot be easily atomic nucleus. Beyond this nucleus, the energy level of created through the fusion process. Since a certain mini- nuclei increases. This is a fundamental result of nuclear mum temperature is required for fusion and since the tem- mass measurements. perature in the core of a star depends primarily on its mass, After the Big Bang, the matter of the universe consisted iron is created only in the most massive stars. It may thus mostly of hydrogen along with a little helium. Because of be said that iron represents the ashes of the nuclear burning the gap between helium and lithium seen in Figure 2, very that took place in the star. Through this process the high little high-Z elements (i.e. elements beyond helium, where Z elements are created and accumulate in a massive star. 940 HIROSHI TSUNEMI

Meanwhile, the energy released in this process is ultimately progenitor must have had a hydrogen envelope, an indication released through the star’s surface, making the star shine. of a massive star. The internal structure of the supernova A star is stable when the gravity of the star, which tends depends on the mass of the progenitor star. toward collapsing the star, is balanced by the pressure in the If the mass of the progenitor star is less than a certain star’s interior, which tends toward expanding the star. This critical mass, a few times M (M represents the mass of outward pressure is maintained by the thermal energy of the the sun, 2 1033 g), it will not become a supernova. It will star which is in turn regulated by the nuclear reactions. evolve instead into a white dwarf. Our sun will therefore Therefore, once the star has exhausted its source of nuclear not explode after it has exhausted its hydrogen about five energy, it can no longer support itself against its own grav- billion years in future. A star whose mass is over the critical ity. If the star has less than a certain mass, another source of mass accumulates high-Z elements in its center forming an pressure will ultimately halt the collapse. However, for stars onion-skin structure. In the deepest layer of the star, we beyond this lower mass, the collapse cannot be stopped and will see that the matter consists mainly of elements from a supernova will result. This supernova will shine as brightly the iron family (Fe, Ni, etc.), which represent the ashes of as the entire Galaxy. During the explosion, more nucleosyn- the nuclear fusion. Above this layer, lighter elements form thesis takes place, creating more high-Z elements. In some layers one by one. The outermost layer consists of hydro- supernova, the entire star is detonated while in others a com- gen, if the star has not lost its hydrogen envelope. Other- pact object Ð a black hole or a neutron star Ð is left behind. wise, it consists primarily of relatively low-Z elements The metal abundance in the universe is referred to as the such as carbon, oxygen etc. ‘cosmic abundance’, which is essentially the same as the If the mass of the progenitor star is in some range (up to abundances of the elements within our solar system. These several times M), it is believed that the entire star will be abundances are determined through studies of the sun as detonated in a supernova. All of the nucleo-synthesized well as of meteorites (Anders and Grevesse 1989). There matter is ejected, creating the building blocks for the next are many elements above iron. The Earth, for example, has generation of stars. If the mass is greater than this critical Ni, Cu, Ag, Au, Pt, U, etc., although their abundances are mass, some inner region of the star is not ejected in the relatively small. These higher-Z elements (elements beyond explosion but instead forms a compact object, i.e. a neutron Fe) are created by a variety of processes. One of these star or a black hole. Whether a neutron star or a black hole processes, known as the rapid-process, occurs only in the is left behind thus depends on the mass of the progenitor. presence of a large flux of neutrons. A supernova has the In this way, we can say that a Type I supernova supplies a necessary high flux so that this rapid-process occurs during large amount of iron since the entire star is blown up, a supernova explosion. The process is able to operate whereas a Type II supernova leaves a compact source because neutrons do not carry an electric charge, so that the behind: a neutron star or a black hole. Coulomb barrier in a nucleus is ignored by them. The elements thus created have large numbers of neutrons, some of 1.3 The supernova remnant which migrate to higher-Z elements via Ðdecay. In this way, the high-Z elements up to uranium are created. These The supernova releases both an enormous amount of energy high-Z elements have a higher energy level than iron, as is and a large mass of high-Z elements into space. The effects seen in Figure 2. Therefore, excess energy can be released of these two components are long-lasting and are seen as a when they are split into two or more lower-Z elements. This supernova remnant (SNR). process is called nuclear fission. We extract energy from As the ejecta expand into the interstellar medium, they these nuclei via the fission process in nuclear power sta- sweep up the surrounding interstellar matter. As long as the tions and in nuclear weapons. material swept up is less than the mass of the ejecta, the expansion is considered to be a free expansion. This phase lasts for a few hundred years, depending on the density of 1.2 The supernova explosion the interstellar matter around the supernova. As the ejecta When the supernova occurs, the high-temperature plasma, expand into the interstellar matter, a violent collision occurs the ejecta, expands outward and mixes with the interstellar between the ejecta and the interstellar matter. The former is matter, giving rise to an increase in its metal abundance. expanding at a velocity of a few thousand km sec1 while Early in the explosion, the ejecta expand adiabatically and the latter is stationary. At the boundary of the collision, a cool very rapidly. In general, there are two types of super- shock front is formed, resulting in shock heating of the nova: Type I and Type II supernovae. The supernova type is matter up to tens of million degree. The shock front repre- phenomenological, depending on whether or not the HÐ sents a boundary at which physical conditions, such as tem- absorption line (in the visible region) is seen in its spectrum perature and density, are discontinuous. After the collision, near the maximum brightness. If the line is present, the the ejecta and the interstellar matter gradually mix. 42

GEORGE GLOECKLER* AND KLAUS-PETER WENZEL** Acceleration processes of heliospheric particle populations

Through space exploration an amazing variety of popula- beyond the boundaries of the Earth’s magnetosphere, tions of energetic particles have been discovered in our Sun’s beginning in the 1960s. surroundings. Wherever we look in the heliosphere, we find In the early years of space exploration, the heliosphere processes that can accelerate ions and electrons of the local was viewed as a passive domain for energetic particles. plasma to energies of 0.01 to 1000 MeV, or even more. Solar energetic particles were known to be injected peri- The heliosphere is that volume of space surrounding the odically at its centre, galactic cosmic rays continually pene- Sun and filled by the solar wind, the ionized gas expanding trated from outside, and occasional energetic particles from the Sun at supersonic speed and embedding all planets. leaked from the Earth’s magnetosphere. The heliosphere This region was named the ‘heliosphere’ by Dessler (1967) was considered to be responsible for merely dispersing and and originally defined ‘as the region of interplanetary space decelerating (cooling) these particle populations. It was a where the solar wind is flowing supersonically’. The current degrading receptacle. definition refers to that region of space where the solar However, in the last thirty years our view of the helios- plasma dominates: the heliosphere extends beyond the phere has changed dramatically. The discoveries of several heliospheric (solar wind) termination shock, believed to lie new energetic particle populations in the 1960s and early about 100 AU from the Sun. Beyond this is the heliopause, 1970s produced convincing evidence that the interplanetary the division between solar and interstellar plasma and mag- medium is the site of large-scale and nearly continuous netic fields. acceleration of charged particles to energies as high as 30 That the space between Sun and Earth (and beyond) is to 100 MeV per atomic mass unit (amu). The heliosphere not completely a vacuum has been recognized for nearly a is not simply a mostly passive medium that transports solar century (or even slightly more), ever since discussions particles, modulates galactic cosmic rays and occasionally started about the observed relationships between solar accelerates particles to modest energies by interplanetary activity and geomagnetic variations (see e.g. historical shocks. sources in Chapman and Bartels 1940). That the helios- The great variety of new energetic particle populations phere is populated with ‘energetic particles’ Ð electrons, accelerated at different sites throughout the heliosphere is protons, and heavier ions with a few tens of keV energy to illustrated in Figure 1: solar energetic particles (SEPs); par- tens of MeV energy Ð was not known until the advent of the ticles associated with travelling shocks (the discovery of flight of appropriate detectors on spacecraft that travelled ‘energetic storm particles’ goes back to the earliest era of space science); ‘upstream particles’ accelerated at the bowshocks of Earth and other planets; ‘corotating’ ion * University of Maryland, College Park, MD, USA ** ESA Ð European Space Research and Technology Centre, Noordwijk, events associated with the forward and reverse shocks The Netherlands bounding ‘corotating interaction regions’ (CIRs) in the

Termination Shock (~100 AU)

High Speed Stream Heliopause Anomalous Cosmic Rays Corotating Particles

Pickup Ions Upstream Particles Interstellar Neutral Gas

Interplanetary Solar Wind Shock Particles Sun Solar Energetic Particles

GGloeckler 2000

Figure 1 Schematic illustration of heliospheric energetic particle populations (bold labels). Solid curves indicate shocks.

solar wind; the anomalous cosmic ray (ACR) component; throughout the Universe, for example, for the problem of and pickup ions. These particles often propagate over great the origin of galactic cosmic rays (see Chapter 29). distances, carrying information on the nature, location and Typical intensity spectra as a function of energy per amu composition of their sources and about the physics of parti- (energy/nucleon) for the different populations of energetic cle acceleration in their energy spectra, ionization states, particles are illustrated in Figure 2. We will discuss these, and abundance of elements and isotopes. with the exception of galactic cosmic rays, in later sections. Most of these heliospheric energetic particle populations are directly associated with collisionless shock waves. The prime acceleration site of the ACR component is thought to SPACECRAFT AND INSTRUMENTATION be the solar wind termination shock. With the exception of impulsive SEP events and interstellar pickup ions, there is The first energetic particle signatures that were found in nearly a one-to-one correspondence between shocks and the heliosphere were detected as a ‘target of opportunity’ energetic (upwards of several hundred keV) particle popu- by spacecraft that had another prime objective, including lations in the heliosphere. Since the solar wind is super- Mariner 2, Explorer 12 and Explorer 14. Later spacecraft sonic, it is not surprising that it is riddled with shock waves. that played a key role in the discovery and study of the vari- But originally it came as a surprise that the shocks are such ous heliospheric ion populations included the Interplanetary prolific particle accelerators. Thus, for more than three Monitoring Platform (IMP) series, in particular IMPs 7 and 8; decades, the heliosphere has provided a gigantic and effec- Helios 1 and 2 in the inner heliosphere; the International tive astrophysical laboratory for intense studies of particle SunÐEarth Explorers (ISEE 1, 2, 3); Pioneers 10 and 11 acceleration processes by means of direct measurements. and Voyagers 1 and 2 in the outer heliosphere; Ulysses, These in situ studies of particle acceleration have formed exploring the three-dimensional heliosphere; and Wind a basis for our understanding of acceleration processes and the Advanced Composition Explorer (ACE). Most of ACCELERATION PROCESSES OF HELIOSPHERIC PARTICLE POPULATIONS 965

these spacecraft were spinning, thus being able to provide information on the arrival direction of the particles measured. By the end of the twentieth century we had explored the heliosphere in three dimensions, mapping the spatial distributions of the solar wind, of CIRs and of coronal mass ejections (CMEs) as Ulysses flew over the solar poles. Energetic particles accelerated at CIR shocks have been followed to latitudes far higher than the shocks themselves, thus serving as probes of the magnetic topology in the solar corona and high-latitude heliosphere. The Voyager spacecraft have tracked the modulation of the spectra of the ACRs out beyond 75 AU. Observations with the large-area detectors on Wind and ACE near 1 AU provide new insights into the physical processes of acceleration of SEPs. And we must not forget those spacecraft that continue to operate for long periods of time, such as IMP 8, which has been providing data for 30 years. These give us a complete perspective on solar-cycle variations that underlie the physical processes we study. Table 1 lists the space missions that have made major contributions to the study of heliospheric particle populations. Detectors for energetic particles used in the very early days of space exploration included Geiger counters, nuclear emulsions, proportional counters and scintillators with photomultiplier tubes. Semiconductor (solid-state) detector Figure 2 Typical oxygen differential spectra for different developments, reducing weight and enhancing reliability of populations of heliospheric particles. (After Figure 2.1 of von instruments, began in the late 1950s and early 1960s. At lower Rosenvinge et al. 1995.) energies space plasma detectors used Faraday cups. These

Table 1 Space missions that played a key role in studies of heliospheric particle populations

Spacecraft Operation Orbit Achievements

Explorer 12 1961Ð1962 Elliptical Earth orbit First detection of ‘energetic storm particles’ Mariner 2 AugÐDecember Reached Venus First observation of an interplanetary shock 1962 in Dec. 1962 IMP 7 1972Ð1978 Circular, 30 RE Discovery of ACR oxygen; first measurement of ionization states of energetic particles IMP 8 1973Ðnow Circular, 30 RE Discovery of ACR He; 1 AU reference mission due to long orbital life Helios 1/2 1974/76Ð Inner heliosphere Spatial structures of SEP events; CIR radial gradients 1986/1980 to 0.3 AU from 1 to 0.3 AU Pioneer 10/11 1972Ð1999 Outer heliosphere ACRs; CIR radial gradient beyond IAU Voyager 1/2 1977Ðnow Outer heliosphere ACRs ISEE 1/2 1977Ð1987 Elliptical Earth orbit Earth’s bow shock and upstream particles ISEE 3 1978Ð1982 215 RE Sunward of Earth Interplanetary shock particles; two classes of SEP events AMPTE/IRM 1984Ð1986 Elliptical Earth orbit Discovery of He pickup ions Ulysses 1990Ðnow Solar polar orbit Discovery of H and heavier pickup ions; (1.3 to 5.4 AU) CIRs at high latitudes SAMPEX 1992Ðnow Earth polar orbit Charge states of ACRs Wind 1995Ðnow Upstream of Earth Large area detector systems ACE 1997Ðnow 215 RE Sunward of Earth High-resolution composition (elemental and isotopic and charge states) with large collection area 966 GEORGE GLOECKLER AND KLAUS-PETER WENZEL early instruments provided a measure of the total ion flux and charge and energy of the particle is obtained. The main dis- could only resolve protons from helium and heavier ions. advantage of plastic track detectors is their lack of good In the course of time, instruments have improved enor- time resolution and the need to recover them. mously, in sensitivity (e.g. larger collecting areas and lower A different approach to measure the composition energy thresholds), in resolution (e.g. better separation of of particles in the low-energy range between about 0.1 different species) and in high-speed on-board processing. to 10 MeV/amu uses the ‘time-of-flight v. energy’ (TOFÐE) Where we once measured event-averaged abundances and technique (Gloeckler and Hsieh 1979). The start detector, a energy spectra in SEP events or across CIR shocks, we can thin foil, is separated from the stop detector (usually a now probe the time-dependent spectral evolution in such solid-state detector) by 10Ð50 cm. Secondary electrons, events for many particle species over 4Ð5 orders of magni- emitted from the respective surfaces of the start and stop tude in energy. detectors, are guided, typically by electric fields, to What led to the many discoveries and provided progress microchannel plates that detect these fast electrons and pro- in the detailed understanding of the different particle popu- duce the start and stop signals for the TOF analysis. lations were Ð besides new spacecraft on different trajec- Measurements of the speed of the particle using TOF analy- tories probing new regions in the heliosphere Ð new sis combined with measurement of its energy (E) in the space-borne instruments capable of measuring the composi- stop detector yields its mass. Composition instruments of tion of particles at increasingly lower energies. The most this type were flown on the Wind (Gloeckler et al. 1995) common composition-resolving particle detectors were so and ACE spacecraft (Mason et al. 1998). called ‘energy loss v. energy’ (dE/dxv. E) telescopes, using Even lower energies are reached by combining TOFÐE stacks of solid-state detectors (see Gloeckler (1970) for with electrostatic analysis, as is done in the Solar Wind descriptions of these and other particle detection tech- Ion Composition Spectrometer (SWICS) instrument on niques). In these particle telescopes, the amplitudes of the Ulysses, Wind and ACE (Gloeckler et al. 1992, 1995, coincident signals from the thin front dE/dx detector and the 1998). Such instruments provide composition measure- thick back E detector (or stack of detectors) were digitized ments from less than 1 to more than 100 keV/charge and (or pulse-height analysed) and transmitted to Earth. When measure, in addition to the mass, the charge or ionization the pulse heights of both detectors were plotted against each state of the particle. This class of instruments is used to other for many recorded particles, distinct tracks would measure solar wind composition and the velocity distribu- appear for each resolved nuclear species, with the separation tion of pickup ions. between tracks depending on the combination of nuclear The most recent set of instruments to measure the com- charge (atomic number) and mass. In this way the atomic position of the solar wind, solar energetic particles and number and mass of the nucleus were determined. galactic cosmic rays with large collection areas and excel- Ever lower energy limits were reached by decreasing the lent elemental and isotopic resolution are assembled on thickness (but not the area) of the dE/dx detectors. This was ACE. It includes, besides SWICS (essentially the same done by, for example, using thin-window proportional coun- instrument as its counterpart on Ulysses; see below), the ters of the Ultra-Low Energy Telescope (ULET) (Hovestadt Ultra Low Energy Isotope Spectrometer (ULEIS), the Solar and Vollmer 1971) provided by the Max-Planck-Institut Energetic Particle Ionic Charge Analyzer (SEPICA), the (Garching) as part of the University of Maryland Solar Isotope Spectrometer (SIS) and the Cosmic Ray Experiments on IMPs 7 and 8 (Tums et al. 1974), or a Isotope Spectrometer (CRIS). These state-of-the-art ACE mosaic of ultra-thin solid-state detectors in the Low Energy instruments are described in Russell et al. (1998). Charged Particle (LECP) experiment on Voyagers 1 and 2 The acceptance of novel and technically advanced exper- (Krimigis et al. 1977). Large-area, position-sensitive solid- iments for spaceflight has not always been easy, and the state detectors made it possible to resolve individual iso- road to the pioneering measurements that were eventually topes of many of the elements in the various particle made was often tortuous and lengthy. For example, there populations (e.g. von Rosenvinge et al. 1995, Stone et al. was considerable resistance to the use of ultra-thin solid- 1998). state detectors that were eventually flown in the LECP To reach even lower energies (tens of kilo-electronvolts), experiment (Krimigis et al. 1977) on Voyagers 1 and 2, yet large-area plastic sheets were exposed in space and then these detectors make it now possible to measure the lowest returned to Earth for analysis (Price et al. 1967). This tech- energy ACRs in the outer heliosphere. nique makes use of the fact that highly ionizing, heavy The TOF technique found it particularly difficult to gain particles will damage the plastic material along their track. acceptance in flight experiments. TOF instruments were Once retrieved from space, these tracks become visible proposed for an Explorer mission and again for Galileo, but after chemical etching, and from the size and shape of the were not selected by NASA because the technology was cones etched into the material, information on the nuclear considered unproven and too risky for flight. However, in 43

GERHARD HAERENDEL* Reconnection

The reconnection of magnetic fields in a conducting fluid or of magnetic configurations and the most powerful conver- gas is one of the most important processes in cosmical plas- sion process of magnetic into kinetic energy in the magnetic mas. It converts energy stored in magnetic fields into kinetic regimes around stars and in interstellar space. Magnetic energy, transforms magnetic configurations, and enables reconnection is, however, also a necessary ingredient of the interactions between two magnetized plasma regimes which dynamos that act in the slow plasma flows in stellar interiors. otherwise would be much weaker. The recognition of the Reconnection is conceptually not a difficult process. existence of this process grew half a century ago out of The main controversy has been not so much its existence the attempt to understand the dramatic energy releases in the in highly conducting fluids, but its speed or efficiency. solar corona and chromosphere during flares. Processes on Postulated in the late 1940s, it took three decades to verify the Sun on all scales of energy release have the been targets its existence in space and two decades more to derive an of reconnection studies ever since. With the beginning of understanding of the microphysical origin of its efficiency. space research and the discovery of the magnetosphere it Although it was first proposed in an attempt to explain the soon became clear that the fundamental process underlying sudden energy release in solar flares, it was the direct solar windÐmagnetosphere interactions and the ensuing access to reconnection situations in the laboratory and in internal dynamics of the magnetosphere must be reconnec- space that laid the foundations of its acceptance by the tion. Furthermore, there was the chance to explore the wider community of plasma physicists and astrophysicists, process by extensive in situ and global response studies. and it was thanks to the rapidly developing art of computer Reconnection is a process that breaks the magnetic simulations during the last two decades of the twentieth connection of plasma elements in situations in which the century that led to a deeper understanding of the micro- magnetic field can be generally considered as frozen in. processes involved. Reconnection is fundamental to the It requires high current densities which are concentrated energetics and dynamics of the solar corona, which is the in thin sheets or filaments and are thus exceptional regions in source of the plasma and magnetic fields that fill the helios- a much larger quasi-inert environment. The formation of phere. Reconnection also plays a key role in transferring reconnection regions Ð that is, of the necessary thin current energy from the solar wind flow to planetary magnetos- sheets Ð may occur over long build-up times. The onset of pheres and atmospheres. It is thus the most important ingre- reconnection can thus have a quasi-explosive character and dient of solarÐterrestrial physics. Although it is most lead to a rapid release of magnetic energy stored during the familiar in the high-beta plasmas of corona, magnetopause, preparatory process. However, in some situations reconnec- and magnetotail, a modified form of reconnection occurs in tion can also be essentially stationary. Since cosmic plasmas the very low-beta plasmas (104) at the bottom of the outside stellar interiors, dense atmospheres, and dense mole- corona or planetary magnetospheres. However, in this con- cular clouds are essentially collisionless and highly conduct- text the term “reconnection” has been mostly avoided: ing, reconnection is the most striking transformation process although magnetic connections are broken and magnetic energy is released, terms such as “auroral acceleration,” “thawing” of field lines, and “magnetic fractures” tend to * International University Bremen, Germany be used instead.

This chapter attempts to trace the evolution of the of the exciting quest for a thorough understanding of this understanding of the concept of reconnection from the first powerful and yet elusive process. conjectures, via the growing body of evidence, to the present-day analysis of the relevant microprocesses. After looking at the early history and the development of basic THE EARLY HISTORY: FROM GIOVANELLI TO reconnection configurations on the Sun and in the magneto- PETSCHEK sphere, we turn to the growth of indirect and direct evidence of its existence and to the insights gained from In a Nature article of 1946, Donald G. Giovanelli (1946) specifically designed reconnection experiments in the labo- proposed a theory of solar flares according to which elec- ratory and from numerical simulations. Finally, we return to trons accelerated by induction electric fields near magnetic our point of origin, the application to the Sun. There are neutral points excite the optical emissions of chromos- still many unanswered questions, and controversies are still pheric atoms. He assumed that a discharge takes place debated. However, the foundations for a successful inter- because, with increasing energy, the electrons undergo pretation of some of the most striking phenomena on the fewer collisions. Consequently, he proposed the existence Sun and in near-Earth space have become increasingly of very high current densities. Cowling (1953) heavily criti- solid, and applications to distant cosmic phenomena are cized Giovanelli’s and Hoyle’s (1949) subsequently devel- gaining credibility. The author is grateful for having been oped ideas on the grounds that this implied current sheets an observer and participant, although not in the front row, a few meters in width “which cannot by any stretch of

A–A 2 2 X E

Y 2 2

(a) (b)

y Y YЈ 2l 2L 2y* x

Figure 1 (a) The direction of the magnetic force, f, near a neutral point, N. (After Dungey 1958.) (b) The collision layer. (After Sweet 1958.) (c) Schematic of a reconnection configuration. (After Parker 1963.) (d) Petschek’s solution: a small diffusion region with standing slow shocks attached. (After Petschek 1964.) RECONNECTION 1009

imagination be regarded as a possible thickness of a flare where Dm is the magnetic diffusivity, layer.” He further concluded that “the discharge is not a regular one, but an irregular dissipation of energy in a vio- Dm (4) 0 lently twisted field.” He then asked how the field can become so heavily twisted. The electromagnetic forces may and is the electrical resistivity. Combining eqns (1)Ð(4) well be there. “But induced currents, whose effect is always provides an expression for the merging or reconnection rate to oppose the changes to which they are due, smother the in terms of the Alfvénic Mach number: effect which was sought and leave only an unsatisfactory v 1/2 vestige of it remaining.” This argument of Cowling, in M R (5) A V m which he invoked Lenz’s law, was disputed by Hoyle’s stu- A dent Jim Dungey (1958), who showed that near neutral where Rm VAL/Dm is the magnetic Reynolds or Lundquist points Lenz’s law is reversed: that the magnetic force den- number. The difficulty, which Parker pointed out and which sity, f j B (Figure 1), “tends to compress the material exists still today (see, e.g., Biskamp 1993), is that for cur- and field in the x-direction and stretch them in the y-direc- rent sheets of macroscopic length L, Rm tends to be a very tion. Since this motion reduces the acute angle between the large number in space and solar plasmas, and the reconnec- limiting lines of force at N, it seems probable that it tion rate v or MA is very small, much too small to account increases the current density.” He added, “While a rigorous for the fast energy release in solar flares. investigation is lacking, then, the indications are that the The solution came from Petschek (1964), in a paper he pressure gradient cannot prevent the discharge.” presented at the AASÐNASA symposium on the physics Dungey’s ideas were taken up by Sweet (1958), who con- of solar flares in 1963. As shown in Figure 1d, he attached cluded that the magnetic forces would flatten the field and standing hydromagnetic waves to a diffusion region of form a thin “collision layer,” as shown in Figure 1b. In anal- length y*, much shorter than the length L of the overall ogy to the hydromagnetic situation, he considered two plates reconnection situation. He determined y* to be being forced together, calculated correctly the outflow velocity, and investigated the dissipation of magnetic energy by Dm * y 2 (6) Joule heating in the current layer of decreasing thickness. vAMA It was Parker (1957, 1963) who, inspired by Sweet’s work, posed the reconnection problem in the simplest possible whereby the merging rate, MA, is considered as an exter- terms, at the same time drawing attention to the fact that nally controlled parameter. His theory then provides a max- none of the then “known mechanisms are sufficiently rapid imum possible flow rate, which (with a correction by a 1 to account for the solar flare from the annihilation of mag- factor of2 , found by Vasyliunas (1975)) is netic fields.” And he was concerned that “there appears to be some popular belief that the necessary annihilation of mag- MA,max (7) 2 8 ln(2 M R ) netic fields has been accounted for by quantitative theory.” In A,max m his papers he elaborated quantitatively on the existing diffi- So, Parker’s merging rate, which depends inversely on the culties, the most striking of which was the rate of reconnec- square root of R , has now been replaced by a logarithmic tion. This can be derived immediately from Parker’s famous m dependence. Even with R 1010, maximum merging rates set of relations. With 2l as the width of the current sheet in m of 0.05 appear to be possible. “The principal effect of Figure 1c and 2L its length, conservation of mass yields including the wave propagation mechanism is to reduce the length over which the diffusion mechanism must operate,” v L V l (1) wrote Petschek. With this reduced length, Parker’s relation where v is the speed of magnetic field merging and V the (eqn (3)) is still valid. efflux velocity, which from pressure equilibrium across the The two waves attached to the diffusion region are slow neutral sheet must be the Alfvén speed: mode shocks which nearly switch off the tangential field component. They propagate with the Alfvén speed based on B 0 the normal magnetic field component. Thus V V (2) A 0 Bn MA (8) where B0 is the upstream magnetic field. According to Sweet B0 and Parker, magnetic diffusion controls the merging speed v: These waves are dissipative, and the pressure increases Dm across the shock so as to maintain continuity of the total v (3) l (gas plus magnetic) pressure. 1010 GERHARD HAERENDEL

As pointed out by Vasyliunas (1975), Petschek’s solution is homogeneous. Other boundary conditions can produce requires a convergent flow toward the diffusion region even higher merging rates. (Figure 2). The magnetic field strength is thus lowered in Petschek’s (1964) paper constituted a major break- the region immediately upstream of the diffusion region. through in the theory of reconnection and in plasma astro- Here the Alfvén speed is lower than it is farther away. Since physics generally. Several alternate models producing even the Alfvén Mach number must not exceed unity just outside higher reconnection rates were subsequently presented, for the diffusion region, it follows that the merging rate must instance by Sonnerup (1970) and Yeh and Axford (1970). be less than unity. This conclusion, however, depends on Their solutions are basically similarity solutions in that, at Petschek’s assumption that the magnetic field far upstream scales between that of the diffusion region (considered to be nearly zero) and the external scale L, the magnetic field and flow velocity depend only on the ratio x/y of the two relevant coordinates. These and other models have been critically analyzed by Vasyliunas (1975). The outcome was: Neither Petschek’s model nor the similarity model predicts a definite merging rate. In both models the speed of plasma inflow (v) is, within limits, a free parameter whose value is assumed to be determined by the boundary conditions; the merging rate fixes the dimensions of the diffusion region in terms of the appropriate microscopic length scale, but neither resistivity nor inertial effects have any influence upon the models outside the reconnection region. The microscopic length scales Vasyliunas was referring to arise from the various terms of the generalized Ohm’s law and are the resistive length, Dm/vA, and the ion inertial length, c/ pi, or the electron inertial length, c/ pe. In a later Figure 2 Convergent flows in the Petschek solution. (After section we return to the role of these scales in the so-called Vasyliunas 1975.) diffusion region according to present-day understanding.

Figure 3 Standing wave solutions for an external field with spatial gradient: (a) for an incompressible fluid, (b) for a compressible one. In (c) the external field is homogeneous. (After Petschek and Thorne 1967.) 44

DOUGLAS O. GOUGH* AND PHILIP H. SCHERRER** The solar interior

The quest to learn the internal structure and dynamics of the of the main scientific topics has followed the development of Sun is motivated by several issues. For most people the most helioseismology, and therefore the discussion of those topics obvious is the desire to understand the source of energy for will be intertwined with the discussion of helioseismology. the Earth, which is essential for the maintenance of life. Astronomers are interested in the Sun because it is an example of a typical main-sequence star that can be studied in STATUS PRIOR TO 1975 enormously greater detail than any other. And in addition, the Sun can be used to investigate fundamental physics: it is an Before 1975 the only observable quantities that yield infor- important source of gravity, providing a testbed for the gen- mation about the solar interior were the global parameters Ð eral theory of relativity, and it contains material at high pres- mass, radius, luminosity, and effective temperature Ð which sure and temperature permitting us to study particle, nuclear were used to calibrate (spherically symmetrical) theoretical and atomic physics, plasma physics, and fluid dynamics models. A measure of the neutrino flux, which reveals energy under conditions that cannot be achieved on Earth. Perhaps generation processes directly, was also available. The age of less fundamental but certainly more important to society, the the Sun is inferred from the age of the Earth, and other inter- solar interior is the source of both secular and cyclic variabil- planetary material. The observed surface differential rotation ity in the electromagnetic and particle fluxes, and of all their and the existence and organization of magnetic fields in the effects on the Earth and human technological systems. photosphere also provided information about processes This chapter describes first our understanding of the solar assumed to be operating in the convection zone. There was interior prior to 1975. It then provides a narrative discussion also a measure of the oblateness of the photosphere, from of developments in each of several major topics since that which one should be able to infer a measure of the mean rota- time. These topics include neutrinos, shape, irradiance, cali- tion of the interior and the oblateness of the exterior gravita- bration of stellar models, rotation, the dynamics of the con- tional equipotentials. But the interpretation of some of those vection zone and of large-scale features within it, magnetic data was in question. Primarily, our understanding of the field generation and the solar cycle, and the nature of active solar interior was based on the application of stellar models. regions and active longitudes. Beginning in about 1975 a new Prior to the observation that there were too few neutri- tool for probing the interior of the Sun, and some day of nos coming from the Sun, theoretical models could be other stars, was discovered. This tool is helioseismology. The adjusted to match the global observables. Explanation of the impact of helioseismological inferences on our understanding low neutrino flux was a big problem. Although in principle of the solar interior is so dominant that a brief review of the the solar data were adequate to determine the uncertain techniques by which it is used will be provided prior to an parameters that specify the simplest theoretical models Ð the examination of the development and present state of each of initial helium abundance Y, the total heavy-element abun- the above topics. For the most part the further development dance Z (the relative proportions of most of the heavy elements can be determined by spectroscopic analysis) and a * University of Cambridge, United Kingdom scaling parameter contained in the mixing-length theory ** Stanford University, CA, USA used to model the convective heat flux Ð the outcome was

Douglas O. Gough and Philip H. Scherrer, The Century of Space Science, 1035Ð1063 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1036 DOUGLAS O. GOUGH AND PHILIP H. SCHERRER generally not believed, for it yielded an implausibly low second assumption is that the core is motionless, and that value of the helium abundance, namely Y 0.16. Even therefore all but the slowest nuclear reactions have reached though it was in the Sun that helium was discovered, a direct local equilibrium. Together, these assumptions impose tight abundance determination is not possible. The atmosphere is theoretical constraints on the balance of reactions and the too cool for a spectroscopic analysis, and the theory of consequent neutrino production rates. radiative transfer in the corona is too uncertain to yield a (ii) Oblateness of gravitational equipotentials and the reliable value. Abundance measurements were available history of solar spin-down: How much greater is the angu- from in situ measurements of the solar wind, but these lar velocity in the core than it is at the surface? There had exhibited temporal and spatial variability, implying that been many studies of potential instabilities arising from chemical differentiation had taken place and that therefore shear presumed to be imposed by spin-down and to which none of the measured values necessarily represents the the Sun was regarded as being neutrally stable. They all value in the Sun. However, there were astronomical esti- implied that increases inwards, which would cause the mates: values of Y determined in the atmospheres of hot oblateness of the Sun to be greater than one might suppose stars believed to be of comparable age to the Sun and mea- from the surface rotation alone. Dicke and Goldenberg surements in ionized interstellar gas clouds suggested a (1967, 1974) had claimed such a greater oblateness which value of about 0.25. Moreover, calibrated theories of Big might have been compatible with some of these theoretical Bang nucleosynthesis yielded values of Y that exceeded studies, but there were problems with interpreting the opti- 0.20 Ð comfortably less than the values observed in the cal measurements of the shape of the Sun, owing to there stars which have condensed from gas clouds that have been being a greater emission of light from the solar atmosphere enriched by nuclear processed material from supernova in equatorial regions making the solar disk appear to be explosions, yet substantially greater than the value from the more oblate than the matter distribution. PoleÐequator varia- solar calibration. Therefore the value of 0.16 was not gener- tion in convective flow could also influence the measure- ally accepted; instead it was assumed that Y 0.25, and that ment. The contribution to the oblateness from the centrifugal it was the neutrino flux that could not be explained. force due to the rotation of the photosphere needs to be sub- Consequently the issue was dubbed “the solar neutrino tracted from the raw measurement: the residual would be problem.” Nevertheless, there remained some doubt about Y. only about 4% of the total if were uniform, rendering the corrected measurement uncertain. A measurement of the oblateness of the surface would have been fine if MOST IMPORTANT ISSUES IN 1975 were much larger in the interior, as Dicke had predicted to be required for the BransÐDicke theory of gravity. (i) Solar neutrino flux: Is it a problem with the structure of The interpretation of the measurements of Dicke and the Sun, or with nuclear physics or even particle physics? Goldenberg were doubted by the community, partly because Spontaneous oscillations between neutrino flavors had been they contradicted general relativity by implying too great a suggested by Pontecorvo (1968), raising the possibility that precession of the perihelion of the orbit of Mercury. A subse- a substantial fraction of the electron neutrinos that are pro- quent contradictory measurement by Hill and Stebbins (1975) duced in the Sun have been transformed into or neutri- was accepted immediately by most astronomers, however, nos by the time they reach the Earth, and thereby had perhaps not as critically as one should expect. Nevertheless, it evaded detection. However, for such a process to occur, neu- still appears that any measurement of the shape of the photos- trinos would need to have mass, and at the time it was phere is an extremely unreliable guide to the oblateness of the almost universally believed that neutrinos were massless. gravitational equipotentials, and that more direct methods are There had been a great industry in adjusting parameters of preferable. As we describe below, the inference from helio- standard solar models, and adding extra ingredients almost seismology is now by far the best, and is likely to remain so always in a spherically symmetrical fashion. That some neu- for a long time. trinos were detected at all was regarded as a triumph for (iii) The source of magnetic fields and the activity cycle nuclear physics. A crucial assumption in the construction of was also a mystery. While a number of competing models the standard models is that they are in thermal balance (the were put forward, the general belief was that a dynamo thermal relaxation time is very much less than the character- process operating in the convection zone and driven by istic time of structural variations arising from chemical com- rotational shear is the source of active-region fields and of position changes produced by nuclear transmutations). This the 22-year magnetic cycle. A phenomenological discussion implies that the integrated rate of generation of thermal invoking differential rotation, buoyancy, and supergranula- energy by nuclear reactions is equal to the “observed” lumi- tion to distribute the fields into the observed patterns nosity of the Sun (at least, the luminosity inferred from irra- (Babcock 1961, Leighton 1969) has held considerable cre- diance measurements, assuming spherical symmetry). A dence, and it is not unlikely that these processes form the THE SOLAR INTERIOR 1037

N N N

W E E W

S S S

STAGE 1 STAGE 2

N N a f P + b a b p – a + E W b b + W – p a + a b b – a S E W S

STAGE 3 STAGE 4 Figure 1 Schematic from Babcock (1961) via Rabin et al. (1991) showing the proposed evolution of the 22-year solar magnetic cycle, which still forms the basis of many modern views. Stage 1 is a state of basically poloidal field. In stage 2 the field lines are stretched into toroidal spirals by the differential rotation (principally in the tachocline, perhaps), and once the field has reached a strength sufficient to become buoyant, parcels of fluid rise through the convection zone drawing with them loops of field which erupt through the photosphere (stage 3) to form bipolar magnetic regions. In stage 4 advection by a combination of differential rotation and the (anisotropic) convection causes leading active regions to migrate equatorward and trailing regions to migrate poleward, creating new poloidal field of opposite polarity; the toroidal field dissipates to produce stage 5, which is like stage 1 but with the sense of the field reversed. basis of much of what is happening. Figure 1 shows the beginning of the twentieth century, had not yielded convinc- essential characteristics of these models. But there was no ing results. In fact, all observations were consistent with an precise model dynamo that could reproduce the observed unchanging Sun, and with measurement variations induced magnetic cycle as shown in Figure 2. The observed fact of by the Earth’s atmosphere through which the measurements “active longitudes” or “zones” could not be accounted for had been made Ð thus the term “solar constant” was in any model. Much progress had been made to understand believed to be a reasonable connotation for the flux of radia- the source of coronal and interplanetary fields in terms of the tion emitted by the Sun (e.g. Smith and Gottlieb 1973). We photospheric fields, but there was no understanding of the had to wait for measurements from space before variations origin of the photospheric fields, nor of their organization. in the solar constant could be measured reliably. (iv) The stability of the “solar constant,” or total irradiance was unclear. There was historical evidence of secular changes, and there was uncertainty about the variability THE STUDY OF SOLAR OSCILLATIONS with activity. The correlation of solar-activity indices with PRIOR TO 1975 northern European climate records suggested strongly that there is at least a long-term coupling of solar activity with In the summers of 1960 and 1961, Robert Leighton climate (Eddy 1976). Attempts to measure total irradiance (1961) and his two students Robert Noyes and George and its variations from the ground, under way since the Simon made observations that fundamentally altered the 1038 DOUGLAS O. GOUGH AND PHILIP H. SCHERRER

Magnetic Field Strength (gauss) –30 –10 –3 –1 –0.3 ±0.1 0.3 1 3 10 30

0 LATITUDE LATITUDE

–25

–50

–75 1996 1997 1998 1999 2000 2001 YEAR

Figure 2 Solar magnetic flux during the beginning of the present activity cycle. The strong fields in darker colors can be seen to migrate toward the equator while the weaker fields share the structure of the large-scale polarity pattern which organizes the corona. Each Carrington rotation is evident in the vertical banded structure, which is a symptom of the variation in field strength (and activity) with longitude. (Courtesy of Dave Hathaway.) study of solar physics (Leighton et al. 1962). By optical At about the same time H.A. Hill announced having seen subtraction of spectroheliograms obtained in the wings oscillations in the solar diameter measurements which were of a spectrum line they discovered both supergranulation interpreted as being global modes of oscillation (e.g. Hill and (Simon and Leighton 1964) and the 5-minute oscillations Caudell, 1979). The diagnostic potential was recognized (Noyes and Leighton 1963). Although we still do not really immediately (Christensen-Dalsgaard and Gough 1976), and understand supergranulation in detail, we do know that it helioseismology was born. Hill et al. (1991) prepared a structures and rearranges magnetic fields and plays a cru- detailed review of these early observations. cial role in the outer atmosphere. The discovery of the 5-minute oscillations spawned helioseismology and the modern study of the solar interior. HELIOSEISMOLOGY IN A NUTSHELL The spatial or temporal coverages of the early studies were limited, masking the essential nature of the oscilla- The Sun is a ball of almost fully ionized gas. Its radius is tions. The true nature of the oscillations as the superposition about 700 Mm. The nuclear reactions that heat the Sun of millions of trapped acoustic waves was at first neither occur mainly in the inner 150 Mm. Up to about 500 Mm understood nor exploited. from the center, the energy is carried outward by radiation. Frazier (1968) claimed that 5-minute solar oscillations An average photon takes about 30,000 years to traverse the were trapped acoustic modes. He suggested to Ulrich that he radiative zone; the thermal cooling time is 1,000 times carry out a calculation which resulted in the understanding longer. The outer 200 Mm is unstable to convection, and that the observations were of evanescent waves that are ves- there the energy is efficiently carried by convection up to tiges of waves propagating in the interior of the Sun and the thin layer at the surface called the photosphere from being reflected beneath the observing layer (Ulrich 1970). At where the energy is radiated into space. It takes several about the same time Leibacher and Stein (1971) reached the months for energy to get through the convection zone and same conclusion. This understanding was not fully accepted about 8 more minutes to get to the Earth. Figure 3a is a until Deubner (1975) observed the predicted ridges in power schematic view of the main features of the Sun. after making an observation with greater spatial and tempo- A number of types of waves can propagate in the Sun. ral resolution. These observations were soon followed by Acoustic waves can propagate throughout the interior. The more detailed observations by Rhodes et al. (1977). sound travel time through the Sun is about 2 hours. Acoustic 45

SAMI K. SOLANKI* AND REINER HAMMER** The solar atmosphere

As a typical star, and the only one that can be spatially in the corona. The boundary between the corona and the TR resolved by direct means, the study of the Sun has provided is often drawn at approximately 106 K. This boundary, like an insight into many of the fundamental processes taking that between chromosphere and TR, is not sharp or well place in stellar atmospheres, often at small scales. A prime defined. At still greater distances from the solar surface the example is magneto-convection or the formation of coronae temperature gradually decreases again, achieving values of and the consequent emission of copious amounts of X-rays. approximately 105 K at 1 AU (whereby electrons and ions In addition, the Sun’s apparent brightness allows measure- need not have the same temperature in the heliosphere). As ments with unprecedented accuracy. Thus the Sun is the stan- we shall see in subsequent sections, the simple plane-parallel dard against which cosmic abundances are compared. Its high representation of the solar gas outlined above is not tenable apparent brightness also means that the Sun is a strong source in any layer of the atmosphere. At any given height more at almost all wavelengths and thus detectable with simple, not than one atmospheric component is present, each having its particularly sensitive equipment such as the early instruments own temperature, density and velocity structure. flown in space. Thus for many wavelengths the Sun was the Features as diverse as granular convection cells in the first (or one of the first) cosmic source(s) detected. photosphere (Figure 1) and magnetic loops in the corona However, only the lowest layers of the Sun’s atmos- (Figure 2) are now known to structure the respective layers phere, the photosphere and chromosphere, can be regularly of the atmosphere. In addition to being spatially inhomoge- observed from the ground over the solar disk. The transition neous at almost all spatial scales, the solar atmosphere is region, corona and the solar wind are best studied from space, also highly dynamic at almost all timescales. Much of the and even many properties of the photosphere (such as the interesting physics to be learnt by studying the solar atmos- variation of solar irradiance with time) had to await space- phere is related to this structuring and dynamics and the based observations for their determination or discovery. associated heating of the chromosphere and corona. In the following we discuss the various atmospheric layers, starting with the photosphere and moving outward. Particular 1 OVERVIEW OF THE SOLAR ATMOSPHERE emphasis is placed on the contributions made by space missions to our knowledge and understanding of the solar atmos- Traditionally the atmosphere of the Sun is divided into four phere. Since these contributions are largest for the transition layers, starting with the photosphere at the bottom, moving region and corona our discussion of these layers will be more up through the chromosphere and transition region to the detailed than of the photosphere and chromosphere. Table 1 corona. The photosphere is the layer in which the tempera- summarizes the space missions mentioned in this chapter. ture drops outwards from around 5800 K at the solar surface to around 4000 K at the temperature minimum. Beyond that point it rises again, first relatively gently (forming the chro- 2 THE PHOTOSPHERE mospheric plateau), but then very rapidly in the transition region (TR). The temperature profile becomes flatter again 2.1 The plane-parallel photosphere

* Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany The solar photosphere is the layer that emits most of the ** Kiepenheuer-Institut für Sonnenphysik, Freiburg i/B, Germany solar radiative energy flux, with the emitted spectrum

80.0

70.0

60.0

50.0

40.0 Arcseconds

30.0

20.0

10.0

0.0 0.0 10.0 20.0 30.0 40.0 50.0 60.070.0 80.0 Arcseconds

Figure 1 A snapshot of a part of the solar photosphere taken with a filter centred on the G band at 430.5 nm by T. Berger and G. Scharmer. The image covers 60 000 60 000 km on the Sun. The most prominent feature is a sunspot. The much smaller dark features are pores. Also visible are granules (bright cells surrounded by dark lanes) and bright points corresponding to magnetic elements. (Courtesy of T. Berger.) having its peak in the visible (in the green part of the wave- greater heights, limb darkening implies a decrease in the length range). As such, the photosphere is the atmospheric temperature with height. Furthermore, the spectral form of layer most easily observed from the ground and conse- the limb darkening provides information on the continuum quently the one to whose investigation spacecraft have con- absorption coefficient. Such observations confirmed the tributed the least. This, however, is changing at a rapid pace, proposal by Wildt (1939) that in the visible the absorption is with the ESAÐNASA Solar and Heliospheric Observatory dominated by the H ion in spite of its low abundance (SOHO; Fleck and Domingo 1995) providing the first (Chalonge and Kourganoff 1946). glimpses of how space-based telescopes can revolutionize Traditionally the limb darkening and the shapes and our understanding of the photosphere. The next major strengths of absorption lines (Fraunhofer lines) have been highlight is expected to be provided by the JapanÐUSÐUK employed to determine the temperature stratification in the Solar B mission. solar photosphere. These diagnostics reveal that the temper- The brightness across the solar disk is not constant but ature decreases outwards in the solar photosphere from rather decreases from the centre of the disk to its edge over 6500 K at the deepest observable layers to around (the solar limb) at visible wavelengths. This is called limb 4000 K at the temperature minimum (e.g. Holweger 1967). darkening. Since at the limb the radiation is emitted at The advent of UV observations from space, in particular Figure 2 Composite of several high-resolution images taken with the Transition Region and Coronal Explorer (TRACE; Handy et al. 1999) in a spectral band near 171 Å , which is dominated by emission from eightfold ionized iron atoms (Fe IX) formed around 106 K. At these temperatures the network is no longer visible, and the disk emission is dominated by active regions, by coronal loops in the quiet corona (i.e. outside of coronal holes and active regions), and by numerous bright points. Plumes extend as ray-shaped density enhancements from the north and south polar coronal holes. (Courtesy of the TRACE team. TRACE is a mission of the Stanford–Lockheed Institute for Space Research, and part of the NASA Small Explorer program.)

Table 1 Space missions mentioned in this chapter

Mission Operation period

Stratoscope several balloon flights 1957 and 1959 OSO 4 (Orbiting Solar Observatory) 1967Ð69 OSO 6 (Orbiting Solar Observatory) 1969Ð72 Skylab 1973Ð74 Spektrostratoskop balloon flight 1975 OSO 8 (Orbiting Solar Observatory) 1975Ð78 HRTS (High Resolution Telescope and Spectrograph) rocket and shuttle flights since 1975 TRC (Transition Region Camera) rocket flights 1979 and 1980 SMM (Solar Maximum Mission) 1980Ð89 SOUP (Solar Optical Universal Polarimeter) experiment on Spacelab 2, 1985 NIXT (Normal-Incidence X-ray Telescope) rocket flights, e.g. 1993 Yohkoh since 1991 SOHO (Solar and Heliospheric Observatory) since 1995 TRACE (Transition Region And Coronal Explorer) since 1998 Solar B launch scheduled for 2005 1068 SAMI K. SOLANKI AND REINER HAMMER from Skylab (Tousey 1977), provided a new diagnostic, the found to be composed of many small magnetic elements, wavelength dependence of the continuum intensity, since at each with a diameter of the order of 100 km. shorter wavelengths the continuum radiation emanates from The major inhomogeneity in photospheric layers is higher layers (e.g. Vernazza et al. 1973, 1981). The advan- introduced by the granulation, which is the surface signa- tage of UV and EUV spectra is that they also contain emis- ture of overshooting convection. The bright granules iden- sion lines belonging to different ions that carry information tify hot upflowing gas overshooting from the convectively on the temperature in the solar chromosphere, transition unstable layers below the solar surface into the stably strati- region and corona. fied photosphere. These are surrounded by multiply con- A reliable knowledge of the thermal stratification is fun- nected cool and hence dark lanes of downflowing gas. damental for the accurate determination of elemental abun- Properties of the granulation have been deciphered using dances. The pioneering work by Russell (1929) and the data obtained with balloon-borne telescopes (with the seminal compilation by Goldberg et al. (1960) have been Stratoscope, Danielson 1961; and the Spektrostratoskop, followed by increasingly detailed and accurate determina- Mehltretter 1978), in space (Solar Optical Universal tions of the abundances of ever more elements. The current Polarimeter (SOUP), Title et al. 1989) and from the ground status of our knowledge of solar abundances (from the solar (Muller 1999). core to its corona) is discussed in the volume edited by A particular success have been detailed two- and three- Fröhlich et al. (1998), with the photospheric abundances dimensional numerical simulations, that is computations of being reviewed therein by Grevesse and Sauval (1998). On the radiation hydrodynamics under conditions correspond- the whole these abundances agree surprisingly well with ing as closely as possible to those present on the Sun, based the meteoritic values, although there are some minor devia- on a minimum of simplifying assumptions. Such simulations and some residual uncertainty. The latter is due partly tions have reproduced a wide variety of observations (e.g. to the inhomogeneity of the solar atmosphere (discussed in Nordlund 1984, Lites et al. 1989), so that they are likely to Sections 2.2 and 2.3), which has generally not been taken include the main physical ingredients necessary to describe into account when determining abundances. However, at solar granulation. Mainly, however, they have led to a better the level of accuracy currently being achieved such inho- physical understanding of solar convection and the influ- mogeneities begin to have a significant effect. ence of granulation on, for example, abundance determinations (e.g. Solanki 1998). Both observations and simulations suggest that the vertical velocity associated with granules 2.2 Convection decreases rapidly with height, while the horizontal velocity It was evident relatively early that a single atmospheric com- becomes increasingly strong, being supersonic over por- ponent cannot adequately describe the solar photosphere. tions of the largest granules. This last fact is one of the rare The dark sunspots and the bright faculae (bright structures predictions made by theory in solar physics that have been most prominent near the limb), already visible with a small subsequently confirmed by observations. telescope, highlight the need for multiple thermal compo- An oscillatory velocity component is also present in the nents. Sunspots and faculae are associated with magnetic photosphere and chromosphere. In the photosphere its power activity (Section 2.3), but even the quiet parts of the Sun peaks occur at a period of around 5 min, while in the chro- are known to be inhomogeneous since the discovery by mosphere the power peak lies near 3 min. The 5 min oscilla- William Herschel of solar granulation, bright structures tions are evanescent in the solar atmosphere, but propagate typically 1000 km in diameter separated by a dark network. in the solar interior. They are used to probe the subsurface Figure 1 shows a snapshot of solar granulation surrounding layers of the Sun (helioseismology). The amplitude of the a sunspot. On a larger scale a bright network (most promi- vertical oscillatory velocity increases with increasing height nent in radiation coming from chromospheric and transition- and dominates over the vertical granular flow field at the region layers) is also known to exist. To account for such top of the photosphere. regions with different brightness, sets of plane-parallel In addition to granulation three larger scales of convec- models have been produced (e.g. Vernazza et al. 1981, tion are known to affect the solar atmosphere, mesogranula- Fontenla et al. 1993). Again, UV spectra taken outside the tion (5Ð7 Mm in size) discovered by November et al. terrestrial atmosphere have played an important role in con- (1981), supergranulation (20Ð30 Mm) discovered by Simon structing such model families. and Leighton (1964) and giant cells (covering 40 in longi- High-resolution observations and the modelling of spec- tude and less than 10 in latitude) discovered by Beck et al. tral lines have shown that at least in the photospheric layers (1998) using Dopplergrams recorded by the Michelson it is mainly inhomogeneities at scales smaller than approxi- Doppler Interferometer (MDI) on SOHO. Granulation has mately 1000 km on the Sun that are of physical relevance. by far the most readily visible signature in the photosphere, For example, faculae, which have sizes of 104Ð105 km, are followed by supergranulation, while the influence of the 46

DAVID ALEXANDER* AND LOREN W. ACTON** The active Sun

Following the sun we left the old world Ð Christopher the late nineteenth and early twentieth centuries are a testi- Columbus mony to the dedication of early solar astronomers. The word “expedition” itself conjures up the effort involved in observ- The title of this chapter presupposes that there exists a Sun ing an eclipse since, more often than not, the eclipse was that is Non-active or Quiet. Modern research, however, is best observed from a remote and inhospitable part of the discovering more and more that there is no such thing as a planet. Despite all of the hardships, the astronomer was often perfectly placid Sun. Activity occurs on all scales at all rewarded with wonderful scientific data and occasionally an times in the solar atmosphere. As solar instrumentation important discovery. Eclipse observations provided a strong improves, the subsequent advances in our knowledge of foundation for the subsequent research into solar activity. what physics controls this continuous activity will lead to a Eclipses always yielded spectacular displays in the dark- better understanding of how stars like the Sun work. Ever ened sky, but until 1836 the coronal phenomena observed since Fabricius and Galileo first confirmed that sunspots were thought to originate on the Moon or even in the were solar phenomena, the nature of our nearest star has Earth’s atmosphere and, therefore, merited little scientific been a subject of scientific investigation. The culmination interest. The last four decades of the nineteenth century, of this line of inquiry is the present-day field of solar however, were witness to many discoveries which laid the physics, the goal of which is to investigate the constantly foundation for the study of solar activity in the solar changing Sun. Solar activity in all of its myriad forms, from corona. Eclipse data, in particular, were responsible for a cyclical variations of the whole Sun to small-scale transient number of these discoveries and significantly advanced our phenomena, is ubiquitous in solar research. Improved knowledge of the Sun and its behavior: the solar origin of instruments now demonstrate that even the quiet Sun is far prominences (1860; Pietro Secchi, Warren De La Rue), the more dynamic than once thought. The intrinsic variablity of discovery of helium (1868; Jules Janssen), the existence the Sun provides a touchstone to the intricate workings of of a chromosphere (1870; Charles Augustus Young), the stars. In this chapter we will explore the active Sun and unequivocal identification of the corona as solar (1871; examine how our knowledge of this dynamic star has blos- Jules Janssen), and the relation between sunspot cycle and somed in this Century of Space Science. corona (1878; Jules Janssen). The eclipse of 1878 was par- An important early tool in the exploration of solar activity ticularly interesting as it occurred during solar minimum was the solar eclipse. The popular attraction of eclipses and exhibited a corona with a marked equatorial extension remains strong today but their scientific value has steadily (in contrast with the nearly circular corona observed in declined since the invention of the coronagraph by French 1871). It was noted that the coronal streamers, as the exten- astronomer Bernard Lyot in 1930. The eclipse expeditions of sions off each limb were called, had a strong resemblance to magnetic lines of force and it was proposed that * Lockheed-Martin Solar and Astrophysics Lab, Palo Alto, CA, USA the Sun must, in fact, be a large magnet (Frank Bigelow ** Montana State University, Bozeman, MT, USA in 1889 and Störmer in 1911). Subsequently in 1912,

