INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 29, May 2012 Editorial

Uranus was the first planet to be regularities. It stood to reason that discovered since ancient times. Such Le Verrier should again come up Impressum was the luck of the young musician with the idea that an unknown William Herschel in 1781. It was planet, which he swiftly called Vul- not just a coincidence, however, be- cain, was shaking Mercury. This is cause Herschel was not only a gifted now the stuff that galvanizes as- composer and organist, but also a tronomers, and it did not last long SPATIUM skilled builder of telescopes which until hasty observers claimed to Published by the he used to observe Britain’s night have seen the hypothetical planet. Association Pro ISSI sky. Later observations of Uranus re- Still, further analyses showed in all vealed strange orbital irregularities cases that the putative Vulcains that could not be explained by were mere products of imagination: ­Kepler’s laws. This is why the planet planet ­Vulcain could not be found. continued to stir up interest in the The mystery was only solved in Association Pro ISSI astronomical community and later 1915, when Albert Einstein ex- Hallerstrasse 6, CH-3012 Bern the French astronomer Urbain Le plained Mercury’s anomalous orbit Phone +41 (0)31 631 48 96 Verrier undertook to unveil its se- with his general theory of see cret. Months of complex calcula- relativity. www.issibern.ch/pro-issi.html tions - at that time the matter of a for the whole Spatium series clear mind, much paper and a sharp That may have been the first, yet pencil - brought him to the con- undoubtedly not the last of Mer- President clusion that the planet’s wobbling cury’s many secrets to be unveiled. Prof. Nicolas Thomas, orbit might be caused by an addi- However, the more scientists look University of Bern tional neighbouring yet unknown at this planet the more they get fas- planet. He quickly announced his cinated by that small, hot sphere. Layout and Publisher findings to the French Academy on Prof. ­Peter Wurz of the Physics Dr. Hansjörg Schlaepfer 31 August 1846; and he also sent ­Institute of the University of Bern, CH-6614 Brissago them to Johann Galle of the Berlin the author of the present issue of Observatory who found the wanted Spatium, is no exception. On Printing planet precisely on the indicated 30 March 2011, he presented the Stämpfli Publikationen AG spot. That marvellous success earned current status­ of Mercury research CH-3001 Bern Le Verrier the nickname of the man to our PRO ISSI association. We who discovered a planet with the point thank Prof. Wurz for his consent and of his pen. support in publishing his enlight- ening talk, and wish our readers a Unknown to Le Verrier, similar cal- hearty portion of that great fasci- culations were made at virtually the nation scientists get when study­ing same time by a student in England; mysterious Mercury. yet, they got lost and re-found again only much later, prompting lengthy Hansjörg Schlaepfer disputes about the true finder of Brissago, May 2012 Neptune, as the newcomer was Front Cover named soon after. Anyway, spurred Craters are Mercury’s trademark so by his success, Le Verrier set out to to speak. The front page shows address a further astronomical mys- the crater Kuiper seen by NASA’s tery: he began observing Mercury, Messenger camera, interpreted by which also exhibits some orbital ir- an artist.

SPATIUM 29 2 Mysterious Mercury1 by Prof. Peter Wurz, Physics Institute, University of Bern

