The Board of Dlrec:tolll The naming of comets can, indeed, be a very difficult matter. Traditionally these small, CARL SAGAN BRUCE MURRAY President Vice President icy solar system bodies were named for their discoverers. But because some people are Director" Laboratory Professor of Planetary very persistent (for example, there are four Comets Meier) a particular name is needed for Planetary Studies. Science, California Camell University Institute of Technology for each individu.al comet. Thus, at discovery a comet is assigned a letter designation LOUIS FRIEDMAN HENRY TANNER based on the order of discovery or recovery in a certain year. So, Comet 1982i was the Executive Director Corporate Secretary and 9th comet found in 1982. Later, comets are assigned new names based on their peri­ Assistant Treasurer, Cafifom;a THOMAS O. PAINE Institute of Technology helion (closest approach to the Sun). 1984 XXll1 was the 23rd comet to pass perihelion Former Administrator. NASA: Chairman, National JOSEPH RYAN in 1984. Confused? Here is a poetic attempt to explain. Commission on Space O'Melveny & Myers Board of Advlsolll DIANE ACKERMAN GARRY E. HUNT poet and author Space -Scientist, THE NAMING OF COMETS (With apologies to T. S. Eliot) United Kingdom ISAAC ASIMOV aulhor HANS MARK BY DAVID H. LEW Chancellor, RICHARD BERENDZEN University of Texas System Presid8nt, American University JAMES MICHENER The naming of Comets is a difficult matter, JACQUES BLAMONT author Chief Scien#st, Centre National It isn't just one of your holiday games; d'Etudes Spatlales, France PHILIP MORRISON Institute Professor, You may think at first I'm mad as a hatter RAY BRADBURY Massachusetts poet and author Institute of Technofogy When I tell you, a comet has THREE DIFFERENT NAMES. ARTHUR C. CLARKE PAUL NEWMAN First of all, there's the name that the family use daily, author actor Such as Whipple, Wilk-Peltier, Wirtanen or Woif, CORNELIS DE JAGER BERNARD M. OLIVER Such as Hubble or Humason, Honda, PlHalley- Professor of Space Research, Chief, SETI Program, The Astronomical Institute at NASA/Ames Research Center All of them sensible everyday names, Utrecht, The Netherlands There are fancier names if you think they sound sweeter, FRANK DRAKE SALLY RIDE Dean. Division of Natural astronaut Some for the gentlemen, some for the dames: Sciences. University of ROALD Z. SAGDEEV Such as Grigg-Skjellerup, de Kock-Paraskevopoulos, Celilornia at Santa Cruz Director. Institute for Cosmic Research, Academy Schwassmann-Wachmann, Herschel-Rigollet, Tsuchinshan 1, LEE A DUBRIDGE of Sciences of the USSR former presidential Churyumov-Solodovnikov, Bappu-Bok-Newkirk- science advisor HARRISON H. SCHMITT former US Senator, NM, But all of them sensible everyday names. JOHN GARDNER and former astronaut founder, Common Cause But I tell you, a comet needs a name that's particular, LEWIS THOMAS THEODORE M. HESBURGH Chancelfor, Memorial Sloan A name that's peculiar, and more dignified, President. K.effering Cancer Center UniverSity of Notre Dame Else how can he keep up his tail antisolar, JAMES VAN ALLEN SHIRLEY M, HUFSTEDLER Professor of Physics, Or spread out emissions, or cherish his pride? educator and jurist University of Iowa Of names of this kind, I can give you a score. The Plenetary Report (ISSN 0736-3680) Is published six 191Oa, '84u, '86b, and such, times yearly at the editorial offices of The Planetary Society, 65 North Catalina Ave., Pasadena, CA 91 106 Or '6Sf, '66b, '83d - there's more- Editor, CHARLENE M. ANDERSON; Names that never belong to more than one comet. Technical Editor, JAMES D. BURKE; Guest Technical Editor, JURGEN' RAHE; But above and beyond there's still one name left over, Associate Editor, LYNDINE McAFEE; And that is the name that at first you can't guess; Assistant Editor, DONNA STEVENS; The name that no human research can discover­ Art Director, BARBARA SMITH Until long after the comet's come and it's gone, Viewpoints expressed in columns or editorials are those of the authors and do not necessarily represent positions of The Planetary Like Nineteen hundred fifty-nine-X- Society, its officers or advisors. C> 1987 by The Planetary Society. In Canada. Second Class Mall Registration NLmber 9597 But the COMET HIMSELF KNOWS, and won't now confess. When you notice a comet in profound meditation, COVER: With Images like thl. one, scientists wete The reason, I tell you, is always the same: at last able to tBlre a close look at the nucleus of Halley's Comet. Here sunlight Is coming from the His mind is engaged in rapt contemplation left, hestJng the Icy body so thst dust IItId gss jets Of the thought, of the thought, of the thought of his name; erupt from Its surface. NeIIr the top are bright spots, His rotational, orbital and below them Is a crater-like festure about 1.5 IcJlometers In dl.meter. This could be a slump feB. Coma-morphological, ture formed by the outgaSSing of nesmy Jets. Below Deep and inscrutable singular Name, and right of the "creter H Is a bright festure about ? . 400 meters high that some scientists call "the David Levy is a member of the International Halley Watch Near Nucleus-iStudies Network mountsln. H I",. t8ken by /he HIIlley Multlcolor CarnenI © 1986 Max Planck InstIIut filr A.ronomle, UndllfllHttrz, in Tucson, Arizona. In January he discovered his second comet, Levy I987A, whose name, F.R.G., provided by H,J. ReItsem/J ttnd w,A. Del.",.,.., B.IIA~ unfortunately, did not fit into the meter of this poem, ·...... Studying Halley's . ) • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • fr"m Earth and from Space • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ~ • • • • • • • • • • • • • • • • t BY JURGEN RAHE