Henri Deslandres suggested that the forms and motions of The study of the Sun’s magnetic field was limited to prominences seen during solar eclipse appeared to be influ- strong-field regions, such as those of sunspots, until the enced by a solar magnetic field. The link between magnetic invention of the solar magnetograph in 1952 (see Figure 1). field and the emitting plasma on the Sun was beginning to Harold Babcock and his son Horace used the first ever solar take shape. magnetograph to discover that the magnetic field existed The key to understanding solar activity is the Sun’s ever- outside of sunspots and was, in fact, distributed across the changing magnetic field. It is now virtually certain that all whole solar surface. The solar magnetograph, refinements solar activity, and perhaps the solar atmosphere itself, is of which are now used in virtually every major solar obser- there because of solar magnetism. However, it is only in vatory in the world (there is even one currently in space), this last century that the fundamental importance of the introduced an improvement of about two orders of magni- magnetic field for solar phenomena has been realized. tude in the sensitivity of magnetic field measurements The epochal discovery of magnetic fields on the Sun by over the contemporary visual or photographic techniques. American astronomer George Ellery Hale in 1908 signalled Without these observational breakthroughs we would know the birth of modern solar physics. This realization led to very little of what turned out to be the active Sun. fundamental progress in our understanding of many physi- Solar physics research was, literally, taken to new heights cal processes occurring on the Sun and set the foundation by the advent of observations from space. The age of space for most of the solar physics advances in the modern age. astronomy has its roots in the flight of a captured World The discovery of solar magnetism arose out of Hale’s War II German V2 rocket on 10 October 1946 from White supposition that the distinctive alignments of penumbral Sands Missile Range in New Mexico. This rocket, carrying filaments in sunspots bore a remarkable resemblance to the instruments to observe for the first time the far ultraviolet iron filing patterns formed around the poles of a bar magnet. spectrum of the Sun, reached an altitude of 90 km above To test his hypothesis Hale looked for the line splittings the Earth’s surface. However, despite the increasing sophis- expected from the newly-discovered Zeeman effect: the first tication of rocket-borne experiments in the following application of this in astronomy. Using the spectrograph at decade and a half, discoveries were hampered by the very the recently completed solar tower telescope at Mount short flight durations attainable by these sounding rockets. Wilson, Hale was immediately rewarded with a clear line- In March of 1953 the National Academy of Sciences triplet indicating the presence of a strong magnetic field. appointed a National Committee to oversee US participation

Figure 1 Two of the original magnetograms of the Sun taken with the Babcock magnetograph in July 1953. (Reproduced with permission of H.W. Babcock and the American Astronomical Society.) THE ACTIVE SUN 1091 in the International Geophysical Year (IGY), a comprehen- our knowledge of solar activity and which changed the sive series of global geophysical activities set up by the nature of solar physics research permanently. In the almost International Council of Scientific Unions the previous year. three decades since the launch of Skylab many solar obser- The IGY spanned 1957Ð58 and included investigations into vatories have been placed in space, each making a step solar activity and the upper atmosphere of the Earth. In con- towards a better understanding of the Sun. The physics and nection with the upper atmosphere research, the USA under- phenomena we will discuss here depend heavily on results took to develop an orbiting satellite program. from these space missions and the legacy left by sounding The advent of orbiting satellites opened the way for con- rockets, OSO, and Skylab. tinuous solar observations at wavelengths unattainable from Much of this chapter will center on what we have the Earth. A series of eight satellites known as the Orbiting learned about the Sun through advances in our knowledge Solar Observatories provided much of the space-based solar of the magnetic field, its variability, and its interaction with observations through the 1960s, although the OSO satellites the solar plasma. While this is of intrinsic interest to the were modest in size with relatively small instruments. Solar solar physics community, the problem posed by the physics received an enormous boost in 1973 when NASA observed magnetic activity of the Sun is of fundamental launched the Skylab space-station mission (Figure 2). importance to all of astrophysics. Activity is observed in Skylab allowed long-focal-length solar telescopes to be many distant stars and galaxies. However, the proximity of flown in space for the first time and the commensurate the Sun makes it the only star whose activity can be seen improvement in observations was so great that the data are in enough detail to guide us toward an understanding. still being used today. Naturally, to do justice to a century of research would tax In terms of versatility and reliability, the performance of even the space limitations of a single dedicated volume. To the Skylab telescopes and instruments exceeded the highest confine such a task to a single chapter necessitates a more aspirations of astronomers at the time. Skylab made a num- circumspect approach. We attempt to highlight the major ber of important discoveries which significantly enhanced discoveries which have led us to where we are today in our

Figure 2 The Skylab space station seen from Skylab 4. (Reproduced with permission of Skylab, NASA.) 1092 DAVID ALEXANDER AND LOREN W. ACTON understanding of solar activity and to describe the phenomena The variability best quantified observationally is that of which best exemplify solar variability. the Sun’s total (spectrally integrated) radiative output. The The generation of the magnetic field in the interior of extremely small amplitude of this variability was unde- the Sun is the subject of a different chapter of this volume tectable until the development of radiometers of sufficient (Chapter 44) but its importance for the observed phenom- sensitivity in the 1970s. Overlapping cross-calibrated ena which have driven solar research over this last century measurements made by active cavity radiometers since provides us with a natural starting point for our discussion November 1978 compose a total irradiance database with into the activity of the Sun. The solar dynamo is the sufficient long-term precision to identify an 11-year total machine that is responsible for this magnetism and whose irradiance cycle of about 0.1% amplitude during solar most evident manifestation is the solar cycle. cycles 21 and 22 (1976Ð1986 and 1986Ð1996, respectively), in phase with solar activity. Although these variations in solar luminosity seem quite small they are of the same order THE CYCLICAL SUN of magnitude as the longer period insolation change that give rise to major ice ages. However, to date it has not been fully With orb and cycle girds the starry throng Ð Wordsworth determined whether cyclical changes in the Sun’s radiative Some two hundred years after the death of Galileo, a pro- output have a marked effect on the Earth’s climate or not. nounced quasi-regular period was discovered in the number While the spectrally integrated solar output provides the of sunspots by a German apothecary and amateur astronomer, best measure of the solar variability, specific wavelengths can Heinrich Schwabe, in 1843. This period demonstrated that provide some insight into the nature and cause of the vari- the Sun had an 11-year cycle of activity which had been ability. A prime example of this is the variation in the X-ray repeated, with a couple of important interruptions, for about irradiance over the course of a solar cycle. The Soft X-ray 250 years. It may seem surprising that it took approxi- Telescope (SXT) on board the Yohkoh spacecraft has demon- mately two centuries for this pattern to become apparent. strated a variation by as much as a factor of 40Ð50 from solar However, it should be pointed out that the cyclical pattern maximum to solar minimum in the wavelength range 3Ð60 A,û in the sunspot behavior is most clearly recognizable after which is to be compared with that seen at shorter wavelengths considerable averaging. Indeed, the oft-quoted cycle of by the (Geostationary Operational Environmental Satellite about 11 years is far from regular, in either duration or (GOES) series of satellites (1Ð8 A)û of about a factor of form. Lengths of cycles vary from a minimum of about 8 to 80Ð100. The advantage of modern grazing (and, more a maximum of about 15 years, the long-term average being recently, normal) incidence optics at X-ray wavelengths is about 11.1 years. While the solar cycle is a discovery of that the imaging allows us to determine the role played by the nineteenth century, it has had a profound effect on the different solar phenomena in the solar variability. The SXT solar physics of the twentieth century, driving much of the has shown that the whole corona participates in the solar advances in our current understanding of the Sun. cycle not just the most active parts (see Figures 3 and 4). Essentially all solar phenomena exhibit 11-year cycles, The solar activity cycle is variable in several aspects. One including radiative outputs, particle and plasma emissions, obvious signature of this variability has shown up in the last interior oscillations, and perhaps even fundamental processes century. The most recent series of solar maxima, spanning in the Sun’s nuclear burning core. Diverse solar parameters 30Ð40 years, exhibited the largest minimum to maximum such as the integrated X-ray emission, UV irradiance, and amplitudes in the 400-year record, suggesting that the overall solar radio flux record this variability over wide ranging activity level is at a historical high. (At the time of writing, spectral and temporal scales. Like the sunspot number, the Cycle 23 appears to have entered its declining phase after total radiative output and the entire solar spectrum exhibit peaking in March 2001 with a maximum which was signifi- pronounced quasi-11-year and 27-day cycles, linking all cantly lower than early predictions.) There are some rules, aspects of solar radiation variability to a common source: the however, that apply to the vast majority of sunspots, demon- magnetic activity of the Sun. The 27-day period corresponds strating large-scale patterns in the activity cycle, which to solar rotation. Detailed investigation of this activity led reveal fundamental properties of the magnetic structure Hale to discover, in 1913, that a predominance of major spot below the atmosphere. Studies of the active Sun can there- groups exhibited a very distinctive pattern. He found that fore complement the recent advances in helio-seismology the preceding and following polarities were opposite in the (see Chapter 44) by providing important clues to the working northern and southern hemispheres and, more importantly, of the solar dynamo. that this pattern was reversed from one 11-year cycle to the Understanding the solar dynamo and its observational next. This ultimately led Hale to conclude (with Seth Barnes effects is now a full-time occupation for an increasing num- Nicholson some 25 years later) that the magnetic cycle of the ber of solar scientists. A network of ground stations (called Sun must have a duration of 22 years. GONG, the Global Oscillation Network Group) and a 47

MARCIA NEUGEBAUER* AND RUDOLF VON STEIGER** The solar wind

PARKER’S THEORY AND THE EARLY consisting of plasma thermally escaping from the corona. MEASUREMENTS OF THE SOLAR WIND Many of the early space investigations therefore included attempts to determine whether interplanetary space was filled 1 Shortly before the beginning of the space age, Eugene with Parker’s supersonic 500 km s solar wind or with 1 N. Parker of the University of Chicago predicted that inter- Chamberlain’s subsonic 10 km s solar breeze. A summary planetary space would be filled with a plasma flowing of those early missions and experiments is given in Table 1. rapidly outward from the Sun (Parker 1958). The likelihood Not surprisingly, the Soviets were the first in space with that the Sun ejects charged particles that cause auroral and instruments capable of measuring the interplanetary magnetic activity on Earth was generally accepted by that plasma. Their “ion traps” were simple Faraday cups with an time. The observation that the plasma tails of active comets inner grid held at 200 V to repel interplanetary electrons always point almost radially away from the Sun led Ludwig and to prevent the escape of photoelectrons from the cup Biermann (1951) to postulate that the solar corpuscular and an outer grid at a positive potential to define the mini- radiation is continuous, rather than intermittent. It was mum energy of the ions entering the cup. Lunik 2 was the also known that the outer atmosphere of the Sun, the solar most successful of four missions, determining that there 8 2 1 corona, was extremely hot, with a temperature exceeding a was indeed a flux of 2 10 cm s of positive ions million degrees. Sidney Chapman (1957) calculated that if with energy/charge 15 eV/charge (Gringauz et al. 1960). the corona was in hydrostatic equilibrium, it must extend Because the speed of a proton with energy 15 eV is 1 throughout the Solar System and cool off to only 2 105 K 53 km s , the Lunik 2 measurements favored Parker’s at the orbit of Earth. Parker (1958) put all these ideas theory over Chamberlain’s, but questions of the extent to together, explaining that the inward pressure of the interstel- which the speed exceeded that limit, the direction of the lar medium was too weak to allow the solar atmosphere to flow, and its persistence were left unanswered. be in hydrostatic equilibrium. He coined the phrase “solar With the Explorer 10 mission in 1961, a group from the wind” to describe the outward flowing solar corona which Massachusetts Institute of Technology (MIT) made the first supplies the pressure required to stand off the local interstel- US measurements of the solar wind (Bonetti et al. 1963). lar medium, to exert the necessary force on cometary plasma Their instrument was an advance over the Soviet ion traps tails, and to transmit solar disturbances to the geomagnetic in that it had an additional grid which carried a positive field. For a more complete theoretical explanation of Parker’s square-wave potential to allow measurement of the ion prediction of the solar wind, see Chapter 9. energy spectrum without confusion between the flux of Parker’s theoretical prediction was not uncontested, how- ions entering the detector (an AC signal) and the flux ever. Most notably, Joseph Chamberlain (1960) proposed that of photoelectrons knocked out of the negative inner grid rather than Parker’s solar wind caused by the hydrodynamic (approximately a DC signal). Before the spacecraft batter- outflow of the solar corona, there was merely a solar breeze, ies died at a distance of 34 Earth radii, the instrument measured an intermittent flux of ions from a direction * Jet Propulsion Laboratory, Pasadena, CA, USA within a 20 by 80 window which included the direction ** International Space Science Institute, Bern, Switzerland from the Sun. When the ions (assumed to be protons) were

Table 1 The earliest attempts (1959–62) to observe the solar wind

Launch date Spacecraft Institution Instrument* Result

2 January 1959 Lunik 1 USSR Four ion traps No publishable data 10 to 15 V

12 September 1959 Lunik 2 USSR Four ion traps 39Ð60 RE 10 to 15 V Flux 15 eV 2 108 cm2 s1 4 October 1959 Lunik 3 USSR Four ion traps One observation of flux 20 eV 4 108 cm2 s1 19 to 25 V Other data threshold (108 cm2 s1) 12 February 1961 Venus Probe USSR Ion traps Very intermittent data 0 and 50 V One observation of flux 4 108 cm2 s1 25 March 1961 Explorer 10 MIT Modulated FC Skimmed magnetopause flank Consistent with flow from Sun Measured n, v, T Supersonic and super-Alfvénic 16 August 1961 Explorer 12 NASA Ames CPA Dayside magnetosheath Did not detect any ions 22 August 1961 Ranger 1 JPL 6 CPAs Failed to get out of parking orbit 18 November 1961 Ranger 2 JPL 6 CPAs Failed to get out of parking orbit 22 July 1962 Mariner 1 JPL CPA Destroyed by range safety 27 August 1962 Mariner 2 JPL CPA 113 days of data Continuous radial flow High-, low-speed streams n, v, T relations v vp; n/np variable; T 4 Tp 2 October 1962 Explorer 14 NASA Ames CPA Mostly magnetosheath UV interference

* FC is Faraday cup and CPA is curved-plate analyzer.

detected, their flux was in the range 1.0Ð2.5 108 cm2 s1, orbit, while the third spacecraft, Mariner 1, went astray and their speed was 280 km s1, and their flow was super- was destroyed by ground command. Finally, after some sonic, as predicted by Parker’s theory. In retrospect, the ion hair-raising misadventures (Neugebauer 1997), Mariner 2 fluxes detected by Explorer 10 were not in the solar wind was safely placed on a trajectory to Venus. The Mariner 2 proper, but downstream of the Earth’s bow shock, in the instrument was a curved-plate analyzer which measured the flank of the magnetosheath. ion current reaching a collector at each of 10 voltages on A group at the NASA Ames Research Center attempted to the deflection electrodes. Mariner 2 obtained a spectrum of measure the solar wind with instruments on Explorers the solar wind every 3.7 minutes almost continuously for 12 and 14 in 1961 and 1962 (Bader 1962, Wolfe and Silva 113 days. There was no longer any doubt that Parker had 1965). These instruments were curved plate analyzers with a been correct; the solar wind exists. voltage applied perpendicular to the ions’ direction of motion Although the ion spectra obtained by Mariner 2 were to bend their trajectories onto a detector. On Explorer 12 very crude by today’s standards, with measurable currents in there was a problem that the field of view of the instrument no more than five energy/charge channels at any time, a lot did not include the solar direction, and on Explorer 14 there of information about the properties of the solar wind could was a problem with contamination of the ion signal by solar be gleaned from the data (Neugebauer and Snyder 1966). ultraviolet radiation when the instrument did face the Sun. The solar wind blew continuously from within a 10 cone Furthermore, on both missions, the spacecraft trajectories centered on the Sun. The wind was organized into low- and were almost entirely downstream of the bow shock. high-speed streams (velocities of 350 and 700 km s1, About the same time as the unsuccessful attempts by respectively), each of about 7 days’ duration. The speed Bader and Wolfe at NASA Ames, one of the authors (MN) versus time profiles were steepened on the leading edges of and her colleague, Conway W. Snyder, flew solar wind the fast streams where the increased density indicated a detectors on four different missions. The first two of those snowplow effect. The proton temperature varied directly spacecraft, Rangers 1 and 2, failed to get out of low-Earth with the speed. These features are illustrated in Figure 1, THE SOLAR WIND 1117

Figure 1 Three-hour averages of solar wind speed (bottom line) and proton temperature with upper and lower limit bars (top) observed by Mariner 2 in 1962. (From Neugebauer and Snyder 1966.) which is a plot of three-hour averages of the solar wind field (Parker 1958). Because of the very high electrical con- speed and temperature over five 27-day rotations of the Sun. ductivity of the solar corona, the plasma and the magnetic The pattern roughly repeated from one rotation to the next. field must move together. That is, the solar field is frozen On average, the ion flux and density varied as the inverse into the solar wind. But at the same time that the field is square of heliocentric distance between 1.0 and 0.7 AU. being dragged nearly radially into space by the solar wind, It was often possible to detect a second spectral peak it is still tied to the rotating Sun, with the result that the which was interpreted as being caused by alpha particles interplanetary field should have a spiral pattern with an (helium nuclei) moving with approximately the same speed angle to the radial direction of 45 near 1 AU. The pre- as the protons. This second peak could not, however, be fit dicted spiral pattern of the field could be discerned in the to a model in which the alpha particles had the same tem- data of the magnetometer on Mariner 2; this is illustrated in perature as the protons; instead, equal thermal speeds were Figure 2, where each point represents a running average of indicated. The abundance of the alpha particles relative to five-hour averages. Although there is a great deal of scatter, the protons was sometimes highly variable from day to day. the points are distributed in the quadrants predicted by the Parker predicted not only the existence of the solar wind, Parker spiral model. The properties of the fluctuations but also the configuration of the interplanetary magnetic about the spiral direction continue to be studied intensively 1118 MARCIA NEUGEBAUER AND RUDOLF VON STEIGER

the solar wind are conveniently divided into recurrent or quasi-stationary streams and transient flows. The discovery of a 27-day (the synodic period of solar rotation) modulation of cosmic rays by Forbush in 1938 was conclusively traced to dynamical phenomena in the interplanetary medium and related to recurring coronal “active regions” (in the terminology of those days) by Simpson in 1954 (Simpson 1998). As shown in Figure 1, such recurrent structure was indeed found in interplanetary space in the form of alternating high- and low-speed streams, each lasting several days. The polarity of the interplanetary magnetic field tended to remain constant throughout each of the high-speed streams, with consecutive streams having opposite polarities. It is important to note that there is not a one-to-one correspondence between fast streams and magnetic sectors. There need not be a fast stream within every magnetic sector, and the position of a fast stream relative to its magnetic sector boundary does not remain fixed in interplanetary space. The leading edge Figure 2. Five-hour sliding averages of hourly averages of of each fast stream, where the solar wind speed increases, the radial (B ) and in-ecliptic transverse (B ) components Z Y is now commonly called a corotating interaction region of the interplanetary magnetic field measured by Mariner 2. The dashed line shows the expected relation for the Parker (CIR). Such interaction regions are an inevitable conse- spiral. (From Smith 1964.) quence if streams of sufficiently different speeds are emitted from the Sun at the same heliographic latitude. The effect of solar rotation is to eventually ram fast solar wind to reveal some of the fundamental processes occurring in into slower wind emitted from more westerly heliographic the solar wind. A change in the direction of the interplan- longitudes. Figure 3 shows an early schematic and a newer etary field from the first to the third quadrant in Figure 2, version of this scenario. The newer version also shows or the reverse, indicates a reversal of the polarity of the how the CIR develops in interplanetary space to engulf the interplanetary field with the field sometimes pointing in magnetic sector boundary. toward and sometimes pointing out from the Sun. Week- As a general rule, two magnetized plasmas cannot inter- long periods of persistent polarity were named “magnetic mix without the benefit of magnetic reconnection or other sectors” by Wilcox and Ness (1965). types of plasma instability. Therefore, the fast and the slow solar wind streams remain separated out to large heliographic distances. Discontinuities separating the two wind MORPHOLOGY types were first studied by Belcher and Davis (1971) using Mariner-5 data.1 Burlaga (1974) introduced the term The solar wind has probably been blowing for at least the “stream interface” for this boundary which is characterized past 3 109 years with essentially the same strength, as can by a decrease in density by a factor of 2, accompanied by be estimated by comparing the flux of xenon ions in today’s a similar increase in kinetic temperature. Sometimes, in solar wind with that deduced from the xenon content of the order to enhance the signal, these two signatures are conve- lunar regolith (Geiss 1973). Observations of comet tails niently combined into the specific entropy argument, T/n1/2, reveal that the solar wind did not stop blowing even during where T is temperature and n is density. As the solar the Maunder minimum, from about 1645 to 1715 when wind expands to 1 AU and beyond, the stream interaction there were essentially no sunspots. becomes progressively more pronounced. The leading It thus seems that the solar wind is a ubiquitous and (slow) plasma becomes accelerated and the trailing (fast) continuous phenomenon, but it is not a structureless one. Its plasma becomes decelerated, building up hydromagnetic density, speed, temperature, ion charge states, elemental stresses which ultimately lead to the development of a composition, and other properties all vary with time and pair of interplanetary shocks, a forward shock at the leading position on timescales from minutes (or less, but knowledge edge of the CIR and a reverse shock at the trailing edge. of fast fluctuations is limited by the typical time resolution of today’s ion sensors) up to decades (or more, limited by the 1 Mariner 5: a NASA Venus flyby mission launched on 14 June 1967 short duration of the space era). The large-scale structures of and operated to 21 November 1967. 48

ANDRÉ BALOGH* AND LENNARD A. FISK** The heliosphere

The heliosphere is the vast region of space surrounding the on cosmic rays in the heliosphere (Fisk et al. 1998), and Sun and the Solar System that is filled with the solar wind. on corotating interaction regions (Balogh et al. 1999). Its inner boundary is the outer atmosphere of the Sun, the A volume that gathered his many contributions to helios- solar corona. Its outer boundary is defined, to a first approx- pheric physics was published by Burlaga (1995). The outer imation, as a surface across which there is a balance of heliosphere has been the subject of a volume edited by pressure between the solar wind and the local interstellar Grzedzielski and Page (1990). In addition, there have been, medium (LISM). This outer boundary is to be found at prob- over the past 15 years, numerous review papers on aspects ably about two and a half to three times the orbital distance of the heliosphere; these, together with many original refer- of the furthest planet, Pluto. The main regions of the helios- ences are listed in this chapter. phere, described in this chapter, are illustrated schematically in Figure 1, although there are many uncertainties concerning this simple picture. The objective of this chapter is to THE EXISTENCE OF THE HELIOSPHERE AND provide an overview of the heliosphere as understood, at the ITS MAIN PROPERTIES beginning of the twenty-first century, complete with our very comprehensive understanding of the solar wind and its large- The existence of the heliosphere, as a volume of space scale dynamic structures that shape the heliosphere. Together controlled by the Sun, was first proposed by Davis (1955), with the development of solar wind studies, the study of the who based this concept on the modulation of cosmic rays in propagation of cosmic rays in the heliosphere has also been anti-phase with the solar activity cycle. Cosmic rays are of great importance in shaping our understanding. These very high-energy particles which are accelerated (by vast topics are reviewed at some length. The space missions that shockwaves generated by supernova explosions) in the particularly contributed to the exploration of the heliosphere Galaxy, at considerable distances from the Solar System. are also summarized. The intensity of cosmic rays observed at the Earth is nearly The many topics related to the heliosphere have been constant, but is a few percent higher at sunspot minimum the subject of several books, as well as numerous review than at sunspot maximum. This 11-year modulation cycle articles. Of particular interest and relevance are the books can only be explained by assuming that the sunspot cycle edited by Schwenn and Marsch (1990, 1991) on the results has an effect on the space surrounding the Sun which, in of the Helios mission in the inner heliosphere; four books turn, affects the propagation of cosmic rays into the inner related to the three-dimensional heliosphere and the Ulysses Solar System, so that it is easier for cosmic rays to reach mission (Marsden 1986, 1995, 2001; Balogh et al. 2001); a the Earth near solar minimum than around solar maximum. volume on cosmic winds and the heliosphere (Jokipii et al. This explanation therefore implies that the medium sur- 1997); three volumes published under the imprint of the rounding the Sun undergoes changes between solar maxi- International Space Science Institute, Bern, on the interstel- mum and minimum. Given that these changes affect the lar medium and the heliosphere (von Steiger et al. 1996), propagation of high-energy cosmic rays (which are electrically charged particles), the changes in the medium around * Imperial College, London, United Kingdom the Sun need to involve magnetic fields and, in effect, ** University of Michigan, Ann Arbor, MI, USA changes in the magnetic field itself, in response to the solar

~200 TO 300 AU shown. The historical background to the early cosmic-ray modulation studies has been reviewed recently by Simpson (1998) and McDonald (2000). It is remarkable that the INTERSTELLAR WIND recognition of the relationship came so early, in the mid- HELIOPAUSE 1950s, even before it had been observed for a complete sunspot cycle. There is also a third quantity plotted in Figure SOLAR 2; this quantity, labelled HCS in the figure, is the calculated WIND maximum heliolatitude of the heliospheric current sheet, in TERMINATION effect the physical extension of the Sun’s magnetic equator SHOCK into the heliosphere. The nature and importance of the HCS in structuring the heliosphere are described below. However, its is clear from the close match of this quantity to the HELIOSPHERIC sunspot numbers (once the two are superimposed, as in the BOW SHOCK (?) figure) that the variation in the heliolatitude of the Sun’s magnetic equator through the solar cycle is an important indicator of solar activity and, given the need for a physical Figure 1 A schematic view of the heliosphere and its description of the solar modulation of cosmic rays, the exten- boundaries. sion of the magnetic equator into the heliosphere is at least a part of the physical mechanism of the modulation process. The necessary advance in understanding the magnetized 4500 Cosmicrays medium surrounding the Sun assumed by Davis (1955) came about at the same time, from the recognition that the 4000 Sun continually emits a ‘corpuscular’ radiation. The existence of such a radiation, to explain the geomagnetic effects that could be related to activity on the Sun, had been 3500 assumed much earlier by Chapman (see Chapman and ° Sunspots 90 200 Bartels 1940, and references therein). The existence of the HCS Cosmic ray flux ++++ ++++ ++ solar wind, as a constant outflow of plasma from the solar 3000 ++++ +++++ ++ ++++ +++++++ + 150 ° + ++++ ++++++ ++ corona was first suggested by Biermann (see Biermann, 60 + ++++ ++ + Heliolatitude + ++++ + + + + ++++ + + + 1957, and references therein) from the study of the ion tails + ++++ + + + 100 + +++++ ++ + + + +++ ++++ +++ of comets. The current understanding of the reason for the +++ ++ ++ +++++ + ° + + +++ + Sunspot numbers 30 ++ + ++ ++++++ existence of the solar wind as a permanent outflow of super- ++ + ++ + ++++++ 50 +++ ++++++ +++++ +++ ++ ++++++ sonic plasma originated with Parker’s theoretical model +++++ +++ 0 (Parker 1958, 1963). Within four years of Parker’s first theo- 1950 1960 1970 1980 1990 2000 retical predictions, the existence of the solar wind was confirmed by the early interplanetary space missions (for a Figure 2 A composite plot showing the monthly number of summary of the early results, see e.g. Lüst 1967). This con- sunspots (black trace), the daily average intensity of cosmic firmation (that included the charting of the interplanetary rays measured on the ground (blue trace) and the maximum heliolatitude of the coronal magnetic neutral line, represent- magnetic fields necessary for affecting the propagation of ing the coronal imprint of the HCS (red symbols). There is cosmic rays) finally placed on a firm foundation the concept clearly a very close correspondence between sunspots and of the heliosphere (e.g. Axford 1972) and opened the way the extension of the coronal magnetic neutral line; the anti- for its exploration in the space age, from the early 1960s. correlation between cosmic-ray intensity and sunspot num- The phenomenology of the heliosphere covers numerous bers is indicative of the modulation of galactic cosmic rays by space plasma phenomena on all scales, from the kinetic solar activity in the heliosphere. plasma scale where the dynamics of particle distributions dominate the physical processes, to the large magnetohy- activity cycle. Given also the large energies of cosmic-ray drodynamic (MHD) scales where the large-scale dynamics particles that are affected, the volume involved in the mod- of the solar wind and the magnetic field define the global ulation also has to be large on the scale of interplanetary properties of the heliosphere. Space plasma processes on distances. many intermediate scales provide a constantly changing, It is this relationship between cosmic-ray fluxes and dynamic environment in the different regions of the sunspots that is illustrated in Figure 2, in which 50 years heliosphere. The relative accessibility of the heliosphere to of observations, covering close to five solar cycles, are direct observations by deep-space probes has made it the THE HELIOSPHERE 1143 largest astrophysical plasma regime that can be studied in between the observed structures and their solar origin rec- detail. Heliospheric physics is now a discipline in its own ognizable only on the largest temporal and spatial scales. right, with obvious and important links to solar physics, There is, however, another equally important way to space plasma physics and, by extrapolation, with many top- divide the heliosphere. For as much as five or six years ics in astrophysics in general. around solar minimum in each solar cycle, the polar regions The size of the heliosphere is not yet known precisely. of the heliosphere have significantly different properties The outer boundary is currently estimated to be somewhere from the equatorial region. Relatively uniform, high-speed between 80 and 120 AU from the Sun. Both the size of the solar wind fills both polar regions, extending down to heliosphere and the nature of the outer boundary depend on between 20 or 30 degrees within the equator. In the equator- the properties of the solar wind at large distances from the ial region, fast and slow solar wind streams intermingle and Sun, on the properties of the LISM and on the physical interact, making this region more structured and dynamic: it processes involved in their interaction. Even though these is this region which can be conveniently subdivided into the properties are not known in sufficient detail to draw defini- three regions, inner, middle and outer heliospheric regions tive conclusions on either the size of the heliosphere or the described above. Due to the absence of direct observations nature of the boundary, there are several estimates based on in the polar regions at different heliocentric distances, its indirect observations which, surprisingly, agree quite well structure and evolution with heliocentric distance can only with each other. Such indirect evidence has also made it be extrapolated from the unique set of observations made at possible to introduce useful models that can be tested as high heliolatitudes by the Ulysses spacecraft at distances of further observations become available. about 2 AU. A great deal is known about the heliosphere from the Sun For several years in each 11-year solar activity cycle, out to the distance of the furthest direct, in situ observa- around maximum activity, the heliosphere becomes consid- tions, currently to 74 AU by the Voyager 2 spacecraft. The erably more complex than around solar minimum. This is in situ observations, made since the early 1960s by deep- due in part to the fragmentation of solar wind streams and space missions as well as by the continuous monitoring of in part to the considerable increase in the occurrence rate the solar wind in the vicinity of the Earth, have provided a and intensity of solar transients, in particular coronal mass vast, even if in some respects selective database for deter- ejections (CMEs). Instead of the relatively simple distribu- mining the structure and dynamics of the inner and middle tion of the solar wind sources in the corona at solar mini- heliosphere. At the same time, progress in understanding mum when they are divided into large-scale coronal holes the physics of the solar corona, the inner boundary of the emitting fast solar wind and the equatorial belt of slow heliosphere, has led to a good understanding of the impor- solar wind, non-uniform but mostly slow solar wind is tant relationships between solar and heliospheric phenom- emitted from the whole corona at solar maximum. Short- ena. Space missions have also played the major role in lived, small-scale coronal holes still emit fast solar wind, increased understanding of the complex dynamics of solar but at lower speeds than wind from the large polar coronal and coronal phenomena relevant to heliospheric physics. holes near solar minimum. Superimposed on the slow solar The role of space missions in exploring the heliosphere wind from the corona, CMEs occur not only more fre- has led to phenomenological descriptions that emphasize quently than at solar minimum, but are distributed more or the dependence of heliospheric phenomena on distance less evenly at all latitudes. Until Ulysses explores the polar from the Sun. Although any division of the heliosphere into regions of the heliosphere during the current solar maxi- different regions is somewhat arbitrary, the inner, the mid- mum activity period in 2000Ð02, it is possible only to dle and the outer heliospheric regions have been distin- extrapolate the near-ecliptic conditions to those regions. guished, based mostly on the dominant dynamic features The terms used in this chapter, referring to the helios- observed. pheric medium and the heliospheric magnetic field, intend The inner heliosphere is the region from the solar corona to underline the three-dimensional nature of the heliosphere to about the orbit of the Earth. In this region the coronal and its phenomena. Historically, the terms ‘interplanetary sources of the different solar wind streams recognizably medium’ and ‘interplanetary magnetic field’ have been dominate the dynamics of the medium. The middle helios- extensively used to describe phenomena in the space phere, from about the Earth’s orbit to Saturn’s orbit at between the orbits of the planets. This use of the terms was 10 AU, is the region where the dynamic evolution of solar justified as most of the space missions that made observa- wind structures forms large-scale structures that begin to tions in the solar wind remained in orbits restricted to close mask the solar origin of the different solar wind streams. In to the ecliptic plane, the Earth’s orbital plane. The main the outer heliosphere, the structures formed in the middle impetus for change came from the Ulysses mission, heliosphere continue to evolve, but dissipative processes described below, with its objective to explore the three- and the intrusion of material from the LISM make the link dimensional properties of the heliosphere in its unique, 1144 ANDRÉ BALOGH AND LENNARD A. FISK polar orbit around the Sun. However, other missions, The first space mission that provided a constant monitor- Pioneer 11 first, when in transit between Jupiter and Saturn, ing of the solar wind, the heliospheric magnetic field, as and, later on, the two Voyager spacecraft left the ecliptic well as energetic charged particles and cosmic rays in the plane, following their final planetary encounters, reaching vicinity of the Earth, yet removed from the influence of ter- now far beyond the furthest planets in the Solar System. It restrial effects, was the International Sun-Earth Explorer has become therefore more appropriate to use the “helios- (ISEE 3) mission. This was the first spacecraft to be pheric” terminology when describing phenomena in the launched to orbit one of the SunÐEarth system’s gravita- three-dimensional volume around the Sun. tional ‘neutral’ points, named after Lagrange. The Lagrange point L1 is situated at about 1% of the distance (or about 1,500,000 km) between the Sun and the Earth, ahead of the THE EXPLORATION OF THE HELIOSPHERE Earth. An orbit around this point in space allows the spacecraft to monitor the solar wind, and even to provide a warn- Space probes have played a key role in the exploration of ing of 30 to 60 minutes if a solar storm (in effect a CME) is the heliosphere. Since the early 1960s, numerous missions about to hit the Earth’s magnetosphere. The ISEE 3 space- have explored different regions in the vicinity of the Earth; craft, launched on 12 August 1978, remained in this orbit in the distant, outer heliosphere; between the Sun and the for four years, through the solar maximum in 1979Ð80, Earth; and above the poles of the Sun. In the following, a before becoming the prototype of space wanderers by being few key missions that contributed in a significant way to retargeted first to the Earth’s distant magnetospheric tail our knowledge of the properties and structures of the and then, following some spectacular manoeuvres, to the heliospheric medium are described, highlighting their con- first spacecraft encounter with a comet in 1985. tributions to our knowledge of the heliosphere. The L1 orbit, first used by ISEE 3, has been used The Soviet space missions to the Moon (Lunik 1 and 2) very successfully, more recently (since the mid-1990s) by carried plasma detectors that first observed the solar wind. the Solar and Heliospheric Observatory (SOHO) and the However, the first extensive set of data confirming the exis- Advanced Charge Composition (ACE) spacecraft. SOHO tence of the solar wind and its main properties, such as its was launched to provide a comprehensive and often spec- velocity, density, temperature, as well as its variability came tacular monitoring of the Sun and its corona, and ACE is from the Mariner 2 mission to Venus in 1962, launched on following up, two solar cycles after ISEE 3, and with a 27 August 1962 (Neugebauer and Snyder 1962). Some of more up-to-date instrumentation, the monitoring of the the properties of the embedded magnetic field were also solar wind and its magnetic field. observed on this mission (Coleman et al. 1962), in particular The Pioneer series of missions, from the early 1960s, the general agreement of the orientation of the magnetic field have explored the region of space between the planets Mars with Parker’s predicted spiral geometry (Davis et al. 1966). and Venus, as well as the distant heliosphere past Jupiter In the 1960s, early Earth-orbiting spacecraft, with and Saturn. The first spacecraft to the outer planets were apogees reaching beyond the magnetosphere, provided the Pioneer 10 and 11 missions. Pioneer 10 was launched further evidence concerning many of the basic properties on 2 March 1972; it reached Jupiter on 3 December 1973 to of the solar wind and the magnetic field embedded in it. All make the first close-up observations of the giant planet. the most important phenomena that shape and characterize After its encounter with Jupiter, the spacecraft followed a the heliospheric medium were first noted by Earth-orbiting trajectory to the outer reaches of the heliosphere. Pioneer 11 spacecraft. In particular, good agreement was found in was launched on 5 April 1973 and it reached Jupiter on general between the Archimedean spiral structure of the 2 December 1974. At Jupiter, the spacecraft was targeted in magnetic field lines proposed by Parker (1958, 1963) and such a way that the flyby would allow it to reach Saturn. the observed orientation of the magnetic field (Davis et al. This involved a trajectory that, for the first time, reached 1966; Ness and Wilcox 1964) on the Interplanetary a heliolatitude of 16 above the ecliptic plane. The two Monitoring Platform (IMP 1) mission. On the same mis- missions were terminated in March 1997 and November sion, the sector structure of the magnetic field, showing a 1995, respectively. At the time when the missions ended, recurring pattern of alternating polarities of the field as a Pioneer 10 was at a heliocentric distance of 67 AU, while function of solar longitude, was also identified (Wilcox and Pioneer 11 was at 43 AU. Ness 1964). A later spacecraft in the same series, the These two spacecraft were the first to make detailed remarkable IMP 8, launched in 1974, has continued to pro- observations of the heliospheric medium out to Saturn and vide observations in the solar wind for nearly 30 years. beyond. The first phase of the Pioneer mission took place These observations now constitute a historic data set that around the minimum activity period in solar cycle 21, in the is used for studying long-term trends in solar wind and mid-1970s. They were the first to observe the recurring cosmic ray parameters. sequence of corotating interaction regions (CIRs) and their 49

EBERHARD GRÜN* The dusty heliosphere

BEGINNINGS examined. Up to now only a handful of meteorite falls have been observed with sufficient accuracy that an orbit could be Zodiacal light observations and meteor studies established. All these cases were ordinary stony meteorites which had orbits with aphelia in the asteroid belt. Dust in space has been recognized for centuries. Three Because of their exotic nature, meteorites belong to the dusty phenomena in space can be observed by the naked best-analysed samples on Earth. While some resemble ordi- eye: comets, meteors and zodiacal light. In the past the spo- nary stones, others consist of almost pure ironÐnickel and radic appearance of bright comets received much attention, a third type consists of a mixture of elements that is and not only from astronomers, because it was linked to believed to be the mean composition (cosmic abundance) of exceptional events in human history such as victory or the whole planetary system except for some volatile ele- defeat in war. The physical nature of comets was illumi- ments, like hydrogen and helium. The latter meteorites nated by the second dusty phenomenon: meteors. Although (carbonaceous chondrites) are considered to consist of several meteors can be observed every clear night, there are primitive material that has not been modified by thermal special periods, so called meteor showers or meteor storms, processes like those that occur in the interior of planets. when the rate is greatly enhanced. It was the coincidence Meteoritic material has been used as a model for the mater- between such meteor showers and the simultaneous appari- ial of which interplanetary dust particles are made. tions of comets that suggested their relationship. From measurements of the entry trajectory and models Triangulation already proved two hundred years ago that of the upper atmosphere it became clear that meteoroids meteors are the luminous trail of pea-sized meteoroids that experience quite variable decelerations. While some slowed enter the Earth’s atmosphere at about a height of 100 km. The within a short time, others kept their speed for much longer speed of meteoroids was determined to range from 11 km s1, 1 times. The concept of loosely bound aggregate particles the escape speed from the Earth, to 72 km s which is the with large cross-section/mass ratios (i.e. low bulk density) maximum collision speed with the Earth that a meteoroid can was developed for those that decelerated rapidly; others have on a bound orbit about the Sun. This speed range clearly behaved more like normal compact stones and even iron identified meteoroids as members of the Solar System. The particles. Meteor physicists like Zdenek Ceplecha (1977) scattering of radio waves by the ionization trail in the atmos- found a correlation between meteoroid densities and their phere allows us to observe meteors even during the day. orbits; namely, some meteoroids that have eccentric, comet- Such radar meteoroids can be as small as 0.1 mm in size. like orbits consist of low-density material, while higher- Football-sized and bigger meteoroids may survive the entry density meteoroids have orbits that just reached the into the atmosphere, and some residual material may fall asteroid belt. to the ground as meteorites, which can be picked up and The asteroid belt has always been suspected to be a source of meteoroids. At the beginning of the twentieth century, * Max-Planck-Institut für Kernphysik, Heidelberg, Germany and Max Wolf detected from the Sternwarte above Heidelberg University of Hawaii, Honolulu, HI, USA such a large number of asteroids that he had a hard time to

find names for all of them. Since then the number of aster- International exchange of latest results in dust research took oids for which precise orbits are known has grown to over place at COSPAR (Committee on Space Research) confer- 20,000. Most asteroids have orbits between Mars and Jupiter. ences. It was Curt Hemenway, from the Dudley Observatory In 1918 Kiyotsugu Hirayama suggested that asteroids with in Albany, USA, who founded the Cosmic Dust Panel within very similar orbits form families that have been generated by COSPAR. At the annual (and, later, biannual) meetings, the break-up of a larger asteroid in a recent (about 100 mil- researchers from around the world met and exchanged their lion years ago) catastrophic collision. Julius Dohnanyi (1970) results and ideas on dust in space. Initially, astronomers met developed a collision model of asteroids in which smaller in parallel at meetings of the International Astronomical asteroids can break up bigger ones. In each collision a flood Union (IAU). Soon it was recognized that it was mutually of smaller fragments is generated. He showed that the size advantageous to bring together these partially overlapping distribution of observed asteroids is compatible with a popu- communities of scientists. In 1967 Jerry Weinberg organized lation of objects in collisional equilibrium. In these collisions the first international dust meeting at the University of a large amount of dust is generated. Hawaii in Honolulu, USA, that produced a significant pro- The third dusty phenomenon is the zodiacal light, visible ceedings volume (Weinberg 1967). After that, specialized to the naked eye in the morning and evening sky in areas free international dust meetings took place every four to five of light pollution. As early as 1683, Cassini presented the cor- years under the sponsorship of IAU and COSPAR. The first rect explanation of this phenomenon: it is sunlight scattered of these meetings, IAU Colloquium No. 13, was organized by dust particles orbiting the Sun. A relation to other “dusty” in 1971 by Curt Hemenway, Peter Millman and Alan Cook interplanetary phenomena, like comets, was soon suspected. (Hemenway et al. 1973) in Albany, USA. It was followed in Comets shed large amounts of dust during their passage 1975 by IAU Colloquium No. 31 at the Max-Planck-Institut through the inner Solar System, which is most visible in their für Kernphysik in Heidelberg, Germany (Elsässer and dust tails. The larger of these dust grains stay for extended Fechtig 1976; the attendees of this meeting are shown in periods close to the orbit of their parent comet, and are recog- Figure 1), in 1979 by IAU Symposium No. 90 at the nized as a meteor shower when they cross the orbit of the Herzberg Institute of Astrophysics in Ottawa, Canada Earth. The finer dust gets injected into the zodiacal cloud. (Halliday and McIntosh 1980), by IAU Colloquium No. 85 Astronomical observations showed that this dust is widely in 1984 at the Laboratoire d’Astronomie Spatiale in distributed in the planetary system, at least out to the asteroid Marseille, France (Giese and Lamy 1985), IAU Colloquium belt. However, the situation of a zodiacal light observer is No. 126 in 1990 at the Kyoto University in Kyoto, Japan similar to that of a person in the midst of a cloud who is try- (Levasseur-Regourd and Hasegawa 1991), IAU Colloquium ing to determine the extent and the distribution of the cloud. No. 150 in 1995 at the University of Florida in Gainesville, Many particles along the line of sight which are at different USA (Gustafson and Hanner 1996), and IAU Colloquium distances from the Sun and have different scattering angles No. 181 organized in 2000 by Ian Williams and Tony (i.e. the angle between Sun and observer) contribute to the McDonnell at the University of Kent in Canterbury, England. zodiacal light brightness. The zodiacal light brightness, there- Proceedings of these meetings are a good record of the fore, depends both on the dust density and the scattering knowledge and an indicator of the advancements in the field. function, both of which are unknown functions of the dis- Besides these international specialist dust meetings, tance from the Sun. A way to resolve this problem, at least there are many national meetings or meetings on broader partially, was developed by René Dumont (1983). He used topics in which dust research plays some role. Among the the fact that an observer (on the Earth or on a spacecraft) who regular meetings are meetings of the Meteoritical Society is moving through the dust cloud will observe in the direction at which analyses of meteoritic and other extraterrestrial of motion different brightnesses at different times. The differ- material are discussed; the Lunar and Planetary Science ence between two brightness values corresponds to the Conferences in Houston, which focus on the study of the amount of dust along the line between the two observations. lunar, Martian and Venusian samples and related topics; Thereby, the observer is able to derive the spatial density AGU (American Geophysical Union) and EGS (European along the line of sight. Geophysical Society) meetings at which a broad range of topics in geophysics and space plasma physics are discussed; DPS (Division of Planetary Science of the American The dust community – astronomers and Astronomical Society) meetings at which discussions on space physicists a wide range of planetary topics take place, from obser- In the early days of space research, the space agencies, vations of planetary phenomena to theories of planetary mainly in the USA, organized conferences and workshops formation; and the Asteroid, Comets, Meteors (ACM) to present and discuss results from space measurements meetings, which were founded in 1983 by Hans Rickman of dust, laboratory simulations and theoretical studies. and Karl Lagerkvist in Upsala, Sweden, and later held at THE DUSTY HELIOSPHERE 1165

Figure 1 Participants of the 1975 Dust Meeting in Heidelberg (Elsässer and Fechtig 1976). 1166 EBERHARD GRÜN different places around the world. There are one or more In everyday terms, “dust” is often a synonym “dirt”– meetings each year to which an active researcher in the dust something to be avoided. This is true as well for interplane- field could go in order not to lose contact with the latest tary dust. Astronomers who want to observe extra-Solar developments in the field. System objects have to contend with foreground obscura- In 1978 Tony McDonnell took up the task to assemble a tion by the zodiacal light. In the extremely empty space comprehensive review of the “cosmic dust” field (Mc Donnell environment a few specks of dust, like paint flakes, cause 1978). With contributions from Fred Whipple, Jerry significant problems to highly sensitive measurements; for Weinberg, David Hughes, Mayo Greenberg, Don Brownlee, example, infrared observers have to struggle with debris D.G. Ashworth, Julius Dohnanyi and Hugo Fechtig as lead particles floating around their telescope. Physicist who authors, a wide range of topics was covered: from zodiacal want to study plasma effects in space have to modify their light over lunar craters to dust particles collected in the “clean” theories if dust is around. Dust is not easily con- atmosphere; from dust dynamics to laboratory simulations. trolled: it follows its own dynamics and disperses rapidly It was more than 20 years before a new version of a com- from its source, like smoke from a fire. This aspect, how- prehensive review of the field was published. The author, Bo ever, has a positive side: dust conveys messages from Gustafson, Stan Dermott and Hugo Fechtig (Grün et al. remote processes and objects by which it was generated. 2001), edited reviews of topics ranging from near-Earth dust Dust has many aspects and is thus difficult to quantify. to circumplanetary to cometary and interstellar dust, from An observer who determines one aspect of a dust particle in instrumentation over properties of interplanetary dust to space, like its size, will find it difficult to determine other orbital evolution of dust. parameters. Dust particles come in many sizes, composi- Table 1 lists major research centres and schools at which tions and shapes. In all cases there is no single size, com- space dust was or is being studied. Key persons, relevant position or shape that represents a certain space-dust methods and space missions are given. Many early and environment. Therefore, many different parameters have to recent dust researchers who made an impact on the field be measured in order to comprehensively characterize dust can be identified in the photograph (Figure 1), taken in grains. Even if a dust particle is predominantly made of a 1975 at the IAU Colloquium No. 31. Since then some key single material, minute surface contamination changes its figures have moved to other institutions, changed to differ- optical appearance so that it can no longer be easily recog- ent fields or terminated their dust research. Long-term nized. If one wants to know the properties of an ensemble of involvement in the field and several researchers at one loca- dust particles, distributions in all relevant parameters have to tion have been used as criteria for inclusion in the table; be determined. Theoreticians modelling interplanetary dust however, the selection is subjective and is not complete in have the difficulty of representing these particles by simpli- neighbouring fields, like comets, zodiacal light, theory, fied models, for example a spherical particle of uniform meteoritics, meteors, laboratory work, and others. composition and having the optical properties of a pure material. The description of the dynamics of dust involves many disciplines: Keplerian dynamics, interactions with the radiation field, and the plasma and magnetic environment. THE EARLY YEARS: FROM TOO MUCH DUST Some of the dust theories involve detailed assumptions TO VERY LITTLE about particle properties and the space environment that cannot be derived from first principles or that are not dis- Space dust: Dangerous, dirty and difficult cernible from direct observations, and hence cause unease in The danger from meteoroids to crewed and uncrewed activ- theoreticians who are trained mostly in, for example, “pure” ities was obvious. Centimetre-sized meteoroids entering the celestial mechanics. Interplanetary dust particles are difficult Earth’s atmosphere at several tens of kilometres per second to characterize and to quantify, and only a small number of will certainly damage any spacecraft they strike. Smaller scientists, both experimenters and theoreticians, have dealt particles could also be dangerous to not-well-shielded satel- with them. However, it is the belief of the author that recent lites and to crewed extravehicular activity. Therefore, all observational and theoretical methods are mature enough for space agencies considered it necessary to understand the us now to consider dust as an important and exciting subject hazard that comes from meteoroids. There were two aspects of astrophysical research. Progress in the field is made by to consider: (1) the flux of meteoroids, which was a ques- taking a multidisciplinary approach involving in situ space tion of observations and measurements in space; and (2) the measurements, astronomical observations, theoretical stud- effect of hypervelocity impacts on space systems, which ies and modelling, and laboratory investigations of basic was best studied in the laboratory. Therefore, as one of the processes. Close cooperation between astronomers, cosmo- first space activities, vigorous research programmes were chemists, dynamicists and experimental physicists has, in instigated to characterize the meteoroid hazard in space. fact, been beneficial in solving dusty problems. 50

ROSINE LALLEMENT* The interaction of the heliosphere with the interstellar medium

Far out in the uncharted backwaters of the unfashionable end radiation, and fields at distances from the Sun varying of the western spiral arm of the Galaxy lies a small unregarded between a few solar radii and tens of parsecs, that is from yellow sun. the solar corona and the birth of the solar wind, to the edges This first sentence of The Hitch Hiker’s Guide to the of the heliosphere at about 100 AU, where the solar wind Galaxy, Douglas Adams’ fiction book, could serve as an stops and comes into equilibrium with the galactic inter- introduction to this chapter, and paraphrasing his style stellar medium, and finally to the stars which are used as (without his talent, I am afraid) I could add: targets for measurements of the local interstellar medium This small and unregarded yellow sun has encountered a properties, located at distances ranging between 1 and 100 pc tiny and insignificant interstellar cloud, which is so tenuous from the Sun (1 pc 200,000 AU, 1 AU 215 solar radii, that the extremely weak wind of the yellow sun blows in the 1 solar radius 700,000 km). tiny cloud a cavity, which extends far beyond the planetary This multidisciplinary aspect is reflected in the large system attached to the yellow sun. Amazingly, trying to number of spacecraft whose instruments have recorded cross the edge of this cavity is one of the favorite games of a relevant data. I remember that during the first meeting held small group of obstinated individuals, who belong to the in the newly created International Space Science Institute in very primitive life forms on one of the orbiting planets. Bern in 1986, a meeting entitled “The Heliosphere in the Here I try to relate the story of the discovery of the small Local Interstellar Cloud,” I counted more than 20 spacecraft cavity carved by the solar wind in the local interstellar whose data were presented or discussed: Ulysses, Prognoz 5 cloud, our heliosphere. Being in the middle of this structure, and 6, Voyager 1 and 2, Pioneer 10 and 11, Pioneer-Venus, i.e. with a very limited and biased view of it, it has not been Galileo, IUE, HST, ISEE 3, IMP 8, AMPTE, Copernicus, easy to infer its existence, and the first steps in the under- EUVE, SOHO, ROSAT, WISCONSIN survey, DXS, and standing of its structure in the 60’s and 70’s have required a SAMPEX (and I must certainly have forgotton some others). strong imagination of the pioneers in this field. Then, during Results obtained from ground using neutron monitors, the last thirty years, the high number of new observations telescopes and radio telescopes are also used in conjunction have made the subject more and more fascinating. with space data. Such a variety (and such a “flotilla”) has made this field so interesting during the last few decades. The expectation of the crossing of the boundary of our 1 INTRODUCTION helio-sphere by one of the deep-space probes adds to the suspense. 1.1 A multidisciplinary field An additional proof of the variety of the field is given by The research area described in this chapter is by essence a the (almost) contemporary historical overviews of S. Suess multidisciplinary field, dealing with the properties of gases, (1990), T. Holzer (1989), V. Baranov (1990), and I. Axford (1990). They give an idea of how different the descriptions * CNRS Ð Service d’Aéronomie, Verrières-le-Buisson, France by experts in solar wind, in plasma physics, and in cosmic

rays can be. At variance with these reviews, the present resonance, creating the so-called interstellar glow, which chapter will have a neutral gas/interstellar medium “flavor.” allows them to be detected from Earth orbit. The glow is Over the years, the number of erroneous interpretations like the patch of light surrounding a street lamp on a foggy in this field has been quite large, but this happens com- night, except that it is not spherically symmetric, but has a monly, and we know from K. Popper that empirical dis- conspicuous maximum on the so-called upwind side, where proof is a seminal event in the scientific process. All these the interstellar wind originates, for the hydrogen Lyman- errors have indeed stimulated the next advances. Also, alpha glow (Figure 3), and on the contrary on the down- while in some cases new instruments have been built wind side for the helium 58.4 nm glow (Figure 5). These specifically for a defined goal, for example the GAS experi- differences arise because helium is ionized much closer to ment on board Ulysses for the “in situ” detection of inter- the Sun as compared to hydrogen, and thus can be focused stellar neutral helium, other findings have occurred in a by the Sun’s gavitational field on the downwind side before totally unexpected way. One of the best examples was the becoming ionized. Actually, the helium density reaches its first detection of the “echoes” from the heliospheric bound- maximum in the focusing cone at about 1 AU. Historically, ary by the Voyager radio experiments in 1983. These such a focusing of interstellar matter was first proposed by echoes today remain, to a large degree, mysterious. Lyttleton in 1950, not for atoms but for dust particles, to explain the formation of comets. Danby and Camm (1957) refuted this idea by showing that random thermal motion in 1.2 The heliosphere: a solar wind “bubble” in the interstellar dust flow prevents the required “perfect” the ambient interstellar gas focusing. Contrary to the case of helium, the ionization Our Sun, an ordinary late-type star in the Local Arm (or time for hydrogen atoms as a function of the distance r to Orion Arm), is presently traveling through a small interstel- the Sun is such that hydrogen atoms traveling at about 20 lar cloud, the Local Cloud, whose size is of the order of a km s1 have a 50% chance of surviving ionization at about few parsecs (Figure 1). This cloud belongs to a small group 5 to 10 AU. Thus there is no focusing cone, and instead a of partially ionized clouds whose temperatures and densi- cavity of this order of magnitude is formed. The maximum ties are of the order of 5,000Ð10,000 K and 0.1 particles per emissivity at Ly-alpha, the resonance wavelength for hydro- cm3. This group of clouds is embedded in a 100 pc wide gen, which varies roughly as the product of the density and volume, the Local Bubble (LB), believed to be filled with r2, is upwind at 1.5Ð2.0 AU, that is, very close to the orbit an extremely tenuous (0.002 cm3) and hot (106 K) gas. The of the Earth (Figure 3). After ionization, the newly formed origin of the LB is still a matter of much debate. particles (the so-called pickup ions) are convected outward The relative motion between the Sun and the interstellar by the solar wind electromagnetic field. medium (ISM) creates a permanent flow of interstellar gas While interstellar helium is mainly photoionized, hydro- and dust around our star, the so-called interstellar wind gen photoionization is a minor effect and hydrogen atoms (Figure 2). The flow velocity is of the order of 25 km s1 are preferentially ionized through charge exchange with the (4 AU per year). When the gas approaches the Sun, the ion- solar wind protons. It is important to note that charge ized part of the interstellar gas and the interstellar neutrals exchange between a hydrogen atom and a proton (H) pro- are expected to behave differently: according to models, the duces a “new” proton and a “new” atom, each one having plasma is decelerated, heated, and deviated, and flows essentially the same momentum as the former particle. In around the heliopause, the contact discontinuity between other words, it is almost exactly as if the electron had just the confined solar wind and the interstellar plasma. At vari- left the hydrogen atom to become attached to the proton, ance with charged species that behave collectively, neutrals, while the two nuclei continue on their way. This means that to a first approximation, flow freely across the interface all newly formed neutral atoms flow radially at the solar between the two plasmas. This behavior is due to their large wind speed (300Ð800 km s1) and leave the inner Solar mean free path with respect to neutralÐion (and Ðelectron) System very rapidly. This is why charge exchange is a loss interactions (of the order of tens of AU) and neutralÐneutral process for the neutral flow. Moreover, these fast atoms no collisions (hundreds of AU). Neutral atoms having success- longer scatter the solar radiation, since, due to their high fully entered the heliosphere approach the Sun, and are velocity, the resonance wavelength in their rest frame is increasingly affected by the solar wind and the solar radia- shifted out from the solar Ly-alpha line which has a finite tion fluxes. width of about 1 Å. They become invisible. From the Around the Sun the so-called ionization cavity is dynamical point of view (trajectories of neutrals having formed, a volume devoid of neutrals, much smaller (about escaped ionization), we know now that even in the absence 10 AU for hydrogen and less than 1 AU for He) than the of a strong ionization, hydrogen atoms would not be gravita- heliosphere. Nevertheless, a fraction of the atoms can tionally focused, since radiation pressure due to the solar approach close enough to scatter the solar UV lines by resonant photons balances the gravitation. All these effects THE INTERACTION OF THE HELIOSPHERE WITH THE INTERSTELLAR MEDIUM 1193