Introduction When the first astronomical tele- cury’s orbit remains a puzzle until scopes appear in the 17th century, 1915, when Albert Einstein solves Mercury is an attractive object in the problem conclusively by means the sky. Even though observation of his general theory of relativity. The solar system’s history begins turns out to be difficult due to the Einstein’s point takes into account with a huge interstellar cloud of gas glare caused by the nearby Sun, the effect of relativity as a ­result of and dust far out in one of the Milky ­Urbain Le Verrier3 notes that its or- the tremendous forces of gravity Way’s several arms. Gravitational bit slightly deviates from what is to reigning in Mercury’s quarters close ­instabilities cause it to collapse into be expected from classical New­ to the Sun. a large flat disk. Most of the matter tonian mechanics. Trying to explain concentrates in its centre where the planet’s mysterious behaviour, A further of Mercury’s many pe- the later Sun will emerge. The re- he stipulates a further hypothetical culiarities is found by Gordon maining material forms the outer planet which by its gravitation Pettengill5 with the help of the parts of the disk, where over a causes the excessive shift of Mer- ­giant 304.8 m diameter Arecibo ra- ­period of some 50 million years cury’s perihelion4, Fig. 1. The un- dar telescope in Puerto Rico in planets form together with all the known planet even gets a name: 1965. He recognizes Mercury’s 3 : 2 other bodies populating the solar Vulcain; yet all attempts to get hold spin/orbit resonance, meaning that system2. of it are bound to fail: the suspected whilst Mercury orbits the Sun planet cannot be found, and Mer- twice, it rotates around its axis ex- Next to the Sun is Mercury, the actly three times. Such resonances smallest of all planets. It is known occur in celestial mechanics as a re- from times immemorial as it can be sult of tidal forces caused by a large seen under good conditions by the body (here the Sun) acting on a naked eye notwithstanding its small smaller companion (here Mercury). size. The Babylonians call it Babo, The latter is slightly deformed by the messenger of the Gods, an at- the larger body’s gravity field which tribute reflecting its fast orbit in turn tends to synchronize it into around the Sun, faster than any a resonant revolution mode. other planet. Civilizations change, Mercury keeps its attribute: the That is about all that is known Greeks call it Hermes and Apollo, ­before the first space probes visit which is reduced to Hermes when Fig. 1: While Newtonian mechanics ­Mercury at close quarters. In 1974 it becomes known that the planet and Kepler’s laws stipulate an ellipti- NASA’s Mariner 10 spacecraft cal orbit, Mercury orbits on a rosette-like in the morning sky next to the Sun pattern around the Sun as a consequence reaches the planet after a half year is the same as that in the evening of – amongst other things – the near Sun’s journey, followed 30 years later by sky. Roman astronomers call it powerful gravity field. The anomalous NASA’s Messenger mission. Both Mercury, the God of dealers and perihelion shift amounts to 43 per cen- programmes provide fascinating tury, which is 120 km/year. This com- thieves, a name the planet bears pares to Earth’s anomalous perihelion new insights into the secrets of down to our day. shift of only 3.8 per century. Mercury, at the same time raising

1 The present issue of Spatium reports on a lecture by Prof. Wurz for the PRO ISSI audience on 30 March 2011. 2 See Spatium no. 6: From Dust to Planets by Willy Benz, October 2000. 3 Urbain Jean Joseph Le Verrier, 1811, Saint-Lô, Manche, France – 1877 Paris, French mathematician and astronomer. 4 The term perihelion designates the point on a planetary orbit which is closest to the Sun. Correspondingly, the aphelion is the farthest point from the Sun. If the reference is not the Sun, then the terms periapsis and apoapsis are used. 5 Gor don Pettengill, 1926, Providence, Rhode Island, USA, US-American radio astronomer and planetary physicist.

SPATIUM 29 3 many new questions. This is why Mysterious inner solar system billions of years the European Space Agency ESA, ago. So, to understand the Earth’s in collaboration with the ­Japanese Mercury history, one also has to understand Space Agency JAXA, is currently Mercury’s evolution. implementing a mission called Be- piColombo in honour of the Ital- ian space scientist Giuseppe Co- The deeper scientists look at Mer- Orbital Characteristics lombo, see text box. This mission is cury the more they get fascinated scheduled for launch in August by the small, yet unusual planet as We have already addressed some pe- 2015. As a space mission to Mer- it turns out to be an outright sto- culiarities of the planet’s orbit. Yet, cury poses uncommon, yet specific rybook telling the early history of there are many more: As the inner- fascinating challenges to engineers the solar system, including that of most planet in the solar system, and scientists, we will treat these the Earth. This property is owed to Mercury’s mean distance to the Sun programmes somewhat more in the fact that the planet has neither is a mere 0.39 AU6. This leads to a depth below. an atmosphere nor tectonic acti­ vities very short year on the planet: Mer- that would tend to erode any traces cury completes one orbit around of the distant past, as is the case for Sun in only 87.97 Earth days, faster instance on Earth. Rather, Mercury than any other planet in the solar preserves faithfully the marks left by system. Furthermore, Mercury’s or- violent events and processes in the bit has the largest eccentricity7 of all planets, see Fig. 2. The perihelion, the point on the orbit closest to the In addition, he participated in research at Sun, is 0.31 AU, while the aphelion the Harvard Smithsonian Center for is 0.47 AU. This feature, together ­Astrophysics, then at Caltech and Jet with the lack of moons, and also its ­Propulsion Laboratory. small size, fuels the speculation that Mercury might not have come into Professor Colombo was a member of var- being as a planet proper, but as the ious advisory committees and national and international academies. He was moon of Venus, which today no awarded NASA’s Gold Medal for out- longer has a moon. standing scientific achievement as well as Giuseppe “Bepi” Colombo was born in several other prizes. He also studied new Padua in 1920 where he attended primary concepts concerning space transportation, and secondary school. After graduating large space structures and evolution of from the University of Pisa in mathemat- space technology for space sciences and ics in 1944, he returned to Padua where applications. He played an important role he worked as an assistant and then asso- in promoting space research at the Italian ciate professor of theoretical mechanics at Space Agency. The Space Geodesy Cen- the University. In 1955 he became full tre in Matera, Italy bears his name. ESA’s professor of applied mechanics at the mission to Mercury has been named Be- faculty of engineering at the University piColombo in honour of this space Fig. 2: Orbital eccentricities of the of Padua. In his career, he lectured on me- pioneer. planets in the solar system. Mercury’s ec- chanical vibrations and celestial mechan- centricity is by far the largest suggesting ics, as well as space vehicles and rockets. Giuseppe Colombo died in 1984. that it might have a different origin to the other planets.