OR CENIURlES, COMETS HAVE BEEN and international science groups endorsed France, Germany and North America, and studied by scientists living and the IHW, the American and West German used these measurements to prove that Fworking in many different countries space agencies agreed to fund it and two comets move in orbits around the Sun and and cooperation has been an essential Lead Centers were established, the Western that they are independent members of the part of their science. Because comets ap­ Hemisphere Office in Pasadena, California solar system. pear only briefly in our sky and because and the Eastern Hemisphere Office in In 1835 F. W. Bessel's extensive observa­ Earth never stops turning, no one scientist Bamberg, Federal Republic of Germany. tions of Halley's Comet clearly showed or observatory can hope to make compre­ The International Halley Watch follows that single observations provided only a hensive observations of any comet. And a long tradition of cooperation across na­ snapshot of the comet and that, for most not all places on Earth are equally good tional boundaries, beginning in the 16th studies, obseJVing sequences were needed. for watching a comet. In the case of Hal­ century. At that time, scientists generally Realizing this, the Astronomical and Astro­ ley's Comet, as it nears the Sun and glows accepted Aristotle's idea that comets were physical Society of America (now the Amer­ its brightest, it swings "south" of Earth, poisonous vapors in Earth's atmosphere. ican Astronomical Society) in 1909 created below the ecliptic (the plane defined by Then, in 1577, Tycho Brahe combined a special Comet Committee to prepare a the planets' orbits) and so appears highest careful position measurements he had photographic history of Halley's Comet: in the southern skies. But on its way in made from his observatory on the island "The ends to be served by these photo­ and out of the inner solar system, it of Hven near Copenhagen with those ob­ graphs and similar ones obtained else­ crosses the ecliptic, and so is best seen tained by Hagecius in Prague. With these where are conceived by the Committee as from the northern hemisphere. widely spaced measurements, he deter­ follows: to give a permanent record, as So, if scientists were to obtain the best mined that the comet displayed a very continuous as possible, of the phenomena possible coverage of Halley's Comet during small parallax. (That is, its position against and changes (0 in the tail of the comet, its 1985-86 apparition, we would have to the star background differed only slightly with special reference to outgoing masses; organize and set up a network of observ­ in the two observations.) He concluded (i0 in the head and nucleus of the comet, atories and observers distributed as un­ that the comet had to be at least 240 Earth particularly as to the formation of envelopes iformly as possible around the world. And radii away - four times farther from Earth and jets." since several countries were planning to than the Moon, and so well beyond the The committee had much in its favor­ send spacecraft to meet the comet, their atmosphere. thorough organization and well-defined observations should be coordinated with By combining observations made in dif­ goals - but it lacked several important in­ those from Earth. ferent countries, Brahe clearly demonstrated gredients: Although it needed and wanted In 1979, Louis D. Friedman, then at that comets move in interplanetary space cooperation from observatories all over the NASA's Jet Propulsion Laboratory and now beyond Earth's orbit. While he did not de­ world, it did not have international mem­ Executive Director of The Planetary Soci­ scribe correctly the orbits of the planets and bership; in addition, it did not have the ety, proposed a worldwide organization of the comet, his measurements profoundly cooperation of participating observatories professional and amateur astronomers, changed the scientific view of comets. in forwarding the material obtained to the called the International Halley Watch About a century later, in 1687, Isaac committee. Nor did it have adequate fund­ OHW), to coordinate observations of the Newton described the motion of a comet ing and staffing to follow up the scientific comet over the entire apparition. With this in Book II of his Philosophiae Naturalis research that it had inspired. Some of the effort, researchers could document the Principia Mathematica. Newton collected problems are illustrated by this statement global properties and evolution of Halley's observations of the Great Comet of 1680 from the committee's 1915 report: Comet in the inner solar system. National made in different locations in England, "Subsequent developments have made it seem inexpedient to cany out the program comet, the data already acquired are being ground-based observations. Agreements to above outlined. The photographs obtained reduced and prepared for inclusion in the exchange ground-based and space data in at the lick Observatory and at Cordoba are Comet Halley Archive, which will eventu­ real time and to include all ground and so numerous and excellent that they must ally hold all ground- and space-gathered space measurements in the Comet Halley have constituted a large part of the mate­ data from the 1986 apparition. Archive are results of the cooperation be­ rial reproduced and, since these observ­ From the formation of the IHW, we plan­ tween the IHW and the IACG. atories have indicated a purpose to repro­ ned to supply the Vega and Giotto missions The Pathfinder project is an important duce their own photographs and a similar