Cobe (H1 and H2)

TO GALACTIC +GP(z) NORTH POLE DENSE GAS CONTOUR from neutral sodium columns

400 TO pc GALACTIC -300 CENTER

-300 LOCAL BUBBLE: HOT GAS I MILLION K VERY TENUOUS 0.01 CM–3 UNCERTAIN ORIGIN δ Cas Altair

Local Cloud LOCAL DIFFUSE Sun -1 CLOUDLETS Capella 26 km s Cloud 5000 – 10000 K N = 0.1 cm–3 α Cen 29 km s-1 10 pc

Procyon Sirius

Figure 1 The solar environment at different scales: the Galaxy (here a combination of COBE results from Hauser et al. 1995), the local arm (local bubble contours from interstellar neutral sodium absorption, results of Sfeir et al. 1999), and the local cloudlets (derived from absorption lines towards very nearby stars (Lallement et al. 1995)). were modeled for the first time in a realistic way by 1.3 Four coincidences concerning the Sun’s motion T.E. Holzer (1977). As we will see, charge exchange plays a through the interstellar medium fundamental role in the structure of the heliosphere, not only Despite our ordinary Sun and ordinary Local Cloud, noth- close to the Sun, but also at the heliospheric interface itself. ing is actually simple in our galactic environment and there 1194 ROSINE LALLEMENT

Figure 2 The solar environment at different scales: schematics of the two local clouds, the heliosphere, the interstellar hydrogen flow around the Sun and the “H glow.” (From Lallement 1998.) are (at least) four peculiarities about the Sun’s motion in the center. For the Sun, the Local Cloud is nothing more than Local Cloud. The first three have caused problems and have one of the numerous interstellar galactic clouds it has somewhat delayed the progress in the field during the last already traveled through. The motion of the Local Cloud 30 years. and the motion of the Sun are thus totally unrelated. It just (1) In the rest frame of the Sun, the interstellar gas of the happens that, by chance, the difference between the two Local Cloud flows from a direction which points at 15 velocity vectors points close to the galactic center (Figure 2). from the galactic center. In other words, for an observer at But this also implies that the interstellar gas emission glow the Sun, the interstellar wind seems to flow from the central has a maximum in the direction of the galactic center. As part of the Galaxy (the Sagittarius area). This is purely we will see, this has had (and still has) some consequences. coincidental. The Sun is too old to be still embedded in its The emission is a maximum on the so-called upwind parent gas (the primordial cloud), it left it billions of years side, simply because it corresponds to the location of clos- ago, and has already circled a few times around the galactic est approach to the Sun of the inflowing interstellar atoms, 6

REIMAR LÜST* Barium cloud experiments in the upper atmosphere

1 INTRODUCTION ionized hydrogen and helium molecules which have an extremely small cross section for light-scattering and so, Space techniques using sounding rockets, satellites and space like the even smaller electrons, do not scatter enough light to probes made it possible to send instruments into space not make their presence visible. only to measure the physical parameters of the surrounding Therefore, it would be interesting to inject into a cosmic atmosphere but also to carry out experiments in order to plasma a suitable material that with has a cross section large learn about matter and fields in space. enough for light-scattering to make the motion of cosmic When injecting barium clouds into space, both measure- plasma visible. For a plasma with very high electrical conduc- ment and experimentation occurs. The barium can be used tivity, this is of particular interest, since every motion perpen- to trace the movement of atmospheric plasma and thus to dicular to the magnetic field lines of force can be described as measure the electric fields. This is only valid if the artificial the motion of the magnetic lines force. H. Alfvén used the plasma cloud does not disturb the surrounding atmosphere image of magnetic lines of force frozen the plasma. too much. By injecting a stronger cloud, it is possible to study the active interaction with the surrounding magnetic field. In this way, one might study interesting general phe- 2 BIERMANN’S THEORY OF THE nomena of a plasma. Experimentation occurs if the pressure INTERACTION OF THE SOLAR WIND WITH THE of the artificial plasma is much stronger than the pressure IONIZED COMETARY TAILS of the magnetic field in space. Experiments with artificial plasma clouds have provided In 1950 Ludwig Biermann taught a course about comets at new possibilities for studying the plasma under conditions the Astronomical Observatory of the University of Göttingen. that cannot be easily set up or may even be impossible to At that time, I was a PhD student attending these lectures. realise in a laboratory. Biermann was puzzled about why tails with ionized mole- These experiments are comparable to methods of observ- cules always pointed away from the Sun, while tails consisting the velocity of a homogeneous fluid. A typical method ing of non-ionized molecules and dust were curved toward involves spreading some coloured particles or metallic dust the Sun? The latter form could be explained by the solar light into the fluid. Normally, one uses only very small amounts pressure and the motion of the comets around the Sun. in order not to disturb the behaviour of the fluid. More than However, explanations of the light pressure as a force to 90 per cent of the cosmic objects are in a plasma state, but blow away the ionized tails failed by orders of magnitude. are also very dilute and therefore not visible except where Biermann developed the theory that the corpuscular radiation concentrated in stars. The cosmic plasma consists mainly of of the Sun was responsible for the high acceleration observed in the ionized tails. That the Sun sporadically emits a corpus- * Max-Planck-Institut für Meteorologie, Hamburg, Germany cular radiation was known from the observed perturbation of

Reimar Lüst, The Century of Space Science, 179Ð187 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 180 REIMAR LÜST the Earth’s magnetic field. Not known at that time was that barium. The required quantities were very low, of the order the Sun emits a corpuscular radiation continuously. This phe- of some 10 to 100 grammes of Ba ions. nomenon was first detected by Russian spacecraft and by the US satellite Explorer X and explained theoretically by Eugene Parker (see Chapter 9). 4 THE DEVELOPMENT OF BARIUM CLOUD Biermann published his theory of the interaction of solar EXPERIMENTS IN THE UPPER ATMOSPHERE corpuscular radiation with the ionized cometary tails in two papers (1951, 1952). In these, he demonstrated the close cor- 4.1 Development of the technique relation of events in the tail of comet Whipple-Fedke with the In 1961 a small, newly formed group in Garching near registration of magnetic storms. In a later paper (dedicated to Munich Ð the nucleus of the future MPI for Extraterrestrial Heisenberg’s 50th birthday) he showed that a similar correla- Physics Ð began to develop the necessary technique for an tion existed for the ionized tails of comet Halley in 1910. artificial cloud experiment in the Earth’s atmosphere with the help of sounding rockets. It was a very lucky coincidence 3 PROPOSAL FOR AN ARTIFICIAL that, during just this period, I met for the first time Jacques Blamont. This was at a meeting at the Royal Society where COMETARY TAIL the first plan for the European Space Research Organisation (ESRO) was discussed. I mentioned to Blamont the idea of After the successful launches of the first artificial satellites creating an artificial cometary tail. He had already used and space probes in the late fifties, I discussed with sounding rockets to release neutral sodium (Na) clouds in Biermann at the beginning of 1960 whether German scien- order to study the upper atmosphere, and invited me to fly a tists should also get involved actively in space research by container of barium on one of these Centaure rockets. using sounding rockets, satellites or space probes and what Blamont (1983) wrote about the first experiments role the Max Planck Institute for Physics and Astrophysics in Munich could play. In the astrophysical part of the The first operation started badly: Two barium burners Institute, only theoretical work was then being performed. (developed by the new group in Garching) were placed During this discussion, the idea of creating an artifi- on Centaures launched in November 1962 from the cial cometary tail in order to understand much better French naval base at the Ile du Levant on the Riviera— Biermann’s theoretical concept of the interaction between both rockets failed (the first failures in the whole devel- the solar wind and the ionized cometary tail was launched. opment history of the rocket …). Two other Centaures Of course, using the same molecules observed in a natural were used with complete success in Hammaguir in May cometary tail for such an artificial one would have been most 1963. Algeria had then become independent, and the attractive. But the calculation showed that several tons of Evian Agreement had authorised the use by the French carbon monoxide (CO) would be needed to create a visible arti- Government for a further five years, until 1967, of the ficial cometary tail (Biermann et al. 1961). Therefore other space complex built in Algeria by the army. The main elements or molecules had to be found. In order to keep the part of the complex was in the town of Colomb-Béchar cost down and the payload of the sounding rocket as light as and the launch sites were located on a base built ex possible, it was clear from the outset of the programme that nihilo 130 km southwest of the town on a flat plateau the best energy source for ionizing and exciting the atoms in covered with stones and sand. Our German friends had an artificial cloud was solar radiation. Furthermore, the cloud the responsibility for the burners, which were integrated had to be observable from the surface of the Earth. These into the nose cones in Hammaguir, and their photo- conditions led to a number of requirements for suitable ele- graphic equipment was manned by a large team sited at ments or molecules: (1) The resonance lines of the ions had the desert outpost, B-1 Nord’, near Colomb-Béchar. I to be within the “optical window” of the Earth’s atmosphere. would like to recall the names of this enthusiastic group (2) The time scales involved in exciting the expected lines of of fine young engineers, technicians and scientists with the ions, and photoionizing the neutral atoms had to be suffi- whom we spent many tense hours: Gerhard Haerendel, ciently short, (3) Since a chemical technique for the release Herbert Bause, Hermann Föppl, Ludwig Heilmeier, was to be used, a low evaporation temperature was highly Hans Loidl, Friedhelm Melzner, Bernhard Myer and desirable. Hans Neuss. It was there (in the bar of the officers’ cafe- The most promising elements to meet these require- teria!) that I learnt from Prof. Reimar Lüst about the ments were some alkaline-earth metals, particularly barium, delicate technique of extracting oneself from a sinking and probably some of the rare-earth elements, namely submarine. europium and ytterbium. We tried strontium and barium, The results of the May 1963 ejection were decisive: we and discovered that visible clouds could be created by using had to communicate to our German friends the results BARIUM CLOUD EXPERIMENTS IN THE UPPER ATMOSPHERE 181

given by our spectrographs: their spectacular clouds … contained no barium atoms or barium ions … but useless barium oxides and strong strontium lines (no strontium had been added to the mixture!! …). The principle of these ejections was changed and subsequent firings at the end of 1964 were a complete success. On 22 April 1966, a high-altitude ejection was performed using a Rubis rocket: the cloud which formed at an altitude of 2000 km was also observed from Germany. From then on the barium-cloud technique proved workable and since it has been used extensively in many scientific programmes. To find an effective way to make barium evaporate, extensive laboratory experiments, as well as theoretical Figure 1 Two barium clouds at an altitude of 260 km. investigations, were carried out. The evaporation of Ba is Photoionization by solar UV light is the cause of the changes achieved by a chemical reaction. We tried several different in color and shape during the first two minutes. The barium means with the most efficient one being a reaction between clouds are shown at 110, 210 and 990 seconds after release. atomic Ba and copper oxide (CuO): The neutral clouds are multicolored and spherical and initially show signs of high optical thickness. The ion clouds (purple) (n 1)Ba CuO l BaO Cu n Ba (vapor) become elongated along the magnetic lines of force and show a field-aligned fine structure. (Source: Max-Planck-Institute of In this reaction part of the barium is burned and provides Extraterrestrial Physics.) the heat necessary for the evaporation of the rest of the barium. An efficiency of about 10 to 20 per cent can finally be achieved: this means that from the volatile barium about changes; the positively charged ions and the negatively 10 to 20 per cent Ba atoms could be observed. The Ba ions charged electrons are trapped by the Earth’s magnetic field, are generated by photoionization with a time scale of about and they begin to spiral around the lines of magnetic force. 30 seconds. Most effective for this time scale is a metastable For this reason the plasma cloud continues to grow only energy level of the atomic barium (Föppl et al. 1965, 1967). along the lines of force. The cloud thus becomes cigar- shaped and can be distinguished readily from the spherical non-ionized cloud (see Figure 1). Later, however, consider- 4.2 The behavior of an artificial plasma cloud able distortion of this typical shape can be caused by an The ionization can be observed not only spectroscopically inhomogeneous electric field. but also directly with the unaided eye, because the barium cloud changes both colour and shape during the transition 4.3 The different sounding-rocket lauchings phase. The non-ionized cloud radiates in several green, yellow and red lines of the visible spectrum. After the ini- During the period (from) 1961 to 1972, the Max Planck tial, optically thick phase, the radiation in the green (spec- Institute for Extraterrestrial Physics participated in trum) is the predominant colour in the neutral cloud. As the 66 sounding-rocket launchings with barium cloud experi- cloud becomes fainter because of photoionization, neutral ments. We used French Centaure, Dragon and Rubis rock- strontium Ð always present as an impurity Ð remains. It ion- ets; the English Skylark rocket; the Canadian Black Brant; izes much more slowly and radiates in the blue. The ionized and the US Javelin, Nike Tomahawk and Nike Apache barium atoms have spectral lines in the violet, blue and red rockets. Launchings took place as far afield as the Algerian regions of the spectrum, resulting in a purple colour. Hence Sahara, at Thumba (India), Sardinia (Italy), Kiruna the ionized cloud can be easily distinguished from the (Sweden), Andoya (Norway), Fort Churchill (Canada) and neutral one because the ionized cloud is purple and the non- Wallops Island (Virginia, USA). ionized one is green and later blue. Most of the clouds were released in the ionosphere at A change of shape takes place as well: the neutral cloud altitudes between 150 and 250 km. There were two princi- is spherical and increases in diameter rather rapidly; the pal reasons for choosing this range of altitude. The first was fast expansion is eventually slowed down by collisions of that these heights can be reached with small and relatively the barium atoms with other atoms and molecules in the inexpensive rockets. Second, the motion of the plasma Earth’s atmosphere; thereafter the neutral cloud increases clouds yields information not only about the region of the in size at a much slower rate by diffusion. Meanwhile, the ionosphere where the clouds are released but also, indirectly, ionized part of the barium cloud undergoes quite different about much higher regions in the magnetosphere. 182 REIMAR LÜST

The experiments were carried out during the twilight period in order to have conditions in which the clouds remained illuminated by the Sun, while the surface of the Earth was dark. The clouds were observed from two or more stations that had to be well separated so that the position of the cloud could be determined by triangulation. The stations were equipped with a variety of cameras, spectrographs and other instruments.

4.4 The drift of the plasma cloud A magnetic field forces the ions and electrons in a helical path along the lines of magnetic force. It is possible for a particle to encounter two kinds of disturbing forces that will deflect it from this helical path. First, it can collide with another particle. However, in the Earth’s atmosphere above 200 km, the frequency of such collisions is very low compared with the frequency of gyration. As a result, the predominant disturbing force is the electric field. In an electric Figure 2 High and inhomogeneous electric fields lead to strong distortions of the ion cloud over hundreds of kilometers. field that is at right angles to the magnetic field, the charged (Source: Max-Planck-Institute of Extraterrestrial Physics.) particles of a plasma tend to drift in the direction perpendicular to both fields. An observer moving with the plasma drift velocity will see only the spiralling of the particles around reasonable length of time without causing strong perturba- the magnetic lines of force. In other words, in this moving tions in the atmosphere (Figure 2). frame of reference, the electric field no longer exists. As mentioned above, Alfvén describes such a case as one in which the lines of force are “frozen into the material”. His 5 MEASUREMENTS IN THE IONOSPHERE description is the same as saying that, during its motion, the plasma distributed along certain lines of force or within a In the following, the results of measurements in the differ- flux-tube stays together, as far as motions at right angles to ent regions of the ionosphere are summarized (for more the magnetic field are concerned. Electric fields aperpendicu- details see Haerendel et al. 1967, Föppl et al. 1968, Lüst lar to the magnetic fields and motions of magnetic field lines 1969 and Haerendel 1987). are consequently interchangeable notions in many situations. The observed velocity of a plasma cloud can be expressed in terms of the strength of an electric field: a 5.1 Low and mid-latitudes velocity of 100 m/s perpendicular to a magnetic field of To investigate the equatorial electrojet, barium clouds were 0.5 gauss (a typical strength for low altitudes) corresponds released over Thumba (India) near the geomagnetic equa- to an electric field strength of about 5 V/km. tor. The observed drift motions demonstrated an upward Of course, some care is necessary in interpreting the electric field of 1.8 to 5.4 V/km after sunset; the associated motion of the plasma cloud in electric fields, since the wind horizontal easterly field was weaker by a factor of about 3. of neutral atoms of the atmosphere might influence the These results on the horizontal fields are in agreement with motion of the cloud and, furthermore, the artificial plasma measurements from radar echoes obtained from the inco- might disturb the surrounding medium by changing the herent backscatter facility in Jicamarca, although vertical electrical conductivity. fields could not be measured by this method. The first effect can be kept small by carrying out the mea- While only a few experiments were done near the geo- surements at altitudes where the collision frequency of the magnetic equator, quite a number of releases took place at barium ions and the neutral particles is small compared with mid-latitudes. In the sixties, all existing low-altitude data the gyration frequency of the barium ions around the lines of on electric fields in this regions were derived from barium force. At altitudes above 180 to 200 km, this condition is cloud experiments. fulfilled and the influence of the neutral wind can be In analyzing the experimental data one has to consider neglected. The perturbation of the cloud in the surrounding Ohm’s law, which in the ionosphere has the following form: medium can be kept small if only small amounts of barium are injected into the atmosphere. Fifteen grammes of i . 1 j E vn B barium ions are sufficient for a cloud to be observable for a c 51

KÁROLY SZEGÖ* Comets: coma and beyond

There is no clear definition of what the coma of a comet is. nucleus can easily mimic the appearance of a jet between Most frequently it means the faint visible halo around the sources, and it is much stronger than those visible above comets, and in other context it denotes material that reflects the actual active regions. This should make us cautious with or emits electromagnetic radiation (including light) back to inferences, but such caution is lacking in many publica- observers. In this chapter we shall use the word ‘coma’ to tions. The complicated rotational motion of the generally describe all materials around the cometary nucleus, solids irregularly shaped nucleus also renders inferences difficult. and volatiles, neutral and charged, irrespective how effec- Even the free rotation of an irregular body strictly speaking tively a certain component is able to scatter or emit radia- is aperiodic in the ecliptic frame of reference; and in the tion. The origin of the coma is the nucleus itself, and we frame of reference attached to the angular momentum vec- shall discuss the processes that eject or emit materials from tor the time variation of two of the Euler angles is charac- the surface to the cometary atmosphere and beyond. We terized by two different periods, and the variation of the consider only physical processes; coma chemistry is not the third Euler angle is aperiodic in general. The rotational topic of this chapter. It is unavoidable to take into account motion under the effect of the torque due to nuclear activity in this context the structure and the physics of the cometary can be even more complex. Filtering out all these details surface layer as well, because this is the source of the from ground-based observations is almost an impossible coma. Comets are believed to contain pristine materials task, especially if the direct inference between coma from the period when the Solar System was born; the and fixed sources on the rotating nuclear surface is also in cometary surface, however, has been exposed to many per- question. turbations (cosmic rays, solar wind, heat cycles, etc.) and it If we browse publications of the 1960s to find out what definitely cannot be considered as pristine. How deep we they contained about planets, we frequently find quite basic have to dig to find unprocessed material is an open ques- statements that were disproved later, after in situ measure- tion. There is only limited experimental evidence on sur- ments were done. When considering comets, we have to face evolution obtained in simulations, and we have to rely bear in mind that nobody has ever landed on a comet, and mostly on guesses. no nucleus has ever been explored from a close distance. At It is also important to note that in situ experimental comet Halley, the Giotto probe took the highest resolution investigation of comets is limited to a few flyby missions; images from a distance of about 1200 km, with a nominal most of our knowledge comes from astronomical observa- resolution of about 27 m; these images covered part of the tions. It is a matter of debate, however, as to what extent the nucleus. The two Vega craft imaged the nucleus from about observed coma features can be directly related to the prop- 8500 km with a resolution of about 150 m. In these mis- erties of the nucleus. If a jet is seen in the coma, astrono- sions the high relative velocity between the nucleus and the mers prefer to connect it with a source below it on the spacecraft made dust analysis difficult, because no dust nucleus surface. However, as Crifo and Rodionov (1999a) detector could be appropriately calibrated before launch. have pointed out, two active sources on an aspherical Formation of a plasma cloud due to the high impact velocity on the target surface, and its influence on data analysis in * Research Institute for Particle and Nuclear Physics, Budapest, Hungary certain cases is still an open issue. Dust size measurements

Károly Szegö, The Century of Space Science, 1217Ð1233 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1218 KÁROLY SZEGÖ made by different techniques differ in fine details. Modelling The solar wind can reach the surface of comets when of the coma and the nucleus surface received a new impetus cometary activity is low, that is at large distances from the when the Rosetta mission of the European Space Agency Sun. It is known that dust can absorb solar wind ions and was approved to explore and land on comet Wirtanen. The re-emit them as neutrals; this process is also likely to occur spectacular appearance of comet Hale-Bopp in 1997 also in comets. However, it is unlikely that the solar wind causes initiated new research. profound physical changes, and therefore we do not discuss In this chapter we first review models of cometary sur- this effect any further. faces, to the level necessary to understand how the coma As a preparatory activity to a landing on comet 46P/ develops and what the boundary constraints are for the Wirtanen, researchers have worked out a nucleus reference models for the acceleration of dust/gas mixtures. Next we model (Möhlmann 1999), which is the best summary of our discuss cometary atmospheres, both the regions where the current knowledge of the thermal, mechanical and electro- properties of neutral gas are dominated by collisions, and magnetic properties of the cometary surface. Ground simu- where collisions cease to be present. We review the results lations of the properties of iceÐdust mixtures (e.g. Thiel of in-flight dust experiments, and we finish with a brief et al. 1995) significantly contributed to this. Our focus overview of the charged particle environment of comets. here, however, is more limited; we are interested only in models that can describe how dust and gas can be released from the surface layer. THE SURFACE OF COMETS The first models concentrated on reproducing the inbound and outbound brightness curves of comets often It is the ‘dirty snowball’ vision of Whipple (1950) that lies at showing an asymmetry at the same heliocentric distance. the heart of all comet models. Accordingly, to a zero-order Whipple (1950) was the first to introduce the mantle as the approximation the surface is like the surface of a dirty snow- likely surface layer of the nucleus, consisting of solid mate- ball. The ‘dirt’ is not homogeneous, the chemical composi- rials after the volatile component evaporated. Mendis and tion of the dirt varies depending on location, which is Brin (1977) suggested that erosion takes place, with parti- convincingly proved by the existence of chemically different cles smaller than a critical size being carried away by the jets in the coma (A’Hearn et al. 1986, Cosmovici outflowing gas. This was developed further by Horanyi et al. 1988, Clairemidi et al. 1990), and it is conceivable that et al. (1984) in the framework of the friable sponge model. there are inhomogeneities in other physical quantities as The key assumptions of this model are that the dust loss well. This entails that cometary activity might not be uni- rate is proportional to the momentum flux of the outflowing form, and though we cautioned against a direct inference vapour, and that erosion takes place only in a thin surface from coma to surface, we are certain that this inhomogene- layer (Figure 1). The gas is released from the surface of the ity is reflected in coma properties as well. Comets are icy core (a frozen dustÐice mixture), covered or uncovered irregularly shaped. The three-dimensional shape of comet by the dust mantle. This implies that the size distribution Halley could be reconstructed based on the Vega flyby of the grains remains constant in the mantle with time, images, and Giotto contributed with a single view only (Szegö et al. 1995). The area of the sunlit nucleus surface 4 varies as the object rotates, this is a natural cause of J To anisotropy. y =0 T(o) =To Comets are always exposed to cosmic rays, which is the ∂T K dominant radiation when they are in the Oort cloud. Their 1 ∂y 3 ∂T kF ∂ interaction with the local interstellar material also affects 2 y y = T ( ) =T surface evolution, but as we do not have much knowledge of ∂ 1 L K T the details, we shall not discuss it here. The penetration depth F 2 ∂y NA of cosmic rays depends on their energy and on the local density; moderately relativistic charged particles lose energy in matter primarily by ionization. From ground experiments it is known (Caso et al. 1998) that the energy loss of protons Figure 1 The ‘friable sponge’ surface model. The upper layer, the dust mantle, consists of degassed dust particles. Under the more energetic than 1 GeV is a few MeVg 1cm2. For less dust layer there is a dust–ice mixture. The heat flows are energetic particles it grows considerably more; for example, indicated, the sublimated vapour is in thermal equilibrium 100 MeV protons penetrate a few centimetres into water ice. with the local mantle. L is the latent heat, NA is the Avogadro Cosmic radiation may trigger chemical reactions leading number, F is the flux of sublimated vapour and K1 and K2 are to surface differentiation, among others to the creation of is the thermal conductivities of the mantle and the core, impermeable chemical layers (Moore et al. 1983). respectively. (After Horanyi et al. 1984, Figure 1.) COMETS: COMA AND BEYOND 1219 independent of the thickness of the mantle. As this requires In this model there is only one volatile component, and that all sizes are removed at the same rate, it is allowed that whereas it reproduces the basic dust-emission process, it larger grains be broken into smaller pieces (friability). evidently cannot elucidate the flux variation of the different The thermal balance on the dusty surface is given by the gases as a function of heliocentric distance. A wide variety following equation: of models has been developed to remedy this and some other simplifications in the thermal properties. The most T (at y 0) 4 a J T D K (1 ) e (1) important modification is that gas is released not only from o y the ice surface, but from deeper layers as well. An excellent where a is the surface albedo, J is the solar energy flux, is summary of these is given by Klinger et al. (1996). We fol- the optical depth of the coma, is the thermal emissivity of low this paper when summarizing the new features: (a) the the surface, is the StephanÐBoltzmann constant, To is the gas phase contributes to the heat transfer to deeper layers; temperature at the surface, D is the heat due to diffuse radi- (b) volatiles are able to diffuse into deeper layers where ation from the coma, K is the thermal conductivity, and they recondense; (c) sublimation can occur at various y is the distance measured from the surface downward. In depths depending on the volatility of the different ices; this model the thermal conductivity can be expressed (d) there is more than one ice phase, the water ice initially as K a bT3; for the meaning of a and b see Horanyi is in an amorphous phase that will become a crystalline et al. (1984). phase; and (e) the pore size varies during the ougassing There is a general agreement that the heat flux, J, due to process. Such a surface model is shown in Figure 2. Most solar irradiation is attenuated by the already existing coma. of these assumptions were verified by the KOSI comet sim- Its intensity depends on the heliospheric distance, the local ulation experiments (see references in Klinger et al. 1996). incidence angle and that part reflected back in proportion to In the following we discuss specifically the model devel- the surface albedo (first term of eqn (1)). The nucleus as a oped by Podolak and Prialnik (1996). They assumed that the black body also irradiates heat proportional to its thermal mantle layer is composed of dust, amorphous ice, crystalline emissivity (second term of eqn (1)), and the existing coma ice, water vapour and gases such as CO and CO2 trapped in not only attenuates the incoming flux, but due to scattering the amorphous ice. The trapped gases are released when the and re-radiation, it contributes to local heating. The amount transition from the amorphous to the crystalline phase takes of heat reaching the surface due to the coma is a matter of place, but the gases released can recondense on pore walls debate. The ‘rule of thumb’ accepted nowadays is that this and their sublimation may occur at a later stage. The gas balances the attenuation (Salo 1988). The back-scattered flow is a free molecular flow, because the mean free path heat from the coma is definitely not enough to maintain in the mantle is much larger than the pore sizes. The dust sizeable surface activity on the dark side; at least there is no in this model is not only liberated from the surface, but may evidence for such processes on the images taken during the slowly move through the pores in the vertical direction, Halley flybys. In all surface models it is of paramount importance as to Sublimating how heat is conducted inside, this accounting for the vari- H2O CO, CO2, ..... ous gas production rates (Rickman 1991). The net heat flux Surface on the surface is conducted inside through the degassed Erosion mantle, warming up the upward-flowing vapours. Models differ as to whether there is a local thermal equilibrium between the local mantle and vapour temperature. At the crust Gas

Crystaline diffusion bottom of the degassed mantle the heat flow reaches the Crystalization pristine core where the iceÐdust mixture resides and is sub- front limated, the sublimation rate is governed by the ClausiusÐ Propagation Clapeyron equation. In the simple friable sponge model all by heating the heat is used on sublimation. The gas production rate remains constant until the mantle thickness reaches a value ice of about 102 cm, and then it decreases sharply with Amorphous Evaporation increasing mantle thickness. The surface temperature for of volatiles ‘pristine iceÐdust’ is the sublimation temperature, and as the mantle thickness increases it reaches the black-body Recondensation temperature. The mantle thickness varies dynamically, and this can account for by the observed inbound/outbound Figure 2 Schematic of a possible surface structure with asymmetry of the coma brightness. different ice phases. (After Rickman 1991, Figure 10.) 1220 KÁROLY SZEGÖ though it is allowed that dust can move horizontally when review of charged dust dynamics in the Solar System. The the vertical path is blocked. All the pores are permeable to motion of a dust particle is determined by the forces acting gas. Based on these and on some more specific assumptions, on it: the Lorenz force due to its own charge, gravitational the probability of a dust particle leaving the mantle can be forces and light pressure. In a coordinate system attached to calculated. The one-dimensional mass and energy equations the rotating nucleus, inertial forces should also be taken are solved only for the gasÐice mixture, the dust basically is into account. Charging is relevant for those cases when the treated separately, and only the dust and gas fluxes are cou- force due to charging is of the same order of magnitude as pled. These considerations lead to a stratified mantle struc- the other forces acting on the dust particle; typically for ture: there is a highly porous dust layer on the top, followed particles of a few micrometres in the vicinity of a few tens deeper by a dense layer of crystalline ice and dust and, of metres of the nucleus. below that there is a layer of amorphous ice and dust, The evolution of the charge created on the surface of an including other frozen-in volatile components at various insulator is described by the following equation (Horanyi depths (the frozen CO2 is closer to the surface than the 1996): frozen CO). This model reproduces very well the observed dQ Jk (2) gas emissions; namely that the CO production rate is higher dt k at large heliospheric distances than that of water, but this where J represents the charging currents. In a stationary ratio changes dramatically as the comet approaches the Sun. k case the left-hand side is zero. This proves that the coma composition does not reflect the In general, the flux of current density of particles, char- composition of the nucleus. acterized by a distribution function f(v), bombarding a sur- These models are one-dimensional, with variations only face with potential , is given as in the radial directions being considered. This was remedied by Enzian et al. (1999) who developed a multidimensional /2 2 rotating nucleus model, though still a spherical one, made up 2 J v dvsin d d v cos f (v) (3) of a porous dustÐice matrix composed of water and CO. Heat * and gas diffusion was allowed both in the radial and merid- v 0 0 ional directions. They found a near-uniform CO production, where v* is chosen for each plasma species so that with water production and surface temperature showing local mv*2/2 e0; that is, the integration volume in velocity variations connected with the incoming heat flux. space depends on the surface potential. It is easy to see that Whereas these models are significant steps towards an the limit of the integration is different if the surface poten- understanding of cometary activity, there are still unresolved tial is positive or negative. If the size, a, of the dust particles problems and controversies; for a review see, for example, is smaller than the characteristic Debye lengths, the surface Crifo and Rodionov (1999b). The model of Enzian et al. we need to take into account for grain charging is the total (1999) can barely account for the observed water production surface, which is 4a2 in the case of a spherical grain. rate, even assuming that the surface is pure ice, and the The actual formulae specifying the different charging radius is at the upper limit of the observations. As a pure ice currents are quite complicated, and are not given here. The surface is very unlikely, one tends to assume that bigger potential distribution in the dusty plasma sheath above the chunks of surface material can be released emitting gas in nucleus can be obtained from the Poisson equation. At flight as well. This scenario is seemingly supported by radar infinity the potential and the electric field should be zero. observations (e.g. Harmon et al. 1997) indicating that the The nightside surface potential differs: obviously there surfaces of several comets are rough on the centimetre scale; is no UV flux and the effect of the solar wind is also differ- however, how such large pieces can be carried away is still ent. Electrons, due to their high thermal velocity, do reach very much a matter of debate. the nightside surface, and the form of the charging current does not differ from that of the dayside. For protons, however, the situation is modified. Though the diameter of the CHARGING OF THE DUST IN FLIGHT AND THE nucleus is negligible compared to the proton gyroradius, COMETARY SURFACE since the flow velocity is higher than the proton thermal velocity, only a fraction of the proton distribution reaches Dust particles and the surface of a cometary nucleus can be the surface, those for which the gyration results in effective charged due to the ultraviolet radiation of the Sun and the backward motion in the cometocentric system. As the escaping flux of photoelectrons. The current carried by the nightside surface potential becomes negative, more protons charged components of the solar wind is important when it can reach the surface. The estimated potential on the night- can reach the cometary surface. In general, secondary ion side can be as large as 1 kV, deflecting the bulk kinetic currents are negligible. Horanyi (1996) gives an excellent energy of the solar wind protons moving past the comet. 52

HORST UWE KELLER* AND LAURENT JORDA* The morphology of cometary nuclei

The sudden appearance of a bright comet stretching over a by perturbations caused by encounters with planets) even of large part of the night sky must have been one of the most the new comets on almost parabolic orbits and typical peri- awesome phenomena for early humans watching the sky. ods of the order of 106 years is short compared to the age of The nature of comets remained obscure well into the the planetary system (4.5 Gy). Therefore, observed comets Middle Ages. Only with the introduction of astronomical could only recently have arrived on their orbits dipping techniques and analyses in Europe was the parallax of a inside the inner Solar System. comet determined by Tycho Brahe for the first time. He This reservoir of comets must have been established proved that comets are not phenomena of the Earth’s atmos- during the formation process of the planetary system itself. phere but are farther away than the Moon; in other words Cometesimals were agglomerated from interstellar/inter- they are interplanetary objects. Later Kepler first predicted planetary gas and dust and scattered out of the inner that comets follow straight lines, then Hevelius suggested Solar System by the giant outer planets (Section 2.3). This parabolic orbits roughly a hundred years later. It was Halley scheme implies that a central part of a comet, its nucleus, is who suggested that the comets of the years 1531, 1607 and stable enough to survive these perturbations. It must also be 1682 were apparitions of one and the same comet that stable enough to pass the vicinity of the sun for many times would return again in 1758. The success of this prediction in the case of a short-period comet. made it clear that comets are members of our Solar System. Comets are bright and large when they are close to the While it was now established that periodic comets are sun and fade quickly when they recede beyond about 2 AU. objects of the planetary system, their origin and nature con- Only with the advent of photography and large astronomical tinued to be debated. Were they formed together with the telescopes could a comet be followed until it becomes a star- planets from the solar nebula (Kant) or were they of extra- like point source. What makes comets active near the Sun, solar origin as suggested by Laplace? This debate lasted for blowing their appearances up to the order of 105 km? Bright 200 years until well into the second half of the last century. comets often develop tails two orders of magnitude longer. Öpik (1932) suggested that a cloud of comets surrounded In an attempt to explain the cometary appearance, our Solar System. This hypothesis was quantified and com- Bredichin (1903) introduced a mechanical model where pared to the observed distribution of orbital parameters repulsive forces drive the particles away from a central con- (essentially the semi-major axes) of new comets by Oort densation. Spectroscopy revealed that dust grains reflect the (1950) (Section 2.1). Comets are scattered into the inner solar irradiation. In addition, simple molecules, radicals and Solar System by perturbations caused by galactic tides, ions were found as constituents of the cometary coma and passing stars and large molecular clouds. tail. The nature of the central condensation remained myste- The Oort cloud would have a radius of 2 105 AU, a rious for a long time because of the observational dilemma. dimension comparable to the distances of stars in our neigh- When the comet is close to the Earth and therefore to the bourhood. The lifetime (limited by decay due to activity and Sun the dense coma obscures the view into its centre. When activity recedes the comet is too far away and too dim for detailed observations of its central condensation. During the * Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany middle of the nineteenth century the connection between

Horst Uwe Keller and Laurent Jorda, The Century of Space Science, 1235Ð1275 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1236 HORST UWE KELLER AND LAURENT JORDA comets and meteor streams was established. Schiaparelli The improvements of radar techniques led to the detec- (1866) calculated the dispersion of cometary dust within tion of reflected signals and finally to the derivation of the orbital plane. From this time on the perception that the nuclear dimensions and rotation rates. The observations, central condensations of comets were agglomerations of however, are also model dependent (rotation and size are dust particles prevailed for about a century. The gas coma similarly interwoven as are albedo and size) and sensitive to was explained by desorption of molecules from dust parti- large dust grains in the vicinity of a nucleus. As an exam- cles with large surfaces (Levin 1943). The storage of highly ple, Kamoun et al. (1982) determined the radius of comet reactive radicals (most observed species (CN, CH, NH2, Encke to 1.5 ( 2.3, 1.0) km using the spin axis determi- etc.) were of this category) posed a major difficulty to nation of Whipple and Sekanina (1979). be explained. The inference that these radicals should be The superb spatial resolution of the Hubble Space dissociation products of stable parent molecules (such as Telescope (HST) is not quite sufficient to resolve a (CN)2,CH4,NH3, etc.) by Wurm (1934, 1935, 1943) led to cometary nucleus. The intensity distribution of the inner our present understanding that these molecules are stored coma, however, can be observed and extrapolated toward as ices within the central nucleus of a comet. Whipple the nucleus based on models of the dust distribution. If this (1950a,b) combined the astrometrical observations of contribution is subtracted from the central brightness the changes of the orbital periods of comets with the existence signal of the nucleus can be derived and hence its product of an icy cometary nucleus. The sublimation of ices cause of albedo times cross-section (Lamy and Toth 1995, reactive (rocket) non-gravitational forces that increase or Rembor 1998, Keller and Rembor 1998; Section 4.3). decrease the orbital period of an active comet according to It has become clear that cometary nuclei are dark, small, the sense of rotation of its nucleus. often irregular bodies with dimensions ranging from about Evidence in support of the icy conglomerate nucleus a kilometre (comet Wirtanen, the target of the Rosetta became more and more compelling by the derived high gas comet rendezvous mission) to about 50 km (comet Hale- production rates that could not be stored by adsorption on Bopp, comet P/Schwassman-Wachmann 1). Their albedos dust grains (Biermann and Trefftz 1964, Huebner 1965, are very low, about 0.04. Their shapes are irregular, axes Keller 1976a,b) and by the same account by the large quan- ratios of 2:1 are often derived. Even though comets are tities of dust moving into the cometary tail (Finson and characterized by their activity, in most cases only a small Probstein 1968b). The ‘sand bank’ model (Lyttleton 1953) fraction of the nuclear surface (in some cases less than 1%) was clearly dismissed in favour of a solid icy nucleus. Its is active. An exception seems to be comet P/Wirtanen formation and origin could now be explored. where all its surface is required to be active in order to While there was some knowledge about the chemical com- explain its production rates (Rickman and Jorda 1998). The position of the nucleus, its physical properties, even the basic detection of trans-Neptunian objects (TNOs) in the Kuiper ones like size, shape and mass, remained largely unknown belt (Jewitt and Luu 1993) reveals a new population of because the nucleus could not be observed. Early attempts to cometary bodies with dimensions an order of magnitude derive the nucleus size from the ‘nuclear’ magnitudes of bigger (100 km and larger) than the typical comet observed comets at large heliocentric distances while they are inactive in the inner planetary system. Little is known about the (Roemer 1966a,b) led to a systematic overestimation of the extent, density, size distribution and physical characteristics size because their residual activity could not be eliminated. of these objects. This region is supposedly the reservoir for The advent of modern detectors and large ground-based short-period comets, manly those controlled by Jupiter telescopes revealed that most comets display residual activ- (Jupiter family comets). ity or clouds of dust grains around their nuclei. Taking the Our present concept of a cometary nucleus has been residual signal into account (mostly using simple models for strongly influenced by the first pictures of the nucleus of the brightness distribution) the size estimates of the nuclei comet Halley achieved during the Giotto flyby in 1986. could be improved. The (nuclear) magnitude of a comet While this revelation seems to be confirmed as typical depends on the product of its albedo and cross-section. Only by modern observations it carries the danger of prototyping in a few cases could the albedo and size of a cometary new observational results and inferences. Missions and nucleus be separated by additional observation of its ther- spacecraft are already on their way (Deep Space, Contour, mal emission at infrared wavelengths. By comparison with Stardust, Deep Impact) or in preparation (Rosetta) to diver- outer Solar System asteroids Cruikshank et al. (1985) sify our knowledge. derived a surprisingly low albedo of about 0.04. A value in The morphology of cometary nuclei is determined by clear contradiction to the perception of an icy surface but their formation process in the early solar nebula, their fully confirmed by the first resolved images of a cometary dynamics and evolution. The physics of the processes lead- nucleus during the flybys of the Vega and Giotto spacecraft ing to their apparent activity while approaching the Sun are of comet Halley (Sagdeev et al. 1986, Keller et al. 1986). still obscure in many details but determine the small- and THE MORPHOLOGY OF COMETARY NUCLEI 1237 intermediate-scale morphology. The large-scale morphol- parameters of the solar nebula the presence of gas induces ogy, the shape, of a cometary nucleus is determined by its drag forces on the dust particles and prevents local gravi- fragility and inner structure and by its generally complex tational instability. Submicrometre- to micronmetre-sized rotational state. These topics will be reviewed in the follow- particles entrained in the gas of the contracting solar nebula ing sections. Chemical and compositional aspects will be grow by coagulation due to Brownian motion and settle only discussed where they are important in the framework toward the central plane. The larger grains decouple from of the physical evolution of cometary nuclei. More details the gas motion and sweep up the smaller grains to grow fast are given in Chapter 53. A brief survey of the current mod- to centimetre sizes. This growth is based on interlocking elling efforts is given. The fate of cometary nuclei and their molecular forces. Before the particles reach the critical den- decay products follows. A summary and outlook ends this sity for gravitational instability to occur they would decou- chapter on the morphology of cometary nuclei. ple from the gas and follow Keplerian orbits. The presence of the gas still influences the motion of the particles by inducing a drag force that is size dependent. The differen- 1 FROM DUST TO COMETS tial rotation relative to the gas causes shear forces that induce turbulence preventing the grain density increasing 1.1 Planet formation in the early Solar System further (Weidenshilling 1980, Cuzzi et al. 1993). Thus cometesimals cannot form by collapse of a cloud of In Whipple’s icy conglomerate model of a cometary centimetre-sized particles but they have to grow by coagula- nucleus, the ices were mixed with differentiated refractory tion and agglomeration to metre size before they decouple matter deduced by analogy to meteoritic materials. The from the shear-induced turbulence. But growth does not nucleus could be quite inhomogeneous, and large refrac- stop there because the gas drag-induced radial velocity dis- tory boulders or even a refractory core were conceivable persion decreases relatively fast for larger bodies (Figure 1). (Sekanina 1972). About 20 years after Whipple’s model of Once they reach dimensions of tens or hundreds of metres a solid nucleus the concept that planet formation was trig- gas drag becomes insignificant. The lag of damping pre- gered by gravitational instabilities of the dust component of vents local gravitational collapse to form solid planetesimals the rotating solar nebula made comets to become building (Weidenshilling 1995). blocks of the planetary system. The growing dust grains settle towards the centre plane of the rotating disk until the density reaches a critical value where gravitational instability occurs (Safronov 1969, Goldreich and Ward 1973). The timescales for formation of these building blocks are short: 105 to 106 years. The size distribution of the resulting nuclei could be estimated based on the scale lengths for the gravitational instability to a few kilometres (Biermann and Michel 1978). Once a larger body is formed it grows very fast by gravitationally attraction (gravitational runaway) to form a planet. The ices of the cometary volatile components (predominantly water) require formation of the comets outside Jupiter’s orbit. The sizes of the homogenous cometary nuclei remain small enough that gravitational compaction is unimportant and the grains from the molecular cloud are hardly altered. The degree of processing of these grains before they agglomerate depends on the physical parameters of the molecular cloud at the location of planetesimal formation such as the optical depth (shielding Figure 1 Particle velocities as a function of size in the model the dust from the central early Sun) and the resulting nebula at 30 AU. Particles are assumed to have a fractal local temperature. A conceivable extreme is the formation structure at size 10 2 cm, and constant density of 0.7 cm 3 at directly from interstellar grains (Greenberg 1977, 1998). d 1 cm. Dotted line: thermal velocity at T 50 K. Solid line: radial velocity, with peak value equal to V 54 cm s1 at d 102 cm. Changes in slope are due to variation of particle 1.2 Accreation of building blocks density (d 1 cm) and transition from Epstein to Stokes drag law (d 105 cm). Dashed line: transverse velocity relative to In contrast to the formation of comets by gravitational pressure-supported gas. Short dashed: escape velocity from instabilities, Weidenshilling (1995) shows that for plausible the particle’s surface. (From Weidenshilling 1997.) 1238 HORST UWE KELLER AND LAURENT JORDA

The radial velocity distribution as a function of particle 2 RESERVOIRS OF COMETS size controls the evolution of the growing bodies. Figure 1 depicts a typical scenario in the solar nebula at a radial 2.1 The Oort cloud distance of 30 AU from the Sun (Weidenshilling 1997). Small particles rotate with the gas velocity but drift radially Comets are rather artificially divided into two classes, the inward, while large particles follow Keplerian orbits and short-period (SP) comets with orbitial periods of less than plough through the slower rotating gas. The peak velocity 200 years and the long-period (LP) comets. The activitity is reached for particle sizes for which the drag-induced of comets near the Sun removes about 0.1 to 1% of the response time t mv/F (m is particle mass, v is relative mass of the nucleus per orbit, so that the lifetime of e D a cometary nucleus is less than 1000 orbital periods. velocity and FD is drag force) is comparable to the orbital 6 period at the radial distance in question. This velocity dis- Obviously even LP comets with orbital periods of 10 years tribution controls the agglomeration of the bodies. As long would not have survived on their present orbits from the as gravitational attraction is unimportant the large bodies beginning of the Solar System. Consequently comets are grow relatively slowly because of their small velocities. either formed or captured episodically. If of primordial ori- They typically grow from bodies at factors 3 to 5 times gin they could not have formed on their present orbits. smaller that still have higher speeds. For example, after Hypotheses based on episodical events include gravitational 8 104 s the largest bodies of 70 m accrete from 20 m ‘par- focusing of passing interstellar cloud material (Lyttleton ticles’; at 1.5 105 s, 500 m from 200 m; at 2 105 s, 6 km 1948), compression of interstellar clouds by shocks from 1 km. Now the velocity becomes gravity influenced (McCrea 1975), or formation in giant molecular clouds and and at 2.5 105 s, 80 km bodies are growing by gravita- subsequent capture by the Solar System (Clube and Napier tional accretion. At any given time most of the mass is 1982) or even formation by eruption of the giant planets concentrated in a narrow size range. and their satellites (Vsekhsvyatskii 1967). These ideas have found little support. Oort (1950) analysed the distribution of orbital energies of new (LP) comets, characterized by 1/a where a is the 1.3 Dust coagulation semi-major axis of the orbit. The more recent compilation These model calculations obviously require the dust parti- by Marsden (1989b) shown in Figure 2 confirms the very 4 1 cles to coagulate, to stick to each other at velocities up to strong peak with 1/a 10 AU . New (in the dynamical several metres per second and agglomeration to prevail over sense) comets come from distances almost comparable to destruction for metre-sized bodies at speeds of 50 m s1. the distances of nearby stars. Once penetrating the inner Then the typical timescales for formation of cometesi- Solar System their orbits are strongly perturbed mainly by 4 1 mals is a few thousand orbital periods so that comets could Jupiter with (1/a) 6 10 AU (Everhart 1968) and form within 106 years even in the Kuiper belt at 40 or rapidly diffuse to small semi-major axes if not expelled 3 50 AU. from the Solar System. Only a minute fraction ( 10 ) of The formation of cometesimals by coagulation implies these new comets will become SP comets, not enough by that the bodies are physically not homogenous but built far to explain the presently known SP comets (about 600; of subnuclei of various sizes and typically about 3 to 10 Williams 2000). To explain the few (less than 10 per year) observed newly detected comets the spherical reservoir of times smaller than the body itself. Theses subnuclei them- 12 selves show a similar structure relative to their overall size. randomly distributed comets must entail more than 10 The speed of collisions are high enough that the building (Weissmann 1980) comets. Oort demonstrated that passing blocks may be partly shattered or have penetrated each stars perturb the cloud of comets repeatedly and change the other. The details will depend on physical parameters such velocities of the comets (change of momentum) but change as density, fluffiness (fractal dimension), stickiness, tensile their orbital energies very little. Most of the comets, there- strength, and so on. These parameters will vary as a func- fore, stay bound to the Solar System even though their tion of body size. One expects voids between building orbital energies are very small. The probability of a comet blocks and volumes of increased density where penetration in the Oort cloud being directed into the inner Solar System took place. ( 30 AU) is controlled by the very small solid angle the inner Solar System encompasses seen from the fringes of The timescale for accretion and the considerable radial 5 velocities of the bodies imply radial migration over sub- the Solar System at about 10 AU. stantial heliocentric distances. Origination of particles from 2.2 The Edgewood–Kuiper belt and TNOs different heliocentric distances at different times during the formation of a cometesimal could lead to chemical differ- While Oort’s hypothesis explains the currently observed entiation, in particular in the dust to gas ratio. numbers of new and LP comets it does not account for the 53

KATHRIN ALTWEGG* AND WESLEY T. HUNTRESS, JR** The constituents of cometary nuclei

Close to the edge of our Solar System is a cloud containing The myth of the special significance of comet apparitions, many tiny objects only a few kilometers in diameter. Even however, has survived into modern times: when the large though their number has been estimated to be as large as and brilliant comet HaleÐBopp appeared in 1998, there ~1013, their total mass is still negligible compared to that of were some who saw the apparition of this comet as a divine the major planets. Their existence would go unnoticed were sign, and hid in shelters or even committed mass-suicide. it not that, from time to time, some of them are gravita- But although comets were reduced from divine objects tional by pertured and move inward, towards the Sun, and to members of the Solar System by the work of Edmund can then be observed as comets. Several space missions Halley, they have retained their special significance for sci- have been aimed uniquely at comets, among them the first entists. It has been recognized that, while comets cannot European deep space mission, Giotto. Rosetta, a corner- tell us about our future, they can at least provide a wealth of stone mission of the Horizon 2000 program of the information about the history of our Solar System. Comets European Space Agency (ESA), is one of several comet provide insights into the origin of planets and the fate of the missions being planned for the first decade of the twenty- dark molecular cloud, long since disappeared, from which first century. Why do comets deserve this special attention? the Solar System Ð including our Earth Ð emerged. Comets streak brilliantly across the night sky, their tiny nucleus obscured behind an immense coma. Their long tails set them apart from all other celestial bodies, and their COMETARY CONSTITUENTS AND THE sometimes sudden, unexpected appearance and unusual ORIGIN OF THE SOLAR SYSTEM paths across the night sky have stimulated the imaginations of skywatchers for thousands of years. In ancient China The origin of solid matter can be divided into three phases: details of comets were meticulously recorded Ð their time the synthesis of the atomic nuclei, the formation of mole- of appearance, location and shape. For a long time the ori- cules from atoms, and the condensation of gaseous atomic gin of comets was believed to be divine. Comets were usu- and molecular matter into solids. The history of cometary ally associated with major happenings in the history of matter is complex, and is shown schematically in Figure 1. nations: lost battles, the death of a king, natural disasters. Atomic nuclei are synthesized in the interiors of stars and In the seventeenth century the English astronomer Edmund released into the interstellar medium at particular stages Halley proved that comets are not doomsayers dispatched of stellar evolution (red giants, supernovae and novae). by a supernatural being, or atmospheric phenomena, but Condensation into solid grains occurs when densities are bodies in our Solar System following their own trajectories. high and temperatures fall, as happens in stellar envelopes or remnants of stellar explosions (producing high-temperature * Universität Bern, Switzerland condensates), molecular clouds (producing low-temperature ** Carnegie Institution of Washington, Washington, DC, USA condensates) and protostellar disks.

Figure 1 The origin of molecules, ice and dust in comets (after Geiss and Altwegg 1998).