6 A U: Astronomical Unit, equalling 149,597,870.7 kilometres, the mean distance between Earth and the Sun. 7 In astronomy, eccentricity  is the measure of deviation of an orbit from the ideal circle with  = 0.

SPATIUM 29 4 Many of Mercury’s peculiarities Planetary Composition was then blown away by the solar have to do with the Sun’s vicinity wind. A third theory, promoted by and its overwhelming strength of Mercury presumably evolved from Prof. Willy Benz of the Physics In- radiation. At perihelion, the surface the same primordial disk of matter stitute, stipulates a catastrophic im- experiences some 14,000 W/m2 of as the other terrestrial planets ­Venus, pact by a large body in the planet’s energy flux which is ten times the Earth and Mars. One would, there- early history that jettisoned much value on the outer layers of the fore, expect their composition to be of the planet’s former mantle con- Earth’s atmosphere. Consequently, roughly the same. Yet, this is not the taining the silicates out to space surface temperatures are extreme: at case. On a plot of density versus size while the remaining material rebuilt noon, they may reach as much as (Fig. 3), the terrestrial planets and the the planet later. The different mod- 450 °C on the equator, while dur- Moon stay roughly on a straight els lead to different chemical com- ing the night they fall down to line, while Mercury has about the positions of the planetary mantle –180 °C. This qualifies Mercury as density of the Earth, but the size of and crust, and as soon as reliable data the body with the highest day/night the Moon. become available, the most pro­m­ temperature differences in the en- ising theory may become apparent, tire solar system. In such an extreme or it might even turn out that a environment no one would expect new model is needed … water. Yet, radar observations of the floors of deep craters near the poles in the shade of sunlight suggest the Surface Geology local occurrence of water ice. Where it comes from is just another of Mercury’s most eye-catching fea- Mercury’s many secrets. ture is its uncountable craters of all sizes, Fig. 4 to 14. In the solar system’s While Mercury is fast in orbiting Fig. 3: Comparing the densities of early phase, more precisely between the Sun, it is very slow in rotating terrestrial planets and the Moon with 4.1 and 3.8 billion years ago, the their radii: Mercury has an exception- around its own axis: it needs 58.65 ally high density value for its size. terrestrial planets experienced what Earth days to complete one revolu- is called the Late Heavy Bombard- 2 tion, exactly ⁄3 of a Mercurian year. Mercury is hence a heavy planet for ment (LHB). At that time, intensity This is the 2: 3 spin/orbit resonance its size. As the core of the terrestrial and frequency of collisions with found by Gordon Pettengill. As a planets is made of iron, this high ­either asteroidal or cometary mate- consequence of the spin/orbit cou- density implies a large iron core at rials reached their last, high values. pling, the planet has surface points the expense of a smaller mantle The impacts created craters on which are always far from the Sun formed by the lighter silicon. Yet, the planetary surfaces, and, if the on the aphelion while on the peri­ even though Mercury possesses collisions were strong enough, helion the opposite point on the much iron in its core, the crust prompted volcanoes to erupt. In this planet’s surface always shows right hardly contains any iron. This comes case, emanating lava filled the cra- to the Sun. This results in locally as a surprise for which the reasons ters again creating large plains with fixed climate zones on the surface are still unknown. A variety of the- smooth surfaces. At the end of the with very hot regions alternating ories have been invoked to explain LHB, the rate of impacts dropped with less hot regions. In any case, this fact. First, it is possible, that the by a factor of 1,000 to the low however, less hot still means very chemical composition of the pri- ­values we experience at present. hot as compared to Earth’s temper- mordial disk was not so homogene- Thus, a heavily cratered surface ature regime … ous as expected. A second possibil- tends to be an old area that suffered ity might be that intense radiation many impacts during the LHB, from the young Sun evaporated while a region exhibiting only few much of the mantle’s silicates which craters tends to be younger.