with data to help them target their space­ example of the international cooperation policy seems to be contemplated else­ craft. We also wanted to make sure that triggered by Halley's Comet. Targeting a wher'e, the Committee deems it unwise to the brief, close-up spacecraft "snapshots" spacecraft to a comet's nucleus is a dif­ undertake a duplicate publication, and of Halley's Comet can be placed into the ficult problem, since the nucleus is hidden equally unwise to make one from which context with the long-duration Earth-orbital by the gas and dust of the coma. Earth­ this material is omitted." and ground-based observations. based IHW observations gave the nucleus' Systematic, worldwide networks have, Three space agencies - the European position with an accuracy of about 500 however, been established and operated Space Agency (ESA), the Soviet-led consor­ kilometers. After the Vegas encountered successfully. Examples are the Carte du Ciel tium Intercosmos and the Japanese Institute the comet, Soviet scientists were able to (Chart of the Sky) Committee, established of Space and Astronautical Science (ISAS) provide ESA with an improved nucleus in the last century or the networks set up - sent spacecraft to Halley's Comet. ESA position so that Giotto could fly within 600 in the last decade which organized obser­ launched Giotto, Intercosmos sent Vegas 1 kilOmeters of the nucleus - successfully vations of an entire rotation of Mars at two­ and 2 and Japan sent Suisei and Sakigake. targeted by spacecraft controllers 144 mil­ hour intervals, with nearly hourly coverage Their flyby distances ranged from 600 lion kilometers away. NASA's Deep Space of the great 1971 dust storm on Mars dur­ kilometers for Giotto to many hundred Network (DSN) supported this effort by ing the Mariner 9 mission. thousand kilometers for Sakigake. And, tracking the Vegas with Very-Long­ Against this background, the International after its encounter with Comet Giacobini­ Baseline Interferometry (VLBI). Ground­ Halley Watch has became the largest inter­ Zinner, NASA's International Cometary based astronomers and space scientists national cooperative program ever under­ Explorer aCE) moved to the vicinity of Hal­ collaborated on this successful endeavor, in­ taken by astronomers. Its scientific objec­ ley's Comet and made observations from dependent of political or language barriers. tives are: to characterize the structure, 30 million kilometers away. When Halley's Comet returned in 1985- physical processes and chemical nature of Several scientific experiments on board 86, scientists around the world were ready the comet's nucleus, atmosphere and tails; these spacecraft complemented each other, for it. The comet was studied from the and to determine the changes that occur while others overlapped. For example, the ground, from Earth orbit, from Venus orbit, over time and as the comet moves through Heidelberg dust particle analyzer was in­ from interplanetary space and from within the solar system. stalled on board the two Vegas and Giotto, the comet itself. This unparalleled cooper­ Internationally selected scientists, the allowing researchers to compare data taken ation triggered by Halley's Comet can Discipline Specialists, organized worldwide at different times. And, having several space­ serve as a model for future international networks of over 1,000 professional astron­ craft passing through the cometary environ­ cooperative programs. omers in 51 countries to assure that Halley's ment meant that we could make measure­ In this special issue of The Planetary Comet was continually monitored by every ments over an extended time. Report we examine the results of this un­ possible technique and at all wavelengths To coordinate planning and data analysis precedented study. D. Asoka Mendis sum­ accessible from the ground. This task re­ and to increase the scientific return, in the marizes the most recent findings as they quired complete longitudinal coverage fall of 1981 the four space agencies formed were presented in late October 1986 at an from around Earth. A network of thousands the Inter-Agency Consultative Group for international conference in Heidelberg, Fed­ of amateur astronomers also contributed Space Science QACG). The IACG coordi­ eral Republic of Germany. Mike Belton ad­ their observations. nated the space missions to Halley's Comet, dresses some of the most fascinating ques­ As scientists continue to observe the similary to the way the IHW coordinated the tions asked at that conference - and which astronomers are still asking: How does the nucleus rotate? Does it rotate in 2 or 7 days? Does it wobble more than rotate? Several cometary experts give us their opinions on the question ''What is a comet's nucleus?" Lou Friedman recounts some of the more amusing reactions to this latest apparition of Halley's Comet. Marcia Neugebauer tells us about the implications of the Halley's Comet missions for NASA's proposed Comet Rendezvous Asteroid Flyby (CRAF) mission. These articles, and many in other publi­ cations, demonstrate that a new era in cometary science has begun. The flyby missions and the Earth-based studies are only the beginning. When CRAF sends its first measurements back to Earth, we ex­ pect to take another major step forward in our understanding of comets, the solar system and its evolution.

Jiirgen Rahe heads the International Halley Watch Eastern Lead Center. He is a co-investigator on Giotto s PIA dust experi­ ment and is now working as a planetary astronomer at NASA headquarters...... • . . .. The...... Science...... of...... •

.. . . A. . . . . Post-Encounter...... Assessment......