Different stellar sources release matter with very differ- from meteorites (e.g. Hoppe et al. 1996). Comets are ent isotopic signatures, which are preserved in grains. expected to have preserved a larger variety of grains with the The stellar origin of grains can therefore be identified from original stellar signature than meteorites. So far, however, isotope abundance determinations, a method that has been such isotopic evidence is scarce. A few grains with high applied with great success to certain refractory grains isolated and variable 13C/12C ratios were found in the coma of THE CONSTITUENTS OF COMETARY NUCLEI 1279

Comet 1P/Halley (Jessberger and Kissel 1989) indicating a flyby mission to be launched on the European rocket multiple stellar origins. More such results should be Ariane 1. The time between the approval of this mission and obtained by the Stardust and Rosetta missions. the launch date was only five years. It was the first European Grains of stellar origin are mixed into the interstellar planetary mission, and it took a great effort from both ESA medium. When molecular clouds are formed, the grains may and the different experiment teams to build the spacecraft serve as nuclei for the condensation of more volatile mole- and its payload on time. The spacecraft was named after the cules. Upon the dispersion of such clouds into the diffuse Italian painter Giotto di Bondone, whose frescoes of the interstellar medium, the mantles of the grains can be adoration of the Magi painted in 1301 at the Scrovegni processed by ultraviolet and particle radiation (Greenberg Chapel, Padua, used Comet Halley as the Star of Bethlehem 1989). Material can be cycled many times between molecu- (the comet was visible in the night sky at that time). lar clouds and the diffuse interstellar medium. About 4.6 Gy The Giotto spacecraft, with a sophisticated payload of ago a galactic local protostellar molecular cloud partially ten instruments, was launched in June 1985 from Kourou in collapsed to form several protostellar disks, among them French Guiana. Giotto was not the only spacecraft to make our own protosolar cloud. During the collapse phase, and in the journey to Halley. A whole fleet, completed by two the solar nebula, many grains and their volatile content Japanese and two Russian spacecraft, was launched Ð other were evaporated and recondensed prior to the accretion of nations had recognized the scientific and public interest of the solid bodies of the Solar System. comets. In addition, NASA reprogrammed its International Fred Whipple (1950) was the first to recognize the Cometary Explorer (ICE) spacecraft (formerly ISEE 3, the importance of comets for the history of the Solar System. In third of the International SunÐEarth Explorer series) the last decades of the twentieth century it became accepted towards the tail of a comet. ICE went through the tail of that comets contain the most pristine material to be found in Comet 21P/GiacobiniÐZinner in 1985, becoming the first any of the bodies in the Solar System. They were created far spacecraft ever to probe a comet in situ, and detect ions from the early Sun, and spent almost their entire lifetime at from the water group (Ogilvie 1985). It was then redirected the edge of the Solar System, where the influence of the Sun towards Halley, passing upstream around 0.2 AU from was minimal, so they provide a reservoir of well-preserved Halley on 27 March 1986, and gathering data on the inter- and minimally altered material from the solar nebula. In action of the comet with the solar wind. studying this material we can deduce physical and chemical The exploration of Halley’s Comet led to an unprece- boundary conditions for the accretion of cometary material dented intensive international cooperation. The International in the solar nebula. We can trace the origin of some Halley Watch, co-ordinating the activities of astronomers and cometary molecules to the dark molecular cloud from which telescopes from all over the world, was initiated. Furthermore, the solar nebula emerged, and thus study the processes that a ‘pathfinder’ concept was established between Russia and led from the molecular cloud, through accretion into the ESA. The two Russian spacecraft, Vega 1 and Vega 2, flew solar nebula, to the present constituents of cometary nuclei. past the comet at distances of 9000 and 8000 km, respectively, a few days before Giotto. They transmitted to ESA the exact position of Halley’s nucleus, allowing Giotto to execute COMETARY SPACE MISSIONS the very precise manoeuvre that would enable it to pass the comet’s nucleus at the predefined distance of 600 km. The Halley fleet During the night of March 13/14 1986, people interested In the 1970s scientists began to prepare for the 1985/6 in cometary science either sat in the control centre in apparition of Comet 1P/Halley. Halley is the most famous Darmstadt intently watching the computer screens as the of all comets, and there were many who still remembered Giotto data were received, or followed the events on televi- its last magnificent apparition in 1910. Since the 1960s, sion. But only the succeeding months and years revealed the space missions had explored many of the planets and the wealth of data acquired during the few hours of the flyby. technology for mounting a mission to a comet was avail- Giotto survived this encounter, although its subsystems able. It was therefore obvious that Halley would present a and the payload were damaged by dust impact. It was scientifically worthwhile and a technically challenging goal decided to redirect Giotto towards a second comet, for a space mission, which at the same time would stimu- 26P/GriggÐSkjellerup. Giotto became the first European late the interest of the public. ESA and NASA planned a spacecraft to enter a long hibernation phase and to make use joint mission in which a NASA-built spacecraft would head of a planetary gravitational assist. In 1990, after four years of for a rendezvous with Comet Tempel 2, releasing en route hibernation, it passed the Earth at a distance of ~23,000 km, an ESA-built probe that would be directed to encounter and finally in July 1992 it passed comet GriggÐSkjellerup Halley. However, when NASA failed to obtain support for at a distance of 95 km. Another ‘night of the comet’ was this mission from the US Government, ESA proceeded celebrated in Darmstadt and on television. GriggÐSkjellerup alone, and the mission was redefined from a rendezvous to is a short-period comet, but not very active. The flyby 1280 KATHRIN ALTWEGG AND WESLEY T. HUNTRESS, JR

geometry was very different from that at Comet Halley, and century, but speculation on the composition of comets has a the scientific results were somewhat less, but the data col- longer history. Ancient popular traditions held that comets lected led to some interesting and novel findings about the are responsible for major effects, generally negative, on interaction between comets and the solar wind. Analysis of earthly life during their apparitions. Isaac Newton Ð whose the Giotto data from the Halley and GriggÐSkjellerup Principia created a major scientific breakthrough in under- encounters continued throughout the 1990s. standing the motion of heavenly bodies, including comets Ð Giotto’s flyby of Comet Halley was certainly one of the was himself influenced by these beliefs, and was convinced highlights of ESA’s space program in the twentieth century. But that emanations from comets could lead to the spontaneous the Russian spacecraft also proved very successful. For Japan, generation of plant life (Oparin 1938). Newton’s more cer- the two spacecraft Suisei and Sakigake were the first success- tain connection with comets was through his relationship ful planetary missions. The absence of NASA’s participation with Edmund Halley, who prompted Newton to write in the Halley space armada was notable, and leadership in the Principia and also funded its publication. space exploration of comets passed to ESA through the Giotto Speculation related to the composition of comets also spacecraft and its particularly close flyby of Comet Halley. appeared during the nineteenth-century debate over Darwin’s theory of the evolution of life. Unlike the rival theory of spontaneous generation, Darwin’s concepts of Future space missions natural selection and common ancestry required a singular In the 1990s the importance of cometary science was defi- and ancient origin for all life. The concept of panspermia nitely established. The two great comets of that decade Ð was brought to bear on this issue by von Helmholtz (1871), Hyakutake (C/1996 B2) in 1996 and HaleÐBopp (C/1995 O1) who wrote, ‘Who can say whether the comets and meteors in 1997 Ð triggered further interest in comets. This led to a big which swarm everywhere through space, may not scatter step forward in remote sensing of cometary constituents. But germs wherever a new world has reached the stage in which it also became evident that in situ measurements were far it is a suitable place for organic beings.’ superior for a full picture of the nature of comets. New comet missions were planned in the 1990s which have not yet been launched or have not yet reached their goal. The US Comet 1900–1950 Rendezvous and Asteroid Flyby (CRAF) mission, planned At the opening of the twentieth century the idea of pansper- just after Giotto, was cancelled in 1991. The European mia, that the Earth was seeded with life from space, partic- Rosetta mission, originally planned as a sample-and-return ularly by comets, was one of the leading explanations for mission jointly with NASA, was redefined to an ESA-built the origin of life on Earth and had many proponents, comet rendezvous mission with a German-built lander. It will including Lord Kelvin and Svante Arrhenius. Thomas be launched in January 2003 for a 2011 rendezvous with Chamberlin (1911) attacked the idea that spores could be Comet 46P/Wirtanen, which will then be near aphelion. The driven across the Universe to transport life from planet to spacecraft will accompany the comet in close vicinity through planet, and proposed an alternate hypothesis that ‘planetesi- its perihelion passage in 2013. NASA launched the Stardust mals’, the small bodies out of which the planets formed, mission in 1998; a small mission aimed at collecting dust in could have been a source of organic molecules on the early the coma of Comet 81P/Wild 2. Stardust will return its sam- Earth. ple to Earth in January 2006. In addition, NASA is redirecting According to Chamberlin and Chamberlin (1908), its Deep Space 1 spacecraft to flyby the comets 19P/Borrelly organic molecules imported to the early Earth by infalling and 107P/WilsonÐHarrington. NASA is planning two addi- planetesimals could have been the basis for abiotic chemi- tional missions, the CONTOUR mission to flyby three cal evolution on the young planet that would produce comets Ð 2P/Encke in 2003, 73P/SchwassmannÐWachmann 3 the chemicals necessary for life. Chamberlin was clearly in 2006 and 6P/d’Arrest in 2008 Ð and Deep Impact, which influenced by the nineteenth-century discoveries of organic will direct an impactor at comet 9P/Tempel 1 and examine compounds in meteorites (Berzelius 1834, Wöhler 1858, the results during a flyby mission in 2005. Wöhler and Hoernes 1859). Chamberlin’s work was aimed more towards an understanding of the origin of the Solar System, but included the original insight linking the origin KNOWLEDGE OF COMETARY CONSTITUENTS of Earth with that of life on the planet. This work went BEFORE THE HALLEY ENCOUNTER essentially unnoticed until the end of this century (Oró and Lazcano 1996). Before 1900 By the end of the first quarter of the twentieth century, The first measurements of comet composition were made by the first detection had been made of molecular species in astronomical spectroscopy in the first half of the twentieth comets. The new technique of astronomical spectroscopy in 54

JAMES W. HEAD III* The Moon and terrestrial planets: geology and geophysics

Before the Space Age, the study of the planets and their things: a tremendous diversity of characteristics, a handful of satellites was the domain of astronomers. Telescopic obser- emerging themes, and a host of new questions. vations of planetary bodies focused on determining their How did planetary geologists and geophysicists approach size, position, orbit, density, average surface composition, these problems? Imagine observing a group of human and the physical state of their surface through photometric beings. You might initially sort them by size and shape, and analysis. With few exceptions, geologists and geophysicists then distinguish them by various other physical attributes. were occupied with the analysis of Earth, the very complex But when the time came to understand the factors and planet beneath their feet. But even in the latter part of the processes that were responsible for these characteristics, you nineteenth century and the first half of the twentieth, a few would need to look more closely, and to understand what geoscientists puzzled over the surface of the Moon, the was going on inside. Beneath the superficial variations in nature of meteorites, and the geochemistry of the Solar surface skin and in hair color and tone, what were the System. G. K. Gilbert, Harold Urey, Ralph Baldwin, Gilbert processes that were responsible for the activity of each of Fielder, and Eugene Shoemaker each contributed to the these organisms, its origin and its evolution? What internal foundation that was to become the basis for modern plane- structure and processes were responsible for its present tary geoscience (Stevenson 2000). Convergence between state? How did the organism regulate its internal heat in the these disciplines began with the Space Age, and the ability to light of such extreme external variations? How did it give see the surfaces of other planetary bodies up close, to probe birth, how has it aged, and how will it die? Similar ques- their interiors, and to analyze them from orbiters and flybys. tions are key to understanding the birth and evolution of the More sophisticated and higher-resolution observations from array of terrestrial planetary bodies. As with humans, plan- spacecraft led to an understanding of the mineralogic ets are complex systems in which most of the driving forces makeup of their crusts through spectroscopic observations, and regulating processes are hidden below the surface. and the distribution of their surface features through geolo- Surface features can give clues as to how the interior works, gical analysis of images taken at visible and radar wave- but a detailed examination of the inside of these bodies is lengths. Sophisticated tracking of spacecraft and direct necessary before any real picture of the evolution of the deployment of instruments led to new insights into planetary planet as a whole can emerge. Of course, planets are not geophysics. In the last half of the twentieth century, intense simple organisms that can be brought into the laboratory, exploration of the Solar System changed planetary bodies studied, and dissected, allowing us to map out the anatomy from solely astronomical objects to geological and geophysi- of the interior and the role this plays in the evolution of cal entities, and the picture that began to emerge was similar its external features. Instead, indirect measurements are to that derived from the examination of any population of required, and indeed the surface features must be studied in detail if we are to infer the nature of the interior and how it * Brown University, Providence, RI, USA may have changed with time.

Thus, among the most fundamental areas of analysis are observations that might lead us to an understanding of their the nature of planetary surfaces, the processes and sequence present and past states. Armed with the basic physical prop- of events implied by their geologic records, and the struc- erties of the terrestrial or Earth-like planets (size, density, ture and state of their interiors and how they have evolved position in the Solar System), we turn to a brief description with time. One of the supreme achievements of the Space of the nature and ages of the surfaces of each terrestrial Age has been the links that have been forged between geol- planetary body, and what is now known about their interi- ogy and geophysics to address these issues and to sketch ors. We start with the smallest, the Moon, and proceed out the story of planetary evolution. upward in size, via Mercury, Mars, and Venus, to Earth. What was the intellectual basis for this phase of planetary What were the major external (e.g., impact cratering) and exploration? We had many questions about planetary interi- internal (e.g., volcanism) processes that were responsible ors: What is their basic structure; are they homogeneous, an for their evolution, and when did most of this activity occur? even mixture of planetary ingredients throughout their inte- What do we know about each planetary body’s interior, and rior? If not, are they like a plum pudding, or layered, perhaps how does this relate to what we see on the surface? Once we in several large layers, or like a torte, with multiple thin lay- have this basic information, we can explore some themes and ers? And what are these layers made of? Are their composi- processes that help to explain the observed characteristics, tions sorted by density, and if so, how does the increase in and address such questions as: What are the major factors pressure with depth inside planets influence the changes in that cause the differences in the interiors and geological his- the state of these materials? When and why did this structure tories of the planets? How much of a planet’s history is pre- develop, and how does it change with time? Volcanic activity determined by its starting conditions (its “genetic” makeup), shows that planets are hotter in their interior than at the sur- and how much is determined by its later history (its “envi- face. Where does this heat come from, how is it distributed in ronment”)? We conclude with several outstanding questions the interior, and what are the processes by which the planet that need to be investigated in the exploration of the Earth redistributes and gets rid of heat? And how has this changed and terrestrial planetary bodies in the twenty-first century. with time, in the course of the planet’s history? Once this type of knowledge is to hand, we can ask even more sophisticated questions: How do planets differ in their basic internal THE NATURE OF PLANETARY INTERIORS AND structure, and how does this determine their evolution? What GEOPHYSICAL PROCESSES role does size, and position in the solar nebula during planetary formation, play in the further evolution of the planets? Present planetary interior structure can be viewed from The approach to addressing these questions was neither two perspectives: that of internal compositional variations, intellectual nor systematic. Missions and experiments were usually configured in layers such as crust, mantle, and core; undertaken or denied for a variety of reasons: national goals, and that of variations in internal temperature and state, national security, proximity to Earth, international competi- producing layers on Earth such as the lithosphere, the tion, international cooperation, technological sophistication, asthenosphere, and the outer molten and inner solid core. financial constraints, politics, professional advocacy, per- Internal compositional variations are produced by differen- sonal advocacy, and scientific rationale. We explored the tiation Ð that is, the segregation of materials of different Moon before we analyzed the surfaces of Venus and composition from more primitive and homogeneous parent Mercury. The Soviet Union sent many missions to Venus, materials. Differentiation can be rapid and catastrophic, as while the United States sent only a few. After the crowning in the case of core formation, a process in which denser achievements of Apollo, most of the Moon’s surface iron-rich material sinks to the deepest interior. Because remained unstudied by spacecraft for over twenty years. The planets are hotter in their early history, core formation is sequential exploration of the Solar System is a complex thought to occur in the first few percent of a planet’s his- story of the historical interplay of these many factors, and tory, and the amount of gravitational potential energy it has been told elsewhere (Stevenson 2000, Chapman 1988, releases is so high that surface melting is implied, at least Cruikshank 1983, Colin 1983, Vaniman et al. 1991, Kieffer for the larger planets. Differentiation can take place over et al. 1992, Snyder and Moroz 1992, Morrison 1999). increasingly longer periods of time as heat in the interior In this chapter we integrate these important historical causes partial melting of the mantle and the ascent of the steps into an overview of the geology and geophysics of hotter, less dense melt products (magma) toward the surface terrestrial planetary bodies, as revealed by exploration of to form crustal materials (intrusions, and extrusions such as the Solar System in the last half of the twentieth century. volcanoes). Depending on the amount of energy involved, We first briefly outline the basic processes that may have and the way it is distributed in the interior, melting and dif- been involved in the formation and evolution of planetary ferentiation can be global (e.g., if the energy is from a high interiors, and then look at the types of measurements and influx of globally distributed random impacts), or local to THE MOON AND TERRESTRIAL PLANETS: GEOLOGY AND GEOPHYSICS 1297 regional (e.g., in the case of mantle plumes producing cir- and heat flow probes, which have so far been emplaced cular hot spots, or a global system of linear cracks at mid- only on the Earth and Moon. oceanic ridges). Variations in the relative proportions and timing of these factors can lead to major differences in the nature and age of the outer differentiated layer, the crust. INTERNAL STRUCTURE AND GEOLOGICAL Energy from early intense bombardment can produce a pri- HISTORY OF THE TERRESTRIAL PLANETARY mary crust, later internal heating and melting can produce BODIES materials for a secondary crust, and reworking of these earlier crusts can yield tertiary crusts (Taylor 1989). The Moon One of the most fundamental aspects of understanding planetary interiors comes from knowledge of heat. What The Moon, as our closest neighbor in space, was the first are the sources of heat, how much of it does a planet have body to attract the attention of geologists studying other at any given time, what is its internal distribution, and how planetary surfaces. Eugene Shoemaker and his co-workers does a planet transfer and get rid of heat over time? These (Don Wilhelms, Jack McCauley, Baerbel Lucchitta, Elliot simple questions are the keys to understanding planetary Morris, Farouk El Baz, and others) applied the basic princi- evolution. How much heat derives from initial accretional ples of terrestrial stratigraphy to lunar surface features and energy, position relative to the Sun, electromagnetic heat- geologic structures, and this enabled them to define geologic ing, core formation and other density instabilities, large units and produce geologic maps, and thus to delineate the impacts, short and long-term radioactive decay of minerals, major surface processes and the sequence of events in lunar and tidal interactions? How is this heat distributed over history. Similar mapping techniques have been applied to time in what is known as a planet’s thermal history? What each successive planet explored over the last 25 years, and is the rate of change of temperature as a function of depth, the collective maps (Carr et al. 1984, Head 1999) provide and how does this relate to the physical state of material the basis for understanding the history of each planet and (e.g., liquid, partially molten, solid)? What is the nature and comparative planetary evolution. The will and determination stability of thermal boundary layers, the transitional layers of a handful of geoscientists in the USA (Harold Urey, separating materials with different temperatures? How is James Arnold, Gerald Wasserburg, Robert Walker, Paul heat transferred through the course of the planet’s evolu- Gast, and George Wetherill and colleagues) and the Soviet tion? What role does conduction play? How important is Union (M. V. Keldysh, A. P. Vinogradov, Roald Sagdeev, convection? How significant is advection Ð the direct trans- Valery Barsukov, Mikhail Marov and colleagues) convinced fer of heat by movement of molten material from the inte- their respective governments that international competition rior to the surface as in the volcanic flooding of a planetary could also produce significant scientific results. surface? How does material behave under the tremendous Pluto and Charon excepted, the Moon is the largest satel- temperatures and pressures typical of planetary interiors? lite, relative to its parent body, in the Solar System, and its What materials change phases (rearrange their internal mode of formation has captivated scientists for years. structure), and how do they then behave? And how does all Current thinking is that the Moon formed very early in Solar this add up? How do different planets lose heat as a func- System history when a Mars-sized object, one-half the tion of time in what is known as their thermal evolution? diameter of the Earth, impacted the proto-Earth, ejecting Are there many paths or only a few? And what are the fac- crust and upper mantle material which re-accreted in Earth tors that determine this? And where are we going Ð if the orbit to form the Moon (e.g., Hartmann and Davis 1975). planets are indeed evolving, where is the Earth heading? Soon afterwards a global crust formed, under a bombard- Our present knowledge of the interior of most planets is ment that lasted for several hundred million years in which a based on their bulk density and on information obtained massive influx of projectiles impacted the newly formed from measurements of surface geochemistry, moment of surface at several kilometers a second, producing impact inertia, gravity, and present and fossil magnetic fields. Also, craters of many sizes. This bombardment fragmented and very significant are inferences made from geologic struc- fractured the upper few kilometers of the Moon’s crust to ture, topography, and geologic history, analogies with form a thick soil layer (megaregolith) and produced interfin- Earth, and assumptions about starting conditions. Deep gering global geologic units of ejecta representing the first drilling, and tectonic uplift and exposure of rocks from few hundred million years of lunar history (e.g., Wilhelms depth, provide some information about the upper few hun- 1987). This so-called late heavy bombardment ended about dredths of a percent of the Earth’s radius, and impact cra- 3.8 billion years ago (Wasserburg et al. 1977), but not tering can expose material from greater depths, perhaps before the largest projectiles had excavated huge depres- even below the crust, on some planets. Detailed assessment sions (as large as 2,000 km in diameter) and spread ejecta of the interior of a planet requires surface seismic networks over immense areas (Spudis 1993), producing the extremely 1298 JAMES W. HEAD III

Figure 1 The heavily cratered lunar farside. The 75 km diameter King crater, with its lobster-claw-like central peaks, is the sharp-rimmed crater just to the lower left of the center. (NASA Apollo 16 image.) rough surface topography typical of the lunar highlands that maria: linear rilles and graben, formed by crustal extension, we see today (Figure 1). Data from the US missions Galileo, were followed by sinuous (wrinkle) ridges formed by con- Clementine, and Lunar Prospector have provided a global traction (Solomon and Head 1980) (Figure 3). Virtually no view of the topography and mineralogy of the lunar crust major internally generated geologic activity for the last (Spudis 1999), and have revealed details of a huge impact 2.5 billion years is manifested on the lunar surface. The basin on the lunar farside that excavated to lower crustal and Moon thus provides a picture of the first half of Solar perhaps mantle depths (e.g., Pieters et al. 2000). System history (characterized by impact bombardment and Volcanic flooding of the surface of the Moon became early volcanism), and serves as a benchmark for the inter- evident during the waning stages of the late heavy bom- pretation of the records preserved on other terrestrial planets. bardment. By about 2.5Ð3.0 billion years ago, basaltic lavas How do these features relate to the nature of the Lunar inte- had covered approximately 17% of the lunar surface, pref- rior? Deployment of seismic instruments on the Moon and erentially filling in the nearside, low-lying basin interiors to analysis of the results by Frank Press, Nafi Toksoz, Gary the lunar maria (Figure 2). Volcanic eruptions on the Moon Latham, and others have helped to address this question. were volumetrically significant, but far and few between, Lunar samples collected by US Apollo and Soviet Luna and occurred predominantly in the first half of its history, missions (e.g., Papike et al. 1998), remote sensing, and sur- under a rapidly decreasing flux (e.g., Head and Wilson face seismic data show that the Moon has been internally dif- 1992). Tectonic activity on the Moon stands in stark con- ferentiated into a crust, mantle, and possibly a small core trast to that on our own planet. The limited array of lunar (Figure 4). The feldspar-rich crust is thinner on the central tectonic features occurs predominantly in and near the nearside, about 55 km, but may reach thicknesses of 100 km 55

LAURENCE E. NYQUIST,* DONALD D. BOGARD* AND CHI-YU SHIH** Radiometric chronology of the Moon and Mars

“How old is the Earth?” was a topic of scientific inquiry in The physical basis for determining the absolute ages of the late nineteenth century. In the twentieth century the ques- objects within our Solar System in the twentieth century is tion became “How old are the Earth and other objects in natural radioactivity, a phenomenon initially discovered in the Solar System?” Related questions are: “How old is the the closing years of the nineteenth century. A collection of Solar System?”; “How has the Earth changed over geologic radioactive atoms can be thought of as being like a clock, time?”; and “How have the planetary bodies in the Solar which is running at a constant and measurable speed. But System changed over time?” Also, “What has made Earth unlike an ordinary clock which tells only the present time, unique in our Solar System?” With the aid of spacecraft- the accumulated daughter products of radioactive decay acquired data and samples, and through the study of lunar give a measure of how long the radioactive clock has been and Martian meteorites, we are beginning to answer some of running since it was initially “set.” these questions as they relate to the Moon and Mars. A century ago the age of the Earth was addressed by diverse approaches. These included calculations of how USING NATURAL RADIOACTIVITY TO TELL long it would take an initially hot Earth to lose its heat by GEOLOGIC TIME thermal conduction and radiation to space; how long it would take the oceans to attain their current load of salt, Historical background assuming an estimated modern rate of weathering of surface rocks; and estimates of the rate of regression of the The discovery of natural radioactivity by H. Becquerel (1896) Moon away from the Earth. The ages obtained by such set scientists on the path to quantitative definition of geologic methods were often of the order of 10Ð100 Ma (million time. In an instance of scientific serendipity, Becquerel found years). Unfortunately, these methods were based on faulty that photographic plates kept near uranium-bearing minerals assumptions. For example, estimates of the age of the Earth became darkened. Marie Sklodowska Curie pursued the based on observable geological processes failed to account observation for her PhD dissertation, noting that both ura- for significant changes in the rate and nature of those nium and thorium were active, and that two minerals of ura- processes acting over very long periods of time. Also, cal- nium, pitchblende and chalcolite, had greater radioactivity culations of the thermal cooling of the Earth failed to con- than uranium itself (Curie 1898). Pursuing the great activity sider the heat generated within the Earth by the natural of pitchblende, she and her husband, Pierre Curie, isolated radioactivity of potassium, uranium, and thorium. the “new” element polonium from this mineral (Curie and Curie 1898). Next, the Curies, with G. Bèmont, also isolated radium from pitchblende (Curie et al. 1898). Although these * NASA Ð Johnson Space Center, Houston, TX, USA shorter-lived daughter elements are far more radioactive ** LockheedÐMartin, Houston, TX, USA than uranium, our interest here remains with uranium, their

Laurence E. Nyquist, Donald D. Bogard and Chi-Yu Shih, The Century of Space Science, 1325Ð1376 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1326 LAURENCE E. NYQUIST, DONALD D. BOGARD AND CHI-YU SHIH radioactive parent. Its decay period, or half-life, was soon variable number of neutrons. The atomic mass of an atom is found to extend over geologic time, and thus its radioactive approximately equal to the total number of protons and decay was recognized as a possible basis for a geologic neutrons it contains. These concepts differ from those of timescale (Rutherford 1906). For historical completeness, we Soddy (1913Ð14) principally in that he envisioned the note that Schmidt (1898) also discovered the radioactivity of nucleus to consist of electrons plus protons, and that the thorium contemporaneously with M. Curie. Within a decade, algebraic sum of charges in the nucleus gave the atomic two more long-lived naturally radioactive elements had been number. Nuclei having differing arithmetical sums of parti- discovered which also could be used to tell geologic time: cles would have differing atomic masses, but could never- potassium (Campbell and Wood 1906, Campbell 1906) and theless share atomic numbers, constituting his definition of rubidium (Campbell and Wood 1906, Campbell 1908). The isotopes. Rutherford later suggested that protons and elec- radioactivity of a fourth element currently used for geologic trons in the nucleus could be so closely bound that they age determinations, samarium, was discovered considerably would in effect constitute a new particle, the neutron. This later (Hevesey et al. 1933). Thus, the fundamental scientific suggestion was confirmed by Chadwick’s work. discoveries leading to a quantitative geologic timescale were The modern term nuclide refers to an atom of specific made near the beginning of the twentieth century. atomic number and specific atomic mass. Radioactive The actual development of rigorous radiometric methods decay results in conversion of a number, P, of radioactive of telling geologic time occupied much of the first half of parent nuclides into a number, D, of daughter nuclides. The the century. Geochronology is the subject of a voluminous daughter nuclide most often is an isotope of a different ele- literature, and of a number of standard texts (e.g. Faure ment than the parent, but sometimes branched-decay into 1986). Here, we hope only to give enough background for more than one type of daughter nuclide also occurs. The the reader to gain an appreciation of the developments that activity, or decay rate, of P parent nuclides is permitted its application to the absolute chronology of the dP P (1) Moon and Mars. In preparing this overview, we rely heav- dt ily, but not exclusively, on “benchmark” papers assembled where is the decay constant, the probability per unit time by Harper (1973), and his associated commentary. that a parent-nuclide will decay. The decay constant is The development of geochronology during the first three related to the half-life, , the time interval required for half decades of the twentieth century was intimately intertwined the parent nuclides to decay by with emerging views of the structure of the atom. The study of radioactive elements showed that atoms of the same chem- (ln 2) (2) ical element could have different atomic weights. For example, we now know that “natural” uranium of atomic number The number of radiogenic daughter nuclides, D*, pro- 92 decays via -particle emission with a very long half-life to duced in the time interval (t t0) is given by the equation: the element of atomic number 90 (thorium), which, after two (t t0) -decays, becomes element 92 (uranium) again, but with a D*(t) P(t) [e 1] (3) much shorter half-life than it had originally. However, its where t is the time when the volume containing the parent 0 atomic mass is lighter by 4 mass units, the mass of the - nuclides became closed, and the daughter nuclides began to particle. This reasoning was expressed by Soddy (1913Ð14), be retained, and t is the time when the numbers of parent who suggested the name isotopes for these two types of ura- and daughter nuclides are measured. In geochronology, the nium. The name stems from the Greek isos (equal) and topos volume of interest is a rock or mineral, and (t t0) T, the (place), meaning that two different kinds of atoms occupy the age of that rock or mineral. same place in the periodic table of the elements. J.J. Thomson It often is convenient to normalize D*(t) to the number also discovered that atoms of a stable element, neon, could of nuclides, N, of a non-radiogenic isotope of the same ele- have differing atomic weights (Thomson 1913). Later, F.W. ment as the daugther to obtain: Aston, a student of Thompson, discovered that many other (t t0) elements also were composed of differing isotopes. Romer D*(t) P(t)[e 1] (4) (1970) provides a historical account of the multiple contribu- N N tions by many investigators to the “discovery” of isotopes. In natural systems, a number, D0, of daughter nuclides A complete understanding of isotopes was not achieved, may be present at time t0. In this case, analytical measure- however, until the discovery of the neutron by Chadwick ments yield not D*(t), but D(t) D*(t) D0. Thus, in (1932). This discovery supported the view of the nucleus as practice, eqn (4) is modified to composed of protons and neutrons. Atoms of a given ele- D(t) D P(t)[e (t t0)1] ment have a fixed number of protons, called the atomic 0 (5) number. However, atoms of a given element may have a N N N RADIOMETRIC CHRONOLOGY OF THE MOON AND MARS 1327 which contains the measured quantities directly. Although photographic plate was exposed to the impact of electri- eqns (4) and (5) are simple modifications of eqn (3), deter- cally accelerated ions. The latter caused ionization of the mining which equation best described various real systems photographic emulsion on the plate proportional to the played a significant role in the development of geochronology. number of incident ions. In later instruments, the magnitude of the positive ion current was measured directly. Such instruments are called mass spectrometers and in modern Development of U–Pb dating geochronology are used exclusively in preference to mass Not surprisingly, the initial application of the above con- spectrographs. cepts involved the -decay of uranium and thorium (e.g. Piggot (1928) records how he wrote to Aston, in October Rutherford 1906). Uranium and thorium decay define sepa- 1926, with the suggestion that several problems encoun- rate -decay series ending with three stable isotopes of lead: tered in the chemical lead method, including the poorly 206Pb, 207Pb, and 208Pb. A fourth, minor, stable isotope, known rate of thorium decay, could be overcome by means 204Pb, is non-radiogenic. The two stable elements resulting of mass spectrograph analysis, which would allow the iden- from uranium and thorium decay are thus lead and helium. tification of those lead isotopes produced by uranium decay Historically, this gave rise to the “chemical lead method” only. He also wrote that it was he who supplied Aston and the “helium method” of telling geologic time, because with the sample of lead tetramethyl used for the analysis only chemical methods were initially available to measure reported in the 1927 Nature article, and that he was “work- the products of radioactive decay. Thus, both methods ing up” a sample of radiogenic lead from some very pure initially attempted to apply eqn (3) with the simplifying Norwegian bröggerite, a mineral containing considerable assumptions that (a) the daughter element was initially proportions of uranium and lead, but a very small propor- absent in the rock or mineral analyzed, and (b) the contribution of thorium. From this sample he hoped to determine tion of thorium decay could be ignored, since with purely directly the uranium-to-lead ratio, “and thereby secure a chemical means there was no way of telling which portion reliable estimate of its age.” The next year, Aston (1929) of the helium or lead came from thorium. These considera- reported the results of the isotopic analysis of the brög- tions initially led to application of the UÐPb method only to gerite with the comment: “These figures have been commu- uranium-rich ores and minerals. nicated to Mr Piggot, and when combined with the analyses Arthur Holmes, in writing the first of several treatises of the mineral should enable its age to be fixed with consid- dealing with the age of the Earth (Holmes 1913), placed erable certainty.” the helium and lead ages of the day into the geological Fenner and Piggot (1929) later reported the age of timescale, placing the oldest geological era, the Archean, at the bröggerite as between 920 and 908 Ma as determined 1400Ð1600 Ma ago according to the lead method. A younger from the 206Pb and (206Pb207Pb) abundances, respectively, age for the Archean of about 700 Ma was indicated by the and 1313 Ma as determined from the 208Pb abundance. helium method, but was attributed to long-term leakage of Rutherford (1929), in a companion paper to the earlier some helium from such ancient minerals, a potential prob- paper by Aston (1929), had assumed the age to be 1000 Ma. lem anticipated by Rutherford (1906). Holmes (1913) wrote: He focused instead on interpreting the spectrographic line “Radioactive minerals, for the geologist, are clocks wound corresponding to mass number 207, rather than the mineral up at the time of their origin. After a few years’ preliminary age. Rutherford, like Aston (1929), interpreted mass 207 as work, we now are confident that the means of reading these the end product of actinium decay, and further inferred its time-keepers is in our possession.” Not all geologists of the ultimate source as an isotope of uranium, of mass 235, time agreed with him, and indeed chemists and physicists which he called actino-uranium. Rutherford estimated “the were still struggling to better understand the phenomenon of period of transformation of the new isotope” to be 3.4 Ga. radioactivity itself. Much more effort would go into refine- He interpreted this period as an upper limit to the age of ment of the methods of telling geologic time. the Earth, noting that it was “about twice the age of the The possibility of mass spectrographic determination of oldest known radioactive minerals.” In doing so, Rutherford the isotopic composition of lead was realized within the (a) inferred the existence of undiscovered 235U from the next decade, and led to significant refinements of the UÐPb existence of 207Pb, (b) estimated the half-life of 235U, (c) uti- method. Aston (1927) reported the first determination of the lized the knowledge that odd-mass isotopes are less abun- isotopic composition of lead in collaboration with C.S. Piggot dant than the main even-mass ones to infer 235U/238U 1 of the Geophysical Laboratory, Washington. Aston’s instru- at production, and (d) estimated an upper limit on the time ment was called a mass spectrograph because it separated of uranium decay, and by assumption on the age of the positively charged ions according to mass, in a manner Earth, from the estimated present-day 235U/238U ratio. similar to that in which a light prism separates sunlight Rutherford’s estimate was within 35% of the age presently into a spectrum of differing wavelengths, and because a accepted, but contained several errors, including the 1328 LAURENCE E. NYQUIST, DONALD D. BOGARD AND CHI-YU SHIH

assumption that uranium was initially produced in the Sun. Furthermore, his estimate (a) used a present-day 235U abundance that was more than twofold too low; and (b) used an estimated ratio of the decay constants for 235U and 238U, 235/ 238 10.6, which was too high by about 70%. This estimate nevertheless illustrates Rutherford’s scientific insight. The next advance in UÐPb dating was made without aid of a mass spectrograph. Rose and Stranathan (1936) determined the ratio of 207Pb (“AcD”) to 206Pb (“RaG”) for a number of radiogenic mineral leads from the hyperfine structure of their emission spectra. They assumed the actual progenitor of AcD was not known with certainty, calling it AcU to distinguish it from UI (238U) and UII (234U). They identified the probable mass of AcU as 235 or 239, most probably the former, following earlier suggestions (Russell 1924, Hahn 1925). Rose and Stranathan (1936) derived an expression for the age of the minerals as a function of 207Pb/ 206Pb and 235U/ 238U, but not explicitly dependent on the Pb/U ratio; that is, they determined the “PbÐPb” ages of the minerals, and showed them to be in reasonable agreement with the UÐPb ages. However, they incorrectly assumed 235/ 238 to lie between 10 and 11, from the earlier suggestions of Rutherford and others. Moreover, the ages of their oldest minerals were about 1 Ga, and did not improve estimates of the age of the Earth. In a pair of classic papers, Nier (1939a,b) simultane- Figure 1 Mass spectrum showing the relative abundance of ously reclaimed UÐPb dating for mass spectrometry, and the isotopes 207Pb and 206Pb, daughter products of decay of put the method on a sound footing by positively identifying 235 238 235 U and U, in pitchblende from Katanga, Africa (Nier AcU as U and by refining the values of the half-lives of 1939b). The heights of the spectral peaks, and thus the 238 235 both U and U. He also measured the isotopic composi- abundances of the isotopes, are proportional to the positive tion of Pb in some U-rich minerals. Figure 1 is reproduced ion current striking the ion-collecting electrode of a mass from Nier (1939b), and shows the relative abundance of spectrometer. In this pioneering work, the positive ion current 207Pb and 206Pb in pitchblende from Katanga, Africa. This was measured with a ballistic galvanometer, as found in many “mass spectrum” shows that the Pb in this mineral, which elementary physics laboratories early in the twentieth century. Nier (1939b) reported as containing 72.2% U, was entirely The magnitude of the current was measured by the torsion of radiogenic, and produced from U decay only, being free of the galvanometer coil. A light beam was directed to a mirror both “common” 204Pb and of 208Pb from 232Th decay. From attached to the coil, and the coil’s torsion measured as meters of deflection of the reflected beam on an opposing laboratory the known -disintegration rate of bulk uranium, Nier wall (A.O.C. Nier, personal communication). derived decay constants for 238U and 235U, respectively, that were within 2% of currently accepted values of 1.551 1010 a1 and 9.85 1010 a1 (Jaffey et al. 1971). superior to other instruments of its day. Those interested in Furthermore, the corresponding ratio of decay constants the historical development of mass spectrometry, the study that he used was within 1% of the currently accepted value of the isotopic abundances of the elements, and geochronol- of 6.35, and significantly different from previous estimates. ogy will find his short autobiographical sketch (Nier 1981) Nier (1939b) calculated ages from the 207Pb/206Pb ratios in fascinating reading. His discovery of 235U not only allowed the minerals he analyzed, and found that one uraninite from it to be identified as the radioactive parent of 207Pb, but also Manitoba, Canada, was 2200 Ma old, older than the then led to its identification as a fissionable uranium isotope accepted age of the Earth. (Nier et al. 1940). Nier often proudly referred to a hand- Nier, a physicist, had entered the field of geochronology written note he had received from E. Fermi in October through work as a postdoctoral fellow at Harvard 1939, to the effect that “deciding whether the slow neutron University in the early 1930s, and, after returning to the fission (of uranium) is or is not due to the 235 isotope … is University of Minnesota, by building a mass spectrometer of considerable theoretical and possibly practical interest.” 56

HEINRICH WÄNKE* Chemical evolution of the Moon and the terrestrial planets

Knowledge of the chemical evolution of the Moon, of Viking 1 on 20 August 1975, landed on Mars on Mercury, Venus, and Mars and the contributions to this 3 September 1976 and transmitted data for 3.5 years. Viking knowledge from space missions vary greatly from body to 1 was launched on 9 September 1975, landed on 20 July body. Without doubt the Moon is a very special case. The 1976 and transmitted data for 6.5 years. Between 1962 and six Apollo missions that landed there returned a total of 1988, the Russians sent many missions to Mars which 381.7 kg of lunar material. The first of these missions was mostly failed or achieved only limited success. The excep- launched on 16 July 1969 and returned to Earth on 24 July tion was Phobos 2, which arrived at Mars orbit on 29 1969. It was followed by Apollo 12, 14, 15, 16, and 17, the January 1989; it identified water vapor in the Martian last of which was launched on 7 December 1972. Apollo 15, atmosphere, returned images of Mars and Phobos as well as 16, and 17 carried a rover vehicle that extended the range of visual and infrared spectra, but stopped transmitting data the astronauts considerably. During the Apollo 17 mission before it was able to deploy surface stations on Phobos. A the rover covered a total of 30.5 km. The Russian uncrewed large increase in our knowledge of the geochemistry of missions Luna 16, Luna 20, and Luna 24 returned about Mars was made by the very successful NASA Pathfinder 370 g of lunar material. The robot rover of the Luna 17 mis- mission. Its rover Sojourner, traveling a total of 104 m, sion traveled 10.5 km in 322 days and that of Luna 21 trav- returned high-quality data on the chemistry of five rocks and eled 37 km in 139 days. In addition, 13 lunar meteorites six soil samples analyzed by the APX-spectrometer on totaling 4.1 kg have been recognized and studied thoroughly. board the rover in addition to a number of close-up images. It should be stressed that with respect to the chemical The second source of information on the chemical evolution evolution of a planet the availability of samples increases of Mars stems from Martian meteorites, of which 14 with a possible insights both in quantity and quality. Hence, it is no total weight of 81 kg have been recognized and studied. surprise that, except for the Earth, the Moon is the best stud- Venus is a most difficult planet to study because of the ied object in the inner Solar System. For the foreseeable very harsh conditions on its surface. Nevertheless valuable future there is no chance that even the most sophisticated data on the chemical composition of the Venusian surface instruments flown to anywhere in the Solar System might were obtained by the Soviet Venera and Vega missions, return as precise and detailed information as that obtainable launched between 1972 and 1984, which transmitted data, from investigations of samples in laboratories on Earth. Venera 13 relaying the first color pictures of the surface. With respect to our understanding of the subject of this However, the returned data do not allow reliable conclu- chapter, next to the Moon comes Mars. Viking 1 and 2 sions to be drawn on the chemical evolution of this planet. returned the first information on the chemical composition Almost nothing is known of the chemistry of Mercury. of the Martian soil which seems to be well mixed on a This is very unfortunate as Mercury has the highest density global scale by strong storms. Viking 2 was launched ahead of all planets and, hence, this must be reflected in its chemical composition. It is hoped that the missions planned both * Max-Planck-Institut für Chemie, Mainz, Germany by ESA and NASA will fill this gap.

Considering the chemical composition of the Moon and The first chemical data obtained in situ on the Moon the planets, we have to bear in mind that the Solar System from Surveyors 5, 6, and 7 by alpha-scattering analyses formed from the solar nebula. To show the fractionation (Turkevich et al. 1970, and references therein), indicated a process involved it is useful to compare individual bulk basaltic composition at the two maria locations and a low- compositions of Solar System objects, or parts of them such iron, basalt-like composition at the third one for the area as individual rocks, to the abundances of elements to solar near Tycho. They proved that Urey and I were wrong. Urey abundance or even better to C1 (carbonaceous chondrites had just written a paper in which he summarized all argu- type 1) abundances which reflect the primordial Solar ments for a lunar origin of some meteorites (Urey 1968). System abundances of all condensable elements. Data for On 26 February 1968, he wrote to me with respect to the C1 or solar abundances are given in the classical paper by Surveyor results: “I have the feeling that the data would Suess and Urey (1956) or in later compilations by Cameron seem to indicate that no meteorites come from the Moon at (1973), Palme et al. (1981), Anders and Ebihara (1982), all. Moreover, I am led to doubt the Mars origin also.” and Anders and Grevesse (1989). In my paper “On the lunar origin of the bronzite chondrites” (H-chondrites), I had mentioned that the other major group of ordinary chondrites Ð the hypersthene chondrites THE MOON (L-chondrites) Ð might come from Mars. The possibility of ejection of rocks from the Moon or The pioneer of modern cosmochemistry, Harold Urey (who Mars was considered to be absolutely impossible by the won the Nobel Prize for Chemistry in 1934 for the discov- experts on cratering mechanisms at that time. When the ery of deuterium), suggested the Moon as a possible place first samples from the Apollo 11 mission were analyzed, I of the origin of stone meteorites (Urey 1959). He was, at had the satisfaction that at least my proposition that the that time, convinced that the Moon was a “primary” object. lunar dust should be loaded with solar wind particles turned To account for the apparent about 10% lower density of the out to be correct. Today, meteorites from the Moon Ð Moon compared with that of ordinary chondrites, he although not of chondritic composition Ð is a not disputed assumed the Moon to contain several percent water and/or fact. A Mars origin of a small group of meteorites (the SNC graphite. It was his belief in a primitive nature of the Moon meteorites) is widely accepted. formed at low temperature, which made the Moon for him When NASA asked for proposals for lunar sample inves- central to the understanding of the formation of the Solar tigations my colleagues and I at the Max-Planck-Institute System. for Chemistry in Mainz, Germany, wrote altogether 12 pro- I, being 35 years younger than Urey, have been from posals addressing different areas of research. As result of early on fascinated by the great old man and his reasoning. these proposals, the Mainz laboratory became a major In the early 1960s, I had found conclusive evidence for the player in the Lunar Sample Analyses Program of NASA. solar wind origin of the solar-type rare gases observed in Our laboratory has received the largest amount of lunar several bronzite chondrites (Wänke 1963, 1965; with material, both in terms of mass and number of sample spec- respect to solar wind implantation in meteorites a number imens, of any laboratory outside the USA. of other papers have to be mentioned: Signer and Suess Apart from rare gas measurements, the multi-element 1963, Suess et al. 1964, Eberhardt et al. 1965, Zähringer analyses of lunar samples at the Mainz laboratory became 1966). To account for this observation, it was a necessity highly appreciated. All major and minor elements and many that individual meteorite grains prior to compaction were geochemically important trace elements (in total up to 54 exposed to solar wind irradiation at the surface of their elements) have been determined in this multi-element analy- parent body, which should not have an atmosphere or mag- sis program. This turned out to be especially valuable for the netic field to hinder the solar wind implantation. I thought investigations of lunar breccias with their considerable het- the Moon would be an ideal body for this implantation and erogeneity. Using this large data set quite a number of together with some weaker arguments on the distribution of element correlations have been observed first or confirmed. cosmic exposure ages, I published a paper (Wänke 1966). To work with lunar samples was very exciting for all of Urey was quite convinced by my arguments and in his letter us involved. Let me describe an incident when we worked to me of 10 February 1967 he wrote: “In fact, I think I am on our first lunar sample, a soil sample no. 10084. Most of probably your most important public relations man in the our analytical work was carried out using neutron activation United States at the present time.’ Later on in this letter: ‘It techniques starting non-destructive by gamma-counting the looks as though I must now begin to caution people not to samples irradiated by thermal neutrons in a nuclear reactor. think that the whole case for meteorites from the moon is The reactor, a TRIGA research reactor of the Gutenberg- completely settled. I keep telling them that only samples University of Mainz, was located next to our building. from the moon will definitely settle the problem.” When we applied the first 6 hour irradiation from 9 a.m. to CHEMICAL EVOLUTION OF THE MOON AND THE TERRESTRIAL PLANETS 1379

3 p.m., the sample was in our gamma-ray spectrometer for The chemical composition of the soil and soil breccias less than about 30 minutes, and we waited for the first spec- was found to mimic that of the igneous rocks from which tra. Our technician, Mr. Bernhard Spettel, came to me they were obviously derived with only small additional highly excited and said: “Very, very strange. I have never components (Wänke et al. 1970). From the amount of seen such a spectrum. There are only three lines, I have to hydrogen evolved during the treatment of lunar soil with look up the isotope table because I do not know these diluted acid under vacuum, the presence of 0.6% metal in lines.” This was indeed very unusual as Mr. Spettel was a sample 10084Ð18 was calculated. Analyses of metal parti- real expert in this field and had most of the gamma-lines in cles separated from the bulk soil by a hand magnet showed his head. A few minutes later, he said: “You would not that practically all of the nickel, cobalt, gold, and iridium in believe, the lines indicate only one element, namely the bulk soil resides in the metal particles, indicating a indium.” I knew that indium, a trace element in rock or soil mainly meteoritic origin. An exception was observed for samples, has a very high cross-section and due to the short tungsten, for which high concentrations of 24 ppm in these half-life of the isotope produced by neutron capture, 116In metal particles proved their equilibration with matter from of only 54 minutes, it would be produced in high quantities lunar basalts at elevated temperatures. (In Figures 1 and 2, even if present only in trace amounts. The short half-life instead of individual mare rocks the soil sample 10084 is meant that the isotope decayed rapidly, and in the evening used for comparison as in this way small differences from of the same day we could see the lines of those elements we rock to rock are avoided.) actually expected. What had happened? In the overall extra- The meteoritic component was extensively studied by ordinarily successful Lunar Sample Analyses Program, Ganapathy et al. (1970) who estimated a 1.9% admixture of NASA had decided to return the samples under vacuum. A carbonaceous chondrite-like material, corresponding to an sample box was designed into which the astronauts had to average influx rate of meteoritic and cometary matter of put the samples and close the lid, which was supposed to be 3.8 108gcm2 yr1. sealed by an indium gadget between the edges of the lid and box. The whole sealing mechanism did not work prop- Rock samples erly, and most of the samples returned from the Moon were The igneous rocks, the mare basalts from Mare Tranquillitatis heavily contaminated with indium. In our sample there (Wänke et al. 1970) compositionally reflected highly dif- must have been a tiny indium grain of about 0.1 mg. In this ferentiated material of clearly basaltic nature (Figure 1). In respect it was an exception, but practically all soil samples contrast to terrestrial rocks, the high titanium concentration showed indium excesses.

The Apollo missions Apollo 11 – Mare Tranquillitatis Soil samples and soil breccias As expected (Wänke 1965, 1966), all soil samples contained large amounts of solar wind-implanted rare gases. 4He concentrations up to 0.5 cm3 STP g1 have been observed. From the ratios 4He/20Ne 91 and 4He/36Ar 505 compared with the ratios of more than three times higher found in magnetically separated metal grains, which hold rare gases more strongly, a considerable loss of helium due to diffusion was evident for the bulk samples (Hintenberger et al. 1970). The amount of hydrogen in the lunar soil, in soil breccias, was found to exceed that of 4He by about a factor of six with a D/H ratio of about three times less than the terrestrial ratio. Laboratories worldwide were engaged in the Lunar Sample Analysis Program. With respect to the Figure 1 Comparison of the concentrations of various solar wind-implanted rare gas isotopes, the studies in Bern elements in lunar fines and carbonaceous chondrites type 1. (Eberhardt et al. 1970) were very comprehensive, while The huge compositional differencies clearly show that the Epstein and Taylor (1970) analyzed hydrogen. The findings Moon is not a primitive but a highly differentiated object. of these groups were identical to those obtained in Mainz as Data for lunar fines from Wänke et al. (1970). (After Wänke well as by other laboratories. et al. 1970, reproduced with permission.) 1380 HEINRICH WÄNKE

Figure 3 Rare earth concentrations in lunar rocks 10057 and 10044 and lunar fines 10084, normalized to chondritic values. Data for chondrites from Schmitt et al. (1963, 1964); data for lunar samples from Wänke et al. (1970). The negative Eu anomaly reflects the low oxygen fugacity of the Moon where Eu becomes divalent and does not fractionate with the Figure 2 Comparison of the concentrations of various other REEs that remain trivalent, but enters feldspar. The Eu elements in lunar fines and eucrites. The higher differentiation anomaly has been reported by various authors (Gast and of the Moon compared to the eucrite parent body is evident. Hubbard 1970, Haskin et al. 1970, Philpotts and Schnetzler, (After Wänke et al. 1970, reproduced with permission.) 1970, Schmitt et al. 1970). (After Wänke et al. 1970, reproduced with permission). in the lunar basalts was striking. Comparing the major and trace element abundances, it was shown that the lunar mare be caught. We started our work around 8 p.m. When we basalts are compositionally close to basaltic achondrites, finished around 11 p.m., I walked back to my office on this the eucrite meteorites (Figure 2), although certain element rather cold but very clear late evening with the Moon high ratios differ considerably excluding a lunar origin of up in the sky. Suddenly I noticed an irritation in my nose eucrites. Striking were also the fractionated rare earth ele- and got out my handkerchief. When I put down the handker- ment (REE) patterns with a large negative europium anom- chief I noticed two black dots on it, which obviously were aly (Figure 3), reflecting a lower oxygen fugacity on the due to some fine dust liberated during the sample prepara- Moon as compared to the Earth. As in the case of rare tion in spite of the plastic tent. I looked up to the Moon on gases, the major and trace element compositions were also which Mare Tranquillitatis was easy visible with the naked studied by many groups. The REEs, for example, were eye, thinking that a rock is missing from it now and a tiny, studied by Wakita et al. (1970). The paper by Gast et al. tiny fraction of it I have just removed from my nose. This (1970) should also be mentioned in this respect. It contains, was the most touching moment of my life. apart from data on REEs and some other trace elements, suggestions on the petrogenesis of the Apollo 11 basalts. In Apollo 12 – Oceanus Procellarum all cases the Mainz data agreed very well with those from other investigators. Soil and rock samples One of the proposals for our investigations on lunar sam- Both the soil and the rock samples returned by the ples to be carried out at Mainz was the determination of Apollo 12 mission showed larger variations in chemical radioactive isotopes produced by the interaction of cosmic- composition than those from Apollo 11, which may just ray particles. For this work sample masses of about 100 g reflect the fact that all the Apollo 11 samples were collected were required. It was necessary to decompose chemically much closer to the landing module. the sample in order to extract the various radioisotopes to be Apollo 12 landed within 200 m of the Surveyor 3 space- studied. The first step in this procedure was reduction and craft. Some distant ejecta rays from the large crater pulverization of the sample. Because of the high value of the Copernicus, located 400 km to the north, crossed the site. sample, my colleague, Prof. Friedrich Begemann, and I had However, none of the returned samples could be proven to decided to do this step ourselves. For the pulverization we contain material originating from Copernicus. used a steel mortar that we had put inside a small plastic tent With respect to their major-element chemistry, the mare so that particles thrown out of the mortar or the sieves could basalts from Oceanus Procellarum were divided into two 57

FRED W. TAYLOR* The atmospheres of the terrestrial planets

It has been known since the end of the nineteenth century theoreticians analysing the carbon dioxide lines in the that the Earth’s nearest planetary neighbours, Venus and Martian spectrum were writing of the surface pressure in Mars, have extensive atmospheres. However, the nature of terms that were more than a factor of 10 too high. It was these atmospheres was not at all well understood until the only with the development of high-resolution ground-based advent of planetary missions by robot probes, beginning spectrometers and the arrival of Mariner 4 in the early with Mariner 2 which encountered Venus in December 1960s that the surprisingly low value of around 6 mbar was 1962. Since then, the spurts of new information from mis- derived and the composition of nearly pure carbon dioxide sions like Pioneer Venus, and Mariner and Viking to Mars, confirmed. plus radically improved Earth-based techniques including Mercury has always been elusive because its position the use of infrared, microwave and radar sensing, have close to the Sun means that telescopic observations from revealed most of the important basic atmospheric properties the Earth and spaceflight in its vicinity are both relatively such as composition, temperature, surface pressure, cloud difficult. Some putative spectroscopic evidence for a dense properties, dynamics and general circulation. Conditions on atmosphere was obtained in the nineteenth century, but this these planets are now known to be, for the most part, very was later called into question on theoretical grounds when different from even the most informed speculation that took it was recognized that gases on such a small, hot body place in the last few decades of the nineteenth century. would readily escape to space. A thin atmosphere like that The bright and featureless aspect of Venus showed early of Mars was still considered a possibility, for example by observers that the planet was cloudy, and it was perhaps nat- those who thought they could discern the occasional obscu- ural to assume that these were water clouds like those on the ration of surface features by airborne dust, well into the Earth, made more copious by the increased evaporation of second half of the twentieth century. The arrival at Mercury surface water due to the greater proximity of Venus to the of Mariner 10 in 1973 finally demonstrated that the surface Sun. From this it seemed logical to expect rain and a swampy, pressure on the planet is less than the most perfect vacuum tropical surface environment with carbonated-water oceans that can be created in laboratories on the Earth. and at least a good chance of life. The searing temperatures The fact that the terrestrial planet atmospheres are now and crushing pressures that we now know lie under clouds of well characterized in general terms does not of course mean concentrated sulphuric acid were recognized only after the that we fully understand them. The curious dynamics of dawn of the space age, and only fully accepted by some after Venus’s cloud layers, the vivid evidence for massive cli- the first landing on the planet by Venera 7 in 1970. matic change on Mars and the possible existence of ice Mars has long enjoyed a reputation as a habitable sort of deposits near the poles on Mercury are just the tip of an place. As early as 1774, Herschel concluded from his enormous iceberg of ignorance and curiosity concerning observations that it had a ‘considerable’ atmosphere con- our planetary neighbours. The flotilla of scientific probes taining clouds; Flammarion in 1896 noted the existence and being aimed towards Mars in the first decade of the twenty- variability of the polar caps. However, as late as the 1950s first century is one testament to the fact that the magnificent journey of discovery that essentially began in the twentieth * University of Oxford, United Kingdom century is set fair to reach its zenith in the next.