SPATIUM 29 5 Craters the crust broke apart. At the faults, Fig. 5: Mercury’s scarred surface. the resulting scarps may reach The image on the opposite top left side shows a high resolution map of Mercury Craters on Mercury range in diam- heights of up to 3,000 m and lengths under uniform lighting conditions based eter from small cavities to multi- of 500 km. on thousands of individual images col- ringed impact basins hundreds of lected by NASA’s Messenger spacecraft. kilometres across. They appear in all (Credit: NASA) states of degradation, from relatively Impact Basins Fig. 6: Mercury forms a beautiful fresh rayed craters to highly de- crescent shape in the image on top graded crater remnants. Mercurian The largest feature on Mercury is right, acquired by the Messenger space- craft. (Credit: NASA) craters differ from lunar craters in the Caloris basin (Fig. 12), a crater that the area blanketed by their excavated by the impact of a huge Fig. 7, bottom left: A double ring ejecta is much smaller, a conse- meteorite. The Caloris basin, fea- crater is an indication of a high-force quence of Mercury’s stronger turing a diameter of some 1,550 km, impact, usually a very massive meteorite, causing a ripple effect in the rock, like gravity. has similar dimensions to the planet dropping a pebble in a pond. (Credit: itself; it is even one of the largest NASA) impact ­basins in the entire solar sys- Fault Lines tem. The violent impact prompted Fig. 8, bottom right: a colourful view of Brontë, the large crater in the top volcanoes to emit magma that filled right corner, and Degas, the blue-hued Just like all proto-planets, Mercury the Caloris basin again leading to crater atop Brontë. These craters are lo- was very hot in the beginning after the large smooth plains we see to- cated in Sobkou Planitia, a plains region formed through past volcanic activity. its formation. As time passed, the day. The impact also left a concen- The colours are artificial; they reflect the core cooled down resulting in a tric ring system over 2,000 m high results of spectroscopic analyses helping shrinking core volume. The solid surrounding the impact crater. Yet, scientists to differentiate various materi- crust began warping, and generat- the most intriguing feature is found als. (Credit: NASA) ing large faults, Fig. 4. Where the at exactly the geographical anti- forces involved were strong enough, pode of the Caloris basin, namely a large region of unusual, hilly terrain known as Weird Terrain, (Fig. 11). One hypothesis for its origin is that the shock waves generated during the Caloris impact travelled around the planet, and converged at the ba- sin’s antipode. The resulting stresses fractured the crust, and created the weird surface.

Fig. 4: When the planet’s liquid core cooled, the solid crust warped up creat- ing long, irregular fault lines that may measure as high as 3,000 m.

SPATIUM 29 6 SPATIUM 29 7 Fig. 9 (top left): The ejecta of the crater in the centre have been deposited on older craters. Straight lines are often caused by a splash kicked up when a me- teorite forms a primary crater, and some of the material tossed up is big enough to make a secondary crater when it comes back down. Instead of just one rock be- ing kicked up and away from the primary impact, several are. They all fly off in ex- actly the same direction and when they come down each one makes a secondary crater – and they all occur in a line. (Credit: NASA)

Fig. 10 (centre left): An unnamed old crater basin has been largely filled by magma from the planet’s interior. Later impacts created a variety of smaller cra- ters in the smooth plain. (Credit: NASA)

Fig. 11 (bottom left): The antipode of the Caloris basin: the Weird Terrain. The giant impact that formed Caloris may have had global consequences for the planet. At the exact antipode is a large area of hilly, grooved terrain, with few small impact craters that are known as the Weird Terrain. It is thought to have been created as seismic waves from the impact converged on the opposite side of the planet. (Credit: NASA)