BY D. ASOKA MENDIS

N 1985 AND 1986, THE INHABITANTS reflects only two to four percent of the sun­ both by spacecraft and Earth-based obser­ of Earth sent spacecraft probes past two light falling on it. This makes it one of the vations. About 80 percent of the cometary Icomets - icy bodies from the outer darkest objects in the solar system. material detected by the spacecraft was solar system that may carry secrets about In 1952 Armand Delsemme and Pol water, with carbon monoxide the next the birth of our system. These missions to Swings significantly advanced our under­ most abundant ice at about 15 percent. Comets Giacobini-Zinner and Halley, coordi­ standing of the nucleus' chemical com­ When the comet appoaches the Sun and its nated with ground-based observations, have position by pointing out that, while water ices begin to sublime away, the produc­ greatly increased our knowledge of comets. is the dominant ice, all other ices, such as tion of this material varies, which is not Careful analyses of the data, with complex carbon monoxide, would be trapped within surprising since most of the nucleus' theoretical modeling, will lead to a much the water lattice as "guests," forming what activity, in throwing off dust and gas, is deeper and more secure understanding of is called a clathrate hydrate. The view that associated with discrete jets [see back these cosmogonically significant objects. water is the dominant ice is supported coverl. These jets erupt mainly on the sun- Yet we still have many questions, and much needs to be done to answer them. The Nucleus The dominant, but not universal, view that a comet's nucleus is an icy body is by no means new. It dates back at least to Pierre Laplace in the early 19th century. However, about 1860, the meteor showers that period­ ically streak the night sky were associated with the ancient tracks of individual com­ ets. This idea had such impact that comets became regarded as the densest parts of meteor streams. The model of a comet as a "flying gravel bank" held sway for almost a century. Then, in 1950, in a seminal paper, Fred Whipple resurrected the idea of a discrete cometary nucleus, with his "icy conglom­ erate" model wherein the nucleus is an aggregate of meteoritic dust and ices, such as water (H20), carbon dioxide (CO~, car­ bon monoxide (CO), methane (CH4) and ammonia (NH:J . Whipple did not merely resurrect Laplace's early idea, he modified, and most important, quantified it so that it not only explained the dynamical effects on comets' orbits from things such as erupting gas jets, but also described the essential features of all cometary observa­ tions. Whipple's theory became the basis of all later work on the dynamics, physics and chemistry of comets. Thus, he put the subject, which had begun to lose its way over a century ago, firmly back on track. So, when Vegas 1 and 2 and Giotto de­ tected an icy nucleus in Halley's Comet, it was no surprise. We expected it to have an irregular shape; it was variously described as a potato, peanut or avocado. It is larger than anticipated, with a volume of some 500 cubic kilometers. Halley's nucleus is vel)' dark - as black as velvet - and ward side of the spinning nucleus and re­ variations seen from Earth. duce or cease activity on its "nightside." However, a period of 7.3 days was Whipple had anticipated that the nuc­ claimed by researchers using two entirely leus would be covered by a dust mantle. If different observations, one based on the RADICALS IN SPACE the mantle were present, the surface tem­ comet's brightness fluctuations and the perature would be around 300 degrees other based on the extended gaseous radi­ n chemistry, a free radical is not someone Kelvin (27 degrees Celsius) at about one cals CN and C2 (see box) in spiral jets exercising constitutional rights. Instead Astronomical Unit from the Sun (an As­ observed by Mike A'Hearn and several col­ Ithe term refers to fragments of molecules, tronomical Unit is the average distance laborators. The authors believe these sub­ such as those that appear when an ultraviolet from Earth to the Sun, about 150 million stances may be released directly from fine photon hits a molecule and breaks it apart. kilometers). The infrared spectrometer on submicron-sized grains, too small to be ob­ Molecules do not like this and they try to re­ Vega 2 measured a surface temperature served optically. (For a further discussion combine or else dissociate farther, into even between 300 and 400 degrees Kelvin (27 of the puzzle of the nucleus' rotation, see smaller bits. and 127 degrees Celsius). While warm dust pages 8-10.) As a result, free radicals are highly reac­ around the nucleus could have contami­ tive and normally do not live long in the lab­ nated this measurement, the temperature Cometary Dust oratory. In Earth's upper atmosphere, how­ of an evaporating surface of dirty ice at this The study of cornets' dust tails has a long ever, or in the tenuous coma of a comet, the distance would be less than 190 degrees history, dating back about 150 years. To molecules are so far apart that long times Kelvin ( - 83 degrees Celsius) . It is therefore explain the tail of Halley's Comet in its may elapse before they can find a partner and 1835 apparition, in 1836 F. W. Bessel recombine. Meanwhile, sunlight and solar derived equations for the motion of dust charged particles continue to bombard the particles emitted from the nucleus and dri­ thin gas, maintaining an equilibrium popula­ TABLE 1: Chemical species identified in ven away from the Sun by some repulsive tion offree radicals. cometary spectra before the encounter force. Much later the postulated force was This is why comet spectroscopists observe (tentative identifications in parenthesis). recognized as the radiation pressure of a zoo of strange molecules, some electrically sunlight. neutral and some bearing positive charges, Atoms Molecules Ions Two questions about cometary dust con­ out in the coma. Using theoretical models, H C2 C+ cern its chemical composition and physi­ they try to determine what were the "parents" 0 12C13C Ca+ cal structure, namely its size distribution, of these molecular fragments and thus to C CH CO+ shape, bulk density and light-reflecting determine the chemical composition of the S CN CH+ properties. Before the Halley's Comet nucleus, whose material is thought to repre­ Na CO CN+ encounters, information about dust com­ sent the primordial matter of the solar system. K CS N2 + position came mainly from the presence of But a comet's coma is a very complicated object. The molecules of ice do not just NH CO + broad emission bands in the infrared spec­ Ca 2 evaporate and split into free radicals. As the V OH H2O+ trum. Data on the dust's structure came from various lines of investigation, such as gases fizzle out of the nucleus, they carry dust Mn C3 H2S+ studies of dust in the tail, and optical and grains with them. An exciting discovery of Fe NH2 the spacecraft encounters with Halley's Co (H O) infrared data. Two less direct sources of 2 information were meteor showers