Table 1 Physical data for the terrestrial planets

Mercury Venus Earth Mars

Orbital and rotational data Mean distance from Sun (km) 5.79 107 1.082 108 1.496 108 2.279 108 Eccentricity 0.2056 0.0068 0.0167 0.0934 Obliquity (deg) 0 177 23.45 23.98 Siderial period (d) 87.97 224.701 365.256 686.980 Rotational period (h) 1407.5 5832.24 23.9345 24.6229 Solar day (d) 115.88 117 1 1.0287 Solar constant (kW m2) See text 2.62 1.38 0.594 Net heat input (kW m2) See text 0.367 0.842 0.499 Solid body data Mass (kg) 3.302 1023 4.870 1024 5.976 1024 6.421 1023 Radius (km) 2439 6051.5 6378 3398 Surface gravity (m s2) 3.70 8.60 9.78 3.72 Atmospheric data Composition He, Na See Table 2 Mean molecular weight — 43.44 28.98 (dry) 43.49 Mean surface temperature (K) 400 730 288 220 Mean surface pressure (N m2) 1012 92 1 0.007 Mass (kg) 8 103 4.77 1020 5.30 1018 1016

Table 2 Composition of the terrestrial planet atmospheres (for Mercury, see Table 4). Values are given as fractional abundances or as parts per million (ppm)

Venus Earth Mars

Carbon dioxide 0.96 0.0003 0.95 Nitrogen 0.035 0.770 0.027 Argon 0.00007 0.0093 0.016 Water vapour 0.0001(?) 0.01 0.0003 Oxygen 0.0013 0.21 0 Sulphur dioxide 0.00015 0.2 ppb 0 Carbon monoxide 0.00004 0.12 ppm 0.0007 Neon 5 ppm 18 ppm 2.5 ppm

Physical data for Mercury, Venus, Earth and Mars are thereby the period of rotation. Maps were drawn, and fea- given in Table 1, and data for the atmospheres of Venus, tures thought to be permanent were assigned romantic Earth and Mars are given in Tables 2 and 3. names. Maps by Schiaparelli in 1889 and Lowell in 1896 showed narrow, linear features reminiscent of those on their charts of Mars and probably of the same chimerical origin. MERCURY Antoniadi, who published a book about Mercury in 1934 and made many observations of his own, was convinced that Having already described the atmosphere of the innermost Mercury exhibited synchronous rotation, with the same face planet as resembling a perfect vacuum, it is perhaps capri- always directed at the Sun. He deduced that Mercury pos- cious to go on at length about its interesting properties. sessed an atmosphere thick enough to support dust storms, Nevertheless, there are a few, and the story of how we got which seemed sometimes to obscure the dark markings he to our present understanding of this wispy envelope, and its saw on the planet. In fact, he was viewing different parts of associated mysteries, is engaging. the disk, as Mercury rotated once every 58.6 days, rather Most of the early observations of Mercury concentrated than the 88 days that would have matched its annual journey on trying to resolve details on the surface and on determining around the Sun. THE ATMOSPHERES OF THE TERRESTRIAL PLANETS 1407

Table 3 Properties of clouds and dust in the terrestrial planet atmospheres*

Venus Earth Mars

Fractional coverage 1.00 0.40 0.05 (cloud) 0Ð1.0 (dust) Typical optical depth 25Ð40 5Ð7 0.2Ð6 (dust) Composition H2SO4 H2OH2O Magnetite etc. (dust); H2O, CO2 Number density (cm3) liquid 50Ð300 100Ð1000 0 solid 10Ð50 0.1Ð50 30Ð1000 Mass loading (g m3) 0.01Ð0.1 0.1Ð10 0.0002Ð0.1 Main production Chemistry Condensation Windblown (dust) process Equivalent depth (mm) 0.1Ð0.2 0.03Ð0.05 1Ð100 Effective radius (m) 2Ð4100.4Ð2.5 (dust) Main forms Stratiform, cumulus stratiform, cumulus stratiform, mixed (dust) Temporal variability Slight (haze), high (deep) High High

* The equivalent depth is the estimated thickness of the cloud material if it were deposited on the surface. The effective radius is the radius of the spherical particles having most nearly the same scattering properties as the cloud at visible wavelengths. After Esposito et al. (1983), with changes and additions

Table 4 Properties of the atmosphere of Mercury, from Mariner 10 results

Surface pressure 1012 bar Average surface temperature 440 K (590Ð725 K, sunward side) Atmospheric composition Detected compounds 42% oxygen, 29% sodium, 22% hydrogen, 6% helium, 0.5% potassium Possible trace constituents Argon, carbon dioxide, water, nitrogen, xenon, krypton, neon

Another way to detect an atmosphere is by analysing the features.’ The spacecraft he refers to is, of course, Mariner polarization of light reflected from the planet. Summarizing 10, the only one to visit Mercury to date. the evidence from a long sequence of such observations at It is to Mariner 10 that we also owe the resolution of the several wavelengths, Dollfus in 1961 concluded that there question of the surface pressure on Mercury. During three was evidence for a pressure at the surface of up to about encounters, on 29 March 1974, 21 September 1974 and 1 mbar, one thousandth of that on Earth. Ingersoll in 1971 16 March 1975, two ultraviolet spectrometers recorded revised this to an upper limit of 0.28 mbar (for carbon diox- atmospheric data. The first observed at a selection of wave- ide; the value depends on the composition assumed). These lengths chosen to correspond to the emission lines of researchers recognized that their analysis depended on the several candidate atmospheric species, specifically helium, assumption that the surface itself does not contribute to the neon, argon, xenon, hydrogen, carbon and oxygen. The polarization of the reflected light; if it does, their values second observed the Sun, and detected absorption by get smaller. In fact, the surface pressure is less than one the atmosphere in four wavelength bands as the Sun billionth of a millibar. went behind the limb of the planet. This is the so-called Reviewing in 1988 the question of surface markings on occultation technique, which is very sensitive to the pres- Mercury, and the various maps that had been drawn over ence of small traces of gas. The results are summarized the years, Patrick Moore wrote ‘My own conclusion is in Table 4. that although it is probable that a few albedo features were The total mass of Mercury’s atmosphere has been esti- glimpsed occasionally, the errors in observation were so mated at only about 8 tonnes. The low surface pressure unavoidably large that, without the spacecraft, we would means that the atoms and molecules can escape from the never have learned anything definite about Mercury’s planet, or collide with the surface, before they collide with 1408 FRED W. TAYLOR each other: by analogy with the outermost layers of the VENUS thicker atmospheres of the other planets, this is called an exosphere. It follows that the atmosphere is transient; to The twentieth century was more than 60 years old before the exist at all, it must be constantly replenished. Since mixing technology was in place to probe beneath the all-enveloping is ineffective, the composition, and the density, may vary cloud veil on Venus. Measurements of emission from the considerably over the globe, depending on the local balance planet at microwave (centimetre) wavelengths, which pene- between sources and sinks for each molecule. trate all but the most massive clouds, showed a source tem- Oxygen, sodium, and potassium in the atmosphere are perature of more than 600 K, a value so high that most probably baked out or otherwise derived from the minerals in planetary scientists assumed at first that the source must be the surface or the crust of Mercury. The hydrogen and helium something other than the surface of the planet. It could, come in as the solar wind and are temporarily trapped. among other possibilities, be emission by some unidentified It seems likely that trace amounts of argon, carbon dioxide, process in the planet’s ionosphere, or from sustained light- water, nitrogen, xenon, krypton and neon are also present. ning activity in the clouds. However, it was pointed out by The metallic ions sodium and potassium are probably pro- Carl Sagan and others that the visible diameter of Venus is duced by sputtering at the surface caused by micromete- several tens of kilometers larger than its radar diameter, so orites, solar wind particles or ions from Mercury’s the atmosphere below the cloud tops must be correspond- magnetosphere. There is observational evidence for localized ingly deep. In an optically thick atmosphere, the vertical concentrations which may imply venting from subsurface temperature gradient cannot deviate much from a value sources or some other mechanism. Accumulations of radar- known as the adiabatic lapse rate, which on Venus is about bright material in craters in the polar regions are thought 10 K km1. The temperature at the cloud tops is relatively by many to be deposits of water ice. They occur in loca- easy to measure, with infrared sensors on ground-based tions that are permanently sheltered from sunlight that telescopes, and was known to be around 240 K. Thus, if this will, in the absence of a substantial atmosphere, be very temperature was, say, 50 km above the ground, the latter cold. If substantial amounts of ice are indeed present (and had to be around 740 K, in agreement with the microwave it remains to be shown that the features are not due to emission temperatures. Sagan and colleagues also went some other radar-bright volatile, such as sulphur or even on to show that it might, under certain conditions that sodium) their origin is probably in meteoritic bombard- remained to be proven, be possible to sustain such a high ment, although again outgassing from a source within the temperature by the greenhouse effect, a well-known atmos- planet cannot be ruled out. pheric phenomenon without which our own planet would Mercury is a prime target for an orbiter mission in the be an icy wasteland. first decade of the new century. NASA has announced plans Still, the idea of as much as 500 K of greenhouse-induced for the Mercury Surface, Space Environment, Geochemistry heating was hard to believe at first. On the Earth, the effect and Ranging mission (Messenger), while the European is only about 35 K; the larger amounts of carbon dioxide on Space Agency (ESA) has definite plans for a ‘cornerstone’ Venus and it being closer to the Sun imply a larger value but mission in 2008 or there abouts, albeit with an unfortu- not, in most people’s minds, so much. For one thing, the nately puerile name ‘Bepi Colombo’. ESA recently com- clouds of Venus are so reflective that about 80% of the Sun’s pleted a technical and scientific study of a mission to return heat is reflected back into space: the amount absorbed is less dust, rock and a core sample from the surface of Mercury, than that of the Earth and actually about the same as that of although no date has been set for this very ambitious mis- the relatively distant, dark and icy planet Mars. sion. While primarily directed towards understanding the The ensuing controversy was resolved only by the flight origin and evolution of Mercury as a planet, key atmos- of Mariner 2, the world’s first successful interplanetary pheric science questions can also be addressed. Firstly, probe. It had on board a microwave radiometer similar to better spectrometers can be used to confirm the composi- those that had detected the hot emission from Venus, the dif- tion given in Table 4, and to search for trace species like ference being that, carried close to the planet, it could resolve water vapour and carbon dioxide. Secondly, the variation in the difference between heat emission from the surface and space and time of these species can be mapped, and any from the upper atmosphere or ionosphere. The results low-pressure ‘volcanoes’, fissures, or other regions where showed the intensity of the emission falling off from the cen- gases are issuing from the interior, located. Finally, the tre of the planet to the limb, clear support for a source at the question of the nature and origin of the reflective deposits surface. Direct temperature sensors on the first successful near the poles can be investigated with observations that Venera entry probe provided the final confirmation in 1967. include infrared temperature measurements. For a planet There followed a flurry of Venus investigation, led by the with a vacuum for an atmosphere, Mercury will remain an spacecraft of the Mariner, Venera and Pioneer programmes. interesting place for many decades to come. Altogether there were 9 instrumented flybys, 15 landers 58

DONALD M. HUNTEN* Jupiter

Jupiter is the largest planet and is the prototype of the would eventually be named Pioneers 10 and 11. At that time giant planets, the others of which are Saturn, Uranus and there was no real Jupiter ‘community’; our body of knowl- Neptune. It possesses four large, icy moons, a huge and edge was expressed in occasional papers but there was little intense magnetosphere, and a unique plasma torus sur- or no discussion at scientific meetings. Jeffreys had shown rounding the orbit of the satellite Io. It has therefore been a that the temperature of the surface [or cloud tops] must be prime target for exploration, and in addition, its huge mass close to the solar-equilibrium value, and Menzel had con- has made it a valuable target for gravity assists. The firmed this by analysis of thermal-infrared observations (see Pioneers and Voyagers used Jupiter in this way to reach Wildt 1969). Jupiter’s small mean density had led to the Saturn, Uranus and Neptune, and Ulysses used it to obtain a inference that hydrogen and helium were major constituents, solar orbit of high inclination. The announcement of the and models of the interior structure had been worked out Pioneer opportunities in 1968 led to the establishment of (DeMarcus 1958). Öpik (1962) had published a detailed the first real Jupiter community, and the measurements by analysis of the composition, leaning heavily on results of a these spacecraft taught us a great deal about the environ- pioneering observation of a stellar occultation by Baum and ment of the asteroid belt and Jupiter’s magnetosphere, as Code (1953). Unfortunately, the scale height obtained from well as the planet itself. Once they had been launched, stud- these data was much too small, and Öpik obtained a helium ies were initiated for an ambitious, highly reliable spacecraft abundance that was correspondingly much too large. Münch called Grand Tour but they were never implemented; instead had pointed out the important role of pressure-induced they were replaced by the less-expensive Voyagers. These absorption by H2 in controlling the thermal opacity, and two spacecraft made history as they visited all four giant models of the thermal structure involving this opacity had planets and also made important measurements of Jupiter’s been worked out (Trafton and Münch 1969). Galilean satellites, as well as Titan and Triton. Galileo Radio emission from the planet and its magnetosphere included an entry Probe to explore Jupiter’s atmosphere and were early discoveries as radio astronomy underwent its an Orbiter to define the magnetosphere and the satellites. In rapid development after the Second World War. Convenient spite of a delay of many years before it could be launched, summaries appear in Berge and Gulkis (1969) and Carr and and a loss of the reflector of the high-gain antenna, it has Desch (1969). Analysis of the synchrotron emission by been a spectacular success. Ulysses made some measure- electrons in the radiation belt allowed an estimate of the ments of the environment, and Cassini will do the same, as strength and orientation of Jupiter’s magnetic field. Intense well as providing remote sensing of the planet and satellites. pulses at decametric wavelengths were eventually found to be linked to the position of the satellite Io, and Goldreich THE PIONEERS and Lynden-Bell (1969) proposed that the mechanism involves a unipolar generator as Jupiter’s rapid rotation In 1968, NASA issued an Announcement of Opportunity sweeps the magnetic field past the satellite. The thermal to solicit experiment proposals for the two spacecraft that emission by Jupiter’s atmosphere appears in the wavelength region from a few mm to tens of cm. The principal opacity * University of Arizona, Tucson, AZ, USA source is ammonia in the pressure region of a few bars, and

Donald M. Hunten, The Century of Space Science, 1425Ð1430 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1426 DONALD M. HUNTEN allows estimates of the ammonia abundance and the tem- Ð The IR Radiometer confirmed that the planet radiates perature profile (Berge and Gulkis 1969, de Pater and approximately twice as much energy as it absorbs from Massie 1985). Measurement of the thermal radiation of the the Sun, and showed that this total radiation is essentially planet had shown that it radiates roughly twice as much independent of latitude (Ingersoll et al. 1976). Because energy as it absorbs from the Sun (Low 1976). the solar heat is deposited primarily at low latitudes, the The principal objectives of the Pioneers would be to mea- internal heat must therefore come out preferentially at sure the zodiacal light, and therefore the density of inter- the higher latitudes, probably as a consequence of the planetary dust; to diagnose the potential hazard to spacecraft special properties of convective heat transport (Ingersoll of the asteroid belt; and to make an initial reconnaissance of and Porco 1985). Jupiter and its radiation belts. One result of the announce- Ð The radio occultation experiment measured the electron ment of opportunity was the formation of a Jupiter commu- densities in the ionospheres of Io and Jupiter. Initial nity, much of which was represented at the Third Arizona results for the density of Jupiter’s neutral atmosphere Conference on Planetary Atmospheres (1969); several of the were very strange; the anomaly was eventually traced to papers presented there have been quoted in the preceding neglect of the planet’s oblateness, and corrected tempera- paragraph. By the time of the Pioneer encounters (December ture profiles are given by Kliore and Woiceshyn (1976). 1973, December 1974) a large body of Jupiter science had The exospheric temperature was found to be an astonish- accumulated. This science and the Pioneer results are repre- ingly hot 1000 K, instead of the 200 K expected for sented in the book ‘Jupiter’ (Gehrels 1976). heating by solar UV. Primary candidates or the additional While Pioneers 1 and 2 were being built and on their heat source are absorption of wave energy that has prop- way to Jupiter, a number of valuable results were obtained agated up from the lower atmosphere (Yelle et al. 1996), from Earth-based studies. First came the 1971 occultation and precipitation of ions and electrons from the magne- of the star Sco by Jupiter. It was successfully observed tosphere (Hunten and Dessler 1977). by three groups: Combes et al. (1971), Hubbard et al. (1972) and Veverka et al. (1974). A critical review by Hunten and Veverka (1976) concludes that the temperature is 170Ð200 K THE VOYAGERS in the mesosphere (number density 5 1014 cm 3 or pressure near 3 bar). Two years later (1973) Brown discovered During 1976 studies were under way at the Jet Propulsion strong emission of the yellow D lines of Na in the vicinity Laboratory for the ‘Grand Tour’. These two spacecraft were of Io (Brown and Yung 1976), and this was followed by designed for the long life and extreme reliability needed to the even more remarkable discovery of the Io Plasma Torus take advantage of a forthcoming opportunity to visit all four (Kupo et al. 1976). This object and its physics are described of the Jovian planets by use of gravity assists. Accompanying in several chapters of the book by Dessler (1993). One of the the spacecraft studies were studies of potential instruments. first discoveries of the Kuiper Airborne Observatory was that Right at the end of 1976, the U.S. Congress withdrew sup- of Jovian water vapor (Larson et al. 1975). The absorptions port for these ambitious missions, and gave it instead to a are strong, but they can only be observed from high alti- pair of missions that would be based on the Mariner line tudes because those of the Earth’s lower atmosphere are that had successfully explored Venus, Mars and Mercury. even stronger. Initially called ‘MJS’ (Mariner Jupiter-Saturn), these space- Returning to the Pioneers, the Jupiter-oriented instru- craft were later re-named ‘Voyagers 1 and 2’. Fortunately ments included an Imaging PhotoPolarimeter, a two-channel the planetary alignment did not go away and the Voyagers UltraViolet Photometer, a Magnetometer, and a 2-channel kept working, so both missions were eventually extended InfraRed Radiometer. Occultation by the ionosphere and to carry out the exploration of the Uranus and Neptune atmosphere was observed by means of the telemetry carrier, systems. and tracking of the spacecraft gave information on the mass The Voyagers arrived at Jupiter in March and July 1979. and gravity field. Charged particles from plasma to cosmic With 3-axis stabilization each spacecraft was able to carry a rays were measured by four instruments, and dust particles by pair of cameras, much more powerful than the spin-scan two. Although Pioneer 10 passed right through the Io Plasma instrument on the Pioneers. Figure 1 shows two false-color Torus, its plasma instrument was oriented in the wrong direc- mosaics, one each for Voyagers 1 and 2, showing one com- tion to readily detect it, and a detection was not reported until plete rotation (actual colors are various shades of yellow). several years after the encounter (Intriligator and Miller 1981). An UltraViolet Spectrometer (UVS) was used in a novel The principal discoveries concerning the planet were: mode, in which it observed occultation of the Sun by the Ð The images showed that the well-known cloud structure upper atmosphere. It was therefore possible to obtain height of belts and zones is supplanted at latitudes above 45 by profiles of the major gases, especially H2, H and CH4. The structures resembling large-scale convection. Pioneer occultation experiments of the telemetry carrier JUPITER 1427

Figure 1 Comparison of two cylindrical projections of Jupiter, made from images taken by Voyagers 1 and 2. Longitudes are 400 to 0 degrees in the top image, and the bottom one is aligned with it. Colours have been enhanced in the processing. Relative motions of features can be seen; for example, the Great Red Spot has moved westward and the white ovals eastward. Regular plume patterns are equidistant around the northern edge of the equator, while a train of small spots at approximately 80 degrees south latitude has moved eastward. Significant changes are evident in the recirculating flow east of the Great Red Spot, in the disturbed region west of the Great Red Spot, and in the brightening of material spreading into the equatorial region from the more southerly latitudes. (P-21772 C.) were repeated. Both measurements confirmed the very high Earth-based work, they found the abundances of the ice- exospheric temperatures, 1000 K, found by the Pioneers, forming vapors CH4,NH3 and H2O to be somewhat greater but the height of the ionosphere was much greater than than ‘solar’, the values they would have if all the C, N and expected (McConnell et al. 1982). Presumably the ionos- O were converted to these molecules. The NH3 and H2O pheric plasma is raised by vertical drifts. were depleted at the higher altitudes and lower tempera- The UVS also observed the airglow from the same tures roughly as expected for condensation into the cloud atmospheric region. Its most striking discovery was of particles that are inferred to be present. The excess is gener- intense far-ultraviolet emissions from the Io Plasma Torus, ally thought to be accounted for by late accretion of icy making it clear that this body is the seat of enormous planetesimals which would be vaporized in the atmosphere. energies, much greater than had been expected from the A careful analysis of the energy balance confirmed that ground-based studies mentioned above. The principal con- Jupiter radiates 1.668 times as much as it receives from the stituents are ions of O and S, each in several charge states. Sun (Hanel et al. 1981). The excess heat from the interior is The source is Io’s atmosphere, of which SO2 was found believed to arise from a combination of residual primordial by IRIS, the InfraRed Interference Spectrometer to be a (or heat and release of gravitational energy by a slight shrink- the) major constituent (Pearl et al. 1979). When this gas age of the planet. escapes from the satellite, it is dissociated and ionized Some of the images of the night side showed bright spots by torus electrons, and the ions are then accelerated to attributed to lightning flashes. Close analysis (Borucki and co-rotation with Jupiter by the latter’s magnetic field. This Williams 1986) showed that they had the size and elliptical acceleration is a major source of energy, which probably shape expected if the flashes had occurred at the 3Ð5 bar powers the entire medium. The plasma temperature is level of the water clouds and the light was diffused by the typically several eV or several hundreds of thousands of ammonia clouds at 0.6 bar. degrees K, with a component of electrons at several hundred eV (Brown et al. 1983). After the ions are neutralized, their velocity is high enough to carry them to distances of GALILEO 100 Jovian radii, where they can again become ionized and join the magnetospheric population. As they work their The idea of a Jupiter Orbiter-Probe (JOP) mission received way back inward, they are accelerated to energies in the a great deal of study even before the launch of the millions of eV and contribute a major part of the magnetos- Voyagers. The Jet Propulsion Laboratory had organized a pheric population. Science Advisory Group as early as 1971 and JOP was A detailed analysis of the infrared spectra has been recommended by the U.S. National Academy’s Space presented by Carlson et al. (1993). Confirming earlier Science Board (SSB) after consideration by its Committee 1428 DONALD M. HUNTEN

on Planetary and Lunar Exploration (COMPLEX). It was taken by and transmitted from the remote-sensing and mag- proposed in the U.S. President’s budget for consideration netospheric instruments on the Orbiter. late in 1977, but was omitted from the version of the budget The Jupiter objectives of Galileo, and the corresponding prepared by the Budget Committee of the House of instruments, were reviewed by Hunten et al. (1986). Many Representatives. A major stated reason, which turned out to of the instruments were carried into the atmosphere by the be all too valid, was that the Space Shuttle and the neces- Probe; they comprised sary Centaur upper stage would not be ready in time to Ð the Atmospheric Structure Instrument, measuring tem- launch the mission. In response, the U.S. planetary commu- perature, pressure and acceleration; nity staged a supporting campaign, which was successful Ð the Mass Spectrometer, measuring composition through- in that the House rejected the recommendation of its own out the descent; committee and appropriated the funds necessary to start the Ð the Helium Abundance Interferometer, dedicated to an mission, which was soon re-named ‘Galileo’. As the date of accurate measurement of this one gas; availability of the Shuttle slipped again and again, the same Ð the Net Flux Radiometer, to measure this quantity in thing happened to Galileo. Development of the Shuttle several IR and visible wavelength bands; version of the Centaur upper stage caused further delays, Ð the Lightning and Radio Detector, also including a detec- but in 1985 the launch system was declared ready and the tor for very energetic electrons and ions in the innermost spacecraft was shipped to Cape Canaveral for launch prepa- magnetosphere; rations. The JPL crew took time off from these prepara- Ð equipment to measure the Doppler shift of the Probe- tions to watch the launch of Challenger and its disastrous Orbiter relay signal and therefore obtain information on breakup. Investigation of the reasons imposed a delay in the the descent rate and winds. launch date of Galileo. Next, NASA cancelled the adapta- tion of the Centaur stage to the shuttle, on the grounds that The Orbiter carried a magnetometer and a suite of it would be too dangerous to launch with a payload contain- instruments to measure the fluxes of ions and electrons as a ing such large quantities of liquid hydrogen and oxygen. function of energy in the torus and magnetosphere. More It appeared that there would be no way to get Galileo to closely related to the planet and the satellites were an Jupiter by means of the available solid-fueled upper stages, imaging system, the Near Infrared Mapping Spectrometer until it was realized that the necessary velocity could be (NIMS), and the Photo-Polarimeter Radiometer (PPR), gained by means of gravity assists, a flyby of Venus fol- with many channels from the visible to the far infrared. lowed by two of the Earth. Such a trajectory required the These three instruments had an unexpected workout in addition of a system of sun shades to Galileo, which had 1994 when the fragments of Comet Shoemaker-Levy 9 not been designed to withstand the additional heating that it entered Jupiter’s atmosphere at locations visible from would encounter so close to the Sun. After a further delay Galileo but not from the Earth (Carlson et al. 1997). Many of three years, the mission was finally launched in October other measurements of the impact sites were obtained from 1989. Because of the three extra orbits, Galileo did not Earth after they had rotated into view. Here only three arrive at Jupiter until December 1995. During its transit, will be mentioned: detection of cometary water vapor from Ulysses obtained its Jupiter gravity assist in February 1992. the KAO (Bjoraker et al. 1996, Sprague et al. 1996) and It made some measurements of the plasma environment, non-detection of seismic waves propagating away from the but was not equipped with any instruments that could sense impacts, which would have given a probe of Jupiter’s inte- the planet itself. rior (Walter et al. 1996). Generally speaking, this event Shortly after the first Earth encounter a command was taught us far more about the comet and the nature of the sent to Galileo to unfurl the reflector of the High-Gain impact phenomena than about Jupiter. Antenna, which was a duplicate of one that had been used The entry site of the Probe was constrained to a very on several Earth satellites operating in geosynchronous low latitude for various reasons, including the need to take orbit. Although the motor responded, something in the advantage of Jupiter’s rapid rotation to minimize the entry system stuck and the reflector only partially opened. In velocity relative to the atmosphere. This velocity, of order spite of many attempts to complete the deployment, none 50 km s1, was much greater than the 11 km s1 experienced was successful and the mission had to be carried out with a by Apollo capsules returning from the Moon, and the low-gain antenna with a bit rate of only a few tens of bits energy per unit mass greater by a factor of 20. The chal- per second. A massive re-programming of the spacecraft’s lenge of designing a successful entry system was therefore central computer was successfully undertaken, providing a major one. Unfortunately, the equatorial region is the site the ability to compress the data to the greatest possible of extensive areas with much less cloud than exists else- extent. There was no loss of the relayed Probe data, but where. These areas are called ‘hot spots’ not because they considerable restriction in the amount of data that could be are any warmer than other areas, but because they appear 59

THÉRÈSE ENCRENAZ* The planets beyond Jupiter

INTRODUCTION main constituent of the giant planets, was observed for the first time in 1960. In the 1970s, the improvement of At large heliocentric distances, the giant planets of the Solar infrared spectroscopy techniques, coupled with the use of system Ð Jupiter, Saturn, Uranus and Neptune Ð have specific larger and larger ground-based telescopes, led to the dis- characteristics: with respect to the terrestrial planets, they covery of a long list of minor atmospheric constituents, on have a large size, a low density, a fast rotation period around both Jupiter and Saturn. We thus had already a good knowl- their axis; they also all have a ring system and a large number edge of the nature and composition of their atmospheres of satellites. The two brightest of them, Jupiter and Saturn, when the first spacecraft devoted to the exploration of the have been known since Antiquity. In contrast Uranus and giant planets, Pioneer 10 and 11, were launched in 1972 Neptune, which are both smaller and farther away from the and 1973 respectively. Sun, have been discovered relatively recently: Uranus was Uranus and Neptune, in contrast, were very poorly known first observed by William Herschel in 1781, following the until the space era, which, in their case, started with the new developments of large telescopes, while the discovery of launch of the second Voyager spacecraft, in 1977. Prior to Neptune, by John Adams and Urbain Le Verrier in 1846, was that date, the available information about these two planets the direct result of celestial mechanics calculations. was limited to poor-quality images, which, still, provided Both Jupiter and Saturn have been monitored from evidence for temporal variability and climatic changes in ground-based telescopes for over three centuries, after the case of Neptune. Hydrogen and methane were for long Galileo Galilei, in 1610, turned to the sky his newly built the only atmospheric species detected from ground-based refractor and discovered, in particular, the four satellites of spectroscopy. Jupiter later called galilean. Concerning the two major Our knowledge of the giant planets has been entirely giant planets, we thus have a huge data base, first made of revised with the two sets of NASA spacecraft, Pioneer 10 drawings, then completed with photographs and spectra. and 11 and later Voyager 1 and 2. The four spacecraft The first images revealed the latitudinal structure of belts encountered Jupiter in 1973 (Pioneer 10), 1974 (Pioneer 11) and zones on both planets, the existence of the Great Red and 1979 (Voyager 1 and 2) respectively; Jupiter was later Spot on Jupiter, and the nature of Saturn’s ring system. extensively studied between 1995 and 2000 by the Galileo Photographs of improving quality provided a monitoring of mission, launched in 1989. Saturn was first encountered by the planets’ meteorological features, showing, in particular, Pioneer 11 in 1979, then by Voyayer 1 and 2 in 1980 and the stability of the Great Red Spot on Jupiter but the vari- 1981. Voyager 2 continued its journey to encounter Uranus ability of the other smaller features. The development of in 1986 and Neptune in 1989, completing the first in situ ground-based spectroscopy, first in the visible, then in the space exploration of the giant planets and their systems. infrared range, provided a quantitative information about This space program turned out to be an outstanding scien- the chemical composition of Jupiter and Saturn. Methane tific success. In the future, scientists are looking for the and ammonia were detected in 1932, and hydrogen, the long-term exploration of Saturn by the Cassini mission arriving there in 2004; for Uranus and Neptune, no further * Observatoire de Paris, Meudon, France space exploration has yet been scheduled.

In addition to in situ space exploration, space observato- the giant planets formed, most of the matter, apart from ries in Earth orbit have greatly contributed to our knowledge hydrogen and helium, was solid, in form of ices (H2O, CH4, of the giant planets. The first of them was the International NH3, …). These ices, incorporated into the planetesimals, Ultraviolet Observer (IUE), a cooperative project of NASA, were abundant enough to form massive cores (about ten to ESA and the UK, launched in 1977, which explored the fifteen terrestrial masses, according to theoretical models), ultraviolet sky for almost twenty years. The next step was the which could in turn accrete the surrounding protosolar Hubble Space Telescope (HST), a NASA/ESA mission in nebula, mostly made of hydrogen and helium. This simple operation since 1989, which has been providing unprece- first-order scenario can account for the large sizes and the dented quality images, as well as UV spectra, of all sorts low densities of the giant planets, as well as their ring and of astronomical sources. The third space observatory to be satellite systems. used for planetology was the Infrared Space Observatory According to the nucleation scenario (Mizuno 1980), all (ISO), an ESA mission with NASA and Japanese participa- giant planets should have formed from an initial core of tions, in operation between 1995 and 1998, which recorded comparable mass. Comparing this number (10Ð15 terres- high sensitivity images and spectra in the infrared range. trial masses) to the actual sizes of the giant planets immedi- Two major missions, currently under development, are ately shows a distinction between the four giants. Jupiter expected to be the next observatories performing planetary and Saturn, whose initial core is 3 to 10% of their total observations: the Far Infra-Red and Submillimeter Telescope mass, are mostly gaseous, while Uranus and Neptune, with (FIRST), an ESA mission with a possible contribution from an initial core of more than 50% of their total mass, can be NASA, will explore the long-wavelength range of the elec- called the icy giants. What can be the origin of this differ- tromagnetic spectrum with unprecedented sensitivity and ence? It has been suggested that Uranus and Neptune, spectral resolution, and the Next Generation Space Telescope located farther from the Sun where the density of the disk (NGST), developed by NASA with a potential ESA partici- was lower, took a longer time than Jupiter and Saturn to pation, will be a follow-up of the HST with an extension of accrete their initial cores. In this case, the accretion of the its spectral range toward the infrared. surrounding gas around Uranus and Neptune may have happened after most of the gas was blown away by the strong solar wind expelled by the early Sun in its T-Tauri THE GIANT PLANETS BEYOND JUPITER phase, and Uranus and Neptune would have found much less protosolar gas available for their accretion. This is no It is possible to understand the general properties of the more than a possible explanation, however; there are still giant planets (such as their large size, low density and large many open questions about the formation scenarios of number of satellites) in the light of their formation scenario. the giant planets, and the recent discovery of many “giant” This so-called “nucleation scenario”, now widely accepted exoplanets in the close vicinity of their star raises new by the scientific community, is based on a whole set of problems, currently unsolved, about their formation. observational data concerning the dynamics of solar-system Apart from the question of their origin, giant planets, bodies and their chemical composition, as well as the as observed today, are far from being fully understood. analysis of star-forming regions and protoplanetary disks, Among the major issues are their internal structure, the which bring stringent constraints to theoretical models. nature of their internal source of heat, the composition of Following the initial concepts proposed by Emmanuel Kant their clouds and their meteorology. The four giant planets and Pierre-Simon de Laplace in the XVIIIth century, the were considered for long as being similar to one another; current scenario assumes that the Solar system was formed however, recent observations, especially through space after the gravitational collapse of a protosolar cloud into exploration, has revealed that the giant planets, like the a disk, in which solid particles accreted together through terrestrial ones, are unique worlds by themselves, and collisions to form first planetesimals, and later larger bodies the reasons for these differences remain to be solved. The (Cassen and Woolum 1999). The chemical composition of main orbital and physical parameters of the giant planets this protosolar cloud, made of interstellar matter, was likely are summarized in Table 1; their chemical composition is to reflect the cosmic abundances, i.e. include first hydrogen given in Table 2. In Table 3, abundance ratios in the giant (about 75%), then helium (about 25%), then traces of other planets are compared to solar and protosolar values. heavier elements (O, C, N, …). In the vicinity of the Sun, where the terrestrial planets formed, the temperature was probably such that only heavy PLANETS BEYOND NEPTUNE elements (silicates and metals) were in solid form; these elements represent only a small fraction of the cosmic Early in the XXth century, it was announced by some matter. In contrast, at large heliocentric distances, where astronomers, including Percival Lowell, that the orbital THE PLANETS BEYOND JUPITER 1433 motions of Uranus and Neptune could not been explained similarities with that of Neptune’s satellite Triton. Finally, without the gravitational perturbations of another planetary ground-based infrared spectroscopy has allowed the detec- body, located at further distances from the Sun. This new tion of several ices on Pluto’s surface (N2,CO,CH4). Water object called “Planet X” was unsuccessfully searched for ice, in contrast, was found on Charon, which confirms that during the following decades. As a result of this observing Pluto and Charon are different in nature, as illustrated by campaign, however, the small Pluto was discovered in 1930 their different densities. The orbital, physical and chemical by Clyde Tombaugh. With a diameter of about 2400 km and properties of Pluto and Charon suggest an origin different a density close to 2 g cm2, Pluto was not able to account from the planetary subnebular material from which the outer for the orbit anomalies mentioned above. In 1978 however, satellites formed. It has been suggested that Pluto and a satellite, called Charon, was detected around Pluto by Charon accreted independently in the outer solar system and James Christy from a study of Pluto’s orbital trajectory. later collided, which led to the binary system observed today Pluto has a distinctly eccentric orbit (e 0.246), that is con- (Mueller and McKinnon 1997; Stern and Yelle 1999). spicuously inclined against the ecliptic plane (i 17 10). In addition to the ground-based exploration of Pluto, sig- No bigger planet was found after Pluto, but a whole family nificant information has come from HST images of Pluto’s of smaller objects, physically similar to Pluto and later called surface which have shown evidence for a differentiation of Trans-Neptunian Objects (TNOs) have been discovered dur- its surface. The same conclusion came from observations of ing the past decade in the Edgeworth-Kuiper Belt, between 30 Pluto’s far-infrared flux with ISO. Both HST and ISO have and 50 AU (Levison and Weissman 1999). The existence of also been used for measuring the visible and far-IR fluxes this belt was predicted independently, on theoretical grounds, of a few TNOs. There has been no in situ space exploration by Kenneth Edgeworth and Gerard Kuiper in the 1940Ð50s. of Pluto so far, but a flyby mission (Pluto Fast Flyby) is They argued that the expected density of the protoplanetary currently under study at NASA. disk, beyond Neptune, should have been too low to allow the formation of large bodies like the giant planets, but should have been sufficient for the formation of planetesimals, THE SPACE MISSIONS comets and possibly very small planets. It was later suggested by Julio Fernandez that this belt could be the reservoir for The flyby missions: Pioneer 10/11 and Voyager short-period comets which have a low inclination. These theoretical studies motivated new observing campaigns which Following a series of Pioneer spacecraft devoted to the benefited from the use of large ground-based telescopes and exploration of the Moon and the solar wind, Pioneer 10 high-sensitivity detectors. The first TNO was detected in 1992 and Pioneer 11 were designed by NASA for a low-cost by David Jewitt and Jane Luu. As of 1 January, 2000, about exploratory mission to Jupiter (Lasher 1997). In addition to 200 objects have been found with diameters larger than the exploration of the jovian environment, the major objec- 100 km. As in the case of comets, their albedo is expected to tives were to study the interplanetary medium beyond the be low (a few percent). It now appears that the TNOs are Mars orbit and to investigate the possible hazards in cross- members of a new family of solar-system objects which ing the asteroidal belt. After the success of the Pioneer 10 could be as many as 100 000 (with a diameter above 100 km) flyby of Jupiter (4 December 1973), Pioneer 11 was retar- in the Edgeworth-Kuiper Belt, with a total mass of a few geted to allow a Saturn flyby after the Jupiter encounter tenths of a terrestrial mass. The existence of this new class (3 December 1974). The Saturn flyby took place on has important implications on physico-chemical and dynami- 1 September 1979. Among the 11 instruments of the cal models of solar-system formation. The search for TNOs scientific payload were an imaging photopolarimeter, a UV and the spectroscopic study of the detected objects have photometer, an IR radiometer, several plasma physics become a new, rapidly exploding field of research. instruments and two impact detectors. Most of our knowledge regarding Pluto and the TNOs These flybys provided the first short-range images of has come from ground-based astronomy (see Stern and Yelle Jupiter, Saturn and the Saturn rings, showing with unprece- 1999, for a review). In particular, in the case of Pluto, two dented detail the structure of the jovian belts and zones, methods have been very powerful. The first one is the obser- the Great Red Spot and Saturn’s ring system. Another high- vation of mutual occultations and transits of Pluto and light of these missions was the first measurement of the Charon, in the beginning of the 1980s, which provided a jovian magnetic field (previously detected from ground- determination of the masses and densities of both objects. based radio-emission of Jupiter) and the first detection of The second method is the observation of a stellar occultation Saturn’s magnetosphere. Pioneer 10 and 11 also demon- by Pluto in 1988, which showed evidence for a stable atmos- strated that spacecraft could survive after crossing the phere around Pluto. This atmosphere, dominated by nitrogen asteroid belt, and thus opened the road to future more N2 with a minor contribution of CH4, shows remarkable sophisticated missions. 1434 THÉRÈSE ENCRENAZ

Figure 2 The planet Uranus as seen from Voyager 2, one week prior to the encounter (January 1986), from a distance of 9 million km. Left side: real colors; right side: false colors Figure 1 The planet Saturn as seen from Voyager 1 on (NASA). 18 October 1980, from a distance of 34 million km. Colors have been enhanced to better show the contrasts in the belt-zone structure (NASA).

Following this success, Voyager 1 and 2, two Mariner- type vehicles, were launched by NASA in September 1977 for a flyby exploration of the four giant planets (Miner 1997). Voyager 1 encountered Jupiter on 5 March 1979 and Saturn on 12 November 1980. Voyager 2 encountered Jupiter on 9 July 1979, Saturn on 25 August 1981, Uranus on 24 January 1986 and Neptune on 25 August 1989. The two probes are expected to send data back to Earth until about 2015, where they will be at 130 AU from the Sun. Their objective is to detect the heliopause, defining the limit between the solar-system plasma and the interstellar medium, which is expected to occur, according to the Voyager data, between 110 and 160 AU from the Sun. The Voyager spacecraft included in their scientific pay- Figure 3 The planet Neptune as seen by Voyager 2 at the load (11 instruments) a camera, IR and UV spectrometers, a time of encounter (August 1989). The image is a false-color magnetometer, and charged-particles, plasma and radio wave combination of 3 exposures in 3 different filters. The deep experiments. The camera observed the cloud structure and blue color (which corresponds to the real color) is due to large abundances of gaseous methane. White spots are the dynamical phenomena of the atmospheres (Figures 1 to believed to be high-altitude CH4 cirrus. The red color, which 3), whose temperature structure and chemical composition is not real, corresponds to high-altitude haze in Neptune’s were inferred from the IR spectrometer IRIS; the UV spec- stratosphere (NASA). trometer probed the stratospheres and the ionospheres. A huge amount of new results and discoveries came The space observatories: IUE, HST and ISO out of the Voyager exploration of the giant planets. Among the highlights are the first detection of active volcanism on The IUE satellite consisted of a 45-cm telescope equipped Io, the detection of Jupiter’s rings and the fine structure of with two spectrographs, giving a resolving power as high the other planetary rings, the composition and structure as 104 over the 1150Ð3200 Å spectral range (Boggess and of Titan’s and Triton’s atmospheres, the first observation of Wilson 1987). UV spectra of the giant planets recorded Uranus and Neptune’s magnetospheres, the first observation by IUE have allowed to probe their upper atmospheres, of the surfaces of Miranda and Triton, and the detection of and in particular their aeronomy and photochemistry. many new small satellites around the four giant planets. Above about 1600 Å , the UV radiation comes from the 60

NICOLAS THOMAS* The satellites of the outer planets

The discovery of the four large satellites of Jupiter by The rest of this chapter is organized as follows. It begins Galileo Galilei in January 1610 was one of the most signifi- with a historical overview of the discoveries of planetary cant events in the history of science. Galileo’s observations satellites in the twentieth century. Included are brief anec- of moons appearing to move from one side of the planet to dotes about some of the astronomers involved in these obser- the other provided support for the Copernican view of the vations and a short description of the spacecraft that made Universe by showing that celestial objects did not all orbit many of the more recent discoveries. In the following sec- the Earth. His claim as the first observer of what he called tions the orbits of planetary satellites are discussed in more the ‘Medicean planets’ was hotly disputed by the German detail, including a look at the limited amount of information astronomer Simon Marius. Marius claimed to have we have about surface composition. This is followed by a observed the satellites in November 1609 and there is little description of the atmospheres of planetary satellites which doubt that he was using a telescope to study the heavens have become a major topic since the early 1980s. The avail- around the same time as Galileo. However, Galileo pub- ability of spacecraft imaging of outer Solar System objects lished his observations while there is no record of those of has also led to an amazing growth in the field of planetary Marius. In Galileo’s honour, the four large satellites of geology which in turn has given us clues as to the evolution Jupiter are often now referred to as the Galilean satellites. of planetary satellites. The chapter concludes with a brief While the scientific and philosophical significance of discussion of what the twenty-first century could hold. Galileo’s discovery cannot be underestimated, the observations also started an enormous expansion in the number of known objects in our Solar System. Prior to Galileo’s dis- HISTORICAL INTRODUCTION covery, eight Solar System objects were known: six planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn), one Status in 1900 moon (the Moon) and the Sun. His observations raised the number of known Solar System objects by 50% at a stroke. By the start of the twentieth century 22 planetary satellites There are now 61 satellites of the 9 planets known (several had been discovered. After the discovery of the Galilean other irregular satellites of Saturn await confirmation), 57 satellites, Christiaan Huygens (1629Ð1695) and Giovanni of them are satellites of the gas giant planets Jupiter, Domenico Cassini (1625Ð1712) found most of the larger Saturn, Uranus and Neptune (Figure 1). Many of the satel- moons of Saturn in the seventeenth century. The larger lites have remarkable phenomena associated with them. satellites of Uranus were found in the eighteenth and nine- There are, for example, volcanoes on Io, geysers on Triton teenth centuries by William Herschel (1738Ð1822) and and a thick atmosphere on Titan. In some senses, therefore, William Lassell (1799Ð1880) while the largest satellite of Galileo’s discovery was also the start of a new discipline in Neptune, Triton, was also discovered by Lassell in 1846. planetary sciences Ð the study of planetary satellites. In his introductory chapter to the book Satellites, Burns (1986) suggested a means of classifiying planetary satellites. * Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany In his classification, regular satellites are large, spherical and

be intermediate between a regular satellite and a collisional Puck shard, was discovered in 1848. Phobos and Deimos were Phoebe discovered by Asaph Hall (1829Ð1907) in 1877 and Edward Hyperion Charon Iapetus Emerson Barnard (1857Ð1923) observed Amalthea, a satel- Nereid Rhea lite inside the orbit of Io, from the Lick Observatory in 1892 Oberon during a systematic search for new Jovian satellites. Mimas Titania Up to this point the discovered satellites were all rela- Proteus tively close to the parent (within 60 planetary radii of the centre of mass). However, the discovery of the irregular Miranda Ganymede Pluto satellite Phoebe, in a retrograde orbit, 215 planetary radii Titan from Saturn by William Henry Pickering (1858Ð1938) in Enceladus Callisto April 1899 (on a plate obtained seven months previously) Triton indicated that many objects at hundreds of planetary radii Umbriel away from the parent planet could exist. Tethys

Initial discoveries Ariel Dione The first significant event of the twentieth century was the discovery in 1904 of the irregular satellite of Jupiter, Himalia, by the American astronomer Charles Dillon Perrine (1878Ð1951). Perrine worked at the Lick Observatory on 500 km Mount Hamilton in northern California and later became director of the Argentinian National Observatory in Cordoba. An 86 km diameter crater on the Moon (at 42.5N, 127.8W) is named after him. Philibert J. Melotte (1880Ð1961) discovered the eighth Figure 1 A montage of the relative sizes and shapes of some moon of Jupiter, Pasiphaë, in 1908. While photographing of the larger planetary satellites in the Solar System. (From Croft and Soderblom 1991, courtesy of University of Arizona Press). Pasiphaë with the 36-inch (0.9 m) Crossley reflector at the Lick Observatory, Seth Barnes Nicholson (1891Ð1963) discovered a ninth moon which was named Sinope. Nicholson relatively close to the parent planet. They are in prograde was born in Springfield, Illinois, and became interested in orbits with low inclination and eccentricity and are often in astronomy as an undergraduate at Drake University in synchronous rotation (that is, the same hemisphere always Iowa. He graduated in 1911 and became a graduate student faces the parent). Their masses are small compared to the at the University of California from which he gained his parent. Collisional shards are small, irregularly shaped doctorate in 1915. His dissertation was based on his discovery objects, extremely close to the planet or co-orbital with a and orbit computation of Sinope. regular satellite. They have prograde orbits with essentially Nicholson subsequently moved to the Mount Wilson zero inclination and eccentricity. They are thought to be the Observatory near Pasadena, California, where he spent the ‘debris’ left over after planet and regular satellite formation. rest of his career. His main job was to observe the Sun with Irregular satellites are far from the parent and have substan- the 150-foot (46 m) solar tower telescope. He produced tial inclination, eccentricity, or both. They are small. They annual reports on sunspot activity (with Hale and St John) are thought to have been captured into their present orbits and magnetism for decades. During this period, however, (although it has to be said that a reasonable mechanism for he also discovered three more Jovian satellites (Lysithea, this process remains to be demonstrated). A fourth group of Ananke and Carme) and a Trojan asteroid (Menelaus) as unusual satellites, which do not fall into any of the above well as computing the orbits of several comets. He also categories, was also suggested (Table 1). worked with Edison Pettit to produce a vacuum thermo- By around 1850 all the regular satellites had been discov- couple which was used to measure the temperatures of the ered. Two of the three satellites we now categorize as Moon and planets in the early 1920s. He was twice presi- unusual (the Moon and Triton) had also been found (the dent of the Astronomical Society of the Pacific and became third, Charon, would not be found until 1978). Shortly the 56th winner of the Catherine Wolfe Bruce medal, before the beginning of the twentieth century, telescopes awarded by the society, shortly before he died. became powerful enough to find some of the smaller The Uranian satellite Miranda was discovered in 1948 by collisional shards. Hyperion, which might be considered to the Dutch-born astronomer Gerrit Pieter Kuiper (1905Ð1973). THE SATELLITES OF THE OUTER PLANETS 1453 1978) (Christy Pluto 1989) 1981) Thalassa (Voyager 2, Thalassa (Voyager Neptune Despina (Voyager 2, Despina (Voyager Triton (Lassel 1846)Triton Charon 1986) 1986) 1997) Ophelia (Voyager 2,Ophelia (Voyager 1989) Uranus Belinda (Voyager 2, Belinda (Voyager 2,Puck (Voyager 1986) Bianca (Voyager 2,Bianca (Voyager 1986) 1989) 2,Desdemona (Voyager Larissa (Reitsema et al . Sycorax (Gladman et al. et al . 1986) Larson/Walker 1966/1980) Laques 1980) G. Bond/Lassell 1848) 1986) Saturn Dione (Cassini 1684) (Huygens 1655) Titan (Herschel 1787) Titania Iapetus (Cassini 1671) Epimetheus (Fountain, 2, Cressida (Voyager Janus (Dollfus 1966) 2, (Yoyager Telesto 1980) et al .1980)Calypso (Pascu 2 ,1986) Juliet (Voyager 2, Galatea (Voyager Helene (Lecacheux, 2 ,1986) Portia (Voyager 1986) 2, Proteus (Voyager and Hyperion (W. 1989) 2, Rosalind (Voyager Pandora (Collins Pandora 1610) 1892) 1938) 1951) 1938) 1914) 1999) Jupiter Io (Galileo 1610)Europa (Galileo 1610) (GalileoGanymede Enceladus (Herschel 1789) Mimas (Herschel 1789) Ariel (Lassell 1851) Callisto (Galileo 1610) (Cassini 1684) Tethys Rhea (Cassini 1672) 1948) Miranda (Kuiper Umbriel (Lassell 1851) Oberon (Herschel 1787) Leda (Kowal 1974)Leda (Kowal 1898) Phoebe (Pickering Caliban (Gladman et al . 1949) Nereid (Kuiper Thebe (Synnott 1979) 1980) Elara (Perrine 1905) (Nicholson, Ananke Carme (Nicholson, ë (Melotte 1908) Pasipha Sinope (Nicholson, S/1999 J1 (Scotti et al . Himalia (Perrine 1904) (Nicholson,Lysithea others await Several confirmation 1997) 1877) 1877) Andrastea (Jewitt) Prometheus (Danielson 1986) Deimos (Hall Amalthea (Barnard 1979) Phobos (Hall Metis (Synnott 1979) 1980) Atlas (Terrile 2, Cordelia (Voyager 2, Naiad (Voyager 1989) Earth Mars Classification of planetary satellites Regular Shards Table 1 Table Irregular Unusual Moon 1454 NICOLAS THOMAS

Kuiper, who is better known by his Americanized name, founded by Percival Lowell (1855Ð1916) in 1894. In subse- Gerard Peter, turned to studies of the Solar System shortly quent years, Lowell studied the orbit of Uranus and pre- before the end of World War II. He discovered the methane dicted that the perturbations in the orbit could be explained atmosphere of Titan in 1944 (see below), the presence of by a large planet outside the orbit of Neptune. According to CO2 in the atmosphere of Mars and the small satellite of Lowell, this planet, called Planet X, needed to be approxi- Neptune, Nereid, in 1949 at the McDonald Observatory. He mately seven Earth masses to fit his observations. The dis- was also instrumental in setting up the first institute dedi- covery of Pluto by Clyde W. Tombaugh (1906Ð1997) in cated to the study of the Solar System. The Lunar and 1930 merely ignited the controversy because Pluto Planetary Laboratory (LPL) of the University of Arizona, appeared to be much fainter than would be expected of at Tucson, remains one of the leading planetary research such a large object. In 1977 Kowal started a photographic institutes in the world. search for other objects outside the orbit of Neptune. He Kuiper not only made ground-based observations. He also studied a region 15 on either side of the ecliptic. He found pioneered the use of telescopes in high-altitude aircraft for nothing attributable to an object beyond Neptune but did infrared observations. The absorption by the Earth’s atmos- discover the first Centaur, 2060 Chiron, which may indicate phere of infrared radiation often prevents the investigation of a link between asteroidal debris in the outer Solar System the compositions of gaseous planets and satellites, the inter- and short-period comets. stellar medium and molecular clouds. Kuiper’s idea led to a In astronomy, the United States Naval Observatory C-141 StarLifter jet aircraft being used as a flying observa- (USNO) is probably best known for being the organization tory. The Kuiper Airborne Observatory (KAO) was put into that, together with the Royal Greenwich Observatory, pro- service in 1975 and is operated by NASA from its Ames duces the Astronomical Almanac. However, in 1978, while Research Center at Moffett Field near San Jose. It is expected measuring the positions of Pluto on a series of images, the to be replaced by a flying observatory called SOFIA, which is USNO’s James W. Christy made an immensely important based upon a Boeing 747, being built in a collaboration discovery. Christy saw that the image of Pluto was between the USA and Germany. Kuiper was also involved in extended to one side. On examining other plates, he saw spaceflight and became chief experimenter for the Ranger that the extension had moved to the opposite side of Pluto. series of missions to the Moon. He also assisted in the selec- Christy realized that he was observing a very close satellite tion of Cerro Tololo in Chile and Mauna Kea in Hawaii as of Pluto. By viewing a series of older plates he noticed that sites for new high-altitude observatories. His name is associ- the extension seemed to move with a period of about a ated with craters on Mercury, the Moon and Mars. The most week. After being reminded by a colleague that Pluto had a prestigious prize of the Division of Planetary Sciences of the light-curve with a period of 6.387 days, he concluded that American Astronomical Society is also named after him. the orbital period was identical to the period of the light- The first satellite of Saturn to be discovered in the curve. His colleague Robert Harrington computed the orbit twentieth century was first observed by Andouin Dollfus in the next day from the detailed analysis of the observations 1966 from the Pic du Midi Observatory in France. Dollfus and found excellent agreement with a 6.387-day period. has had a long and distinguished career as a ground-based This discovery was important because it allowed a determi- observer of planets and comets. Much of his more recent nation of the mass of Pluto that demonstrated that Pluto’s work has concerned the investigation of the polarization of mass was very much less than that of the Earth and there- light reflected by dust grains ejected from comets. The satel- fore could not be the Planet X predicted by Percival lite of Saturn he observed is called Janus. However, there Lowell. Interestingly, Seth Nicholson was also involved was considerable uncertainty about the orbit of the new satel- these studies. He and others had investigated perturbations lite. In 1978 John Fountain and Stephen Larson clarified the in Neptune’s orbit and concluded that the mass of Pluto uncertainty by re-analysing the results of Dollfus’s observa- should be only 0.8Ð0.9 Earth masses. Although this was tions and demonstrating that there were two objects in the moving in the right direction, Christy’s discovery led to a same orbit about Saturn. The smaller of the two co-orbital mass estimate of only 1/400th of the Earth’s mass. objects was subsequently called Epimetheus. In March 1980, just prior to the arrival of Voyager 1 at Saturn, Lecacheux The space age and Lacques who, like Dollfus, worked at the Pic du Midi Observatory detected a small satellite (Helene) in the same By the time of Christy’s discovery the exploration of the orbit as Dione. Hence, prior to the arrival of the Voyager outer Solar System with robotic spacecraft was already in spacecraft, Saturn was known to have at least 12 satellites. full swing. On 3 March 1972 Pioneer 10 became the first In 1974 Charles Kowal discovered the 13th satellite of probe to be launched to Jupiter. It made its closest approach Jupiter (Leda). Kowal worked at the Lowell Observatory in to Jupiter (132 000 km) on 3 December 1973. Pioneer 11 was Flagstaff, Arizona. This privately endowed observatory was launched on 5 April 1973 and passed Jupiter (42 800 km) on 7