Fig.12 (right): The Caloris basin in a false colour representation. The large yel- low area is the Caloris basin, featuring a diameter­ of some 1,550 km, among the largest impact basins in the solar system. The impact which created the Caloris basin must have occurred towards the end of the Last Heavy Bombardment, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon, such as the Mare Imbrium and Mare Orientale, are believed to have formed at about the same time between 3.8 and 3.9 billion years ago. This image was coloured based on spectroscopic in- formation to identify different rock types. Impact craters serve as probes into a plan- et’s subsurface, excavating and exposing material from depth that would be other­ wise unobservable. Thus the study of ­impact crater deposits can help to eluci- date the geological history of the target region. (Credit: Science/AAAS using data from Messenger’s Dual Imaging System)

Exosphere

Mercury is too small and too hot to retain a significant atmosphere over long periods of time. Yet, it possesses an exosphere. The term exosphere ­relates to a thin gaseous medium where the mean distance before an atmospheric particle hits another one is comparable to or larger than the ­atmospheric thickness. As a con- sequence of the exosphere’s low den- sity, a molecule travelling upward fast Fig. 13: The Kuiper crater: This enhanced colour view of Kuiper crater shows not enough can escape to space before just the bright rays that extend out from this relatively young crater but also the red- colliding with another particle. Hence der colour of Kuiper’s ejecta blanket. The redder colour may be due to a composi- tionally distinct material excavated from depth by the impact that formed Kuiper. the exosphere continuously dissipates (Credit: NASA) material out to space. In a steady state, it is replenished by released material from the planet’s surface.

As Earth formerly had a CO2 rich atmosphere, and Venus still possesses such an atmosphere, Mercury was

expected to have a CO2-dominated exosphere as well. Yet, this was not

the case: no CO2 was found, but rather hydrogen, oxygen and he- lium from out-gassing surface ma- terial. Upon a certain filtering func- tion, the exosphere reflects therefore the chemical composition of the surface. The solar wind continu- ously blows a fraction of the dissi- pated material from the exosphere deep out to space. This can be seen in the Na-D lines8 in Fig. 15. An enormous tail passes away from the planet, up to some 2,000 planetary radii into interplanetary space. ­Mercury, therefore, is continuously ­losing material, the surface gets de- Fig. 14: Scarps in the Rembrandt impact basin. The long scarp trending verti- pleted of certain components with cally on the left-side of this image is located in the interior of the large 715-kilometer­ time, and it may well be that in the diameter basin Rembrandt. (Credit: NASA) beginning this process had been much stronger which could ac- 8 The sodium-D doublet at wavelengths of 589.592 nm and 588.995 nm count for the lack of a thick man- is the dominant spectral feature of sodium. tle as we know it from the other terrestrial planets.

SPATIUM 29 10 Magnetic Field and Preparing for the unpredictable, planetary magnetic field that results Magnetosphere NASA decided to install a mag- in a cavity void of solar wind plasma netometer on its Mariner 10 space- around the planet, which extend as A small planet like Mercury is con- craft. The decision was rewarded: a long tail away from the Sun. As sidered geologically dead, i.e. with- against all expectations, the instru- Mercury has no atmosphere, and out any geological activity. Specifi- ment detected an outright magne- therefore no ionosphere, this inter- cally, this planet was not expected tosphere, which is, however, small action is very direct, and undis- to possess a global magnetic field, as as compared to Earth’s. turbed, thereby offering an ideal such a field requires a dynamo to laboratory for in­vestigations in generate it, which in turn needs a The magnetosphere results from the space plasma physics. liquid iron core to rotate at a differ- interaction of the stream of ionized ent speed to the crust. particles in the solar wind with the

Fig. 15: Mercury’s tail of sodium gas in the Na-D lines in front of the Sun meas- ures some 2.5 million km. The inserts show where on the planet the tail gases come from. (Credit: Center for Space Physics, Boston University)