and Comet is that these dust grains themselves Ni HCN may contain (or be coated with) ices whose Cu CH CN Brownlee particles collected in Earth's 3 stratosphere, which some believe to be of molecules contribute to the gaseous zoo. And 52 cometary origin. the whole picture is further complicated by HCO electromagnetic interactions with the fast­ The dust composition analyzers on the NH3 flowing plasma of the solar wind, to which Vegas indicated at least three classes of (H CO) the comet's coma is a large but tenuous 2 grains: one of compounds of light elements (NH4) obstacle. such as carbon, hydrogen, oxygen and nitro­ In the accompanying article, we are given gen (CHON particles), a second similar to a first glimpse of what spacecraft have now certain meteorites enriched in carbon, and told us about these phenomena. Scientists will a .third similar to the second but more en­ have happy and productive years ahead as reasonable to assume that what we see is riched in hydrogen. they try to untangle the meanings of these not a bare, icy nucleus, but a surface layer The small, submicron-sized CHON grains data from humanity's first brief encounters of dark, warm dust. may be the "parents" of the CN and C2 jets with Comets Giacobini-Zinner and Halley. seen around the comet. These observa­ - JAMES D. BURKE Rotation Period tions indicate that the "parents" of some of Some photos of Halley's Comet taken in the observed radicals need not be in the 1910, when treated by modem image pro­ gas phase, but could be dust grains. This cessing, show spiral jets emerging from new concept has emerged since the Hal­ the nucleus. Recently Zdenek Sekanina ley's Comet encounters. the question arises "What causes this and Steve Larson used these jets to deter­ Dust detectors on the Vegas and Giotto breakup?" Do the aggregates' come apart mine the nucleus' rotation period (52 found large quantities of very small parti­ due to the sublimation of their icy glue? Or hours) and spin axis. Assuming that these cles, below 0.1 micron in size. The distri­ are they electrostatically disrupted? spiral jets arise from active regions that bution of the various sizes of particles was While the spacecraft provided us with turn on at sunrise and off at sunset, very interesting. The smallest particles much new information about the dust Sekanina and Larson have also mapped were detected much farther away than we particles, including their elemental abun­ these regions on the comet's surface. The expected. Scientists have suggested sev­ dances and their size and spatial distribu­ locations of many of the dust jets seen by eral possibilities for this anomalous spatial tions, these data are complemented by the Vegas reasonably support their predic­ distribution. One is that the grains have Earth-based observations. The infrared tions. Sekanina and Larson's deduced spin very unusual electrical properties, another observations of the Kuiper Airborne Obser­ period is also supported both by the Vega is that larger grains leaving the nucleus get vatory (KAO) tell us about the large dust 1 and 2 observations (53 ±0.5 hours) and broken up and finally, these electrically grains. On the basis of infrared maps, by the Suisei images showing a period of charged grains are accelerated in the mag­ some have already claimed that Halley's 52 to 53 hours. This spin period observa­ netized plasma environment. If larger Comet released fewer large particles than 6 tion is further buttressed by brightness grains are broken up far from the nucleus, Comet Giacobini-Zinner. Gas: Neutrals and Ions When Halley's Comet appeared in 1910, comet spectroscopy (the science of detect­ ing atoms and molecules by the colors of light that they emit or absorb) was still in its infancy. Even so, several chemical species, both neutral and ionized (CH, C2, C3, CN, Na, CO+ and N2 +), were identified in the comet's atmosphere and plasma tail. Advances In spectrophotometric tech­ niques, and the extension of the spectral range into the ultraviolet, infrared and radio regions, have enabled us to detect many more chemical species in later com­ ets. (See Table 1 on page 6.) At present, the study of cometary atmos­ pheres is the only way to tell the chemical composition of the icy component of the nucleus. However, most of the observed species in Table 1 are radicals, that is, chemically reactive fragments of presuma­ bly more stable molecules once stored in the nucleus. One major discovery of the ion mass spectrometers aboard the International Cometary Explorer (ICE) at Giacobini-Zin­ ner and the Vegas and Giotto at Halley's Comet was the detection of H30 + as pre­ dicted by theoretical studies. This estab­ lished the general validity of atmosphere! ionosphere models. The neutral and ion mass spectrometers solar wind interactions observed by the some old questions as well as new ones on Giotto and the Vegas, as well as the spacecraft at Comets Giacobini-Zinner and that arose from the ICE, Vega and Giotto photon spectrometers on the Vegas, de­ Halley were very close to predictions. We missions. tected most species identified earlier, and were surprised, however, by the high level Scientists know that determining the a number of new ones. The ion mass and large extent of the turbulent motions chemical composition of a comet's nuc­ spectrometers clearly identified the domin­ in the plasma. Besides large variations of leus could lead to far-reaching inferences ant H30 +, H20+, OH+ and 0 + ions of the magnetic field strength and plasma about the primordial dusty nebula from the H20 group. A clear feature in the spec­ parameters over many minutes, a plethora which comets and the rest of the solar sys­ trum is almost certainly C +. A composite of plasma waves was observed. These have tem formed. Knowing a comet's internal list of other possible identifications in­ focused our attention on the microphysics structure, perhaps unchanged since birth, cludes CO2+, CO+, CS2+, S2+, CS +, S+ , of the comet-solar wind interaction. Clearly could help us understand that event. How­ CH +, Fe+ and Na+. Heavier, unidentified central to all this activity is the pick-up of ever, our spacecraft did not see the ions were also observed. The Giotto neu­ heavy cometary ions by the inflowing evaporating surface of Halley's Comet, but tral mass spectrometer also detected the solar wind. The distributions of the pick­ a dust mantle insulating the ices under­ dominant neutral species H20, 0 and OH, up ions have been measured in several neath. A future mission should sample a and probably also CO2, experiments on the various spacecraft. comet's subsurface material with a pene­ The role that dust played both in the As a result of all these considerations, trator, as in the proposed Comet Rendez­ dynamics and the thermodynamics of the the theoretical modeling of the comet­ vous Asteroid Flyby (CRAF) mission (see atmosphere was appreciated before the solar wind interaction is moving away pages 16-17). return of Halley's Comet. But the important from the fluid approach to a more kinetic We have observed the solar