JACQUES BLAMONT* Alkali metal cloud experiments in the upper atmosphere

Prior to the space age, scientists knew practically nothing the sodium D doublet at 5890 and 5896 Å. The explana- about the neutral atmosphere above 30 km. The early sound- tion which immediately suggests itself is that there is reso- ing rockets greatly advanced our understanding of the nance scattering of solar radiation by free sodium atoms: physics of this region in a relatively short time. During the 2 2 period 1954Ð1970, ejections of sodium atoms by rockets Na ( S) h (5890, 5896 Å) → Na ( P) (1) created clouds that remained observable for tens of minutes. Na (2P) → Na (2S) h (5890, 5896 Å) (2) Sodium atoms in the atmosphere above 90 km have a relatively long lifetime. They are visible at twilight from optical An alternative explanation, advocated by Vegard (1947), resonance induced by sunlight at D doublet wavelengths. was that the excited atoms are released in the photodissoci- The first sodium trail experiments gave entirely new insights ation of some compound of the element. into the physics, dynamics and structure of the neutral Bricard and Kastler (1944) studied the absorption of the atmosphere from 90 to 400 km. The turbopause was atmospheric D doublet by a sodium vapour cell kept at vari- discovered, winds were measured, and quantitative data on ous temperatures, and from their results were able to show eddy diffusion and the first vertical structure of the neutral that the linewidth of the emission is of the order of 102 Å, temperature were obtained. which can be interpreted as indicating a temperature of the emitting atoms in the atmosphere of about 240 50 K. This favours the resonance scattering theory, since in general the THE SODIUM TWILIGHT AIRGLOW fragments of a molecule that has suffered photodissociation initially have considerable kinetic energy. Any doubt remain- Following a suggestion by Otto Struve (Elvey 1950), the ing was removed in 1949 when Bricard et al. (1949) proved word ‘airglow’ was adopted as a convenient designation for that the 5893 Å twilight emission is polarized. The the radiation emitted by the Earth’s upper atmosphere, extent of the polarization for an observing direction making other than that due to aurorae. If it is desired to specify the a right angle with the direction of the Sun was found to be nocturnal emission alone, ‘nightglow’ is used. ‘Dayglow’ about 9%, as predicted, with resonance scattering assumed. and ‘twilightglow’ are defined analogously. Photodissociation would of course yield no polarization. As discovered independently by Currie and Edwards The variation is as would be expected. In the case of the (1936), Chernaev and Vuks (1937) and Bernard (1938a), morning twilight, for example, the intensity first increases as the intensity of the nightglow yellow emission line is the Sun approaches the horizon, since the solar radiation can greatly enhanced at twilight. Bernard (1938b) and, simulta- reach more sodium without attenuation in the lower atmos- neously, Cabannes et al. (1938) identified the emission as phere; it next reaches a plateau, and finally decreases again (Blamont 1956), this decrease being due to absorption of the * Centre National d’Etudes Spatiales, Paris, France solar radiation by sodium on the dayward side (Chamberlain

1956). The behaviour during evening twilight is similar but height reversed in sequence. km Sanford (1950) was able to show that the total number of free sodium atoms present at night does not exceed 1010 in a 400 column with a 1-cm2 cross-section that extends from the ground to the top of the atmosphere. Hunten and Shepherd 300 (1954) investigated the distribution. They found that the 200 concentration of free sodium atoms is greatest at 85 3 km, 1 , 2 direction of 2 3 3 1 observation with a maximum around 10 cm , that below this level it 100 Natural falls off quite rapidly, and that above, up to at least 100 km solar excitation Sodium and probably to 115 km, it falls off with a scale height of Layer 7.5 2 km. Hunten (1956) reported that the seasonal change observer in this altitude is not more than 1 or 2 km. The presence of free atoms of sodium near the mesopause level can be explained (Chapman 1939, Bates earth 1947, Hunten 1954) by an equilibrium between creation processes by reduction of oxides:

3 → NaO2 O ( P) NaO O2 (3) 2 3 → NaO (X ) O ( P) Na O2 (4) and destruction processes which are of two kinds, oxidation and ionization. Oxidation occurs through: Figure 1 Geometry of a sodium cloud experiment; for exam- 2 3 → Na ( S) O2 (X g) M NaO2 M (5) ple, at Wallops Island, on 13 September 1961, two clouds were formed one around 200 km, the other at 400 km. (From which Bawn and Evans (1937) have shown to be extremely Blamont and Lory 1963b.) rapid, and through

2 → Na ( S) O3 NaO O2 (6) The existence of free atoms of sodium at 85 km, and the proof by Bricard and Kastler that their emission is due to Photoionization optical resonance excited by solar light, which renders them visible at twilight when the lower atmosphere is in shadow, 2 → 1 Na ( S) h Na ( S) e (7) led D.R. Bates in 1951 to suggest that the ejection of atomic tends to reduce the number of neutral atoms. It is opposed sodium by a rocket in the high atmosphere could create a by several recombination processes, of which cloud visible at twilight (Figure 1). The experiment was performed successfully in 1954 by Edwards et al. (1956), and a 1 2 → number of Aerobee rockets were devoted to the creation of Na ( S) O ( P) Na O (8) sodium clouds by the US Air Force Cambridge Research is thought to be the most effective in the upper part of the Center (Bedinger et al. 1957, 1958; Manring et al. 1959). region where sodium can be detected. The rate coefficient The method was to ignite a mixture of Al Fe2O3, called for reaction (7) is about 1 105 s1 (Bates and Seaton thermite, in which sodium pellets had been dispersed. The 1950), and that for reaction (8) is probably of the order oxidation reaction of aluminium by iron oxide is extremely of 108 cm3 s1 (Bates and Boyd 1956). Hence, the daytime exothermic but stable; the mixture, placed in a container equilibrium value of the ratio of the concentration of sodium fixed in the rocket nose cone, melts, and sodium vapour is ions to the concentration of sodium atoms is given by ejected in a steady stream. However, apart from the observation of strong wind shears which quickly deformed the 3 n(Na ) 10 clouds, no scientific results were obtained. The existence of (9) n(Na) n(O ) intrinsic atmospheric turbulence was not recognized. Similar results were obtained by Groves (1960) with a Skylark and may thus exceed unity. The two processes concerned rocket launched on 3 December 1958 at Woomera, Australia. are very slow, however, and consequently equilibrium can In 1957, France, having developed and built 15 Véronique scarcely be reached. rockets to be used for scientific purposes during the (a)

(c)

(d)

Figure 2 (a) The historic sodium cloud of 10 March 1959. From this picture, obtained at Colomb Bechar, Algeria, the existence of the turbopause was recognized for the first time. In order to understand this image, start at the widest part of the cloud (near the bottom of the picture), which corresponds to the maximum altitude reached by the rocket (130 km). From there, the trail divides into two identical, parallel smooth parts corresponding to the ascent and descent of the rocket. These parts are distorted by the wind. At the lowest altitude (102 km and below) there appears, both at ascent and descent, the sharp separation between the laminar atmosphere (smooth trail) and the turbulent atmosphere (globulous trail). (From Blamont (1966).) (b) Schematic drawing of sodium trail. The dotted line is the path of the rocket. (From Blamont and de Jager 1961.) (c) Evolution in time of the sodium trail obtained on 10 March 1959 at Hammaguir. From left to right: first picture is taken at L (time of launch) 5 min; second at L 8 min; third at L 11 min; fourth at L 12 min; fifth at L 15 min. The turbopause already seen in the left-hand picture can be followed through the cloud’s development. (From Blamont and Baguette 1961b.) (d) The March 1959 sodium cloud experiment team pictured at Colomb Béchar, Algeria. From left to right: (seated) M. Maguery, F. Roddier and J.Y. Gal; (standing) J. Blamont, C. Cohen-Tannoudji, J.P. Schneider, G. Courtès, M.L. Lory, P. Lena and P. Delache. 192 JACQUES BLAMONT

International Geophysical Year (IGY), had practically no hand, and measurements of the temperature of the neutral budget for payloads, no science group, no experimented engi- atmosphere on the other, could be undertaken. The decision neers and no equipment for telemetry, command or tracking. was made by the French IGY Programme Office not only to The author proposed to the French IGY Programme Office devote eight Véronique rockets to the ejection of various that a few of the available rockets should be devoted to chemicals, but also a number of the solid-fuel rockets under sodium ejections. The on-board hardware would be extremely development, called Centaure (reaching 200 km) and subse- simple and cheap, and the rocket was ideally adapted to quently Dragon (reaching 400 km). The programme was sodium ejections, since it was designed to carry a 100 kg pay- extended to sodium and potassium launches at various lati- load to an altitude of 200 km. The launches would take place tudes (central Sahara, Argentina, Canada, Sweden, USSR, on the French Army proving ground of Hammaguir (31N, India), to clouds created by explosives, which were found to 5W), near Colomb Bechar (in what was then French Algeria) exhibit the resonance fluorescence of the oxide AlO, and to with the help of military resources and personnel. barium clouds, in cooperation with R. Lüst’s laboratory The scientific objectives would be to study the atmos- (Chapter 6). A Panel for Simultaneous Rocket Sounding pheric horizontal winds as a function of altitude, and to Launches was created by COSPAR to support the effort. measure the temperature of the neutral atmosphere as a These experiments were carried out during the 1960s. function of altitude. The observations, all performed on the ground, would be: for dynamical studies, triangulation of the cloud by cameras placed at four sites, each separated STUDY OF THE DYNAMICS OF THE from 10 to about 100 km, forming roughly an equilateral ATMOSPHERE BETWEEN 90 AND 200 km triangle; and for temperature measurements, the absorption of the cloud’s D line emission by three cells containing A sodium cloud created by the continuous ejection of vapour sodium vapour at different optical thicknesses. by a rocket as it ascends from 80 km to its maximum altitude The first launches took place on 10 and 12 March 1959 (around 200 km) and descends is subject to three types of from Hammaguir, at dusk and dawn, respectively. On both motions (Blamont 1959, Blamont and Barat 1967a): occasions a sodium cloud was created above 85 km, the first one up to 130 km and the second up to 180 km. They 1. Diffusion of sodium in the medium in a horizontal plane. remained visible for 20 minutes (Figure 2). Two major The diffusion velocity varies inversely with the atmos- results were obtained from these flights (Blamont 1959, pheric density, and therefore increases with altitude. 1960): 2. Horizontal winds with a velocity up to 200 m s1, a ran- 1. A spectacular feature of the clouds was an abrupt domly distributed direction and horizontal scale above change in their structure around 102 km: below this alti- 100 km. tude, the clouds showed a small-scale motion field con- 3. Turbulence in the lower part of the cloud (below sisting of elements with an average diameter of about 102Ð105 km), which gives it the appearance of a cumu- 0.5 km; above this altitude, the trails displayed a smooth lus cloud, and stops abruptly above this altitude. character, not showing the slightest trace of small-scale motions. The region of transition was not thicker than 1 km. This was, as we will see, a major discovery. Diffusion of sodium 2. The temperature of the sodium atoms in the clouds measured with the absorption sodium cell technique were The sodium emitted at one point diffuses in two dimensions found to be very high (3000 K). This was obviously in a horizontal plane. A determination of the growth rate of spurious and due to the multiple scattering of the reso- the horizontal dimensions of the cloud (i.e. of the diffusion nance line inside the cloud: for the first experiments, velocity of sodium in the medium) shows the following. 10 kg of sodium had been included in the thermite con- 1. On the smooth part of the cloud, above 102 km, mea- tainer! It was therefore necessary to reduce greatly the surements show that the law of molecular diffusion is amount of sodium ejected, and also to measure the opti- obeyed perfectly. The diffusion of sodium follows the cal thickness of the cloud in order to make certain that atmospheric diffusion because the molecular (or atomic) there would be regions in the cloud where the optical masses are nearly identical. Therefore the distribution of thickness would be small enough to allow linewidth sodium atoms at any given time as a function of the dis- measurements. tance (r) to the centre of the cloud should be a Gaussian 2 2 The potential for scientific purposes of sodium trails gen- function exp( r /L0), where L0 is given by: erated by rockets had therefore been demonstrated: studies 2 of the dynamical structure of the atmosphere on the one L0 4 Dt (10) 61

NORMAN F. NESS* Planetary and lunar magnetism

In situ studies by spacecraft of the magnetic fields of Earth data provide much firmer evidence of localized regions of and all the planets except for Pluto began with the USSR’s remanent crustal magnetization by the MAGER (MAGnetic launch of Sputnik 3 in 1958. The study of the geomagnetic field Electron Reflectometer) instrument on Lunar Prospector field by the USA followed with Vanguard 3 in 1959. Since (Lin et al. 1998). then the US Explorer, Mariner, Pioneer, Voyager, Ulysses, The latest detailed study of these lunar magnetized regions and Galileo missions have surveyed all the planets from from Lunar Prospector data suggest a special geometrical Mercury to Neptune as well as the Earth’s Moon, the relationship and correlation with impact craters which are Galilean moons of Jupiter, and Saturn’s largest moon, Titan. antipodal in location to them (Halekas et al. 2001). The The USSR repeatedly studied Earth’s Moon, Venus, and source of the magnetic field for these magnetized regions is Mars with their Luna, Venera, Mars, and Phobos missions. unclear at present. They may indicate that there was an Mariner 10 discovered in 1974 and 1975 that Mercury ancient lunar dynamo that had magnetized the lunar crust, or was globally magnetized but with such a small intrinsic perhaps there is a magnetization process that occurs as a result magnetic field that no durably trapped radiation belts were of the high-velocity impacts which generate the observed permanently associated with the magnetosphere, formed by craters. It is also possible that both processes are important. the solar wind. Venus appears to be globally unmagnetized, In 1964 the US Mariner IV, and subsequently several according to results from Venera and Mariner spacecraft Soviet Mars and Phobos probes, had placed upper limits on since 1961. The solar wind interaction with its atmosphere any global intrinsic Martian field, although the existence of and ionosphere creates an induced magnetosphere, but a detached bow shock and magnetic tail were observed devoid of any trapped radiation belts. (Ness 1979a). Since achieving orbit in 1997, the US orbiter The magnetic field of the Moon has been studied by sev- Mars Global Surveyor has finally clarified the long-stand- eral spacecraft, especially Explorer 35 in 1968 (Ness 1969). ing enigma of the status of the Martian magnetic field with Measurements on the lunar surface were conducted by the the discovery of localized intense remanent crustal magne- robotic rover Lunokhod in 1968 (Dolginov et al. 1975) and tization but no significant global field (Acuña et al. 1999, with the three Apollo Lunar Surface Experiments Packages Connerney et al. 1999). (ALSEP) that were placed there during the Apollo program Pioneers 10 (1974) and 11 (1975) and Voyagers 1 (1979) of the late 1960s and early 1970s (Dyal et al. 1972, 1974). and 2 (1979) examined in situ the magnetic field and mag- The Apollo 15 and 16 missions each carried a sub-satellite netosphere of Jupiter, which had been inferred from its which was launched into a 100 km altitude, low-latitude non-thermal radio emissions, first detected in 1955. orbit. The data they gathered suggested the existence of Jupiter’s magnetic field is much stronger than Earth’s and is magnetized crustal regions (Hood et al. 1981). More recent distinctly non-dipolar close to the planet. More recently, the US Galileo and ESA Ulysses spacecraft have added to the * Bartol Research Institute, Newark, DE, USA database of in situ studies of the main field of Jupiter.

Galileo has also studied the four Galilean moons during distance of the spacecraft trajectories relative to the center multiple close flybys, and results suggest that some of these of the planet, in units of planetary radii. moons possess an intrinsic or induced field. In 1979 the US Pioneer 11 discovered a significant global magnetic field and trapped radiation belt at Saturn. GEOMAGNETISM AND PLANETARY DYNAMOS This was studied in much more detail by Voyagers 1 and 2 in 1980 and 1981. Saturn was found to have a weaker field The most plausible explanation for the origin of planetary than Jupiter, and one that appears to be remarkably axisym- magnetic fields, and the observed and well-documented ter- metric (to order and degree n 3 in a spherical harmonic restrial secular variation, is that there is a coherent regener- or multipole representation) about its rotation axis. ative dynamo motion of an electrically conducting fluid in a The Voyager 2 flybys of Uranus and Neptune in 1986 planet’s interior. This motion is a result, most likely, of and 1989 discovered significant global magnetic fields and thermally driven convection. There are also suggestions that trapped energetic particle radiation belts. Both Uranus and coupling of precessional torques, which arise in the differ- Neptune display quite similar magnetic fields, but very dif- ential motion between the cores and mantles of the planets, ferent from those of Jupiter, Saturn, and Earth. As astro- may drive this coherent motion. Finally, this coherent physical objects, Uranus and Neptune are best described as motion may also be driven by tidal forces associated with oblique rotators because of the large angular offsets of their other celestial bodies in close proximity, for example Jovian magnetic axes from their rotation axes (59 and 47, respec- tides in the Galilean moons, which display spinÐorbit tively). Additionally, their magnetic centers are offset from coupling as a result of tidal dissipation. their center of figure by substantial fractions of a planetary Study of the Earth’s magnetic field has been under way radius (0.31RU and 0.55RN, respectively). ever since William Gilbert published his famous treatise, This chapter summarizes our present knowledge of the De magnete, in 1600. Geomagnetism is a well-established quantitative characteristics of the magnetic fields of the discipline with a long and distinguished history, but one planets and certain of their moons. An early review of space that lies outside the scope of this chapter. Interested readers magnetometry by Ness (1970) summarized the first decade are referred to any of the many texts that address the many of in situ studies of the magnetic fields of Earth, Mars, special problems of geomagnetism, such as Busse (1978), Venus, and the Moon, discussed the technical problems of Campbell (1997), Backus et al. (1996), Jacobs (1994), measuring weak interplanetary fields, and presented techni- Merrill et al. (1996), Stevenson (1983), and Soward (1992). cal details of instrumentation. A more recent review of the The internal motion in the Earth’s electrically conduct- author’s role in space exploration has appeared (Ness 1996). ing core is regenerative in the sense that it leads to the The dates of significant discoveries of planetary magnetic maintenance of electrical currents and associated magnetic fields and the associated spacecraft are given in Table 1. fields by the dynamo process. The observed secular varia- An additional relevant mission parameter in these studies tion of the geomagnetic field (Cain 1995) adds substantial of solar wind interactions and the quantitative analysis of support to this concept of dynamic fluid motion in the the intrinsic magnetic fields is the closest flyby or periapsis interior.

Table 1 Summary of US spacecraft, 1971–2004, which have discovered and investigated in situ the magnetic fields of the planets

Year of encounter or orbit/periapis in Rp Mercury Mars Jupiter Saturn Uranus Neptune

Mariner 10 ¥1974/1.30 1975/1.13 Pioneer 10 1973/2.84 Pioneer 11 1974/1.31 ¥1979/1.35 Voyager 1 1979/4.88 1980/3.07 Voyager 2 1979/10.1 1981/2.69 ¥1986/4.18 ¥1989/1.18 Ulysses 1992/6.3 Galileo (orbiter) 1995/6Ð15 Cassini (orbiter) [2004/1] Mars Global Surveyor ¥1997/1.03

¥Discovery encounter. PLANETARY AND LUNAR MAGNETISM 1481

The history of the Earth’s magnetic field is recorded in Thus, the oceanographic magnetic data were analogous magnetized rocks and sediments, which bear magnetizable to a tape recording since the hot upwelling molten material materials. The study of these ancient samples, which is the from the mantle preserved a memory of the status of the discipline of paleomagnetism, indicates that the geomag- geomagnetic field as it cooled below its Curie point. The netic field has reversed itself many times in the Earth’s geo- concept and the theory of continental drift was proposed in logical past (McElhinney 1973, Piper 1987). These 1912 by the German meteorologist Alfred Wegener. He reversals are dated through the study of the relative abun- became Professor of Geophysics and Meteorology at the dances of certain isotopes of the radioactive elements. University of Graz, Austria, from 1924 to 1930, the year of The physical properties of the interiors of the other plan- his death. He had based his theory originally on the geo- ets can only be estimated, whereas the study of earthquake metrical similarities of the coastlines of the South American generated seismic signals provides accurate estimates of the and African continents, and matching geological formations structure and physical properties of Earth’s interior. There on opposite sides of the South Atlantic. Wegener’s ideas appears to be no obstacle to the assumption that all plane- and theory of such moving continents were initially met tary global magnetic fields are generated within electrically with skepticism, which continued for a long time, with conducting core or shell regions. However, a continuing some geophysicists “proving” that such motion was physi- challenge is the development of a comprehensive and quan- cally impossible. titative theory of how rapidly rotating, self-gravitating, and The first orbital flight of a magnetometer was aboard the highly condensed bodies throughout the Universe generate Soviet satellite Sputnik 3 in 1958; this was followed by the their magnetic fields. US Vanguard 3 in 1959. It was not until 1979 that the USA Dynamo theory remains one of the continuing basic dedicated a special mission to study the geomagnetic field, challenges in theoretical planetary physics and astrophysics MAGSAT. This spacecraft carried both an ultra-precise tri- (Roberts 1995, Proctor and Gilbert 1996). Recent super- axial fluxgate vector magnetometer and a cesium vapor computer simulations of the Earth’s dynamo process that total field unit to measure accurately the magnitude of the reveal intrinsic reversals have been reported by Glatzmaier field. Use of these spacecraft data and standard ground sta- and Roberts (1995, 1996) and Glatzmaier et al. (1999). tion magnetic observatories allowed the development of Interesting laboratory experiments using molten sodium models of the Earth’s magnetic field up to degree and order have recently been conducted which demonstrate the 13 and beyond (McLeod 1996). dynamo effect (Gailitis et al. 2000).

GLOBAL MAGNETIC FIELDS AND THE GEOMAGNETIC FIELD SOLAR WIND RAMIFICATIONS

Studies of the geomagnetic field prior to the space era were The existence of a global planetary magnetic field with suf- conducted on land and sea, sometimes with specially con- ficient strength to deflect the solar wind leads to several structed sailing vessels such as the one used in the seven- unique features of the planetary environment (see the teenth century by Edmund Halley in an investigation of the reviews by Stern and Ness 1982, Schulz 1995). The main deviation of the compass direction from true north in the feature is the formation of a magnetic cavity, or magnetos- Atlantic Ocean near South America. The major advance in phere, from which undisturbed solar wind plasma is studies of the geomagnetic field resulting from land and sea excluded and within which a distorted planetary field con- measurements occurred just as the space age began. This was trols the motion of energetic charged particles Ð mostly the development of the theory of plate tectonics in the early electrons and protons, but some heavier ions too. Examples 1960s. This was based upon paleomagnetic studies of crustal of the latter are found in the Jovian magnetosphere, which rocks and a vast database of total field measurements by is populated with heavier ions such as S2 and SO that instruments towed behind oceanographic research vessels. originate in the SO2 ejections from Io’s active volcanoes. Paleomagnetic results had shown that the geomagnetic The neutral atoms become ionized by charge exchange with field had reversed its polarity many times in the past. These the co-rotating magnetosphere of Jupiter. reversals are features which mark the time of these events Outside this cavity a detached bow shock wave develops and were accurately dated from radioisotopes present in in the supersonic and super-Alfvénic solar wind flow. common rock-forming minerals. The oceanographic data Finally, a magnetotail is formed which trails far behind the were combined to demonstrate conclusively that ocean planet in the anti-solar-wind direction. The distance to the floors were spreading apart from central regions, called sunward stagnation point of solar wind flow, measured in ridges, as a result of convection in the mantle driving the units of planetary radii, varies from the low values of 1.2 at relative motion of the continents. Mars (Vignes et al. 2000) and 1.4 at Mercury (Ness 1976) 1482 NORMAN F. NESS to as great as 70 or more at Jupiter. This parameter, and develops as a transverse planar structure in the magnetotail, those describing the global field to lowest order and degree which separates the bipolar lobes or flux tubes of the as a simple offset tilted dipole, are summarized in Table 2. magnetotail. The traditional way of describing a planetary magnetic At Jupiter and Saturn a bipolar magnetotail and plasma field quantitatively is based upon a method developed by sheet similarly develops. Important consequences of the Gauss. This uses spherical harmonic coefficients to represent solar wind interaction, trapped radiation belts, and plasma magnetic multipoles. Modern methods of deriving planetary sheets are that there are a number of electrical current sys- field representations from spacecraft data using generalized tems external to the planet. The magnetic fields of these inversion techniques have been developed by Connerney currents have a significant influence on in situ measure- (1981). The Gaussian coefficients for all of the planetary mag- ments of the magnetic fields by flyby or orbiting spacecraft. netic fields known to date are given in Table 3. The lowest- These currents must be carefully considered and, if appro- order magnetic field multipole moment (n 1) is a dipole. priate, explicitly modeled when attempts are made to Thus, a basic result of the solar wind interaction is that extract accurate estimates of the intrinsic field of the planet. a bipolar magnetotail is developed. Within this magnetotail is a field reversal region, referred to as a plasma or neutral sheet, which contains an embedded plasma. The MERCURY origin of this plasma is the atmosphere of the parent planet, any natural moons, and the shock-modified solar wind. The Only one spacecraft, the US Mariner 10, has studied the strength of the magnetic field in the magnetotails of planets planet Mercury. Launched in 1973, it used a gravity-assist depends primarily upon the properties of the solar wind. maneuver at Venus to encounter Mercury. There were three The magnetotail appears to be an important energy store, encounters, due to a serendipitous exact-integer relationship playing an important role in the overall dynamics of the between the heliocentric orbital periods of Mercury and magnetosphere. See Ness (1987) for a review of the discov- Mariner 10. Successful encounters in March 1974 (Ness ery in 1964 and early space age studies of the Earth’s et al. 1974) and again in March 1975 provided direct obser- magnetotail. vational evidence for a global magnetic field at Mercury A unique feature of the magnetotails of Uranus and (Ness 1978). The second one, in September 1974, was ded- Neptune is the obliquity of their rotational axes combined icated to imaging the surface and did not penetrate the mag- with the large angular offsets of the magnetic axes from the netosphere or cross the magnetopause and detached bow rotational axes. This combination produces a unique mag- shock wave. netic pole-on interaction with the solar wind at certain peri- Data from the magnetic field experiment on board ods during their orbits. As the planet rotates, the magnetic Mariner 10 during the 1975 encounter are shown in Figure 1. axis of the planetary dipole field becomes colinear with that The positions of the detached bow shock and magne- of the solar wind velocity. This is theorized to lead to a topause, scaled from Earth data, are shown superimposed magnetotail configuration in which the plasma sheet on the plot of the magnetic field vectors projected on the becomes cylindrical in shape, surrounding a unipolar flux plane of the spacecraft’s orbit (Connerney and Ness 1988). tube (Voigt et al. 1987, Voigt and Ness 1990). This is The discovery of an intrinsic magnetic field at Mercury the opposite of the case at Earth, where the plasma sheet was a great surprise, since there was neither any prior

Table 2 Summary of magnetic fields, solar wind sunward stagnation point, and rotation periods of the planets (Note: 1 G cm3103 Am2; 1 nT105 G109 T)

Planet Dipole moment Tilt and sense Dipole equatorial Average stagnation Rotation period 3 (G cm ) field (nT) point distance (Rp) Mercury 5 1022 14 330 1.4 58.7d Venus 4 1021 — 2 1.0243d Earth 8.0 1025 11.7 31 000 10.4 23.9h Moon 1 1019 — 0.2 none Mars 2 1020 — 0.5 1.29 24.6h Jupiter 1.6 1030 9.6 428 000 65 15 9.92h Saturn 4.7 1028 0.0 21 200 20 3 10.66h Uranus 3.8 1027 58.6 23 000 20 17.24h Neptune 2.0 1027 46.9 14 000 26 16.1h 62

RUDOLF A. TREUMANN* AND MANFRED SCHOLER* The magnetosphere as a plasma laboratory

INTRODUCTION after the advent of the space era. For astronomy, which had been previously restricted to the optical and long- Most of the baryonic matter in the Universe is in the plasma wavelength radio wavebands, the space era opened up the state. This comes about when the temperature of the matter areas of UV-, X-ray, Gamma-ray and Infrared Astronomy. becomes so hot that the atoms spontaneously dissociate into However, from the point of view of physics, the greatest positively charged ions and electrons. Because of charge success in space exploration must be attributed to Space conservation in this dissociation process, plasmas are usu- Plasma Physics. This domain profited directly from the ally quasineutral with equal numbers of negative and posi- possibility of in situ observation of the processes, the struc- tive charges. In other words, the number densities, ne,i, ture, and the particle and field distributions in the plasma of the oppositely charged particles are equal, ne Zini, environment of Earth. It discovered its richness and beauty where the indices e, i indicate electrons and ions, and Zi and uncovered a wealth of new and unknown physical phe- is the nuclear charge number. Since the overwhelming nomena and processes which could not have been produced majority of baryonic matter is hydrogen, one usually has in the laboratory but are of utmost importance for the equal numbers of electrons and protons in the volume, understanding of the properties of remote plasmas in astro- ne ni. Even when the fraction of the ionized component is physics. Because, however, space plasma physics as a field small the ionized component shows a totally different of research does not, by its very nature, produce a multitude behaviour than neutral matter. It is governed by the laws of nice pictures and photographs which please a broad pub- of electrodynamics and many particle theory rather than lic, the success of space plasma physics has been by far less by gravitation. It is these interactions which lead to the par- spectacular than that of both groundbased and spacebound ticular properties of a plasma and distinguish it from other astronomy, where each technical progress literally opens up matter states. new vistas out into the Universe. Information about remote plasmas is obtained solely The plasma in near Earth space, which is the only one from the electromagnetic radiation that they emit. At tem- accessible to in situ measurements, is, with a few excep- peratures on the order of eV (1 eV 1.6 1019 Joule cor- tions, invisible. The most spectacular of these exceptions is responds to a temperature of 11 000 K) and much higher, the Aurora. As it turns out, however, this phenomenon pro- this radiation is in the UV, X-ray, and radio wave ranges, vides only a marginal indication of other much more vio- the latter indicating the presence of magnetic fields. Of lent processes taking place in the depths of near Earth space all this radiation, only long-wavelength radio waves can which would never have been discovered nor understood reach the surface of Earth, because all short wavelengths without direct access to these regions. One may easily con- are absorbed by the atmosphere. The study of collisionless clude from this fact that many astrophysical observations processes in plasmas in space thus became possible only are nothing but marginal indications of processes to which we have not the slightest access and which are hidden forever to our knowledge, forcing us into interpretations * Max-Planck-Institut für Extraterrestrische Physik, Garching bei which may have little to do with what actually takes place München, Germany out there.

In this respect, space plasma physics is of undeniable wind, geomagnetic field and ionosphere at hand, one is in the value. It provides the only possibility of testing models and fortunate position to monitor many processes to which we processes which are believed to take place in plasma but otherwise would not have any access. cannot yet be directly measured. In fact, astrophysics has, In the present essay we review a number of the magne- probably more than any other science, benefited from the tospheric processes in the context of their history and their insights reached in space plasma physics. Many of its more physical importance. When identifying such processes we successful models and descriptions are based on extensions have to be very selective. We therefore focus on only a few and rescaling of the processes inferred from measurements most obvious ones: the investigation of particle confine- in the plasma in near Earth space and in particular the ment in a magnetic mirror geometry as naturally given in Earth’s magnetosphere, the easiest accessible part of all the radiation belt dipolar geomagnetic field region, the for- the cosmos. mation of the Earth’s bow shock wave, the surface of vio- To mention only one example, we refer to the very exis- lent deceleration of the solar wind by the presence of tence of Earth’s magnetosphere (cf. Chapter 63). The possi- Earth’s magnetic field, on transport processes at the magne- bility of a magnetosphere surrounding Earth and confining tospheric boundary, the magnetopause, on reconnection, on its magnetic field into a drop-like region was first suggested the generation of electric fields and particle acceleration, and by T. Gold in 1949 as a theoretical idea lacking any experi- on the most interesting processes of wave generation and mental evidence and without any mathematical explication. radiation. Wherever possible, we also in passing attempt to It was picked up by the Russian plasma physics school at point on the key historical events. However, as our histori- the end of the fifties, when it was already possible to per- cal knowledge and abilities are rather restricted we apolo- form measurements in space, using rockets and satellites. gize for that the historical aspect will naturally fall short By the time when astrophysicists (Pacini 1967, Gold 1968, compared with the scientific description. Unfortunately, his- Goldreich and Julian 1969) made the first use of the notion torical recollections by its main actors on these subjects are of a magnetosphere in 1967Ð1969 in order to understand rather sparse. We refer the more historically interested the physics of pulsars in analogy to Earth, the Earth’s mag- reader to the article by Stern (1996) and references therein. netosphere was already a well established and also, in its Of interest in this respect are also the books by Van Allen basic properties, a well understood fact. (1983) and Kennel (1995), and an essay by Van Allen The plasma in the magnetosphere and its closer environ- (1990). Additional historical information can be found in ment constitutes an invaluable huge laboratory for the inves- Gillmor and Spreiter (1997). tigation of the processes taking place in a collisionless (or at the best weakly collisional) dilute warm plasma. This has been immediately recognized after the discovery of the mag- PLASMA CONFINEMENT: THE RADIATION netosphere. Thus most of the efforts to explore the processes BELTS taking place in the magnetospheric plasma are driven by two complementary intentions: the purely explorational wish to Historically, the phenomenon of plasma confinement in a map and understand the space-time evolution of the near- magnetic mirror geometry was the first physical problem Earth environment with the highest available precision and, where space physics became aware of the existence of the at the same time, to use this environment in order to infer magnetosphere. about the physical processes which take place in an extended The first few Russian spacecraft launched at the end of dilute natural plasma. The former intention is the more geo- the fifties and in the early sixties of the twentieth century physically and environmentally oriented one, which is of the first one, including the famous Sputnik I, launched on enormous importance in understanding the more extended 4 October 1957 in honour of the fiftieth anniversary of the human living space. The latter intention instead is entirely Bol’shevyk October Revolution (for a recollection of the physically oriented and can be considered as the contribution shock on the American public and US administration and of space physics to basic plasma research. These processes the immediate science-political activity following it see arise in the first place in the interaction between the high McDougall 1985), carried on board a few simple particle speed collisionless solar wind stream and the dipolar almost detectors which were sensitive to energies higher than a few stationary geomagnetic field confined to the magnetosphere. 10 eV (or temperature of a few 100 000 K). One of us (R.T.) They also arise in the dynamical mixing of plasma of solar vividly remembers his feelings when, as a young high- wind origin with plasma of atmospheric origin. Thus the school student during the autumn holidays in early October three main actors in the physical spectacle of space plasma 1957, he saw the faint glimmering light of Sputnik I pass- physics are the solar wind, the geomagnetic field confined to ing slowly and quietly over the dark late evening sky. No- the magnetosphere, and the electrically conducting upper one ever before had witnessed a man-made moon even as layer of Earth’s atmosphere, the ionosphere. With the solar tiny as this one flying around Earth across the vast infinity THE MAGNETOSPHERE AS A PLASMA LABORATORY 1497 of space. It was the sole privilege of our generation to be the first to visually experience and enjoy this demonstration and manifestation of the enormous achievements, dimensions and potentialities of human scientific and technological endeavour. Very little was known at that time about the existence, density, particle flux and temperature of the plasma component in space. It was in fact widely believed that space was essentially empty, sometimes only crossed by high-energy cosmic rays, asteroids, quasi-regularly traversed by meteoric showers, occasionally by comets, and from time to time also by solar magnetic cloud ejections, which then caused strong geomagnetic storms the physics of which was largely unclear. Following the simple pioneering theory of Chapman and Ferraro, put forward nearly thirty years earlier in 1931, storms were boldly attributed to a brief compression of the geomagnetic field when one of these clouds, during its passage over Earth, transitionally compressed the geomagnetic field. The suddenness by which such storms begin did support this theory quite well, as did Ð in an entirely wrong interpretation Ð the long-lasting main phase of the storm, when the geomagnetic field on the Earth’s surface falls below its undisturbed value, until it, Figure 1 William Pickering, James Van Allen, and Wernher finally, gradually recovers in the storm recovery phase. von Braun (from left) proudly lifting a scale copy of the first This undershoot of the field was, in its simplistic interpreta- U.S. satellite Explorer 1 and its launcher at a press conference after its successful launch into space by one of von Braun’s tion, attributed to an over-expanding geomagnetic field Juno rockets a model of which is standing on the right of the after the magnetic cloud had passed the Earth. One should table. (Courtesy, NASA, JPL photo, see also Murray 1990.) note this disturbing and long prevailing lack of physical understanding of the storm mechanism and the even less well understood smaller sister of magnetic storms, the magnetic bay disturbance or, as it is called today, the substorm! The latter one was attributed solely to variations of currents American satellite Explorer 1, of which a scale model flowing in the ionosphere. The only indication of space mounted on its carrier could be lifted by its three principle being filled continuously with plasma came from observa- experimenters, Wernher von Braun, James Van Allen, and tion of comet tails, which are directed radially away from William Pickering (Figure 1). Explorer 1 was carried into the Sun (Biermann 1951). This indication anticipated the space in 1958 by one of von Braun’s Army rockets “Juno”, solar wind. which was developed from the Second War German V2/V3 Which instrument to put on a spacecraft depends on the family also designed by von Braun. expectation of what could be measured, and on the avail- Explorer 1 had on board just one single Geiger counter, ability of instruments. The early Soviet spacecraft carried which was provided in the last minute by James Van Allen some low-energy particle traps. As it was known that space and his collaborators, Carl McIlwain, Georg Ludwig and is continuously traversed by cosmic rays, the hope of the Ernest Ray, as the sole scientific instrument available at that investigators was to measure in situ some of the lower time expected to work under space conditions (for a per- energy cosmic rays which could not pass through the sonal recollection of the history of Van Allen’s contribution, atmosphere, and possibly or occasionally some hypotheti- see Van Allen 1996). Each time when the spacecraft passed cal medium energy solar particles, which, since the early a certain range of low magnetic latitudes at an altitude of a theoretical endeavors of Störmer (1907), were thought to few hundred kilometers the counter fell into saturation. be responsible for the optical auroral emissions. They High particle flux had been detected permanently in those detected relatively strong charged particle fluxes at low latitudes. It was Van Allen who immediately realized that altitudes and medium latitudes, the origin of which was dif- these latitudes corresponded to a particular restricted range ficult to explain (Gringauz et al. 1961, see also Lemaire of dipolar geomagnetic field lines. The high particle flux and Gringauz 1998). The interpretation of these findings detected here corresponded to particles that had been about remained a mystery until the launch of the first tiny permanently trapped in the dipolar geomagnetic field, a 1498 RUDOLF A. TREUMANN AND MANFRED SCHOLER

property of charged energetic particles that had been be trapped for infinitely long times because the trapped theoretically predicted long ago by H. Alfvén, E. Teller, component does not experience any collisions. Thus the T. Northrop and others, based on the theory of adiabatic discovery of the radiation belts provided the immediate motion of charged particles (cf. Northrop 1963). These opportunity to study the properties of an ideally trapped regions were later called the Earth’s Radiation Belts or Van plasma. This study led to a large number of hitherto unex- Allen Belts, and are since publicly known as some regions pected results and to a deep understanding of the trapping which are biologically dangerous for astronauts to pass process, one of the greatest successes in the early period of through. This is partially true, but the radiation belts are of space science. danger in the first place for modern spacecraft like Hubble, The early spacecraft carried instrumentation which was Chandra, Rosat, SOHO and others because the high inten- unable to distinguish between electrons and ions. Though it sity of particle radiation can affect the highly sensitive was clear that both kinds of particles could be trapped in the instrumentation on board. radiation belts, determination of the real distribution of elec- The origin of the radiation belts can be understood trons and ions in the belts had to wait until around 1975 for resulting from an interplay between the Earth’s dense investigation and confirmation. At about that time, it was atmosphere and the flux of energetic galactic cosmic rays confirmed that protons above 100 keV energy populated one coming from the depths of the Universe and hitting the single large radiation belt, while the radiation belt electrons atmosphere. The collisions between the cosmic radia- above 40 keV distributed themselves into two spatially sepa- tion and the atmosphere cause the cosmic ray albedo neu- rated belts, the inner and outer electron belts. The region tron decay. Neutrons, produced in the nuclear interaction between those two is called the slot of the radiation belts. between the cosmic rays and the nuclei of the atmosphere are reflected upward from the atmosphere. These free neutrons are unstable and decay into energetic protons and Pitch-angle diffusion electrons which become trapped in the closed geomagnetic Stably trapped particles possess three adiabatic invariants: field configuration near the Earth, forming an inner and an the magnetic moment, the bounce and the azimuthal drift outer radiation belt. The spectrum of the trapped protons invariant. Typical gyration times for electrons and protons typically decreases approximately inversely with energy from 1 MeV to 1 GeV. A third innermost belt is produced by the energetic interstellar ions of the anomalous 6 cosmic ray component in our solar system. These ions have Electrons > 1 MeV a high rigidity, ignore the geomagnetic field and penetrate deep into it. In collisions with the upper atmosphere they Inner Belt lead to ionization and become themselves stably trapped. 3 Radiation belts have also been artificially generated during the enormous charged particle releases from atmospheric

) 5

nuclear tests like the “Starfish” nuclear explosion in 1962. E 3x10

R 2 ( 6 Sometimes similar injections of particles have been 0 3x10 10 observed during very strong magnetic storms. Thus the ori- 3x104 gin of the radiation belts is well understood. However their persistent confinement, spatial structure and temporal varia- Distance tion requires explanation and provides an important and -3 5 interesting exercise in plasma physics. 3x10 Plasma confinement is particularly important in fusion Outer Belt research, where a hot plasma with a temperature of 107Ð108 K (or energy 1Ð10 keV) must be magnetically -6 confined for a sufficiently long time in order to make possi- 036 912 ble the ignition and self-sustainment of the nuclear fusion Distance (R ) reaction. At the high densities of a fusion plasma, these E times are rather short and limited by collisions. At the low Figure 2 Spatial distribution of omnidirectional radiation densities of the trapped particles in the Earth’s (Van Allen) belt electron flux (in units of electrons cm2 s1) following the radiation belts, as these regions have been called, the trap- 1962 “Starfish” atmospheric nuclear test explosion. The flux ping times can become very long, of the order of years, distribution indicates the presence of an inner and an outer allowing for a detailed investigation of the injection and electron radiation belt. Constant flux contours follow the loss processes of trapped particles. Ideally, the particles can geomagnetic dipole field flux tubes. (After Walt 1994.) 63

BENGT HULTQVIST* Earth’s magnetosphere

INTRODUCTION early satellites and in addition they were not used to build own instruments suitable for launch into space. Besides The magnetosphere was discovered in the early years of the from auroral and ionospheric researchers, the magnetos- space era. Magnetospheric physics became the first new phere recruited its workers from nuclear physics Ð gener- scientific discipline conceived in the space age, although it ally via the cosmic rays and solar energetic particles fields was in the beginning called space plasma physics, auroral Ð and laboratory plasma physics (ionized gases) groups. physics, or ionospheric physics, as the concept magnetos- The magnetospheric physics community is still one of the phere did not exist yet. Auroral and ionospheric physics largest in the solar system sciences. were important items in the research program of the In the four decades of the space era magnetospheric ‘International Geophysical Year’ (IGY), which, after long research has reached a certain degree of maturity. It is, preparations, was implemented in the 18 months starting however, still a young field in the sense that unexpected, on 1 July 1957 and thereafter was followed up in a number ‘surprising’ results continue to constitute the most impor- of international research programs. Being the largest inter- tant new results from practically all new space missions. national co-operative research program ever, it resulted Earth’s magnetosphere has become the main ‘laboratory’ in important progress of magnetosphere-related research for studies of space plasma physics. We rely on nature to fields Ð such as the aurora Ð based on worldwide ground- make most of the ‘experiments’ in that laboratory, but some based observations. IGY had also in its program the launch- ‘active’ experiments have been made by researchers. ing and operation of the first man-made satellites, both That the concept magnetosphere did not exist when Soviet Sputniks and American Explorers and Vanguards, Sputnik 1 was launched does not mean that no physical for in situ measurements in space. Still, Sputnik 1 caused concepts of major importance in magnetospheric physics an enormous sensation when it appeared in the sky on were known before the space age. A first attempt to October 4, 1957. describe what happens when a plasma cloud from the Sun The physics of near space came to dominate space reaches Earth with its strong internal magnetic field was research in the first decade or more after Sputnik. That had made by Chapman and Ferraro already in the early thirties to do with the fact that the early satellites, which were sta- and in the late thirties and early forties Alfvén introduced bilized by spinning, were well suited for accommodating large-scale electric fields around Earth and magnetic fields the instruments of the space physicists, who were also very into the solar plasma interacting with Earth, as well as mag- eager to send their instruments into space to measure netohydrodynamics and the guiding center approximation. directly on that hot plasma, the effects of which in the form Still, we must conclude that before the satellite era our of aurora, ionspheric variations and energetic particles at knowledge of magnetospheric physics was very limited Earth’s surface, they had studied for long from the ground. and many early ideas have turned out to be very far from The astronomers could generally not be well served by the reality. This is not astonishing considering what we now know of the complexity of magnetospheres. The complex- * Swedish Institute of Space Physics, Kiruna, Sweden ity is so great that it is very difficult, if not impossible, to

Bengt Hultqvist, The Century of Space Science, 1529Ð1557 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1530 BENGT HULTQVIST derive more than a very limited amount of information solar wind was discovered by Biermann (1951) from his from basic principles. Only in situ observations can gener- observations of comet tails and Parker (1958) gave a theory ally tell what happens and theory needs a strong guidance for it. From the presence of an always-present solar wind from experiments in order to find its right directions. But follows an always-present boundary to the geomagnetic the unavoidable limitations of observational possibilities in field. The name ‘magnetosphere’ for the elongated volume the huge magnetospheric system makes theoretical and within this boundary was used for the first time by Gold numerical models of the system essential for the interpreta- (1959). Several of the early US satellites with eccentric tion of the few scattered observations available when nature orbits showed a fluctuating and irregular magnetic field makes its ‘experiments’ around and within the magnetos- beyond certain distances (Pioneers 1 and 5 and Explorer phere. Thus, theory and experiments depend strongly on 10), then considered as a broad, unstable boundary region. each other for progress. That the most important results Explorer 12, launched in August 1961, was the first to proof new magnetospheric missions are unexpected, as men- vide a clear determination of the boundary of the magnetos- tioned above, means that many theoretical and numerical phere in the subsolar region (Cahill and Amazeen, 1963). models are in an early, incomplete, or even wrong state. The sharp boundary, called the magnetopause, was found to To learn to understand what are the prevailing processes be located in the range from 8 to 12 RE near noon. The among the many possible ones in the complex magnetos- expected compression of the outer subsolar geomagnetic phere is, of course, an important task from a basic scientific field was observed and the magnetic fluctuations were point of view and it is important also because many of the found to be larger outside the magnetopause than inside. basic magnetospheric processes, which can be studied in Less than two years after Cahill and Amazeen had made detail only in our own magnetosphere, certainly play impor- the first observations of the sharp boundary of the magne- tant roles in the physics of the plasma universe in general, tosphere, Ness et al. (1964), in data from IMP-1, succeeded thus not only around the other bodies in our solar system in interpreting the irregular variations in the magnetic field and in the heliosphere as a whole, but around other stars and outside of the magnetopause and identified for the first time in the interstellar space. To the basic scientific interest adds the bowshock of the magnetosphere in the solar wind. Their that understanding of magnetospheric physics is a require- results are reproduced in Figure 1. That there would exist a ment for the exploration of space for various applications. bowshock in the collisionless plasma of the solar wind had ‘Space weather’ is affecting an increasing amount of human been suggested by Axford (1962) and Kellogg (1962) activities in the environment of humankind. shortly before, however without specifying the processes The present review deals with some of the most important which replace the collisions in an ordinary gas. A collis- results of magnetospheric research from the point of view sionless shock wave had never been seen before. Collective not only of understanding the magnetosphere but for basic plasma processes have in them taken the place of collisions plasma physics and astrophysics in addition. The early part in ordinary gases for the scattering and thermalization of of the paper is organized mainly regionally. The first section the ions and electrons. describes the discovery of the magnetosphere, i.e. of the magnetopause and the bow shock and the physical processes –30 there. Thereafter come in turn the inner magnetosphere, the Yse high-latitude magnetosphere with the aurora-related phenomena, the magnetotail, the boundary layers. These regional Explorer 10 sections are followed by some on processes: magnetospheric Shock wave –20 convection, substorms and storms, plasma sources and losses, and wave-particle interactions. Finally, future direc- Aberration of solar wind tions of research and the roles of magnetospheric research in Magnetopause the investigation of the universe are discussed briefly. 5° –10 IMP–1 rectified boundary THE CELLULAR STRUCTURE crossings Magnetosphere Chapman and Ferraro (1931, 1932), in a study of the cause Xse of magnetic storms on Earth, had shown that a plasma To sun 10 Earth –10 –20 cloud emitted by the disturbed Sun when reaching Earth Geocentric distance (Earth radii) would enclose the geomagnetic field and compress it in the Figure 1 Observations of spacecraft crossings of the direction of the Sun. This effect was only expected during magnetopause and the standing shock wave in front of Earth’s magnetic storms. The existence of a permanently occurring magnetosphere (after Ness et al. 1964). EARTH’S MAGNETOSPHERE 1531

Figure 2 Schematic representation of magnetic field profiles at different locations of Earth’s bowshock (after Greenstadt and Fredricks 1979). Field magnitude is plotted vertically. The superposed three-dimensional sketches represent solar wind proton thermal properties as number distributions in velocity space.

Collisionless shockwaves in hot magnetized plasmas are The magnetopause separates the comparatively dense difficult to produce and investigate in the laboratory and and not too hot plasma of the solar wind from the thinner most of what is known about them has been learnt from and hotter plasma in the magnetosphere. The transition space measurements. Major efforts, both experimental and takes place over a thin boundary layer. Also the magnetic theoretical, have been devoted to the investigation of such fields in the two regions are different. An important result shock waves since the middle of the sixties. A representa- of space plasma physics research is the demonstration that tion of observational results from Earth’s bowshock is this kind of thin boundary between plasmas of quite differ- shown in Figure 2 (after Greenstadt and Fredricks 1979). It ent properties characterizes the solar system as a whole and illustrates the complexity of the shock. not only the vicinity of Earth. All planets have similar Collisionless shock waves are quite important for the boundary layers and the interplanetary magnetic field sector acceleration of particles in the solar system and in the boundaries are other examples. The solar system thus has a whole universe. Besides the bow shocks in front of planets, kind of cellular structure and it is most likely that this is many travelling shocks generated near the Sun have been true for the universe as a whole. The ‘cell walls’ are not observed. There are still many aspects of the shock waves possible to observe by remote sensing techniques but only that are poorly understood and the experimental investiga- by in situ measurements. tion of them in space will certainly go on for a long time. Such a cellular structure of the plasma universe may be The reader is referred to e.g. Tsurutani and Stone (1985) for of major astrophysical importance. Alfvén (1981) has even detailed information. suggested that a consequence of the ability of plasma to 1532 BENGT HULTQVIST concentrate differences between different populations into can, however, be drawn, and are being drawn, without a very thin boundary layers between them may be that a detailed understanding of what happens in the diffusion matter Ð antimatter symmetric universe may possibly exist region. That reconnection occurs under certain conditions in spite of lacking observational evidence, with matter and must be considered as experimentally demonstrated beyond antimatter separated from each other by thin ‘cell walls’ doubt, but a lot of work remains before the process is well and therefore the interaction between matter and antimatter understood. The magnetopause region of Earth will cer- in these thin boundaries being too weak to be observable tainly be the main region for experimental investigations from Earth. also in the future. The interaction of solar wind and magnetospheric plas- Another kind of interaction between the solar wind and mas and fields at the magnetopause is an interaction the magnetosphere was proposed in the same year as between two collisionless, high-temperature plasma popu- Dungey published his reconnection-produced open magne- lations containing different magnetic fields and moving rel- tosphere, namely some kind of viscous processes at the ative to each other with supersonic and super-Alfvénic magnetopause (Axford and Hines 1961). Although most velocities. Such interaction is of basic interest and impor- evidence suggests that reconnection is quantitatively more tance from both a general plasma physics and an astro- important than viscous processes for the transfer of plasma physics point of view. It is a complicated and difficult and energy across the magnetopause, the latter processes matter, still far from being understood in all its aspects. are likely to occur and may even dominate the interaction Practically all existing experimental knowledge about it has when the interplanetary magnetic field is pointing north- been obtained from space plasma physics investigations at ward. What micro-processes are most important for the vis- the boundary between the solar wind and the magnetos- cous effects is still unknown. phere of Earth. The investigations have shown that the mag- That the differences between the properties of two netosphere is not closed but open in a way proposed by plasma populations which are streaming relative to each Dungey (1961) in the early space era and illustrated in other are concentrated to a thin boundary between them, Figure 3. How the interconnection of the magnetic fields of does not mean that they do not affect each other beyond the the solar wind and the magnetosphere really occurs, a boundary layer. All the phenomena to be reviewed below process generally called reconnection and discussed in a are in fact driven by the streaming of the solar wind around special chapter of this book, is still a front line item of mag- the magnetosphere. The characteristic solar wind speed of netospheric research. The dependence of the interaction on 400 km/s corresponds to a voltage difference over the width the direction of the interplanetary magnetic field being of the magnetosphere of several hundred kilovolts. Part of opposite to the direction of the magnetospheric field, or at this voltage is transferred into the magnetosphere and least having a strong such component, was demonstrated causes convection and acceleration of the plasma. The solar observationally already a few years after the discovery of plasma dynamic pressure on the magnetopause also has the magnetospause (Fairfield and Cahill 1966). The recon- direct effects in the form of variable compression of the nection process has become an important tool in magnetos- day-side magnetosphere caused by the variable solar wind pheric research as well as in many fields of astrophysics, and on the penetration of plasma in the polar cusp regions although what really happens in the reconnection volume Ð (see Figure 23) and possibly elsewhere. the diffusion region Ð has not yet been demonstrated exper- The transient re-organizations of the inner magnetos- imentally. Many conclusions about effects of reconnection phere that give rise to magnetospheric substorms and storms (see later section) involve power levels of the order of 1012 Watt or even more for the intense cases. Those processes use, at least partly, energy accumulated within the magnetosphere during longer periods than it is dissipated in, but the supply of energy by the solar wind is more than enough for running such processes continuously. That it does not happen is not because of too little energy in the streaming solar wind. In Earth’s magnetosphere the rotation of the planet contributes significant energy to the magnetosphere but much less than the solar wind. For Jupiter, the Figure 3 The ‘open’ magnetosphere proposed by Dungey opposite is true: the planetary rotation is a more important (1961), in which the magnetic field lines of the solar wind energy source for the magnetosphere than the solar wind. plasma are connected with the geomagnetic field lines on the A comparison: For an average rate of dissipation of sunward side of Earth and they are disconnected again in the energy within the near-Earth magnetosphere and the ionos- geomagnetic tail. phere of one tenth of the figure mentioned above for 64

TOR HAGFORS* AND KRISTIAN SCHLEGEL* Earth’s ionosphere

1 INTRODUCTION (of the magnetosphere) as well as from below (of the lower atmosphere) will be discussed as well. A brief summary of In this chapter we will deal with the ionosphere, a part of the current status of our knowledge of ionospheric processes the terrestrial atmosphere ranging from about 80 km to completes this section. more than 1000 km height. Its lower boundary lies in the The techniques for probing the ionosphere from the mesosphere, at large heights a transition into the magnetos- ground and in situ are the subjects of Section 4. phere occurs. In altitude the ionosphere approximately Experiments by Appleton and Barnett (1925) and by Breit coincides with the thermosphere a part of the neutral atmos- and Tuve (1926) designed to determine the height of reflec- phere (Figure 1). The term thermosphere is used when the tion of radio waves from the ionospheric layers signaled the properties of the neutral gas are under consideration, the beginning of ionospheric research as a science. They in term ionosphere when the electric properties and processes many ways can be said to represent the start of modern are regarded. Although the charged particle concentration space science. They also, as it turned out, had an enormous in the ionosphere is at most around 1/1000 of the total par- influence on other fields. The development of the magneto- ticle density (around the F-layer peak, see Figure 1), the ionic theory to describe the propagation of radio waves in ions and electrons enable fundamentally different processes magnetized plasmas (Lassen 1927; Appleton 1932) marked to take place in the ionosphere as compared to those occur- a large step forward in plasma physics which before ring in the neutral atmosphere. Electric currents, electric primarily had been confined to studies of gas discharges. fields, particle drifts in these fields and the reflection of The later studies of reflection and scattering from and radio waves are among these phenomena. transmission through irregular ionospheric layers and their In Section 2 we shall first follow the exploration of interpretation had a profound influence on radio and radar ionospheric structure in its historical context. We will deal astronomy (Ratcliffe 1948; Booker and Clemmow 1950; with the different layers (see Figure 1), will discuss pecu- Booker et al. 1950). They helped to establish relations liarities thereof and geographic as well as temporal anom- between the behavior of radio signals transmitted through alies. A separate subsection is devoted to ionospheric the interplanetary and the interstellar medium, and led to irregularities and some consequences of their existence. increased understanding of the properties of plasmas in Section 3 describes the development of our understand- our solar system, in our galaxy and in other galaxies. ing of the fundamental physical processes leading to the Studies designed to detect small radio sources by observing formation and spatial and temporal behavior of the ionos- scintillation caused by interplanetary plasma irregularities phere. We shall deal with production and loss processes, led to the discovery of pulsars (Hewish et al. 1968). They chemistry, dynamics and transport, winds and waves in contributed to the interpretation of radar echoes from the ionosphere and thermosphere. Influences from above planets, asteroids and comets in terms of their degree of surface irregularity. The studies of the ionosphere even brought with them important technological advances in * Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany that all modern radar systems trace their roots to Breit

log. neutral particle density [m-3] Speaking about important contributions, the most outstanding scientist in our field was certainly Professor Edward V. Appleton (1892Ð1965) as will become evident in the following sections. He received the Nobel Price for his achievements in 1947, the only scientist thus honored for ionospheric research.