SPATIUM 29 11 Exploring ­Mercury in the Space Age

Mercury is difficult to observe from Earth due to its vicinity to the Sun, which tends to outshine the small planet. On the other hand, inspect- ing Mercury at close range with space probes is also not an easy task, as the immense heat from the near Sun poses a critical challenge to the Fig. 16: The Mariner 10 spacecraft, the first space probe ever to visit Mercury in spacecraft’s thermal control systems. 1974/1975. (Credit: NASA) Even worse: the Sun’s gravity field accelerates the probes during their resonance with Mercury. This orbit­ scientist Giuseppe Colombo. Mar- long journey leading to velocities brings the spacecraft once around iner 10 implemented three succes- far above the orbital speeds around the Sun while Mercury executes sive fly-bys to Mercury at different Mercury. So, mission engineers have two orbits. The innovative trajec- distances. As a result of the resonant to implement appropriate braking tory was inspired by orbital me- orbit, however, the planet’s lighted actions en route, which may last as chanics calculations by the Italian surface was almost the same on the long as seven years. three fly-bys, restricting the imaged area to somewhat less than half of the planet’s surface. Therefore, large Mariner 10 portions of the planet remained unobserved. NASA’s Mariner 10 (Fig. 16) was the first mission to Mercury. It was also Notwithstanding Mariner 10’s in- the first spacecraft to use the grav- herent limitations, the mission itational pull of a planet to appro- ­delivered impressive amounts of priately change its trajectory to ­scientific data from which research- reach Mercury, and the first to use ers had to live for the subsequent the solar radiation pressure on its 30 years until the second space solar panels and its high-gain an- probe, Messenger, came along. tenna as a means of attitude control during flight. This works just like the sails of a boat, which, when Messenger properly aligned to the wind, pro- vide the ship with thrust in the Messenger, an acronym for Mercury ­desired direction. Surface, Space Environment, Geo- Fig. 17: The Messenger space probe chemistry and Ranging (Fig. 17), was On 3 November 1973 Mariner 10 during testing at Johns Hopkins Univer- launched on 3 August 2004. Based started to fly-by Venus to bring its sity. Clearly visible is the large ceramic- on the experience with planetary fabric sun shield that protects the space- perihelion down to the level of craft against solar radiation. (Credit: fly-bys gained in the meantime, a Mercury’s orbit in a 1: 2 orbit/orbit- NASA) much more sophisticated trajectory

SPATIUM 29 12 was chosen including a gravity as- BepiColombo will be fixed together as part of the sist with Earth, two gravity assists Mercury Composite Spacecraft with Venus, three times with Mer- Even though Messenger is a great (MCS). In addition to the two cury, to appropriately lower its leap forward regarding our knowl- probes, the composite comprises speed before insertion into a polar edge of Mercury, it will leave open the Mercury Transfer Module orbit around Mercury on 17 March questions, and certainly raise new (MTM), providing solar-electric 2011. That was after a voyage of ones as well. Together with the propulsion to secure the required nearly seven years over some 8 bil- ­Japanese Space Agency JAXA , the braking action on the trajectory. lion km with 15 round trips around European Space Agency ESA is Shortly before Mercury orbit inser- the Sun. currently preparing a mission to tion, the Transfer Module is jetti- Mercury. Honouring the Italian soned from the spacecraft stack. The In contrast to Mariner 10, Messen- space pioneer it bears the name Be- MPO provides the MMO with the ger was scheduled to enter a close piColombo, Fig. 18. The mission is necessary resources and services orbit around Mercury with the scheduled for start in August 2015, ­until it is delivered into its own corresponding challenges for its and to arrive at destination about ­mission orbit, when control will be temperature control system. To this six years later. taken over by JAXA. end, Messenger is equipped with a heat shield that is constantly aligned BepiColombo features a new mis- BepiColombo is a high risk mission: toward the Sun providing shade to sion concept including two separate the Planetary Orbiter will visit Mer- the spacecraft proper. The orbit is spacecraft: the Mercury Planetary cury over its hottest zones, where highly elliptical bringing the probe Orbiter (MPO) will circle the thermal radiation from the hot sur- as close as 200 km to the planet’s planet on a low orbit to allow for face adds to solar radiation to sum surface, and as far out as 15,193 km, high resolution imaging of the up to some 20 kW/m2. In contrast, where excessive heat can be dissi- ­entire surface, while the Mercury the MMO will be better off: it will pated. At the time of writing, Mes- Magnetospheric Orbiter (MMO) is reach the planet’s hot inner zone senger continues to be operational, intended to enter a wide elliptical only on its periapsis, while toward and is delivering fascinating science orbit. During launch and the jour- its apoapsis it can radiate off some of data. ney to Mercury, the two spacecraft the heat received close to the planet.