wind interac­ role it also plays in the atmospheric chem­ one (that is, dealing with free particles). tion with a well-developed atmosphere istry was overlooked. Some dust grains While partial models are being developed to because the comets were fairly close to may be the long-sought parents for some of explain various aspects, we are still very the Sun. Comets produce a whole range of the observed molecular fragments. Clearly far from a complete kinetic description of atmospheres, from a fledgling one when we will have to consider this in future the comet-solar wind interaction. they are out beyond Mars' orbit, to a dense atmospheric modeling. one when they are close to the Sun. CRAF Wrapping Up would enable us to study the interaction of Solar-Wind Interaction The missions to Comets Giacobini-Zinner the solar wind with the entire range of Since 1951, when Ludwig Biermann used and Halley, with the coordinated Earth­ atmospheres, as well as with the bare nuc­ observations of comets' plasma tails to based studies, have greatly advanced our leus, which may cause interesting effects infer the continuous outflow of plasma knowledge of comets. We are only begin­ such as electrostatic levitation and blow­ from the Sun (now called the solar wind), ning to appreciate the implications of all off of fine surface dust. Halley's Comet comets have been used to delineate the the new discoveries. This will, in time, has given up some of its mysteries, but it flow of the solar wind. Dramatic changes lead to a much deeper understanding of and other comets still hold many for us in the plasma tail, such as its sudden, total comets and their interaction with solar to explore. disconnection from the comet's head, radiation and the solar wind. D. Asoka Mendis is a professor in the de­ have recently been associated with discon­ Much remains to be done. While we con­ partment of electrical engineering and tinuous changes in the solar wind as it tinue to analyze the existing data and build computer science at the University of flows past the comet. (See the May/June more elaborate intellectual models to ex­ California at San Diego. He specializes in 1986 Planetary Report.) plain them, we should not forget the need the study of solar system physics, and The large-scale structures of the comet- for more cometary missions to answer particularly, in cometary physics. 7 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • The Wobbling Nucleus • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

~f Halley's BY MICHAEL J. s. BELTON ·......

N THE FNE YEARS SINCE HALLEY'S be regular or "pure." If it spins about structure. Energy is dissipated through fric­ Comet was spotted on its way back to some other axis in the object, it will have tion and escapes to space as radiated heat. Ithe inner solar system, scientists have more energy in the rotational motion and Electrical currents may be induced in the collected a rich and varied set of high generally (but not always) the spin axis comet as it spins in the interplanetary quality data covering every aspect of the will wobble in a complex way, that is, it magnetic field; this too can lead to the dis­ comet's behavior. We have made many is said to nutate. This is because the rota­ sipation of energy through internal heating. exciting discoveries, such as the nucleus' tional momentum of the object, fixed by the Long-term conservation of rotational unexpectedly large size and a new class of laws of physics, depends on both the spin momentum is equally difficult because cometary dust rich in organic material, axis and the distribution of mass in the ob­ there are, at each perihelion passage, called CHON (for carbon, hydrogen, oxy­ ject. As a result, the direction of the spin mechanical torques due to the escaping gen and nitrogen) particles. These dis­ axis is not always the same as the direc­ gas and dust. Gravitational forces, mainly coveries, almost instantaneously, led to tion of the rotational momentum and, due to the Sun, apply torques to the nuc­ major changes in our perception of what since the latter is fixed, the spin axis must leus and cause its spin axis to precess. comets are like and what they can tell us move around as the object rotates. However, these precessional motions are about nature. If, in addition, the object is not spinning very slow (with periods of tens of thousands Ultimately, insights from detailed scien­ freely and finds itself torqued by forces of years), and I doubt that they are impor­ tific analyses of the various data sets and that are coherent over time, the spin may tant in the case of Halley's nucleus. their subtle relationships will have the increase or decrease and the spin axis Finally, collisions with other objects in most profound effect. Such work will extract may precess, that is, describe a circular space, as Allan Harris and Joseph Burns whatever there is to be learned about the path around a direction associated with have shown, should quite effectively add fundamental chemical and physical pro­ the applied forces. or subtract to the energy and rotational cesses by which cometary nuclei evolve momentum of asteroids on these time­ and, perhaps more significant, the pro­ Obeying the Laws scales. This may also be true for the nuclei cesses that created them four or more bil­ Whatever the motion, the object has to of comets. lion years ago. obey all of the usual laws of physics and Detailed studies of the rotational charac­ Successful integration of ground and two of these - that the total energy of the teristics of many asteroids tell us that they space-based measurements is not simple system is conserved, and that, unless are in their lowest energy spin state, that and will require - mainly because of the acted on by external torques, such as jet is, they are spinning about their shortest unpredictability of the comet's activity­ reactions, the total rotational momentum axes. There are, with few exceptions, such precise knowledge of the rotation of the is conserved - will help us understand as asteroid 433 Eros, no indications of any comet's nucleus. Only when we have a what is going on. We begin by investigat­ complex wobbling or precession in their precise timetable, or ephemeris, for the il­ ing what the most likely state of rotation rotational motion. Asteroids apparently lumination of different parts of the comet's of the comet's nucleus might be. dissipate energy quickly enough (by the surface, can we expect to relate long-term Over times as short as a single orbit (75 activity of the comet's atmosphere, as seen years for Halley's Comet) the rotational from the ground, to active regions on the energy and rotational momentum will be comet's surface or in its atmosphere as little changed by interactions with its envi­ seen by Vega and Giotto. ronment. The energy is determined by the Now it has turned out that the rotational speed with which it spins and this must state of Halley's Comet's nucleus is proba­ remain constant. With this spin we can bly quite complicated, and possibly is associate a definite period - the