2 EXPLORATION OF IONOSPHERIC STRUCTURE

2.1 Early ideas The first ideas of an electrically conducting layer in the atmosphere were advanced by Gauss who in 1839 argued that small daily variations of the geomagnetic field could be explained by electric currents flowing in such a layer (see Kaiser 1962). Later Stewart (1882) suggested that these currents may be produced by the dynamo actions of winds blowing across the geomagnetic field. The basis of the electric conductivity however, remained unclear before the discovery of the electron in 1897 and ions around 1900. After the successful reception of a radio signal transmitted from Pudhu/Cornwall at St. Johns/Newfoundland by Figure 1 Schematic diagram of the ionosphere, one of the Marconi on 12 December 1901, it became obvious that “spheres” of terrestrial atmosphere, between the mesosphere these waves did not propagate on a rectilinear path. Electro- and the magnetosphere. It is characterized by its electron magnetic waves discovered by Hertz in 1887 were expected density height profile, subdivided into different layers and to travel along straight rays like visible light in vacuum, regions as described in the following sections (right hand part but could also undergo reflection and diffraction. Either of the figure). The ionosphere covers approximately the same one of these processes had to be involved to explain the altitude range as the neutral thermosphere (left hand part of propagation over such long distance. The first clear the figure), characterized by the neutral gas temperature. The neutral particle number density is plotted as a dashed line, its hypotheses of an electrical conducting layer and of a reflec- scale is on the top of the figure. tion of the signal by this layer were published independently by Kennelly (1902) and Heaviside (1902). The diffraction of radio waves around a conducting Earth was and Tuve’s experiments with pulsed radar to determine the treated by MacDonald (1903), Rayleigh (1903), Poincare height of the ionosphere from the time delay of the echoes. (1903), Zenneck (1907) and Sommerfeld (1909) and was Finally, in Section 5 we briefly describe the role of the discarded as possible explanation of Marconi’s results. ionosphere and of ionospheric research in our modern Although radio transmissions were more and more society. Impacts on communication, geodesy and space extensively used for broadcasting and information transfer technology are discussed in the context of solar-terrestrial in the following years, it took more than two decades relations and space weather. before the height of the layer could be established. The literature on the ionosphere is very extensive com- Appleton’s famous wavelength change experiment was per- prising several tens of thousands of papers, and it will be formed in December 1924 (Appleton and Barnett 1925). It impossible to refer to them all in a book on the ionosphere, proved the existence of a ground wave (direct path) and a and let alone in such a brief chapter on the subject. The sky-wave (reflected at the layer) which are subject to inter- choice of references in this chapter is highly subjective and ference at the reception point, causing a fading of the sig- to some extent reflects our own interests and areas of nal. From the fading characteristics the authors were able to research. We apologize for the omission of many important derive a first height estimate of the conducting layer of contributions to the field. These omissions must be ascribed about 90 km. From our present knowledge (see Figure 1) to our ignorance of important contributions rather than to we know that this very low height had two causes. The ill intent. We sincerely apologize for this limitation in sounding frequency of 0.75 MHz used was very low and acknowledging all those who have contributed to the field. therefore the reflection height was well below the E-region EARTH’S IONOSPHERE 1561 peak and, the measurements were made during night derived the same formula even a few years earlier (Lassen (because the BBC transmitter used was available for this 1926), but the name of the formula remained. research purpose only after the end of the normal broad- On the basis of eqn (2.1) it was obvious that the waves casting at midnight). About a year later the so-called do not propagate with the velocity of light through the Peterborough experiment, gave the first height variation: plasma and that measurements using the signal travel time during the time one hour before sunrise to sunrise the layer only give a ‘virtual’ height h. In the general case the virtual descended from about 115 to 95 km. The interference pat- height is related to the true height h by tern formed on the ground between transmitter and receiver h h was also utilized in another experiment to estimate the layer c n height. Hollingworth (1926) measured the variation of sig- h dz n dz (2.2) Vg nal strength of a transmitter near Paris at various receiving 0 0 locations between London and Edinburgh from which he deduced the height of reflection. At the same time, as men- where Vg is the group velocity of the wave and n the index tioned in the introduction, Tuve and Breit (1925) used a of refraction (the simplified form is given in eqn (2.1)) transmitting method still applied today, the pulse technique. depending on the height profile of the electron density N(h) It gave similar results, namely around 100 km for the and the sounding frequency . reflection height. Experiments with vertically reflected waves on a higher frequency led Appleton (1927) to the conclusion that another layer existed above the E layer. He called it F layer 2.2 The ionospheric layers and found later that during day time it splits into an F1 and The name of the layer was at that time still associated with an F2 layer. Both layers (cf. Figure 1) were systematically researchers in the field, i.e. ‘Heaviside layer’ or ‘Appleton studied in the following years making use of ionosondes layer’. After the British National Committee for Radio- developed at the beginning of the thirties (see Section 4). It Telegraphy in 1926 had suggested that a non-personal name was found that the ionization followed the solar zenith should be found, the term ‘ionosphere’ was introduced by angle (Appleton and Naismith 1932) suggesting that the R.A. Watson-Watt and apparently also independently by ionizing radiation came from the sun. The prevalence of the Appleton. Appleton used the letter ‘E’ first to denote the F layer ionization during night remained a puzzle for many electric field of signals reflected from the layer. Only later years (see Section 3). Information about the layer thickness the letter was used to indicate the layer itself. was first obtained in 1937, when Appleton showed that the A thorough interpretation of these early radio experi- E and F layer had a thickness ratio of 1 : 4. ments was only possible after a suitable theory of radio A layer below the E layer escaped detection for quite a wave propagation was developed. Larmour (1924) was the long time, because of measuring difficulties. The early first to realize that a plasma has a refractive index less than ionosondes were not able to measure it, because of the low unity causing incoming waves to be bent away from the electron densities in this layer and the consequently necessary normal to the layer and consequently reflect radio waves. long wavelength to probe it (cf. eqn (1)). First indication of He derived the formula for the index of refraction this ‘D layer’ came from fadeouts accompanying solar flares (Mögel 1931), later also from the study of the interference

2 pattern between ground wave and sky wave near long-wave- 2 p n 1 1 X (2.1) length transmitters. Best and coworkers (1936) were able to derive reflection heights of 74 km during day and 92 km at with the plasma frequency night. More precise data about the electron density distribution were not obtained before the 1950s when the partial 2 Ne reflection (Gardner and Pawsey 1953) and the cross modula- p me 0 tion techniques (Fejer 1955) were introduced (cf. Section 4). The F1 and the D layers are not really layers with a where N and me are the electron density and electron mass, well-defined peak as is the case for the E and the F2 layers respectively, e the elementary charge, 0 the dielectric con- (Figure 1). They form a ledge of enhanced ionization just stant of free space, and the angular frequency of the trans- below the E and the F2 layer, respectively. During the mitted signal. Appleton realized that the influence of the last twenty years, however, it became clear that they geomagnetic field on the motion of electrons and ions had to are really different from the layers above in terms of the be taken into account. This finally led to the derivation of physics involved (ionization, recombination, dynamics, see the so-called Appleton-Hartree formula (eqn (4.1)), (Appleton Section 3). Today we speak of D and F1 regions, rather 1932; Hartree 1931) for the index of refraction. Lassen had than using the term layer. 1562 TOR HAGFORS AND KRISTIAN SCHLEGEL

It should be remembered that ionosondes do not give 2.3 Special effects and geographic peculiarities electron density data above the peak of a layer. This led to a The study of diurnal and seasonal variations of the F2 layer gap in the measurements between the E and the F layer only revealed several anomalies. The diurnal anomaly addresses closed after incoherent scatter facilities and rocket and satel- the fact that the highest electron density does occur not at lite measurements became available. The electron density local noon (smallest solar zenith angle) but in the after- above the F2-layer peak was first observed with topside noon. The winter anomaly refers to the higher ionization in sounders from satellites in 1962 and also later with the inco- winter than in summer at mid latitudes (Berkner and Wells herent scatter technique. The latter provides the most accu- 1938). The former can be explained when dynamic effects rate measurements from the ground between about 80 km are included in the physical description of the layers rather and several hundreds of km altitude (Figure 2 and Section 4). than only ionization, production and recombination (see At very large heights (above 1000 km) the ionospheric Section 3), the latter by seasonal changes of the O/N con- ion composition changes from O to H and He ions, there- 2 centration ratio. fore this outer part of the ionosphere is sometimes called Already in the thirties a special form of the E layer was protonosphere. The whole near-Earth space filled with ther- detected and later named ‘sporadic E’, or E (Appleton and mal plasma was named plasmasphere. This ‘sphere’ is coro- s Naismith 1940). It showed high electron densities with a tating with the Earth and the thermal plasma within is peak often below the normal E-region peak, it was very transported upward and downward along magnetic field lines, narrow, and it occurred only sporadically. At mid latitudes even from one hemisphere to the other (Lemaire and observations showed a very pronounced maximum in the Gringauz 1998). The plasmasphere exhibits a relatively sharp probability of occurrence during summer, whereas no clear outer boundary, a steep drop in plasma density from about 109 seasonal variation was found at low and high latitudes to about 107 m 3 at 3Ð5 Earth radii. This plasmapause was (Smith 1957). Later, through measurements with sounding detected almost simultaneously from early satellite measure- rockets, it was shown that E layers contain a considerable ments (Gringauz et al. 1961) and from ground-based whistler s content of metallic ions, i.e. Fe and Mg besides O and observations (Carpenter and Park 1963). The plasmapause 2 NO, the usual dominant ions at E-region height (Narcisi can also be regarded as the ultimate border of the ionosphere. et al. 1967). In the sixties a wind-shear theory of sporadic E was developed by several authors (e.g. Whitehead 1961; Axford 1963). An altitude variation of the wind speed acts in the presence of the geomagnetic field to compress the ionization. This wind-shear theory is generally accepted today as one cause of mid latitude sporadic E. A special form of Es with peak electron densities well exceeding those of the F2 layer was observed at high latitudes (Appleton et al. 1933) and extensively studied particularly by Scandinavian scientists in the following years. After rocket and satellite measurements became available in the sixties, it was shown that these very pronounced layers (Figure 3) are created by electrons with energies of several keV precipitating from the magnetosphere along geomagnetic field lines. The particle energy determines the height of these layers, the particle flux its peak density (see Section 3). In recent years it became clear that the precipitation of these electrons (and sometimes protons) is controlled by the solar wind. The morphology of these layers is determined by magnetospheric processes also associated with geomagnetic disturbances. During these events signifi- Figure 2 Example of an incoherent scatter measurement of cant changes in the F layer were also observed: so-called the ionospheric electron density at mid latitudes with the positive and negative storm effects, denoting an increase Millstone Hill radar. It covers most of the ionosphere from the or decrease of the F-layer ionization, respectively (see D layer to the topside above the F2-layer peak. The F1 layer is Section 3). visible as well. With the help of advanced transmitter pulse Enhanced electron densities in the high latitude D layer schemes the height resolution can be kept variable, i.e. high were inferred from radio wave propagation disturbances in regions with small scale details (D, E, F1 region) and low in (Mögel 1931; Dellinger 1937). These ‘sudden ionospheric the F2 region and above. (J. Holt, private communication.) 65

JOHNNY A. JOHANNESSEN*, STEIN SANDVEN* AND DOMINIQUE DURAND* Oceanography

1 INTRODUCTION Ð Agenda 21 and the UN Commission on Sustainable Development. At the onset of this new millennium increased awareness of Ð The Intergovernmental Panel on Climate Change. the stresses being placed on the Earth system, often induced by human activities, has intensified the need for informa- These commitments require substantial economic, tech- tion on the present state of the Earth system and for nical and scientific resources for their execution, and action enhanced capability to assess its evolution such as associ- at many levels, including significant programmes of global ated with environmental pollution, natural resource man- observations. In this context it is recognised that Earth agement, sustainable development, and global climate observation satellites provide an important and unique change. source of information. The most well established interna- This realisation has resulted in increased political and tional forum for coordinating the operational provision of legal obligations on governments and on national and data is without doubt the World Weather Watch (WWW) of regional agencies to address Earth system topics of global the World Meteorological Organisation (WMO). Another concern. These obligations are often encapsulated within prominent, though less established forum is the Global international treaties, whose signatories have explicit Ocean Observing System (GOOS). Common for these inter- requirements placed upon them. national forum and observing system is the role of satellite Many of these treaties call for systematic observations of Earth Observation. Here it is worth mentioning the World the Earth to increase our understanding of its processes and Climate Research Program (WCRP), International our ability to monitor them: Geopshere and Biosphere Program (IGBP) and Intergover- mental Oceanographic Committee (IOC) in which research Ð The UN Framework Convention on Climate Change projects and observing systems highlight the importance of (FCCC). continuous and regular access to Earth Observation data. Ð The UN Convention to Combat Desertification in those Earth observations from satellite are highly complemen- Countries experiencing Serious Drought and/or tary to those collected by in-situ systems. Whereas in-situ Desertification. measurements are necessary for underwater observations, Ð The Montreal Protocol of the Vienna Convention on the for high accuracy local observations, for the calibration Protection of the Ozone Layer. of observations made by satellite and as input to numerical models, satellite observations provide an inherent wide area unique capability to obtain regular quantitative information of surface variables and upper layer phenomena at global, * Nansen Environmental and Remote Sensing Center, Bergen, Norway regional and local scales.

Johnny A. Johannessen, Stein Sandven and Dominique Durand. The Century of Space Science, 1585Ð1622 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1586 JOHNNY A. JOHANNESSEN, STEIN SANDVEN AND DOMINIQUE DURAND

Present-day applications of satellite data are widespread agencies (CEOS 97). In addition, Space Agencies are cur- and cover research, operational and commercial activities. rently implementing their new strategy for defining future These activities are of interest in the global context and in Earth Observing satellites, both dedicated research missions the regional, national, and local context where Earth obser- and continuous (operational) monitoring missions. vation data are successfully applied in support of a range of In this article we will review the status of satellite different sectors, including (not exclusive): oceanography at the onset of the new millennium. In Section 2 the principal methods, instruments and basic Ð climate change research, measured surface parameters are addressed. Examples of Ð stratospheric chemistry, particularly related to the ozone contribution to climate monitoring and operational hole, oceanography are then given in Section 3 and Section 4, Ð weather forecasts based on Numerical Weather Prediction respectively. In Section 5 an outlook towards the near (NWP), future satellite observing system is provided followed by a Ð agriculture and forestry services, summary in Section 6. Ð resource mapping, Ð hazard monitoring and disaster assessment, 2 PRINCIPAL METHODS, INSTRUMENTS Ð sea ice monitoring, AND SURFACE CHARACTERISTICS Ð coastal zone management, Ð oceanographic applications. Satellite oceanography is primarily using three domains Details of the outstanding scientific advances made pos- within the electromagnetic (EM) spectrum, notably radia- sible by satellite observations of the ocean and the associ- tion in: Ð the visible/near-infrared (VNIR); Ð the thermal ated societal benefits are provided by Halpern (2000). infrared (TIR); Ð and the microwave bands of the EM spec- The number of Earth Observing satellites are growing trum (Figure 1). rapidly for both scientific research and operational applica- The visible and infrared channels utilize intervals of the tion within fields of land, atmosphere, marine meteorology EM spectrum with high atmospheric transmission, such as and oceanography including sea ice covered regions. in the bands from 0.4Ð2.5 m, 3.5Ð4.0m and 10Ð13 m International investment in satellite platforms, instruments (Figure 1, top). The EM waves in these bands do not gener- and associated ground segments is already substantial, and ally penetrate clouds, so remote-sensing observations of the more investment is planned over the coming decade. There Earth’s surface in these bands can only be done satisfacto- are currently over 45 missions operating, and around 70 rily under cloud free conditions. As such this is posing more missions, carrying over 230 instruments, planned for severe limitations in regions where clouds are frequently operation during the next 15 years by the world’s civil space present. In the microwave area, on the other hand, at

Figure 1 The electromagnetic (EM) spectrum showing the bands used in remote sensing together with the operating area for some sensors (upper graph). The atmospheric transmission of the EM spectrum is shown in the lower graph. Note that the operating areas for satellite oceanography are located in parts of the spectrum where atmospheric transmission is high. OCEANOGRAPHY 1587 wavelengths above 0.3 cm, EM waves generally penetrate In such waters, which represent about 90% of the World clouds which makes it feasible to obtain regular, daily ocean, the variation in the color of the upper water column observations of ocean and sea ice surfaces (Figure 1, could be related to the variation of the concentration of bottom). These characteristic spectral domains are further chlorophyll pigments contained in phytoplankton cells. The addressed in the following. color of oceanic case I waters shifts from deep blue in oligotrophic waters (very low chlorophyll concentration) to dark green in eutrophic waters (high concentration). This 2.1 Visible/near infrared shift results from the strong absorption by algae pigments The basic quantity observed in the VNIR domain is the (chlorophyll pigments and carotenoids) in the blue part of albedo or alternatively the fraction of the incident sunlight the visible spectrum, with a maximum around 445 nm, that has been scattered and/or reflected in the atmosphere/ compared with the weak absorption around 550Ð580 nm ocean system. The incident solar radiation undergoes a (Morel, 1998). In case I type water, quite robust empirical number of interactions (absorption and scattering) with relationships can be derived, linking the chlorophyll con- molecules and particles in the atmosphere and in the water, in centration to the ratio of water leaving radiance (and/or the addition to the reflection that occurs at the air Ð sea interface. reflectance of the sea) at these wavelengths. Only a fraction of the incident radiation penetrates the Case II waters are more optically complex. In such water body. Absorption by water molecules becomes criti- waters the satellite derived water-leaving radiance, in addi- cal at wavelength greater than 700 nm. Therefore water tion to being dependent upon chlorophyll pigments and appears black at such wavelength, except when a high load derived products, is also sensitive and modified by at least of suspended particles is present near the surface. The visi- one other optically-active component, e.g., suspended sedi- ble light (400Ð700 nm) may propagate in the water medium ment and/or colored dissolved organic matter. Most coastal and interact with water molecules, organic and inorganic waters are classified as case II waters. Furthermore the particles in suspension, dissolved optically active sub- various optically active components do not display typical stances, and possibly the sea-floor in optically shallow- linear relationship. Therefore simple empirical models may waters. The penetration depth depends upon the wavelength no longer be used, and more sophisticated approach, such and the water column absorption properties. Only a small as inverse modeling must be considered. fraction of the visible-light spectrum is scattered upwards to the surface giving rise to the so-called water-leaving radiance, L . This is expressed as: w 2.2 Thermal infrared Lw (Ls La Lr)/ (2.1) For TIR (and PMW) the measured quantity is the emitted energy as function of surface temperature and emissivity. where Ls is the radiance reaching the sensor, La is the atmos- The emissivity is a dimensionless coefficient, e, and can pheric radiance, Lr is reflected radiation from the sea surface be computed from the complex dielectric constant (or the and is the atmospheric diffuse transmittance. In most relative permettivity) e eie, which characterizes oceanic waters, Lw represents less than 10% of the total signal the electrical properties of the media. e is referred to as the measured by a spaceborne sensor. Typically, 90% of radiation dielectric constant and e as the dielectric loss factor. has been scattered in the atmosphere, without interaction with Alternatively, e can be estimated from the complex index of subsurface waters. Therefore any quantitative estimation of refraction such as n2 e. water-column optically active constituents depends upon an In the thermal infrared part of the spectrum, the surface adequate retrieval of the water-leaving radiance Lw emanating signal expressed as the radiance observed by remote sens- from the water column. This requires reliable correction of ing can be used as input to Planck’s law of radiation to find the remotely sensed signal scattered in the atmosphere and the sea surface temperature (SST) if the emissivity e of the reflected at the airÐsea interface (Gordon, 1997). surface is known. For water, the value of e in the most used Furthermore the capability of deriving the accurate thermal spectral band of 10 mÐ12 m is very high and sta- quantity of a particular water quality parameter depends ble, about 0.99. At a given wavelength the blackbody upon the complexity of the water column in terms of num- radiance, L, can be expressed as: ber and properties of components that interact with the electromagnetic signal. According to the optical complexity L (2hc25)(hcekSST 1) (2.2) of the water body, two types of water are defined, namely case I and case II waters (Morel and Prieur, 1977). The Planck’s law is usually expanded in a Taylor series Case I waters are natural open-ocean water bodies, for from which the linear term is maintained: which water-leaving radiance measured by remote sensors 1 are only dependent on chlorophyll pigment concentration. Ln(L) aTb b (2.3) 1588 JOHNNY A. JOHANNESSEN, STEIN SANDVEN AND DOMINIQUE DURAND

where Tb is the brightness temperature in the TIR part of is antenna area. The energy of the outward propagating the spectrum, and the coefficients a, b are constant values wave, which is spherically expanding, is given by the first for each spectral band. ratio. This spherically expanding wave is focused down to an angular beamwidth by the antenna so that the fluxes becomes higher by a factor of G over that of a spherically 2.3 Passive microwaves expanding wave. The focused energy impinges on an object In the microwave domain the brightness temperature Tb pro- which has a radar cross section , which is defined as the vides the measurement of the microwave emission from the equivalent of a perfectly reflecting object of a given area surface. The brightness temperature is defined by the real which reflects isotropically (spherically) as shown by the surface temperature Ts and the emissivity e by the relation second ratio. Finally, the antenna area, A, term intercepts a portion of the reflected wave so that this portion of the flux Tb Ts * e (2.4) defines the power received by the antenna. The basic radar equation is general; it can be applied to Spatial variations in Tb observed over the surface of the Earth are due primarily to variations in the emissivity of the any object of any shape or composition. For imaging over surface material and secondly to variations in surface tem- areas of terrain or ocean, a reflection coefficient is defined, perature. For the most frequently used frequencies between 0, which is the radar cross section, , per unit area. The about 6 GHz and 90 GHz, the emissivity of ice, snow and radar equation (2.5) can then be expressed as water show large variations allowing observations of a wide 2 P G2 t 0 range of multidisciplinary parameters spanning land, ocean, P dA (2.6) R 3 4 cryosphere and atmosphere. (4 ) R While e for calm water can be calculated quite accu- The averaged received power for a radar can then be deter- rately from the electric properties (Stogryn, 1971), the mined by examining the integral radar equation for distrib- value and variation of e for the various forms of ice and uted targets. The radar scattering coefficient, 0, also called snow is less accurately known, and therefore often has to be backscatter coefficient, expresses a measure (usually in dB empirically measured. For sea ice, the dielectric constant e units) of the energy scattered back towards the antenna. It is is relatively constant with frequency above 1 MHz, but e is a function of frequency, incidence angle, polarization and not. There is a minimum in e at 3Ð8 GHz with higher value the scattering characteristics of the illuminated area. for lower and higher frequencies. Radar frequencies are identified by letter designations, For firstyear ice at 283 K and 8‰ salinity, the minimum and the most commonly used are K-band (30 GHz, 1 cm), e is approximately 0.3. As temperature decreases e will X-band (9.4 GHz, 3.2 cm), C-band (5.3 GHz, 5.7 cm), increase because precipitated salt will go back into solution. L-band (1.25 GHz, 23.5 cm) and P-band (450 MHz, 62 cm). Furthermore, e will decrease with decreasing salt content. At these wavelengths the EM-waves are not appreciably Multiyear ice has a lower e than firstyear ice and its tem- attenuated by clouds, precipitation or the Earth’s atmos- perature dependence is weaker. Thus, microwave radiation phere (see Figure 1). Therefore, good quality radar data can penetrates deeper into multiyear ice than firstyear ice. be obtained during all kind of weather and light conditions. The three main classes of satellite radars (altimeter, synthetic aperture radar, and scatterometer) are further described in 2.4 Active microwaves the following sections. Radar instruments provide their own source of illumination in the microwave portion of the EM spectrum, at wave- 2.4.1 Radar altimeter lengths on the order of 104 longer than those in the visible part of the spectrum (Figure 1). Because of this, radars can The radar altimeter measures the transit time and backscatter operate independent of solar illumination, cloud cover and power of individual transmitted pulses. The transit time is precipitation conditions. proportional to the satellite’s altitude above the ocean, land All radar measurements can be described by a basic equa- or ice surfaces. The pulse propagates toward the surface tion, which relates transmitted power, distance, reflectivity at time t1 with a speed of light c, is backscattered by the and antenna characteristics. The equation can be formulated as surface, and an echo is received by the sensor at a time t2. The time difference td t2 t1 is equal to the round trip P t distance to the reflecting surface divided by the propagation P G A (2.5) R 4R2 4R2 speed where P is power received, P is power transmitted, G is R t 2h the gain of the antenna, is the radar cross section, and A td c (2.7) 66

FRANÇOIS BARLIER* AND MICHEL LEFEBVRE** A new look at planet Earth: Satellite geodesy and geosciences

INTRODUCTION main problem was the poor quality of observations. The need for a global perspective was such that a dedicated planetary Yes! The Earth is also a planet! In the recent past, looking program was undertaken and implemented in 1957Ð1958; back at the long list of scientific space missions launched the objective of this IGY (International Geophysical Year) by space agencies, you have the impression that neither any was to collect as many geophysical measurements as possi- celestial body, nor astronomical or geophysical theme has ble from world-wide well distributed sites. Although success- not been covered. But looking more carefully, you soon ful in some fields it also showed the limits of this approach realize that something is missing … the Earth! over the long term. As a coincidence, the first SPUTNIK It is not the case that space is ignoring the Earth; indeed, satellite was launched in October 1957 and the government huge programs are devoted to Earth observations, but they of USSR claimed officially that it had to be considered as a are designed as if there was no need for a scientific study of contribution to IGY, not a bad vision indeed. our planet, itself. The space agencies recently reconsidered their programs This equivocation is not just a semantic one. It seems and made the knowledge of the Earth a top priority. ESA that the Moon or other planets in the solar system deserve (European Space Agency) started a new program called more attention than the Earth. As an example, the Magellan “Earth Living Planet” while NASA (National Aeronautics mission made a complete mapping of the surface of Venus and Space Administration) started a huge program devoted which took five years with a SAR instrument (Synthetic explicitly to Earth sciences. To be even more explicit, they Aperture Radar); several years before such an instrument named it “Destination Earth”; this movement was world- could have very usefully flown around the Earth. wide. Today the space agencies try to optimise their participa- Now, there is a new appeal for understanding the Earth, tion in a common endeavour through such ad hoc committees and the situation has changed drastically. The Chernobyl as CEOS (Committee on Earth Observation Satellites). accident caused a strong public reaction and created a After 35 years of satellite geodesy and oceanography, our widespread feeling that the atmosphere knows no borders. objective is to show that in spite of the absence of dedicated The recent large-scale ocean-atmosphere movements, like important programs in Earth physics from space, there was the ENSO (El Niño), widely discussed on by TV channels, a de facto strategy. It began at the time when the necessity to contributes to public awareness. undertake the study of the Earth became apparent and has The scientific community was perfectly aware of the lack produced significant results, as well as developed both tools of knowledge of the Earth as a planet and recognised that the and a living and multi-disciplinary community ready to go. We will show this development that through a historical * Observatoire de la Côte d’Azur, Grasse, France overview of activities over the past 35 years, presenting the ** Centre National d’ Etudes Spatiales, Toulouse, France different phases and major turns in Earth-space science.

1. FROM GEODESY TO SATELLITE GEODESY neighbouring countries differed in several cases by an BEFORE 1957 amount larger than the internal error; it was simply coming from the use of different reference systems. The above Geodesy is one of the oldest disciplines in the IUGG comments do not aim at making any assessment of classical (International Union of Geodesy and Geophysics) and has geodesy but rather at putting emphasis on the limitations of its roots in the early stages of civilization. When you live classical geodetic systems at the scale of the Earth. Now, somewhere, the first thing to do is to know where you are, satellite is there but how to use it at the best? what is your local environment like, then to share your views with your neighbours, extend your perspective from local to regional and finally to planetary scales. 2. TRANSITION EXPLORATORY PHASE The Chamber’s dictionary gives the following definition 1957–1970 of geodesy: “it measures the Earth and its parts on a larger scale”. The etymology from Greek tells us that geodesy 2.1 The geometrical optical phase: A too obvious comes from GE, the Earth and DAIEIN to share. Indeed approach what we have now to share is the EARTH. First of all, how to observe? At first, the favoured method When Sputnik was launched in October 4, 1957, geodesy of observation was optical. Observatories had or developed was confined to improving the accuracy of measurements as big cameras and made photographs of artificial satellites well as increasing the resolution of the grid of networks. A lightened by the Sun, the spatial reference being provided high level of expertise existed and was exercised in special- by the surrounding stars. The first obvious idea was to use a ized geodetic institutes, most of them sponsored by govern- satellite as a target, high enough to make intercontinental ments. The links with equivalent bodies working on military links that seem the most obviously missing element in geo- objectives have been varied but on average were well estab- desy and to have access to the 3D dimension. Everybody lished, such that the release of data and results was often pre- was enthusiastic; nobody realised that a satellite indeed vented. Concerning the shape of the Earth and its gravity ignores national boundaries: we remember the shock of field, numbers will give some ideas of the status. At the level some authorities, military or otherwise, when scientists of the continental formations, there were some efforts to get published the co-ordinates of Malvern (UK) and Nice (FR), homogeneous sets of parameters representing at best the breaking for ever the long tradition to keep such data and results obtained by triangulation and by astrogeodesy on the positions secret. shape of the Earth and to get coherent geodetic systems like The geometrical approach was limited by the magnitude the NAD datum (North American Datum) or the EUR 50 of the satellite. A first solution was to launch some dedi- datum (European datum). The relative accuracy, at least for cated satellites shaped like balloons with a large diameter, the horizontal components, was acceptable inside a given about 25 meters, such as the ECHO 1 and 2 satellites (Note: system and when close to the main fundamental networks 5 6 it was the common approach used in a telecommunication was of the order of 10 or 10 (1 meter over 1000 kilome- experiment, the expectation being to use the satellite as a ters). However, it was uncertain in the limits of networks reflector for electromagnetic waves) The visual magnitude when no closure was available (as an example the south of of ECHO satellites was around, 1, so that they were acces- Spain relative to EUR 50). The precision of vertical angle sible to many small existing cameras, allowing the increase measurements was also limited by the atmospheric refraction. in the number of stations within the nets to link. The knowledge of the relationships between the origin of these systems and the center of mass of the Earth was The two ECHO satellites were so bright that it was possible poor, such that the absolute positions of the stations were for anybody to watch them visually with the naked eye, and to maybe in error up to several hundred meters. No data were follow their motion across the stars. It was attractive enough available over the oceans that are over 70% of the Earth to drive the newspapers to publish the times that these “New surface. Over the continents, the data were sparse in many Stars”, passed over head. areas. Moreover, the networks were measured by discrete Several million people watched and acquired a personal campaigns at different times with different instruments by physical feeling for the existence of satellites and became different teams. Despite the high quality of the actors, there aware that we were entering the space age. The visual obser- were some intrinsic limitations. The users had no other way vations were not just curiosities; networks of amateurs observed with some optical instruments and provided the than asking to their national geodetic institutes to make the directions, elevation and azimuth to some centers using these geodetic links; the advantage was to have them made observations, especially from very low altitude satellites. The by professionals and well controlled. But the lack of a uni- result was the first models of the Earth’s gravity field and of fied system was still a problem. Even in the 1960s the posi- the upper atmosphere density. tion of the same radar antenna given by 2 institutes from A NEW LOOK AT PLANET EARTH 1625

Several geodetic institutes were thus able, through inten- Ð the acting forces on the satellite (gravity field of the sive campaigns, to make geodetic intercontinental links. Earth, solid and ocean tides, air drag, direct and reflected One on the most successful campaign was the geodetic solar radiation pressure …). connection between Europe and Africa. So the GSM game is easy to explain. Knowing the func- Going beyond, a more optimized dedicated program was tions relating quantities (the measurements) to some para- undertaken; PAGEOS, a better designed and more stable meters considered as unknowns, you have to compare your balloon satellite was put in orbit at a higher altitude. The observation with a value computed by marking a first guess existing BC Ð 4 cameras from the US Coast and Geodetic about the unknowns. In a linearized approach, you have to Survey were deployed in networks occupying 40 sites well compute the partial derivatives of the observation relative to distributed around the Earth; the geocentric positions of the the unknowns. Then you have to minimize the differences 40 stations were published and were considered by this time between all computed and observed values using statistical as making one of the first homogeneous global Earth refer- assumptions and an algorithm of adjustment. The problem is ence systems. It was in fact a dead-end and it is interesting in fact not linear, so you need to process by using iterations. to understand why. First the accuracy was not good enough. Among the unknowns, there are possible systematic biases The best result obtained for the positioning was at the 10Ð15 in your system or errors in the physics of your model. You meters level, but there is also a major disadvantage; this new expect to converge on accurate and reliable values. reference system was not accessible to the common user. In the dynamical approach, the motion of the satellite is an important component in many respects. Firstly, it is the 2.2 The space geodetic scheme natural way to scan any part of the planet. This sampling can be optimized using the orbital parameters that can be The general principles of satellite geodesy may be depicted adjusted accordingly. Secondly, the perturbations of the with an elementary diagram, the GSM scheme (Figure 1). satellite motion provide the determination of the acting G is the center of mass of the Earth, forces and in priority the gravity field of the Earth. S is the position of a tracking station, Conversely, a perfect knowledge of the forces provide a M is the position of the moving satellite. powerful constraint on the orbit determination. Then the At any time GM GS SM: accurate knowledge of the position and velocity of the satellite in a well-controlled reference system allows us to SM corresponds to the observation. It may be the measure- have the benefit of other measurements performed by ment of range, range rate, directions whatever. onboard instruments; such as, for instance, those provided GS corresponds to the position of the tracking station. It is by radar altimeters in oceanography. an unknown to be determined and it must be referred to a unified homogeneous Earth reference system. GS moves (due to tides, tectonic motions, and local 2.3 First look at observing technique candidates effects). The terrestrial reference frame, the position of the 2.3.1 Photographic observations axis of rotation as well as the speed of rotation (the parameters of the Earth rotation) vary over time. Photographic observations were the only precise material GM characterises the motion of the satellite. available for a while; nowadays, these types of observations are no longer used (except for specific applications, such The unknowns are: the observation of space debris); but it is still interesting to Ð the initial conditions (position and velocity of the satel- examine all the efforts made. lite) at a reference time. Photographic Observations Mobil z The photography of satellites with respect to stars was the first type of precise measurements of the angular positions of satellites. Station The procedure requires a capability of observing even satellites with faint magnitudes. The only way was to integrate enough light by a longer time of exposure, that means to use G y cameras able to track the satellite during its pass. The fully optimized system was the American network of BAKER-NUNN x cameras that were modified by adding an extra degree of freedom. It was successful and a world-wide network was Figure 1 A simplified scheme for geodesy. (continued) 1626 FRANÇOIS BARLIER AND MICHEL LEFEBVRE

implemented. There was a large effort in automation not only First Laser Returns, A Hunting Party to observe but also to measure the films and provide in a few During the winter of 1964, we had the good luck to be weeks better right ascensions and declinations of the satellite, involved in the first attempts to get some returns from BEB just the reference being provided by the surrounding stars. to help R. and M. Bivas in charge of this experiment at the These photographic observations were the core of the first Haute Provence Observatory. global determination of the gravity field (see Standard Earth It was like a hunting party. At this time, the game was to below). The accuracy was limited to about 2 arc seconds, that view the satellite with binoculars, in a position as low as pos- is to say around 10 to 20 meters. sible on the horizon, the best situation to get your prey in case In parallel, other countries were developing their own sys- of non-accurate predictions. So the game was to watch and, as tems, like the AFU 75 camera in USSR (aperture of 20 cm and soon as the satellite was in view, to transmit orally useful focal length of 75 cm), the tracking camera ANTARES at the information to the Bivas. They were seated in an old turret that Nice Observatory, the HEWITT camera at the Malvern they manoeuvred around two axes to maintain the instrument Observatory and the ZEISS camera in Germany. in the direction of the satellite. In the meantime, they shot with For the purpose of developing the geometrical space geo- the laser transmitter. The overall system was heating, request- desy with satellites like ECHO or PAGEOS, many smaller ing some cooling. The subsequent leakage of oil was evapo- cameras were used: as examples the Wild BC4 camera of the rated with Mrs Bivas’s hairdryer! Coast and Geodetic Survey in USA (aperture 11.7 cm, focal When you participate in such a venture, you become more length 30.5 cm), the IGN camera in France (aperture 10 cm, respectful of the data, though without falling into exotic com- focal length 30 cm). Schmidt Telescopes were also successfully ments such as made by a newsman: shooting at a laser target used for observing flashing satellites (ANNA Ð 1B, GEOS-A is equivalent to firing at the eye of a bee flying around with a and -B satellites launched by the USA) or for observing laser speed of 10 kilometers per second. returns on the satellites and obtaining the 3 components of the Thanks to celestial mechanics it was not as hard! station-to-satellite vector. But nowadays, all these techniques are generally abandoned, taking into account the progress realized with the laser techniques and the radio techniques. However, they played a major role in the beginning and old 2.3.3 Radio frequency tracking data: photographic data continued to be used in gravity field model- The TRANSIT system ing to ensure a good decorrelation between the different harmonics. During the 60s, some radio-electric systems were developed and research undertaken to better understand the different components in order to identify the key points for the design of permanent and weather independent accurate 2.3.2 Satellite laser ranging system of tracking. Laser technology is able to emit highly concentrated phased The TRANSIT system was developed very early by the optical energy in very narrow beams. This capability was US Navy to provide an improved navigation system for used in transmitting energy from the ground towards the their fleet. The core was a one way Doppler downlink satellite equipped with corner cubes that reflect the light mode. In such a system, the transmitter is onboard the satel- back in the same direction. The returned energy is detected lite, the receiver on the ground in tracking stations where and the time elapsed between emission and reception, after the Doppler effect is measured and dated in the station time some corrections, provides the range. The first satellite scale. The system is global. equipped by the USA with laser corner cubes was BEB The three main components are: (1964). There was a competition to get first returns. The Ð a network of tracking stations well distributed around the Goddard Space Flight Center (GSFC) team in the USA got Earth (TRANET network), the first ones in December 1964. The French CNRS team Ð a fleet of orbiting satellites, the TRANSIT satellites, with (Centre National de la Recherche Scientifique) at Verrières- enough redundancy to provide a global coverage, le-Buisson obtained the first pass at the Haute Provence Ð a main operating center that collects the Doppler mea- Observatory in January 1965. They only presented the first surements from the ground stations, computes orbits and orbit computed with laser observations at the COSPAR enters these coded orbits onboard the satellites. meeting in Buenos-Aires in spring 1965. The claimed precision from the rms (root mean square) of the measure- In 1966, CNES (The French Space Agency), launched a ments was of about 1.0Ð1.5 meters. small satellite named Diapason with a USO (ultra stable Today, laser ranging is the most accurate technique, and quartz oscillator) onboard to test accurate one-way Doppler it is still open to many improvements. One of the advan- downlink measurements. In 1967, two other satellites, tages is that the onboard equipment is light, cheap, has an named DIADEME 1 and 2, equipped with USO and laser infinite lifetime and does not consume any energy. reflectors were launched. The data from DIADEME were 67

JOHN E. HARRIES* Chemistry and physics of the atmosphere

1 INTRODUCTION anthropogenic aspects when it called the Earth science envelope programme “Living Earth.” The advent of space techniques has had a profound effect on how mankind views the place of the Earth and of the human race in the universe. To see the planet Earth, in images taken 2 HISTORICAL PERSPECTIVE from orbiting spacecraft such as the Space Shuttle, or from greater distances (as in the case of those famous images of the April Fool’s day, 1960, was perhaps not the best choice of Earth from the surface of the Moon) tells us something about date for the launch of the World’s great adventure in moni- the fragility of the planet, and the importance of caring for our toring how the atmosphere of our planet Earth works, from environment. Scientists were quick to recognise that this new a space platform. Nevertheless, on that date, the United perspective could be turned to great advantage in the drive to States of America, acting through the new National Aero- understand how the physical, chemical and indeed biological, nautics and Space Administration (NASA), launched the systems of the Earth worked. From about 1960, a series of TIROS 1 (the Television and InfraRed Observation Satellite). space missions began, aimed not outwards towards the stars, The satellite was basically a cylinder with 18 flattened but inwards towards our atmosphere, oceans and land. In this sides to mount solar power cells. The satellite was approxi- chapter, we will follow the story of how space techniques mately 1.07 m in diameter, 0.56 m high (including the pro- have been applied in the 20th century to studies of: jecting television camera lens), and had a launch weight of approximately 128.4 kg, including fuel for small solid rock- ¥ The meteorology and physics of the atmosphere and ets to control the satellite’s spin over time. (For compari- climate; son, the latest generation of Earth observing satellites often ¥ The chemistry of the atmosphere. reach 4 m in length, and weigh several metric tonnes!). We will also describe some of the scientific ideas that TIROS I ceased operating in mid-June 1960 due to an elec- have been tested from space, as well as the satellites and trical failure. During the 77 days it operated, the satellite instruments that have been developed: we will also mention sent back 19,389 usable pictures that were used in weather some of the people involved in this exciting enterprise. It operations. TIROS II was launched on November 23, 1960. will be seen from this review that space has had a profound The main sensors that provided the cloud pictures were effect not only on mankind’s perspective of the planet Earth, television cameras. The TIROS cameras were slow-scan but also on the scientific understanding of the processes at devices that took snapshots of the scene below; one “snap- work in the atmosphere and climate on the Earth. NASA shot” was taken every ten seconds. These were rugged, captured just the right phrase when it created its “Mission to lightweight devices weighing only about 2 kg, including the Planet Earth”1 and ESA brought in the biological and camera lens. TIROS I was equipped with two cameras, wide angle and narrow angle. Progress since these simple early experiments has been enormous, as we shall see later. * Imperial College, London, United Kingdom 1 Despite subsequently changing this to the more prosaic “Earth Science NASA had been formed from the National Advisory Enterprise”. Committee for Aeronautics (NACA) on October 1, 1958,

John E. Harries, The Century of Space Science, 1653Ð1668 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 1654 JOHN E. HARRIES and achieved many startling successes across the whole In the rest of this chapter we have been faced with a range of space activities, not only in studies of the Earth. choice. We might either attempt to provide an absolutely Nevertheless, Earth observation was clearly one of the comprehensive listing of all the missions that have most important applications of the new space technol- occurred, along with relevant technical details; or to be ogy and systems that were being developed. In recognition more selective, and try only to illustrate the range of obser- of this, the National Oceanographic and Atmospheric vations, and to pick out a few highlights of scientific Administration (NOAA) was formed, to become a major advance that has come about, perhaps mentioning one or partner to NASA (primarily a technology agency) in the two individuals along the way, in an attempt to provide exploitation of the new observations. These two agencies a little more colour to our story. Despite the fact that this between them would go on to dominate the observation, latter approach will not be comprehensive, and will leave study and monitoring of planet Earth from space for the gaps (for which the author apologises right away), we have remainder of the century. decided on the latter course. What is about to be served Elsewhere, other countries were quick to recognise the up is a subjective view of the highlights of efforts to study the importance of Earth observation from space for studies of atmosphere from space in the latter part of the 20th century. the environment. In Europe, the European Space Agency (ESA) was formed in 1975, and quickly developed the Meteosat series of spinning satellites, the first of which was 3 THE METEOROLOGY AND PHYSICS OF launched in 1977 into an equatorial geostationary orbit at THE ATMOSPHERE AND CLIMATE the Greenwich meridian position. This was just two years after the first NASA GOES (Geostationary Operational 3.1 Background Environmental Satellite: a 3-axis stabilised spacecraft) had been placed into geostationary orbit over the Indian Ocean. The measurement of basic atmospheric parameters, such as ESA went on to develop the ERS-1 and -2 polar orbiting temperature and humidity, by instruments on the meteoro- satellites, which carried a variety of visible, infrared and logical satellites flown by several agencies has been of microwave instrumentation. Within Europe, 1986 saw the great value in two ways. First, by providing images of creation of what is in some ways a European equivalent cloud patterns and their developments, or by providing of NOAA, the EUMETSAT (European Meteorological vertical profiles of temperature and humidity in the atmos- Satellite organisation), to exploit the new observations for phere, these data have had a direct and immediate impact meteorology and later for climate studies. on the accuracy of weather forecasts and extreme weather Across the world, other countries were becoming events. A second, extremely powerful use of the data has involved, including Japan, which through NASDA (National only more recently been appreciated, however. This is that, Aeronautics and Space Development Agency), and with the by providing a long term series of accurate, well inter- involvement of other national agencies, has developed a calibrated measurements, extending over several decades, series of polar and geostationary satellites. Also, India, Brazil, these missions have provided data on the state of the atmos- Australia and other nations have set up national agencies phere which is absolutely vital in studies of how the climate and become active. might be varying. Data from one-off dedicated “scientific” Now, at the beginning of the 21st century, a wide variety missions are of limited value in this sense, since they have of observations of the Earth’s atmosphere from space are very limited duration (with a few exceptions, such as UARS Ð available, including operational observations, made regu- see below). There is huge scientific value in this longevity larly, with good sampling over long periods; and also dedi- and continuity of observation. The exceptional value of cated, more specific missions, designed to address specific operational meteorological satellites in providing the basic, issues. Progress has been enormous in just 40 years. Our long term description of the state of the atmosphere neces- understanding of the Earth and its atmosphere has advanced sary for a wide range of atmospheric problems has become amazingly, often helped greatly by the new, global observa- apparent in the closing decades of the 20th century. In tions from space. In one way, this advent of global observa- addition, the activities of other agencies such as NASA that tions has come just in time as far as present worries over have funded long-term observations by instruments measur- climate change are concerned: it is already the case that ing simple, but important physical parameters like the Earth researchers are able to go back over much of those 40 years Radiation Budget, have also been most important. and study how planet Earth has actually developed, based Geostationary satellites provide a continuous view of the on satellite observations. In another way, perhaps we have earth disc from an apparently stationary position in space. become more sensitive to global variations, partly by hav- The instruments on polar orbiting satellites, flying at a ing more data available. Either way, space has made a much lower altitude, provide higher resolution details about major impact. atmospheric temperature and moisture profiles, ozone CHEMISTRY AND PHYSICS OF THE ATMOSPHERE 1655 amounts and radiation budget, although with a less frequent global coverage. The combination of the two types of measurement has proven to be very powerful. We have already heard how TIROS-1 was put into orbit on April 1, 1960. There followed a rush of satellites and new developments of instruments, too numerous to account for fully here. Between 1960 and 1973, the ESSA series (first operational TIROS) was developed and flown; the Improved TIROS Operational System (ITOS) began in January 1970, carrying a second generation of visible and IR sensors; later in 1970, NOAA-1/ITOS-A heralded the first in the long series of NOAA satellites that operates up to the present day; and in 1973 the NOAA-3 spacecraft was the first satellite which provided direct broadcast VTPR (Vertical Temperature Profile Radiometer) data. The NOAA series has operated up to the end of the century, carrying instruments which have been gradually improved and developed. Then, in 1975 GOES-1 heralded the beginning of geostationary observations, joined by Meteosat-1 in 1977. Let us look a little more closely at some of these developments. Figure 1 GOES visible image of Hurricanes Madeline and Lester, off the coast of Mexico, October 1998. (Courtesy of 3.2 NOAA’s geostationary weather satellites2 NASA–Goddard Space Flight Center, data from NOAA GOES.) The idea of using the geostationary orbit generally has been ascribed to the science fiction writer, Arthur C Clarke, and overall extent of snow cover. Such data help meteorolo- but the power of the geostationary orbit for meteorology, gists issue severe weather warnings. Figure 1 shows a equipped with a variety of visible and infrared imagers GOES image of hurricanes Lester and Madeline, off the and sounders was advocated strongly by Professor Verner coast of Mexico on October 17, 1998, illustrating the use Suomi3 of the University of Wisconsin. His vision led to the of geostationary observations for one of these purposes, enormous application that we see every day of geostation- tracking storms. ary observations to weather and climate research (Suomi The United States normally operates two meteorological and Vonder Haar 1969). satellites in geostationary orbit over the equator. Each satellite GOES satellites provide the kind of continuous monitor- views almost a third of the Earth’s surface: one monitors ing necessary for intensive data analysis. They circle the North and South America and most of the Atlantic Ocean, the Earth in a geosynchronous orbit, which means they orbit in other North America and the Pacific Ocean basin. GOES-8 the equatorial plane of the Earth at a speed matching the (or GOES-East) is positioned at 75W longitude and the Earth’s rotation. This allows them to remain over one posi- equator, while GOES-10 (or GOES-West) is positioned at tion on the surface. The geosynchronous orbit is about 135W longitude and the equator. Coverage extends approxi- 35,800 km above the Earth, high enough to allow the satel- mately from 20W longitude to 165E longitude. lites a full-disc view of the Earth. Because they stay above a The main mission is carried out by the primary instru- fixed spot on the surface, they provide a constant vigil for ments, the Imager and the Sounder. The imager is a multi- the atmospheric “triggers” for severe weather conditions channel instrument that senses radiant energy and reflected such as tornadoes, flash floods, hail storms, and hurricanes. solar energy from the Earth’s surface and atmosphere. The When these conditions develop, the GOES satellites are able Sounder provides data to determine the vertical temperature to monitor storm development and track their movements. and moisture profile of the atmosphere, surface and cloud GOES satellite imagery is also used to estimate rainfall top temperatures, and ozone distribution. during the thunderstorms and hurricanes for flash flood Other instruments on board the spacecraft include a warnings, as well as to estimate snowfall accumulations Search and Rescue transponder, a data collection and relay system for ground-based data platforms, and a space environment monitor. The latter consists of a magnetometer, an 2For more information about NOAA geostationary and polar orbiting satellites, see: http:\\www.noaa.gov. X-ray sensor, a high energy proton and alpha detector, and 3For more about the life and work of Verner Suomi, see: http://profhorn. an energetic particles sensor. All are used for monitoring meteor.wisc.edu/wxwise/museum/a1main.html the near-Earth space environment or solar “weather.” 1656 JOHN E. HARRIES

Table 1 GOES-10 characteristics Table 2 NOAA-15 characteristics

Main body 2.0 m by 2.1 m by 2.3 m Main body 4.2 m long, 1.88 m diameter Solar array 4.8 m by 2.7 m Solar array 2.73 m by 6.14 m Weight at liftoff 2105 kg Weight at liftoff 2231.7 kg including 756.7 kg of Launch vehicle Atlas I expendable fuel Launch date April 25, 1997, Cape Canaveral Launch vehicle Lockheed Martin Titan II Air Station, FL Launch date May 13, 1998, Vandenburg Air Force Orbital information Type: Geosynchronous Base, CA Altitude: 35,786 km Orbital information Type: sun synchronous Period: 1,436 minutes Altitude: 833 km Inclination: 0.41 degrees Period: 101.2 minutes Sensors Imager Inclination: 98.70 degrees Sounder Sensors Advanced Very High Resolution Space Environment Monitor (SEM) Radiometer (AVHRR/3) Data Collection System (DCS) Advanced Microwave Sounding Unit-A Search and Rescue (S&R) (AMSU-A) transponder Advanced Microwave Sounding Unit-B (AMSU-B) High Resolution Infrared Radiation Some typical characteristics of the GOES spacecraft are Sounder (HIRS/3) shown in Table 1. Space Environment Monitor (SEM/2) Data from GOES satellites aid forecasters in providing Search and Rescue (S&R) Repeater and Processor better advanced warnings of thunderstorms, flash floods, Data Collection System (DCS/2) hurricanes, and other severe weather. The GOES-I series provide meteorologists and hydrologists with detailed weather measurements, more frequent imagery, and new or aboard free-floating balloons. The primary instrument types of atmospheric soundings. The data gathered by the aboard the satellite is the Advanced Very High Resolution GOES satellites, combined with that from new Doppler Radiometer or AVHRR. radars and sophisticated communications systems make for Data from all the satellite sensors are transmitted to the improved forecasts and weather warnings. ground via a broadcast called the High Resolution Picture Transmission (HRPT). A second data transmission consists of only image data from two of the AVHRR channels, 3.3 NOAA’s polar orbiting weather satellites called Automatic Picture Transmission (APT). For users Complementing the geostationary satellites are two polar- who want to establish their own direct readout receiving orbiting satellites, constantly circling the Earth in an almost station, low resolution imagery data in the APT service can northÐsouth orbit, passing close to both poles. The orbits be received with inexpensive equipment, while the highest are circular and sun synchronous, with an altitude between resolution data transmitted in the HRPT service utilises a 830 (morning orbit) and 870 (afternoon orbit) km. One more complex receiver. satellite crosses the equator at 7:30 a.m. local time, the Table 2 contains some characteristics of the NOAA-15 other at 1:40 p.m. local time. The circular orbit permits uni- satellite. form data acquisition by the satellite and efficient control of The polar orbiters are able to monitor the entire Earth, the satellite by the NOAA Command and Data Acquisition tracking atmospheric variables and providing atmospheric (CDA) stations located near Fairbanks, Alaska and Wallops data and cloud images. The satellites provide visible and Island, Virginia. Operating as pair, these satellites ensure infrared radiometer data that are used for imaging purposes, that data for any region of the Earth are no more than six radiation measurements, and temperature profiles. The hours old. polar orbiters’ ultraviolet sensors also provide ozone levels A suite of instruments is able to measure many parame- in the atmosphere and are able to detect the “ozone hole” ters of the Earth’s atmosphere, its surface, cloud cover, over Antarctica during mid-September to mid-November. incoming solar protons, positive ions, electron-flux density, These satellites send more than 16,000 global measure- and the energy spectrum at the satellite altitude. As a part of ments daily via NOAA’s CDA station to NOAA computers, the mission, the satellites can receive, process and retrans- adding valuable information for forecasting models, espe- mit data from Search and Rescue beacon transmitters, and cially for remote ocean areas, where conventional data are automatic data collection platforms on land, ocean buoys, lacking. 4

JOHN A. SIMPSON† The cosmic radiation

INTRODUCTORY REMARKS Ray Observatory satellite have revealed a dramatically more energetic view of our galaxy. The light in the upper Cosmic ray research has developed as one of the most spec- panel in Figure 1 is from gamma-rays of approximately tacular and vital contributors to science in the Twentieth one-hundred million electron volts. Century. From a humble beginning early in this Century, Nuclear interactions arising from accelerated cosmic rays the quest for an understanding of this radiation and the with atoms in the interstellar medium result in the creation challenges confronting investigators led to the rise of new of many fundamental particles, among them pions of zero scientific disciplines, technologies and astrophysical con- electrical charge that promptly decay into the gamma rays cepts. Particle and high energy physics, radiocarbon dating observed in the upper panel of Figure 1. Thus, it is clear that and magnetic fields and plasmas of astrophysical origin are the cosmic rays propagate throughout the galactic disc. representative of the many research fields born from cosmic The early years of cosmic ray research were both excit- ray research. Examples of technological contributions ing and romantic since the investigators worked alone or in include radiation instruments and balloon and space flight small groups world-wide. In the 1930’s K.K. Darrow, for concepts required to reach the cosmic radiation beyond many years the secretary of the American Physical Society Earth’s atmosphere. captured the spirit of these early years by noting: From our concept of a quiet universe at the beginning of [The study of cosmic rays] is unique in modern physics the century to the recognition of a violent universe today, for the minuteness of the phenomena, the delicacy of the the cosmic radiations have Ð and continue to be Ð a vital observations, the adventurous excursions of the obser- component for understanding these dynamical phenomena vers, the subtlety of the analysis and the grandeur of the on all astrophysical scales. Indeed, cosmic ray particles inferences. (Darrow 1932) have become, towards the end of this century, part of what is now called astroparticle physics. This culture of personal involvement of physicists in carry- At night, when we look at the Milky Way with naked ing out experiments and personally recording data on eyes, we observe an overall glow like that shown in the mountains, in aircraft or balloons continued into the 1950’s. lower panel in Figure 1. This is the visible light from the In return for the personal risks were the rewards of instant excitation of atoms by very low energy processes Ð i.e., discovery. tens of electron volts. On the other hand, recent observa- With the opening of the so-called Space Age it became tions by the EGRET instrument on the Compton Gamma possible to study directly the acceleration mechanisms and the propagation of charged particles in interplanetary mag- † University of Chicago, Chicago, IL, USA. John Simpson died on netic fields. It also became necessary to include engineers August 31, 2000. The editors are indebted to Bruce McKibben for and other technologists to assist in the design, construction helping them to put the paper into its final form. and testing of space flight instrumentation.