Mission Objectives

The dual BepiColombo spacecraft is intended to further our insight in a range of directions: It is to inves- tigate the origin and evolution of a planet close to the Sun. Other re- search directions call for the study of Mercury as a planet, its form, in- terior structure, geology, composi- tion and craters, and the composi- Fig. 18: The joint ESA/JAXA BepiColombo mission in cruise configuration. tion and origins of its polar deposits. It consists of a stack of four units that travel together to Mercury. The Mercury Plan- The Magnetospheric Orbiter will etary Orbiter is an ESA contribution intended for exploring Mercury on a low orbit. probe the planet’s magnetic field The Mercury Magnetospheric Orbiter is a JAXA contribution aiming at probing and its origins, as well as the struc- ­Mercury’s magnetosphere. The Mercury Transfer Module will provide the appro- priate braking during the entire flight by means of solar electric propulsion, a tech- ture and dynamics of the magneto- nology demonstrated earlier on ESA’s SMART-1 mission. (Credit: ESA) sphere. Mercury’s vestigial atmos-

SPATIUM 29 13 phere will be an objective for both do two identical masses really have Swiss Contributions parts of the tandem spacecraft. double the effect as compared to one single mass? BepiColombo is a cornerstone mis- Beyond planetary sciences, the mis- sion in the Agency’s Horizon 2000 sion also offers opportunities for Last, but not least, an orbit around programme, and as such also an im- fundamental physics experiments: Mercury also provides a vantage portant endeavour for the Swiss as Mercury is far away from disturb- ­position from which to observe the space community. The Planetary ing bodies, such as the asteroid belt solar system from a different and Orbiter spacecraft features an ex- and the giant planets Jupiter and most attractive perspective, as it tremely lightweight composite Saturn, it is also intended to per- ­allows to have the Sun in the back structure based on an aluminium form tests of Einstein’s general and the objects illuminated frontally. honeycomb core and plastic face ­theory of relativity. Scientists do not Of special interest in this respect are sheets designed and manufactured exclude the possibility that Bepi- small bodies with a size in the order in Switzerland. The structure con- Colombo could yield results that of 100 m, which are difficult to ob- tains heat pipes to dissipate exces- might require amendments to Ein- serve from Earth in the inner solar sive heat from the spacecraft out to stein’s relativity theory which for system. Such bodies are called vol- space. A further critical Swiss made science would be nothing less than canoids. They are made of pristine subsystem is the Solar Array Drive a full-blown revolution. Another material, which may hold further Mechanism, controlling the appro- fundamental physics issue pertains clues to the evolution of the solar priate position of the solar arrays to to the old question of whether system. In addition, Bepi­Colombo generate sufficient electrical energy gravitational mass is ­really equiva- will also provide a unique opportu- while protecting them from lent to inertial mass, which is still nity to search for Near Earth Ob- overheating. assumed to be the case without, jects (NEO’s) that eventually may however, knowing exactly why. A cross the Earth’s orbit posing a seri- Switzerland is also playing a prom- third topic amongst the mission’s ous danger of a catastrophic colli- inent role on the level of scientific many objectives is testing the valid- sion with our home planet. instruments for the Planetary ity of the superposition principle: Orbiter:

Fig. 19: The BepiColombo Laser tion on the planetary surface it reaches nated point on the surface can be calcu- Altimeter (BELA): On the left an the large receiver entrance on the left. lated. The electronics are contained in the ­engineering drawing shows the entire The time lag between the laser pulse and boxes behind the laser system. The right system. The laser is mounted in the fore- the received signal provides precise in­ image depicts the optical elements of the ground. The laser pulse leaves the system formation on the distance to the surface engineering model. (Credit: University through the small aperture. Upon reflec- from which the elevation of the illumi- of Bern)

SPATIUM 29 14 – The BepiColombo Laser Alti­ with Prof. Tilman Spohn of the highly accurate elevation map of meter (BELA), (Fig. 19). For this Deutsche Luft- und Raumfahrt, the entire surface of Mercury. Var- instrument PRO ISSI’s current Berlin. BELA is a joint effort be- ious partners of the Swiss space President, Prof. Nick Thomas, acts tween Switzerland and Germany community are currently involved as Principal Investigator together intended to produce a detailed, in designing and building subsys- tems for the BELA instrument.