length of similar to the complex motions seen in a a day on the comet. The polar, or spin, disturbed toy gyroscope or a child's whip­ axis may move around, but how it moves ping-top - gyrations which have fascinated and its relationship to a fixed direction in and confounded people for hundreds of space defined by the rotational momen­ years. This has quite abruptly become a tum will be described later. very interesting area of research. On astronomical timescales, say a Individual pictures of the nucleus, when hundred million years, it's impossible to they are combined, indicate that Halley's conserve the rotational energy, and it is Comet is shaped somewhat like an avocado transformed to other energy forms. A small (Vega experimenter Karoly Szego's favorite fraction may be lost at each passage analogy). The pictures also tell us that the through the inner solar system, as intense nucleus' length is roughly twice its width. sunlight heats the comet and sublimed When a rigid object with a compli­ gases and entrained dust flow off into cated shape spins freely in space, it can space. Other processes, investigated by execute some peculiar motions. If it planetary scientists interested in asteroids, spins about its shortest axis, it will be in are also at work: as the object spins it will 8 its lowest energy state and the spin will flex and periodically distort its internal ' ~ "

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internal processes described above) to ble may build up, through a random, or damp out any wobbling motion resulting "drunkard's" walk, to some sizable value. from collisions with other objects. Accord­ A quick calculation shows that, in the case ing to work done by Joseph Burns and of Halley, the spin axis may be disturbed V. Safronov this damping should occur by a modest few tenths of a degree each (for something the size of Halley's Comet) time. But after a thousand orbits (a mere in times as short as a few hundred 76,000 years for Halley's Comet), the net thousand years. disturbance to the direction of the spin The asteroids are not, however, as axis could be amplified by more than a fragile and active as comets. Active comets factor of ten. frequently form multiple nuclei, indicating Similarly, at every perihelion passage that small pieces have broken off the pri­ the cometary nucleus will experience an mary nucleus. When this happens, the incessant stream of short-lived (hours­ nucleus' mass distribution will instantane­ long) torques from the periodic flow of ously change and cause a small wobble. material from active regions. Again we For a single event this change will be may expect some randomness in these small, but because such events occur impulses and their accumulative effect quite often (perhaps every few orbits or may also add up slowly over time, espe­ tens to hundreds of years) there is no time cially if the nucleus is already wobbling. in between events to damp out the induced motion. After hundreds of orbits the wob- Nutating Nucleus With both of the above effects operating, it should come as no surprise that comet­ ary nuclei, particularly of active comets like Halley's, are nutating. Using pictures of the comet taken in 1910, Zdenek Sekanina and Steve Larson made a rough estimate for Halley's rota­ tion period (about 50 hours) well before no appreciable atmosphere and the light the Vega and Giotto encounters. Unfortu­ reflected from the rotating nucleus should nately, uncertainties in this determination, easily dominate the total brightness. which made use of the repetition of struc­ Glimpses of periodicity were noted in the tures in the comet's atmosphere, were un­ data, but none seemed to make much avoidably large and the results, elegant as sense. In three separate investigations dif­ they were, had to be viewed with consider­ ferent research teams failed to find a con­ able caution. vincing periodicity. In retrospect it now When the comet was recovered in Octo­ seems very evident why - the data were ber 1982, ground-based astronomers be­ too few and sparsely sampled to stand on gan work to improve the precision of this their own. In spite of tremendous efforts by estimate. It was thought that fluctuations the International Halley Watch to obtain in the comet's total brightness would show the observations, the deficiencies of the a periodic component. data set, caused by too little observing The prime time to do this work was time on large telescopes, overwhelmed the when the comet was still far from the Sun information content. (beyond the orbit of Jupiter), even though After the Vega and Giotto encounters, it would be very faint. It would then have spacecraft experimenters made their prelim- 9 much longer-some 7.37 days. This picture of the comet's rotational Millis and Schleicher very politely pro­ motion seems very attractive and mayex­ posed that the spacecraft experimenters plain most facts as they are now known. must have made a mistake in the way they Similar results have been announced by interpreted the encounter images and that other investigators, particularly Jack Lis­ 7.37 days was the true spin period. I be­ sauer, who was the first to apply Euler's lieve that this is incorrect. In my view, the dynamical equations to the problem - in most likely explanation is that the nucleus "real" time at the conference when Millis is in a state of nutation. That is, as it spins and Schleicher announced their results. it also wobbles rather badly. However, in my view, it seems unlikely that such a simple situation is the actual Wobbly Spin state of affairs. There is no good reason Let us see how we can explain the two why nature should distribute Halley's (possibly more) basic periods appearing Comet's mass exactly like that in a sym­ in Halley's Comet's brightness fluctua­ metric top . It is much more likely that the tions. If the comet's mass were distributed nucleus will behave like an asymmetric top. in a spherical ball, there would be no pre­ Since the pictures show that the shorter ferred direction and the spin axis would dimensions of the comet's nucleus are not always be parallel to the fixed direction too different, I have looked at the possibil­ defined by the rotational momentum. It ity that the comet may rotate as a slightly would have only one apparent period and asymmetric top. In this case a third period­ this would be the period of spin. icity should appear which has exactly half In the next more complicated situation, the Euler period, 3.7 days. As the spin axis Halley's Comet might have its mass distri­ rotates around the fixed axis, it will now buted like a football (or an avocado). In "nod" up and down with a period exactly this case, the mass is effectively distri­ half that of the apparent spin about the buted in the same way about two axes, but long axis. In addition, neither of the appar­ in a different way around the third longer ent periods discussed earlier will be con­ axis, the axis of symmetry .. stant but will rapidly modulate about their In physics we call this a symmetric top mean value with the