Figure 1 Lower: The galaxy in visible light. Upper: The 100 MeV Galaxy (NASA).

This cultural change now largely pervades experimental cosmic ray physics. Experimental research in many areas is a big science effort with some young investigators having contact with experiments only by way of computer screens. For many theorists there also has been a cultural shift to massive computer code modeling. The editors of this publication have requested this author’s personal account of the history of cosmic ray research and of his early entry as a cosmic ray researcher into the space era, to be directed to a general scientific readership, and to be carried through to the 1970’s or 1980’s. Accordingly, this account is divided into two parts: the first part being mainly historical efforts to understand the radiation through the atmosphere; followed in the second part by both the international and the author’s research in space beginning in 1958. A guide of references to our present knowledge of cosmic ray physics at the end of the 20th century is also included.

PART A: REACHING TOWARDS SPACE

1. THE DISCOVERY OF EXTRATERRESTRIAL RADIATION

The events leading to the discovery of the cosmic radiation Figure 2 Electroscope and ionization chamber (Th. Wulf were, in many respects, mysterious and misleading. By the 1909). late 1890’s the basic instrument for measuring electric charge was the electroscope often used to demonstrate the This explanation had become popular as a result of presence of an electrostatic charge. Aside from leakage of the discovery of X-ray by Roentgen and discovery by H. stored charge from imperfect insulators, the electric charge Becquerel in 1896 of radioactivity in a compound of imparted on the electroscope should hold this charge indefi- Uranium and with the realization that radioactive materials nitely. However, even the most highly developed electro- were to be found everywhere on Earth. Even with 5 cm of scopes slowly lost their charge due to ionization in the lead surrounding the electroscope the leakage of charge atmosphere. persisted. At that time the explanation for this leakage of electric The Dutch physicist, Th. Wulf (1909) had developed charge appeared obvious Ð it must arise from radioactivity a highly stable electroscope and ionization chamber in the atmosphere ionizing the air around the electroscope. (Figure 2) that he used to test this hypothesis. He carried THE COSMIC RADIATION 119

It seemed to me necessary to measure accurately the absorption of gamma rays from radium in air in order to find out how far above the ground gamma rays could act as an ionizing agency. The next step was the construction of an air-tight ionization apparatus which could be used during balloon flights and fitted with a sensitive electrometric system which was not influenced by the large fluctuations of temperature occurring in the flights. I used a modification of Th. Wulf’s apparatus with walls of zinc, thick enough to withstand the excess pressure of one atmosphere and a temperature compensation for the fiber electrometer. Furthermore, I found it very important always to use two or three of the instruments simultaneously in order to avoid errors from instrumental defects. With such instruments, I made ten balloon ascents: two in 1911, seven in 1912 and one in 1913. Five of them were carried out at night, and some of them continued during the following morning. One flight was made during a solar eclipse, in April 1912. By taking successive readings of the ionization with two or three instruments at a time, much more reliable data were obtained. I found that at 500 meters above the ground the ionization was, on the average, about 2 I (I ion pairs-cm3 sec1) lower than on the ground and that, from about 1800 meters upwards, an increase of ionization is undoubtedly in evidence. At 1500 meters, the ionization increased to the same value as had been found on the ground. At 3500 meters, the increase Figure 3 Viktor Hess after a balloon landing (1912). amounted to no less than 4 I, at 5000 meters to 16 I above the ground value. No difference between day and night observations was noticed. his electroscope up the Eiffel Tower and found a radiation An explanation of the increase of ionization with level of only 64% below the value on the Earth’s surface for increasing altitude on account of the action of radioac- the leakage rate Ð much lower than he had calculated. He tive substances was impossible.… surmised, therefore, that there was either an additional The only possible way to interpret my experimental source of radiation from the upper atmosphere, or that the findings was to conclude to the existence of a hitherto absorption of gamma rays in air was much smaller than had unknown and very penetrating radiation, coming mainly been assumed. At about the same time Gockel (1911) in from above and being most probably of extra-terrestrial Switzerland carried a Wulf instrument in a balloon flight to (cosmic) origin … 4500 m. He also questioned from his qualitative measurements whether there might exist a new radiation superposed Within a year Kolhörster (1913) confirmed Hess’s observa- on the radiation from Earth. tions and conclusions. Nevertheless, many investigators dis- The 28 year old Victor Hess had been following these puted their measurements and conclusion with arguments reports on the source of radiation producing the leakage concerning the stability of the electroscopes. It was not charge in electroscopes. In a recounting of those years until after World War I that experiments were resumed. (Figure 3) he stated (Hess 1912, 1940): Millikan and Bowen (1923) developed a low mass (190 g) … At that time in the Spring of 1911, after reading an electrometer and ion chamber for unmanned balloon flights account of Father Wulf’s Eiffel-Tower experiments, I using radiosonar technology developed during World War I. was inclined to believe that a hitherto unknown source In balloon flights to 15,000 m in Texas they were surprised of ionization may have been in evidence in all these to find a radiation intensity not more than one-fourth the experiments; and I decided to attack the problem by intensity reported by Hess and Kolhörster. They attributed direct experiments of my own. this difference to a turnover in the intensity at higher 120 JOHN A. SIMPSON altitude, being unaware that a geomagnetic latitude cutoff the American Association for the Advancement of Science existed between the measurement in Europe and Texas. in 1932. Thus, Millikan believed that there was no extraterrestrial The electroscope did not fail on the Millikan party’s radiation until he and Cameron (1926) carried out absorp- return northward across the equator. This dispute ended in tion measurements of the radiation at various depths in February 1933 when Millikan admitted that there was a snow-fed lakes at high altitudes. Based upon the absorption latitude effect and that the cosmic rays must be charged coefficients and altitude dependence of the radiation, they particles. concluded that the radiation was high energy gamma rays Since Compton and Millikan were supported for their and that “these rays shoot through space equally in all research by the Carnegie Corporation through its Depart- directions” calling them “cosmic rays”. They argued that ment of Terrestrial Magnetism (DTM), the DTM was eager the radiations are “… generated by nuclear changes having to settle the acrimonious debate by deciding which instru- energy values not far from [those that they recorded] in ment was superior Ð that is, which instrument (Compton’s nebulous matter in space.” or Millikan’s) was the better one to use as a world standard. Millikan than proclaimed that this cosmic radiation was In the early 1930’s, Auguste and Jean Picard received the “birth cries of atoms” in our galaxy. His lectures drew much publicity for their manned balloon flights in the considerable attention from, among others, Eddington and stratosphere that claimed to study the origin of cosmic rays. Jeans, who struggled unsuccessfully to describe processes In 1933 the promoters of the Chicago “Century of that could account for Millikan’s claim (cf. De Maria and Progress” World’s Fair engaged them to make such a flight, Russo 1990). with Compton agreeing to arrange all the scientific instrumentation to be carried in the manned gondola. Compton seized on this opportunity to seek a collaboration with 2. COSMIC RAY CHARGED PARTICLES Millikan. “It would seem too bad to let an expensive flight of this kind occur without making use of it for some high A key experiment, which would decide whether the incom- altitude measurements,” he wrote (DeVorkin 1989). He ing radiation was electrically charged or uncharged, was the invited Millikan to supply his automatic recording electro- measurement of the dependence of cosmic ray intensity on scope to be flown with the Chicago instruments in order to geomagnetic latitude. If the radiation was electrically neu- reconcile differences in their performance. Millikan, who tral, such as gamma rays, there would be no dependence on had claimed his instrument was superior, agreed to meet the strength of the magnetic field. On the other hand, if the Compton’s challenge. cosmic rays were electrically charged particles Ð say elec- The Compton-Millikan venture appeared to have no trons or protons Ð their deflection in the Earth’s magnetic impact on the advancement of cosmic ray physics. However, field would limit access at all but the highest latitudes. Thus the development of the Compton-Bennett model-C ioniza- the cosmic ray intensity would be less at the equator than at tion chamber (Figure 4) (Compton et al. 1934) became high latitudes. in the long run the standard adopted by the Carnegie In 1927, J. Clay (1928) from the Netherlands Ð by carry- Institution when a worldwide network of stations to search ing ionization detectors on ships that traveled over a large for cosmic ray variations with time was set up with latitude range Ð observed a geomagnetic latitude effect in the Chicago-built instruments at world-wide sites of cosmic ray intensity. If confirmed, the radiation must con- the Institution’s Department of Terrestrial Magnetism. The sist, at least in part, of charged particles interacting in the work of Scott E. Forbush, who was responsible for the external geomagnetic field. Although Clay’s work was dis- stations, led to the proof that the observed intensity of cos- puted by Millikan, A.H. Compton (1933) carried out in mic rays within the atmosphere of Earth varied with time 1932 a world-wide survey to settle the dispute. The Earth (Forbush 1938). was divided into nine zones and teams, with all investigators using identical ion chambers. He then reported (1933) that there was a latitude effect, that cosmic rays were 3. CHARGED PARTICLES, OR ? charged particles and that Millikan was wrong. At about the same time, Millikan had sent Victor Neher In the period 1928Ð1932 it was clear that cosmic rays (his junior colleague from the California Institute of included charged particles, but what was the sign of the Technology) on another latitude survey to South America. electrical charge that they carried Ð plus or minus? Unknown to the survey party, their electroscope had mal- Until about 1930 the only property of cosmic rays that functioned on the way down and they reported no latitude could be measured was their specific ionization (ions effect. This was triumphantly proclaimed by Millikan who cm3 sec1). Fortunately, Geiger and Müller (1928) had attacked Compton in a debate at the Christmas meeting of invented a cylindrical charged particle detector which made 8

CORNELIS DE JAGER* Early solar space research

The Sun, the most luminous celestial object as viewed and by the first Ð mostly fairly simple Ð satellites. We con- from Earth, was the first target for space experiments when clude with a description of the major results of Skylab, but such possibilities opened up. It is convenient to distinguish it should be realized that the scientific influence of this between the quasi-permanent and slowly changing solar space laboratory persisted for a long time after it had been features, for which incidental rocket observations suffice, put out of service. and the rapidly changing, transient phenomena, which need satellite monitoring. In the first category are objects such as the photosphere, including the fairly slowly changing THE WARTIME ERA: KIEPENHEUER features, like centres of activity, the basic features of the AND THE V-2 chromosphere and of the coronal regions. Solar flares and coronal mass ejections come into the second category. During World War II German military authorities, under It is therefore only natural that in the rocket era the Wernher von Braun, developed the A-IV rocket (later called emphasis was on the (quasi-)stationary parts of the Sun, V2, from Vergeltung-zwei, ‘Revenge 2’), in order to be able while, once satellites were orbiting the Earth, the rapidly to bombard targets many hundreds of kilometres away. The varying aspects could be studied. These two different V2 was able to climb to altitudes of over 100 km. This abil- aspects Ð stationary versus transient Ð govern the organiza- ity attracted the attention of scientists who realized that Ð in tion of this chapter. That distinction does not lead to a strict principle Ð the V2 could make it possible to study both the chronological description, though we attempt to stick more structure of the upper atmosphere and solar radiation from or less to chronology. Rocket observations persisted until wavelengths that are inaccessible from ground-based sites. long into the satellite era, while there were occasional flare This fascinating episode is described by Hufbauer (1991, observations being made in the rocket era; for example, pp. 120Ð124) and Wolfschmidt (1993). when a rocket was fired accidentally or deliberately during Wolfschmidt starts her review by quoting Heraclitus: the occurrence of a solar flare. ‘War is the father of all things.’ This evidently exaggerated Like any history, ours has a prelude. During World War II statement does indeed apply to many scientific develop- the first ideas for observing the Sun by means of rockets ments that began in that period and would come to domi- were developed in Germany. These ideas, though at one nate the second part of the twentieth century. During that time very close to their realization, never materialized. The period the technique of radio astronomy was developed, story, though, constitutes an essential part of the history of and the foundations of space research were laid. In the space research. USA, Walter Orr Roberts founded, under war pressure, the This chapter deals with the period between about 1940 High Altitude Observatory for solar monitoring, and on and 1970Ð75, which was dominated by rocket observations the European continent Karl Kiepenheuer founded six solar * SRON Ð National Institute for Space Research, Utrecht, observatories. The impetus behind these developments was The Netherlands the knowledge that the Sun influences long-distance radio

Cornelis de Jager, The Century of Space Science, 203Ð223 © 2001 Kluwer Academic Publishers. Printed in The Netherlands. 204 CORNELIS DE JAGER communications, and the expectation that understanding balloons did not significantly improve detection. The rea- the origins of solar variability would lead to greater control son was rapidly found in absorption by terrestrial ozone at over long-distance radio communications. The military heights above a few tens of kilometres. importance of the latter is obvious. With some scientific An immediate challenge resulting from these investiga- opportunism, solar scientists on either side of the front line tions was therefore to try going higher, above the ozone thus hoped that military funding would enable them to con- layer, which means above about 50 km. When the military tinue and intensify their research. authorities started developing the A-IV (V2) rocket, the The fact that the Sun influences the ionosphere and thus project leader, von Braun, was eager to collect information the efficiency of radio propagation was known by the early on the medium through which the rockets were supposed to twentieth century, but the origin of the disturbances was fly: the upper atmosphere. He succeeded in interesting unknown, despite well-organized worldwide solar monitoring Erich Regener, who had strong anti-Nazi sentiments, but since the IAU General Assemblies of 1924 and 1928. At that who was attracted by the prospects of doing pioneering time a large co-operative project started, to which the names research with the new vehicle. Overcoming his political of George Ellery Hale, Sydney Chapman, Charles St John, sentiments, Regener started developing a scientific payload Giorgio Abetti and others are associated. It had been known for measuring atmospheric densities and temperatures with for some years that radio disturbances were associated with the A-IV. Somewhat later, Regener realized the importance the occurrence of sunspot groups, and Howard Dellinger of flying a spectrograph in order to study the absorption of (1935) found, on the basis of synoptic solar data (obtained by solar radiation by the residual atmosphere above the rocket. Henri d’Azambuja) that the just-discovered solar flares were To that end he asked for the assistance of Paetzold, a responsible for at least part of the radio disturbances. specialist in optics, and Paetzold recruited Kiepenheuer into As a post-World War I punishment by the victorious the project. From a letter dated 8 July 1942 we quote: allied nations, Germany was excluded from international The A-IV offers the possibility of using the newly devel- scientific co-operation during the period between the wars, oped methods to carry out measurements in the upper a humiliation to which decent scientists should never have atmosphere. [There follows a description of proposed lent support. German scientific institutions were therefore instrumentation] … above all, an ultraviolet spectro- not included in the campaigns for global solar monitoring. graph. This must be above the absorbing ozone layer, Before the war, Germany had few solar observatories and which is to say at an altitude of 50 km or more, if it is to no modern monitoring equipment. Apparently, in order to record the solar spectrum. (Author’s translation) cope with that, Grotrian started systematic sunspot observations in Rechlin, some 100 km north of Berlin, at the end of At Peenemünde, Paetzold developed a prototype of the the 1930s. To improve the quality and continuity of solar spectrograph. A second, more sophisticated flight instru- monitoring, Hans Plendl and Kiepenheuer, the latter then in ment, conceived by Kiepenheuer and constructed by Göttingen, proposed at the end of 1939 to establish a solar Paetzold, was a spectrograph based on LiF optics (in order observatory at the Wendelstein mountain. to be able to observe wavelengths less than 2000 Å). In At about the same time (1940) Kiepenheuer, guided by spite of the increasingly difficult war situation, von Braun Grotrian and Otto Heckmann, constructed a spectrohelio- planned a prototype flight in the winter of 1944Ð45, to be graph in Göttingen. In co-operation with Plendl he estab- followed by a scientific flight in the spring. The instrument lished during wartime a network of solar monitoring stations and nose cone arrived in Peenemünde in January 1945, in Germany and Italy, thus providing those involved in radio where they were subsequently integrated and underwent communication with reliable solar data. By including obser- their pre-launch tests. No launch ever took place. vatories in occupied countries in the network (for example, Ultimately, the increasing severity of the war prohibited the Meudon, Pic du Midi, Ondrejov)ˇ he provided these institutes use of V2 rockets for scientific experiments. with some protection against military interference with their other scientific work. Earlier (1936Ð39), Kiepenheuer had realized the importance of the solar UV flux in controlling the ionization of FIRST SOLAR NEAR-UV RADIATION the ionosphere. In order to obtain information about solar DETECTED BY ROCKETS UV radiation, and particularly its variations with time, he made observations at the Jungfraujoch observatory (altitude After the war the USA took the lead in space research 3600 m) and (from 1939) with stratospheric balloons with rockets. The first years after the war set the direction (Kiepenheuer 1938). The instrument was a double mono- for the subsequent development of solar space research, chromator based on quartz optics. The Jungfraujoch obser- and showed all the elements that would prove to be so vations showed him that even at that altitude the solar characteristic of the initial phases of various subdisciplines UV could not be detected, and even using stratospheric of space research. A slightly humorous aspect was that the EARLY SOLAR SPACE RESEARCH 205 scene in the USA was complicated by competition between outside the Earth’s atmosphere … I would like nothing army, navy and air force. more than to be involved in such a project, even if it After the military collapse of Germany in the spring of meant shaving my head and working in a cell for the 1945, a US army intelligence unit under Colonel Holger N. next ten or fifteen years. Toftoy rapidly rounded up von Braun’s team and captured, almost from under the nose of the approaching Soviet army, But already in 1948 Goldberg was having doubts when he sufficient material for assembling around a hundred V2 wrote to Lyman Spitzer (Hufbauer 1991, p. 139): rockets. Toftoy, realizing the importance of establishing good relations between the military and scientific commu- I have been somewhat disappointed in the development nities, charged Major James G. Bain with the task of of the rocket project since its inception. It had been my finding scientists who might be interested in assembling hope and I think also yours, that the rockets would open payloads for rocket-borne scientific experiments that could a new field of solar research … At least thus far … they be launched from the newly established range at White have hardly opened a new field of investigation … at the Sands. moment I would not want to ask for a renewal of the Initially there was great enthusiasm, particularly among contract solely on the basis of the V2 investigations. the leading scientists, about the new potential for doing research in a spectral region that had so far been inaccessible. Goldberg’s disappointment has been echoed many times in Later, though, when confronted with the large technical the initial period of development of space research, particu- obstacles and the new and unconventional techniques, along larly by theoretically inclined scientists who are essentially with the timescale for realizing the goals set, scientists interested in the scientific results, and have difficulty in showed a tendency to step back and resume their previous grasping the time and effort needed for solving the technical modes of research. In the meantime, physicists and engineers problems encountered when a new field of research is facing who were familiar with the new techniques entered the field unconventional and often new technological challenges. from other areas of science or engineering. They seized the In the hectic period immediately after the war, things in initiative and became the main people responsible for the the USA went ahead remarkably well, under able leader- experiments. In later phases, when the scientific results ship. An informal V2 panel set up under Ernest H. Krause became available and were reaching the level of specifica- identified in a relatively short time institutions and scientists tions set by the astronomers, the latter returned. Then, how- willing and able to develop scientific experiments for the ever, they often found that the physicists had in the meantime new vehicle. It is remarkable that at that time most of acquired sufficient knowledge of the field and hardly needed the solar scientists were from two institutions only. One was assistance from traditional astronomers. This development the Naval Research Laboratory (NRL), directed by Edward could be witnessed in several disciplines, particularly in O. Hulburt, who had for a long time been deeply interested space research. In addition to solar space physics this same in the relationship between solar energetic radiation and the dynamic of community development could also be observed ionosphere; the other was the Applied Physics Laboratory of in the disciplines of X-ray and gamma-ray astronomy, where Johns Hopkins University, directed by Merle Tuve. cosmic ray physicists came to dominate this typically astro- Richard Tousey’s group at the NRL had the first success: physical field. To a lesser degree this development also took on 10 October 1946 a rocket-borne camera took the first place in radio astronomy. photograph of the solar UV spectrum down to 2200 Å. The Ultimately, this process appeared to be enriching astron- spectrum was obtained from a height of 55 km. omy, because of the entry of eminent physicists into the In 1947 the chairmanship of the V2 panel passed to field. There were Bernard Lovell, Martin Ryle, Wilbur James Van Allen, while Homer A. Newell became head of Christiansen and others in radio astronomy, and a multitude the section for rocket research of the NRL. These appoint- of scientists in space research: Bruno Rossi, Richard Tousey, ments greatly strengthened the programme. Herbert Friedman, Sergei Mandel’shtam, Saito Hayakawa, When, in 1948, the stock of V2s became exhausted, US- Guiseppe (Beppo) Occhialini, and many others. made rockets were introduced, first the Aerobees and We illustrate the situation with an example. In September Vikings. The first Aerobees did not reach sufficient altitude 1945, confronted for the first time with the possibility of to be scientifically useful, but after increasing the length of observing the Sun from space, Leo Goldberg wrote to the rockets they attained heights of 150 km or more, thus Donald Menzel (Hufbauer 1991, p. 136): surpassing the greater altitudes reached by the V2s. None of the people involved in the first rounds of solar If anyone asked you what technological development experiments had much prior knowledge of solar physics. could, at one stroke, make obsolete all our textbooks Richard Tousey’s background was in UV spectroscopy, written in astronomy, I am sure your answer and mine and this made him well suited for the new tasks. But new would be the same, namely the spectroscopy of the Sun problems arose. In the first few years, when Sun-pointing 206 CORNELIS DE JAGER devices had still to be developed, methods had to be gen- position usually occupied by the spectrograph slit. A slit erated for directing sunlight into the spinning, yawing spec- was then no longer needed. The introduction of two spheres trograph. One remedy was to use large-field spectrographs, was needed to compensate partly for the rocket’s spinning. but that was not a sufficient solution. Various methods The drawback was that the ‘slit’ was smeared out, causing (Tousey 1953) were tried of placing reflecting objects in degradation of resolution. Another method, used by the front of the spectrograph slit. In one of the most successful Johns Hopkins group, was based on a diffuse reflector con- versions, introduced by the NRL group, two polished sisting of rough aluminized glass. In that case a slit was spheres of LiF, 2 mm in diameter, were positioned some necessary. Other variants are described by Tousey (1953) distance in front of the entrance aperture of the spectro- and are shown in Figure 1. In all cases the spectrographs graph. Thus a point-like solar image was produced, at the had luminous efficiencies below 1%.

Figure 1 Wide-field-of-view entrance aperture systems for rocket spectrographs: (a) diffuse reflector and slit; (b) corrugated cylindrical mirror and slit; (c) lithium fluoride sphere (diameter 2 mm); (d) mirror-jawed slit. (From Tousey 1953. Reproduced with the permission of the University of Chicago Press.) 9

EUGENE N. PARKER* A history of the solar wind concept

One presumes that the supersonic solar wind has been blowing The presence of the solar wind is directly indicated by since the formation of the Solar System, providing the the continuing fluctuations of the geomagnetic field at the dynamical plasma and magnetic field throughout interplane- surface of Earth, particularly at high latitudes, the continuing tary space. It appears that the early massive solar wind and aurora at high latitudes, the varying intensity of the galactic simultaneous strong solar magnetic fields carried away the cosmic rays, and the antisolar orientation of the gaseous initial angular momentum of the young Sun during the first tails of comets. However, these effects, known individually 3108 years or so, leaving the leisurely 25-day equatorial for decades and centuries, are evidence only of some form rotation period that we see today (Schatzman 1959, 1973; of external disturbance, and the scientific challenge over Biermann 1973). Nowadays the tenuous wind carries no sig- the last century has been to work out precisely what that nificant angular momentum or mass from the Sun, but it disturbance really is. continues to be the dominating condition in interplanetary Historically there has been no end of ambiguities and space, providing the outward sweeping spiral magnetic distractions. For instance, starting from the classical point field, impacting the terrestrial magnetosphere, drastically of view that space is completely empty, Kelvin proved that reducing the intensity of the galactic cosmic rays, and push- the Sun is not capable of such large magnetic variations as ing the interstellar gas and galactic magnetic field out would extrapolate to the observed geomagnetic fluctuations beyond the farthest planets. The consequences for the terres- at a distance of 1 AU. From this he asserted that there can trial environment are profound, but not immediately obvious be no connection between the activity of the Sun and the to us who dwell at the surface of Earth. So the history of the magnetic fluctuations at Earth. With the same hypothesis thoughts leading to the recognition of the solar wind extends that space is completely empty it was thought possible that back more than two millennia. Progress has been paced the observed variations in the cosmic ray intensity are a largely by the development of physics from the days of consequence of large electrostatic potential differences Gilbert (1544Ð1603) and Galileo (1562Ð1642). That is to across interplanetary space. The antisolar orientation of say, with the exception of the aurora, the effects of the solar gaseous comet tails was attributed to the radiation pressure wind are not available to our biological senses, but require of sunlight. The implications of the continuing aurora and observations and measurements that can be made only with high-latitude geomagnetic fluctuations were largely ignored, specially devised scientific instruments. And then it requires their origin perhaps meteorological. an advanced state of physics to infer the implications of the To go back to the beginning of the concepts on which observations. So the history of the solar wind follows both the solar wind is based, it must be recalled that the concept the advances in fundamental classical physics and the stud- of space itself is essential and is only a relatively recent ies of the natural phenomena pertinent to the solar wind. development in human thought. In primitive times the sky was not viewed as the window looking out into space. Rather * University of Chicago, IL, USA theskywastheabodeofgodsandspirits, who looked after, or

were represented by, the Sun and the Moon and the various oxide, magnetite). He recognized that the magnetic field planets and stars, which in turn had a powerful influence on is a special stress system in the space surrounding the our personal destinies. The sky was “that inverted bowl magnetized sphere Ð in other words, that the phenomenon they call the sky whereunder crawling coop’d we live and of forces between magnetic objects involves a force-field die” (Omar Khayyam). Knowledge of geography was as extending through the space between them. He was the limited as the concept of the sky. Some mythologies would first, then, to recognize the magnetic field as a physical have humans descending to Earth from a beginning in the entity throughout the space around a magnet. sky, and not so very long ago at that. The concept of the The accumulating records of magnetic declination and span of time was as stunted as the localized concept of inclination from extensive ocean voyages were enough for the sky and space. The idea that the whole world was cre- Gilbert to show that Earth itself is a magnetized sphere, and ated around us primarily to accommodate us humans is still he pointed out that the surrounding space is a special region alive and well today, demonstrating the reluctance of the as a consequence of the magnetic field extending out from human mind to relinquish the egocentric world of fantasy. Earth. In short, Gilbert recognized the terrestrial magneto- So it is not surprising that the realization of the vast sphere, as seen from its lower boundary at the surface space in which the tiny Earth resides developed only slowly of Earth. and sporadically. We owe great respect and admiration to Aristarchus of Samos (circa 275 BC), as the first ancient philosopher-scientist of record to recognize the central GEOMAGNETIC ACTIVITY AND position of the Sun in our local system and the vastness of CLASSICAL PHYSICS the surrounding space. Armed with a clear understanding of geometry he realized from the converging umbral shadow It was more than a hundred years after Gilbert’s work that of Earth during an eclipse of the Moon that the Sun is Graham (1724) observed a delicately suspended magnetic larger than Earth. From this he recognized that it is more needle with a microscope and discovered the slight agita- likely that little Earth orbits the large Sun, rather than vice tion of the needle, implying fluctuations in the magnetic versa. He viewed the Solar System much as Copernicus did field of Earth. One can appreciate the care that must have 1800 years later, recognizing the great distance to the stars. been taken to avoid vibrations and air currents to be sure Needless to say, the contemporary experts found his ideas that the agitation was of geomagnetic origin. Celsius (1741) disturbing, undermining their infallibility, so he was offi- became engaged in similar observations and noticed that cially rejected. Fortunately he was not entirely ignored, and the appearance of the aurora was accompanied by enhanced his penetrating insights, although lost in the original, have magnetic fluctuations. Celsius and Graham corresponded come down to us through the writings of others. and soon established that the observed periods of enhanced Copernicus (1473Ð1543), beginning in 1530 and finally fluctuation were often simultaneous, thereby showing their published in his De revolutionibus orbium coelestium in large geographic scale. 1543, swept away the Earth-centered crystal spheres of the Wilcke (1777) recognized that the auroral rays are ori- Ptolemaic geocentric system (first developed by Hipparchus ented along Gilbert’s dipole geomagnetic field. Canton c. 150 BC), thereby clearing space of its classical intellec- (1759) introduced the first observational connection to the tual rubbish and making way for the concept of local terres- Sun, pointing out that the quiet-time geomagnetic fluctua- trial effects from distant astronomical sources. Equally tions are stronger in the summer when the hemisphere is important, of course, Copernicus, followed by Kepler tilted toward the Sun. (1571Ð1630) and by Galileo with the application of the It is interesting to note that de Mairan (1754) proposed telescope, began to make sense out of celestial mechanics. that the aurora arises from the entry of particles from the This prepared the way for Newton (1642Ð1727) to develop Sun into the terrestrial magnetosphere. His idea was based the Newtonian theory of gravitation and mechanics pub- on the contemporary interpretation of the zodiacal light as lished in his Principia in 1686. an extension of the solar corona. Thus Earth orbits through The gradual recognition of the magnetic field of Earth, this extended corona, and he proposed that the aurora and with the invention of the magnetic compass in China during the associated geomagnetic fluctuations are the result. It the Han dynasty about two millennia ago, was another was an inspired conjecture, but he could not do more important step on the long road leading to an understanding because the physics of charged particles and magnetic of the solar wind. The scientific credit here goes particu- fields was unknown at the time. larly to Gilbert (1600) whose ingenious, careful, and quan- Cardan in 1551 had clearly distinguished magnetic and titative laboratory experiments first established the precise electric effects, and Gilbert emphasized the fundamental form of the dipole magnetic field around a more or less difference. In 1733 Du Fay distinguished between positive uniformly magnetized sphere (composed of magnetic iron and negative charge, and a decade later Franklin carried out A HISTORY OF THE SOLAR WIND CONCEPT 227 a number of experiments establishing the conservation of remarks on Maxwell’s contributions to electricity and total electric charge. He described positive and negative magnetism, citing Maxwell’s Treatise on Electricity and charge in terms of a surplus or deficit, respectively, of some Magnetism (1873) and noting that the primary test of fundamental electric fluid. We recognize today that elec- Maxwell’s theory of electromagnetism is whether the veloc- trons make up the mobile electric fluid in most circum- ity of light is accurately given by the ratio of the units of stances, so that positive charge actually represents a deficit charge in e.m.u. and c.g.s. A few, such as Heaviside, recog- and negative charge a surplus, but one can see how Franklin nized the importance of Maxwell’s complete electro- advanced the concept. magnetic equations, as did Helmholtz and Rowland, and Priestley discovered the inverse square force law particularly Hertz whose experiments on electromagnetic between electric charges in 1767, and Coulomb rediscov- waves eventually convinced the world of the fundamental ered the law in 1785, with the result that today the name importance of Maxwell’s equations. Lorentz (1895) demon- Coulomb is associated with the electrostatic inverse square strated the implications of Maxwell’s equations for trans- law 1/r2. It was not until 1820, however, that Oersted made forming E and B from one moving coordinate frame to the experimental connection between electricity and mag- another, which became part of Einstein’s (1905) special rel- netism, demonstrating the magnetic field around an electric ativity theory 10 years later. current. With this physics in hand, one could understand This is perhaps an appropriate place to note an addi- how electrically charged particles might produce magnetic tional fundamental contribution of Faraday, who, in the fluctuations. Only three years later Ampère demonstrated course of his epic experiments on magnetic induction, rec- the equivalent magnetic fields of a closed electric circuit ognized the central importance of the concept of field lines, and a uniformly magnetized shell whose periphery is or magnetic and electric lines of force, as the most direct bounded by the electric circuit. Today we write Ampère’s and effective means for visualizing the form and behavior law in the familiar differential form cB 4j, where j of a field. The concept is widely applicable today, particu- is the electric current density and B is the magnetic field. larly in magnetohydrodynamics (MHD), wherein the mag- By 1831 Faraday had shown that an electric current is dri- netic field is transported bodily with any medium that ven through a conductor by a time-varying magnetic field, cannot support a significant electric field in its own moving indicating the presence of an electric field whenever B/t frame of reference. The magnetic field and, of course, the is nonvanishing. The differential form of Faraday’s law of magnetic energy and stress as well, move with the fluid, as induction is the familiar B/t cE, of course. if each field line were a thin line of ink entrained in the Maxwell completed the electromagnetic equations in 1864, fluid, thereby providing a direct visualization of the defor- 1 introducing the displacement current 4 E/ t into Ampère’s mation and transport of the magnetic field. law to obtain 4jE/t cB. Maxwell pointed out The theoretical basis for this magnetic transport arises that the complete equations predict the electromagnetic from the researches of Faraday and Lorentz, already men- wave propagating through vacuum at a speed c given by the tioned. Beginning with the nonrelativistic Lorentz transfor- ratio of the e.m.u. unit of charge to the c.g.s. unit of charge. mations for the electric field E and magnetic field B in the Or, as we would say today, the ratio of the units of charge is frame of reference moving with the fluid velocity u relative given by the speed of light, the speed of light being mea- to the laboratory frame, in which the fields are E and B,we sured directly in the laboratory. have It is an interesting, and not entirely unexpected, historical fact that Maxwell’s extension of Ampère’s law, based on Ј u E Ј u B B B , E E a direct analogy with an electric current in a dielectric, and c c the consequent prediction of electromagnetic waves, was ≅ largely ignored in England for the next several decades. Then since E 0 in the conducting fluid, it follows that Kelvin declared that he did not understand Maxwell’s elec- E u B/c, so that B B upon neglecting terms sec- tromagnetic theory (which was certainly the case), did not ond order in u/c compared to one, and Faraday’s induction need Maxwell’s electromagnetic theory because light and equation reduces to the familiar MHD result (Alfvén 1950) electromagnetic waves could just as well be explained by B (u B) an elastic aether, and could see no benefit from Maxwell’s t theory (an immediate consequence of his lack of understanding of the theory). For instance, no reference is given Poynting (1884, 1885) showed the mutual compatibility to Maxwell’s electromagnetic theory in the extensive arti- of Newtonian mechanics and Maxwellian electromagnetic cles on electricity, magnetism, and light to be found in the theory, with the concept of the electromagnetic stress 2 2 1894 edition of the Encyclopaedia Britannica, except for tensor ij(E B )/8(EiEjBiBj)/4 representing the some sentences in Maxwell’s biography where the writer isotropic pressure (E2B2)/8 and the tension E2/4 along 228 EUGENE N. PARKER

the electric field lines and the tension B2/4 along the mag- Robert Marsden, using alpha particles from the newly netic field lines. Thus, static equilibrium of an electric field discovered radioactive decay of radium, were performed in involves a balance between the pressure gradient and the 1909, from which Rutherford pointed out in 1911 that the tension along curved field lines. The same applies for static distribution of deflected alpha particles indicated that the equilibrium of a magnetic field. The energy densities of the atoms from which they scattered contained a massive con- electric and magnetic fields are E2/8 and B2/8, respec- centrated central nucleus surrounded by a diffuse electron tively. At this point in time one could begin to appreciate cloud. Millikan’s first measurements of the charge of the Gilbert’s concept that a magnetic field represents a physi- electron (4.810 10 esu) were carried out in the same year. cally real medium Ð or stress system (Gauss 1839). The field is not a ponderable medium in that it is not made up of matter, but it is an elastic physical medium with its own THE NATURE OF AURORAE AND energy and, therefore, its own mass density. Thus, for GEOMAGNETIC ACTIVITY instance, the energy of Gilbert’s geomagnetic field above the solid surface of Earth is 8 1024 erg, contributing 9 kg At this point the physics was ready for interpreting the to the inertial and gravitational mass of Earth Ð equivalent aurora and the geomagnetic fluctuations, although the scant to a sack of flour. observational data, limited to instruments at the surface of The magnetic field becomes a ponderable elastic Earth, held out no brilliant guiding light. Franklin in about medium, of course, when a collisionless plasma or equiva- 1750, Dalton (1828, 1834), and Gauss (1839) had all come lent highly conducting medium is present. In particular, to the view that the aurora is an electrical phenomenon, Poynting showed that electromagnetic energy is transported based on analogy with such electrostatic phenomena as 2 with a flux density cEB/4, which becomes u⊥B /4 in high-voltage coronal streamers and lightning discharges. MHD, where u⊥ is the fluid velocity perpendicular to B and By about 1890 the shifting ray structure of the aurora B2/4 represents the magnetic enthalpy. Thus the electro- reminded physicists of the cathode ray streamers in the par- magnetic transport under MHD conditions (E0) is the tially evacuated Crookes tube, from which Fitzgerald and convective transport of the magnetic field and the enthalpy others suggested that the aurora is a similar electrical dis- of that field, representing the convective transport of the charge. That is to say, the aurora was recognized as being 2 magnetic energy density B /8 plus the work done by u⊥ caused by fast particles, and it was presumed that the asso- pushing the magnetic pressure B2/8 against the fluid ciated geomagnetic fluctuations were a product of the same ahead. In the nonrelativistic case the electromagnetic particles through Oersted’s effect and Ampère’s law. momentum density EB/4c is small to second order in Fortunately the observational studies were moving ahead u/c compared to the momentum density in the moving during the nineteenth century, however slowly. Gauss fluid, so that it can be neglected. (1839) became interested in the geomagnetic anomalies and We can see how toward the end of the nineteenth century fluctuations, and, beginning in 1832, established the stan- the basic classical physics was falling into place for inter- dard magnetic observatory instrumentation. Backed by preting the geomagnetic fluctuations. The final piece of Humboldt he began organizing the building of magnetic physics was the realization that electric charge is not a fluid observatories around the world over the next two decades. but consists of discrete particles. The quantized nature of His instruments used a small mirror attached to a magnetic electric charge was indicated by Faraday’s experiments compass needle, onto which a collimated beam of light was with electrolysis around 1831. A small integer times a basic directed. The reflected pencil of light intersected a suitable total charge is always associated with the electrolysis of a distant screen, thereby vividly displaying any agitation of mole of any substance. Then, noting that the electrolysis the needle without resorting to the cumbersome close-up experiments showed that matter is composed of discrete microscopic scrutiny (see Chapman (1967) for a review of particles, Helmholtz in 1881 emphasized that the electric the geomagnetic studies). charge, intimately associated with the discrete ions, must Humboldt publicized Schwabe’s discovery of the also come in discrete units. Johnston Stoney was the first to decadal periodicity of sunspots to the scientific world (see apply the term “electron” to the basic charge in 1874. In von Humboldt 1858). Sabine (1852) pointed out the impor- 1897 J.J. Thomson used a Crookes tube (cathode ray tube) tant fact that geomagnetic disturbances at Toronto are to show that the particles emitted by the cathode have a strongly correlated with the number of spots on the Sun. mass that is the fraction 1/1840 of the mass of the hydrogen Lamont noted the same correlation for the magnetic varia- atom. Thus the idea of the individual lightweight electron tions at München. and the relatively massive ion as individual free particles The connection of geomagnetic activity with sunspots was born. The actual structure of the atom was not estab- was reinforced when the four-week variation of the activity lished until scattering experiments by Hans Geiger and was recognized by Broun (1858, 1874), and shown by 10

WILLIAM K. HARTMANN* The terrestrial planets at the dawn of the space age

It is difficult to overstate the profundity of the change in our but rather “fizzled out” as scientific interests shifted to new perception of the planets during a single generation in the observations. For example, much effort was spent trying to latter part of the twentieth century. The revolution is second map the markings of Mercury, but this effort virtually died only to that in the generation from Copernicus to Kepler at when the first photographs were returned; curiously, it is the end the sixteenth century. The very character of the sci- still unclear to what extent early observers, such as Eugène entific questions changed as the planets went from being Antoniadi, were recording real albedo features, and how astronomical objects to geological objects. these features match the geology of Mercury. A primary goal of this chapter is to document some of This is also an interesting issue from the point of view of the changes in perception and scientific issues involving pursuing one’s own career. For example, major parts of terrestrial planets that happened as we went from the purely careers and grants were spent on issues such as mapping and telescopic to the spacecraft era. While I have documented interpreting markings, photographing Venusian clouds, trying some technical material, I have allowed myself, as one who to estimate surface pressures from inadequate data, and so on. lived through this change, to include some anecdotal mater- Were these valuable steps toward a goal, or poor choices of ial. For example, I grew up making backyard telescope research activity? Should the researchers and funding agen- observations of planets when such observations could cies simply have waited for better data Ð especially once it exceed the best photographic imagery; I experienced became clear that probes would be a reality? Often, research Sputnik 1 while at college; I was at graduate school under time and expenditures on such issues increased dramatically Professor Gerard Kuiper as he supervised a lunar mapping before missions, because of interest in what the probe would program for the Apollo landings, before a graduate program find, even though the results would soon be rendered obsolete in planetary sciences was available; I was a co-investigator by a single click of a shutter on a spacecraft. With hindsight, in the first orbital mapping of Mars; and I attended profes- we can see that some efforts, such as the first proof of CO2 on sional meetings where international friends and colleagues Venus or Mars, became fundamental advances to be cited in announced the first close-up results from Mercury, Venus, many future textbooks, while for others whole subjects were Jupiter, Saturn, Uranus, Neptune, Halley’s Comet, Gaspra, swept away by flyby probes or landers. Ida, Mathilda, and so on. A related subject is the subtle abandonment of early An additional aspect of this chapter involves the philoso- “burning questions.” Probably the most argued photo- phy of science and exploration. The change from an astro- graphicÐvisualÐtelescopic question about Mars in the first nomical to a geological approach is interesting because half of the twentieth century was the fundamental underly- some of the burning questions of the astronomical era were ing cause of the dark markings. Mariner 9 showed that they not so much answered with great fanfare by space probes, involved windblown dust, and the case seemed to be closed. But then the question was, why is the dust not mixed? What * Planetary Science Institute, Tucson, AZ, USA is the nature and source of the dark or light material? Do

bare rock outcrops exist and are they a source of spectrally subsequently settled by overwhelming and diverse lines of fresher and less weathered material, perhaps of lower albedo evidence, including radiometric dating. Yet spokespersons than the weathered dust? Spectral and thermal observations for these factions routinely explain in the US media that seem to show that the dark material is coarser, fresher basalt there is plenty of scientific evidence in their favor. One and the light material is finer, oxidized, and weathered member of the Kansas Board of Education, a veterinarian material. With this finding, the issue is currently on a back- who voted to remove evolutionary concepts from the edu- burner. Still unresolved is where the fresh basalt material is cation standards, was described in the New York Times and coming from (especially since lava flows are more evident International Herald Tribune citing “legitimate scientific in light areas like Tharsis and Amazonis). In that sense, the doubts about whether the Universe is more than several original, underlying question that drove much of early Mars thousand years old” (Glanz 1999), and this view is wide- research is still unanswered. An “answer” was given, based spread in fundamentalist literature. This in spite of the inde- on results obtained with instruments then available, but it pendent and international work by researchers such as did not answer the original question. This example illus- Gerling (1942) in Leningrad, Holmes (1946) in Edinburgh, trates a slightly unsettling characteristic of science: that the and Houtermans (1947) in Göttingen, who pioneered the phenomena we measure and debate are not necessarily the modern view of the age of Earth, not to mention the “most important” phenomena of nature, identified objec- acknowledgment by Pope Pius XII (1951) that the ages of tively, but rather are heavily weighted by the particular the Earth and Universe must be measured not instruments we happen to have and the subdisciplines that in thousands, but billions of years. Although the have emerged through socio-scientific processes. Copernican Revolution started 450 years ago, it has not It is unsettling to discover that one’s students or younger been won Ð and this is an indictment of science teachers as colleagues don’t share certain pivotal experiences that well as fundamentalists. shaped planetary science, and that to them, these experiences are prehistory, or even unknown! Many of the contributors to this book have passed through various moments SETTING THE STAGE: THE 1940S AND 1950S of shock with various successive waves of students: the ones who don’t remember Sputnik, the ones who don’t The terrestrial planets were not major objects of study in remember Gagarin’s flight or President Kennedy’s the 1940s and 1950s, and only a handful of scientists announcement a few weeks later of the Moon landing pro- worked professionally on the Solar System. These included gram, the ones who don’t remember the first photographs astronomer Gerard Kuiper, who discovered the atmosphere of Mars, and, finally, the ones who weren’t even born when of Titan spectroscopically (Kuiper 1944) and trained newly Neil Armstrong stepped onto the Moon. For these reasons, developed infrared technology on the Venusian and Martian I shall try to capture some of the character of the early atmospheres after World War II (Kuiper 1952); the Nobel developments as experienced at the time. laureate geochemist Harold Urey, who published the epoch- At the same time, as the twenty-first century begins, one marking book The Planets (Urey 1952); astronomer Fred is only too aware that the future progress of planetary Whipple, who published seminal papers on the nature of exploration and research in general is under threat from comets (Whipple 1950, 1951); and Ernst Öpik, who pub- anti-intellectual, anti-science, fundamentalist sources. In lished amazingly prescient papers on a variety of planetary the USA, the battle over the teaching of biological evolu- issues, such as impact cratering by interplanetary bodies tion in schools not only continues, but has expanded into an (Öpik 1951, 1963, 1964) and the surface of Mars (Öpik attack on evolutionary processes, even in physical systems 1965). Urey and Kuiper both developed far-ranging ideas such as stars and galaxies. In 1999, the Kansas state Board on the origin of planets and planetary surface features, of Education, under pressure from fundamentalists who had but rarely saw eye to eye. It is important to realize that gained a majority in elections, withdrew not only biological although researchers such as Otto Schmidt and, later, Viktor evolution but coverage of the big bang cosmological theory Safronov (1972) developed a strong school of studies on from the subjects covered by the state’s educational stan- planetary origins, their work was virtually unknown outside dards and testing program, effectively excluding them from the Soviet Union until the 1970s. the curriculum. Generally speaking, the proponents of these The nature of meteorite craters was wholly unappreci- views are not well educated about the history of science or ated during this period. Much of the literature about the the nature of the modern international scientific literature, Moon during the preceding century was consumed with but take refuge in their own gray literature and national arguments over whether the lunar craters were volcanic or networks of radio programming. Ironically, they wage ghostly impact features. Scattered suggestions that there were many echoes of battles already fought and settled nearly two cen- eroded impact features on Earth had about them the aura turies ago by pioneers such as Lamarck and Cuvier Ð battles of fringe science. Arizona’s so-called “Meteor Crater” was THE TERRESTRIAL PLANETS AT THE DAWN OF THE SPACE AGE 259 still listed as “Crater Mound” by the US Board of existing military rockets, but would develop a new civilian Geographical Names, even though it was not a mound rocket for a fledgling, non-military space program. As is (Hoyt 1987). This was allegedly because the first director now known publicly, the Soviet Union had also begun of the survey, G.K. Gilbert, believed it was a volcanic fea- development of an artificial satellite project under Sergei ture and not an impact crater. This was ironic, because P. Korolev, with initial authorization given in January 1956 Gilbert himself later championed the impact theory for (Harford 1997, p. 125 ff.). lunar craters. As late as 1945, US Geological Survey On 4 October 1957, before the Americans were ready to (USGS) scientist N.H. Darton delivered a paper decrying launch, the Soviet engineers launched Sputnik 1 into Earth any use of the terms “meteor” or “meteorite” in the crater’s orbit. It was followed a month later by the much bigger title, because “I am convinced that no meteorite is present” Sputnik 2 and its booster, which were very prominent in due to the lack of discovery of an iron mass during drilling skies around the world. Sputnik 1 was very faint, but the (Hoyt 1987, p. 333). This in spite of the fact that the surface booster of Sputnik 2 was bright and tumbled in its orbit, around the crater is strewn with small iron meteorites! leading to dramatic variations in brightness with a period of In 1949, Ralph Baldwin, a businessman with a PhD in the order of tens of seconds, flashing to about first or sec- astronomy from the University of Chicago, published The ond magnitude. This produced profound shock in the USA, Face of the Moon (Baldwin 1949), a highly original work in leading to massive reorganization of school science pro- which he used his experience with bomb craters in World grams, and for the first time moved the popular concept of War II to argue methodically that the geometric properties spaceflight from the realm of science fiction to reality. At of lunar craters matched impact explosion features, not vol- the same time, the Sputniks galvanized the imagination of canic features. This was the turning point in the argument young science students in the USA. The spirit of this period about the origin of craters, and led to wide acceptance of is well captured in the film October Sky, based on the mem- the view that the Moon had been peppered with impacts. oir Rocket Boys by Apollo engineer Homer Hickam (1998). (The book, from the University of Chicago Press, was not a In retrospect, the competing political philosophies of big seller; I bought my copy on a remainder table of a secrecy v. scientific openness provide us with important favorite bookstore in Pittsburgh for $1.49 when I was a boy, lessons about scientific progress in planetary exploration. attracted by its cover picture of the Moon.) The discovery of Earth’s radiation belts was credited to As for the terrestrial planets, amateurs and professionals James Van Allen from data received by the US Explorer 1 alike argued about the cause and significance of the various satellite in 1958, but Dessler (1984) reviews how the detec- faint, dusky markings that could be glimpsed on Mars, tors installed in the earlier Soviet satellite, Sputnik 2, actu- Mercury, and probably Venus. Frank E. Ross (1928) had ally measured the radiation belts in 1957. However, shown that the faint, dusky markings of Venus could be according to Dessler, the Soviets chose for reasons of photographed with much more clarity and contrast by using secrecy not to arrange for other countries to detect or ultraviolet filters; they were evidently cloud features, but decode their satellite signals or pass them on to the Sputnik little more was known about them. Walter Adams and team. Thus, they did not get enough tracking data from Theodore Dunham (1932) had shown that carbon dioxide Sputnik 2 to recognize the belts. Dessler remarks that, was a major atmospheric constituent. “Because of their perceived need for secrecy, the Russians There was a curious flavor to interest in spaceflight or missed making one of the most dramatic discoveries in planetary exploration at this time, difficult to recapture space science.” today. Popular literature, such as comic books, were full of articles about “post-war marvels.” Boys (and a few girls) who read science fiction “knew” that spaceflight was com- THE LOW ATMOSPHERIC PRESSURE AND RED ing, but adults rolled their eyes at such fantasies. In COLOR OF MARS: 1947–64 1952Ð54, Collier’s magazine ran a now-famous series of articles by Wernher von Braun, with paintings by the dean In the twentieth century, the estimated atmospheric pres- of astronomical art, Chesley Bonestell, detailing how we sure and habitability of Mars fell progressively. Percival could launch artificial satellites, build a wheel-shaped space Lowell’s theories at the beginning of the century suggested station (the design later immortalized in Stanley Kubrick’s a thin but Earth-like atmosphere. Although Lowell has been 1968 film 2001), and explore the Moon and Mars. In 1956, vilified, and even ridiculed, for his theories about civiliza- President Eisenhower surprised many Americans and galva- tions and artificial canals on Mars, he started by propound- nized science students’ imaginations by announcing that the ing underlying ideas that still have validity. He pointed USA, as part of the International Geophysical Year, would out that smaller planets lose their internal heat faster than attempt to launch an artificial satellite around the Earth. larger planets, and are also more subject to the escape of Eisenhower emphasized that the USA would not use their atmospheres. Thus, Mars was geologically dying and 260 WILLIAM K. HARTMANN

drying, and the atmosphere was being lost to space. But long-sought confirmation that the dark markings were Lowell thought there was evidence for a thicker atmosphere caused by simple plant forms. However, a few years later than really exists. He argued that the warm equatorial Sinton himself and other observers showed that these bands regions were the most habitable in terms of temperature but were due to other causes. This was as close as ground- that most of the planet’s water was locked in the polar ice. based telescopic observers ever came to claiming definitive Thus, Martian civilizations had built canals to deliver melt evidence for life on Mars. water from the polar ice caps to their equatorial settlements. Eventually, a clearer view emerged from the ground- This view of Mars electrified not only scientists, but writers based infrared work. Kaplan et al. (1964) studied weak, and intellectuals from Alfred Tennyson to H.G. Wells, who unsaturated CO2 bands and concluded that the mean surface combined it with the ideas of Charles Darwin to speculate pressure was 2.515 mbar (Horowitz (1986) refers to this upon the implications for humanity if we were not alone in paper as the true beginning of the post-Lowellian under- the Solar System. This was the Mars that colored much of standing of Mars.) In the same year, Owen and Kuiper twentieth-century Martian research, and served as the (1964) used their own spectra and improved laboratory cali- framework against which to react. In the first decades of the bration (long-pathlength spectra through low-pressure CO2) century, researchers began to realize that the planet must be to derive a surface pressure of 1.73 mbar, though they still less habitable than Lowell had thought and the surface con- thought that the atmosphere was likely to be dominated by ditions were likened to a winter day at the top of a moun- nitrogen (see also Kuiper 1964). tain on the island of Spitzbergen in the Arctic Ocean. The same period saw the emergence of the basic under- The changing ideas of Mars were also reflected in popu- standing of the red color of Mars. By the 1960s, broadband lar culture. Edgar Rice Burroughs, in his science fantasy infrared spectroscopy had advanced to the point where it novels, peopled Mars with evil queens and great beasts. could begin to be used to identify major absorption bands Ray Bradbury, in his 1940s classic The Martian Chronicles, due to minerals on the Martian surface. Two graduate stu- portrayed the last remnants of Martian civilization, eleing dents in Kuiper’s infrared spectroscopy laboratory, Binder out an existence by the languid canals. and Cruikshank (1966), showed that there was a good fit By the middle of the century, the estimated surface between Mars spectra and red, oxidized basalts found in the pressure had dropped to 100Ð200 mbar. Kuiper (1947) first desert west of Tucson, Arizona. They proposed, essentially detected the carbon dioxide of Mars and erroneously reduced correctly, that the color of Mars comes from oxidized iron the data to give a surface CO2 partial pressure of only minerals that exist as stains on the surface of rocks and dust 0.35 mbar. Because this was so small, most researchers particles; they emphasized limonite and other iron oxide assumed that CO2 was only a minor constituent in the atmos- minerals. This work was generally supported by Adams phere and that nitrogen or some other gas was dominant, (1968) and McCord and Adams (1969), who examined with a moderate surface pressure. Gerard de Vaucouleurs additional iron oxide minerals. (1954), summarizing the available evidence, gave the surface Philosophically, work of this sort was an extension of the pressure as 85 4 mbar. The erroneously low estimate of Copernican Revolution and early ideas about the plurality of CO2 abundance was a factor in Kuiper’s further conclusion, worlds. The Copernican Revolution revealed that Earth was now disproved, that water ice was the main constituent of the not a unique center, but merely one of many planetary polar ice caps. worlds. The idea of the plurality of worlds introduced an These views persisted for some years. Horowitz (1986), assumption that the other worlds might be other Earths, with with some amusement, cites an advisory report to NASA environments similar to our own. William Herschel had from a prestigious space science board in 1961, stating that believed that Mars, on which he saw clouds and polar caps, “infrared reflection spectra of the polar caps show conclu- was Earth-like. Only in the 1950s and 1960s did solid obser- sively that they are not composed of frozen carbon diox- vations begin to accumulate that allowed these ideas to be ide,” but were water ice and frost. They also advised that assessed. The emerging answer, with profound and still the total surface pressure was within a factor two of under-appreciated consequences, was that many other plan- 85 mbar Ð exactly the value, as Horowitz points out, that ets are indeed worlds with Earth-like minerals and geology, Percival Lowell (1910) had published in his book Mars as and yet each has a personality of its own. the Abode of Life. During the first years of infrared studies, another interesting episode occurred. William Sinton (1957) announced EARTH: IMPACT CRATERS LEGITIMIZED, confirmation of three faint bands around 3.4 m wave- C. 1960 length, which he identified with the CÐH bond, specifically chlorophyll. In the title of his paper he called this Research on planetary geology began to be legitimized “Spectroscopic evidence for vegetation on Mars”– the around this same time. In the late 1950s geologist Eugene