– The Search for Exosphere Refilling and Emitted Neutral Abundances (SERENA), Fig. 20, is a mass spec- trometer, for which the Univer- sity of Bern contributes to the Start from a Rotating Field mass spectrometer (STROFIO) instru- ment under the responsibility of Prof. Peter Wurz. STROFIO has been designed to determine the chemical composition of Mercu- ry’s exosphere (to derive the sur- face composition), providing a unique tool to study the planet’s geological history.

Outlook

Missions of the size and complex- ity of BepiColombo are always joint endeavours involving a large num- ber of scientific­ , industrial and man- agerial partners. Beyond the fasci- nation space research has per se, inter­national and interdisciplinary co-operation provides an extremely rewarding field for space scientists and engineers. The only drop of bit- terness is the long wait required un- Fig. 20: The STROFIO instrument is part of the SERENA package. It is a mass spectrograph that determines the particle mass-per-charge ratio by the time-of-flight til the spacecraft reaches its distant technique. (Credit: University of Bern) destination.

SPATIUM 29 15 SPATIUM

The Author

Peter Wurz was Argonne was fullerene molecules for the detection of neutral ener- born in Vienna, (for example the famous C60), getic atoms. The first implementa- Austria, in 1961. which were discovered just at that tion of this technology was IMAGE, Early on he time. Since the topic was new lots a NASA mission to investigate the showed a high in- of information needed to be re- magnetosphere of the Earth. ESA terest in mathe- searched: the efficient production missions to Mars and Venus were the matics and tech- of these molecules, the separation next where this technology was nical subjects and of individual fullerene species, the used. All these missions returned so it was natural that he obtained an spectrometric characterisation, and exciting data on the interaction of engineering degree in electronics. synthesis of new compounds, for the solar wind with planetary at- However, working as an engineer example superconducting rubidium mospheres. The next to come is the he soon discovered that there had and potassium fullerenes, and crys- BepiColombo mission of ESA for to be more so he started to study tals, for example placing an alkaline- which the instrumentation is cur- physics at the Technical University earth atom inside a fullerene cage. rently being developed in his labo- of Vienna. He did his diploma in ratory. Also, in preparation for fu- the field of solid-state physics In fall 1992 he became research as- ture investigations, he has done ­investigating lattice defects in alkali sociate at the Physics Institute of theoretical studies on Mercury’s ex- halides and alkali-earth halide crys- the University of Bern in the solar osphere and ­surface, on the lunar tals. He continued his stay at the physics group. The field of space sci- exosphere, on Mars and Titan’s at- Technical University of Vienna with ence was new to him, but the com- mospheres, and a few more. a doctoral thesis in the field of sur- mon theme was again mass spec- face physics; he was asked to build trometry. Solar research was at a In many areas of space research a highly sensitive mass spectrome- high at that time in the Bernese Swiss scientists are at the forefront, ter for the analysis of trace ­elements group because of the SOHO mis- for instance in the development of on surfaces, which he then used to sion of ESA with the CELIAS in- highly sophisticated instrumenta- study metal alloys. During this work strument to measure the chemical tion in Swiss research institutions. he had the opportunity to spend composition of the solar wind From a small country, and a small time for research at the University (principal investigator D. Hovestadt, institute, it is difficult to become an of Tennessee, Nashville, USA, and Max-Planck-Institut, Garching, appreciated partner by big players at Stanford, California, USA. Germany,and later P. Bochsler, Uni- like NASA. Given the good support versity of Bern, Switzerland). He in Switzerland collaborations with After his doctorate he went to Ar- studied the abundance of several many space agencies have been gonne National Laboratory, Chi- minor ions in the solar wind, and in made possible to him: with the cago, USA, to hold a post-doctoral transient events, the so-called coro- American and ­European space position in the Chemistry division. nal mass ejections. This work al- agencies (NASA and ESA), the To move from surface physics to lowed him to obtain the habilitation Russian space agency (ROSKO- chemistry might seem to be a large at the University of Bern in 1999. MOS), the Indian (ISRO) and the step, but the common theme was Japanese (JAXA). mass spectrometry at a high level. In parallel to the work in solar phys- The research topic in the group at ics, he developed a new technology