same frequency as the inary estimates of the shape, size and rota­ and when it is in its most general state of "nodding." The amplitude of these effects tional period of the nucleus. It appeared motion, the outside observer (such as a is related (in a very complex way) to the that the spin axis was aligned roughly ground-based astronomer) would discern comet's internal mass distribution and parallel to the shortest dimension of the two periods: one related to an apparent would be of considerable interest to deter­ nucleus, giving confidence that the nuc­ motion of the object about its axis of sym­ mine. A periodicity near 3.7 days appears leus was in a low energy, pure spin, state metry, and one related to the apparent strongly in Millis' and Schleicher's data. of rotation. The dimensions and shape of motion of the object about a fixed direc­ They ascribe it to an artifact in the data, the nucleus also made it clear that, if it tion in space. Both apparent rotational but I believe it may originate in the "nod­ were visible in the distant brightness data, motions are composed of two parts: one ding" motion described above. then the brightness fluctuations should re­ associated with the nutation and the other The story of the motion of Halley's flect its rotation. with components of the spin itself. The Comet's nucleus is thus far from complete. Using these results, I went back to the first is sometimes referred to as the Euler It is a hot research topic and we will soon early ground-based brightness data and, in period after the scientist who first gave a learn much more. We hope to generate an spite of the data's deficiencies, retrieved satisfactory mathematical description of accurate ephemeris so that we can fully precise information on the spin rate. The the rotational motion of a rigid body, and understand the relationships between the period was either 53.96 or 54.125 hours­ I associate Millis and Schleicher's 7.37 day different kinds of ground-based and space­ it was impossible to be sure which - but result with this period. craft data. We also urgently need many the precision of each possibility was better Both of these apparent spin periods are more brightness observations, made with than two minutes - good enough to con­ constants of the motion; however neither is large telescopes, as Halley's Comet moves struct an accurate ephemeris for the rota­ equal to the "real" spin period, that is, the away from the Sun and becomes less active. tion. As a check, it was possible to show period of rotation about the spin axis itself. These should begin in the spring of 1987, that the brightness fluctuations had re­ We can, however, calculate Halley's when the comet once again moves beyond peated over a year and showed two max­ Comet's true period if we also know the 5 Astronomical Units (1 Astronomical Unit ima per full period, as would be expected nucleus' mass distribution. We can roughly equals the distance from Earth to the Sun, for an elongated object like Halley's estimate this from the nucleus' shape as about 150 million kilometers) and we can Comet. Based on the data, Vega 1 and seen in the spacecraft pictures. If the separate the light of the nucleus from that Giotto should have seen opposite ends of comet is indeed a symmetric top, then this reflected by the comet's atmosphere. the nucleus; while Vega 2 would have interpretation yields a "true" spin period of A great effort will be needed to coordinate seen the illuminated side broadside. Since about 49 hours, and the polar, or spin, these observations. The International Halley this was the case, many, including myself, axis will move so that it is inclined about Watch, already so successful in organizing believed (mistakenly) that the nucleus' 16 degrees to a fixed direction in space. It observations when the comet was active, spin state must be regular with no appreci­ will appear to trace out a cone as it moves. still has a challenge before it to see that the able nutation or precession. The long axis of the nucleus would make needed measurements are made. [f they are, But then a truly remarkable discovery an angle of about 78 degrees with the fixed we can look forward to having a vivid and was announced by Robert Millis and direction, and the component of spin (not correct mental image of Halley's dark, ob­ Douglas Schleicher of the Lowell Observa­ the apparent rotation) about the long axis long, pock-marked nucleus tumbling away tory in Arizona. Brightness data, which would have a period of about 106 hours. inexorably on its long path toward the/cold they obtained at the Cerro Tololo Inter­ This latter result is interesting because it outer reach of our solar system. American Observatory in Chile near the happens to be almost equal to the interval Michael 1. S. Belton is an astronomer at the Vega Giotto time of the spacecraft encounters and between the 2 and encoun­ Kitt Peak National Observatory in Tucson, under the best of circumstances, clearly ters. As a consequence, it may be hard to Arizona, specializing in ground-based obser­ showed a periodicity - but not with a find evidence of this component of the vations of comets and space-based observa­ 10 period near 54 hours. Their period was spin in the spacecraft images. tions of the outer planets. I ...... ·1

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Humans haye been watching comets for thousands of years, tracking the glowing apparitions across the sky and attempting to explain what caused the bright comas and the streaming tails. In 1950, Fred Whipple I proposed that a small, dusty, icy nucleus lies at a comet's heart, and that effects such as the coma and tails can all be explained using this model. But to test the model, the comet would haye to be seen up close. In 1986, three spacecraft were sent into Halley's Comet to photograph the nucleus and to help answer the question "What is it?" We asked a few of the world's leading comet experts to tackle that question for us. Here are their answers, along with the best close·ups of the nucleus taken by Intercosmos' Vegas and the European Space Agency's Giotto spacecraft.

THE HALLEY NUCLEUS IS THE HOLY GRAIL OF SOLAR system astronomy. Since the space program began nearly 30 years ago, one of its most basic goals has been to under­ stand how our planetary system formed and to find sam­ ples of primitive material from the solar nebula. The Halley nucleus is an icy planetesimal, a primitive agglomeration of dust, ice and gas that came together in the solar nebula 4.5 billion years ago. It is the sort of basic building block out of which the planets were made. The Vega and Giotto images are tremendously exciting because, for the first time, we can see something of what the nucleus is like, yet it seems that most decisive details are just beyond the resolution of the best images, or are obscured by the plumes of dust. What we can tell is that the nucleus is a very irregular object, not just because of its very extended, potato-like shape, but also because of the roughness we see along the limb and terminator (the boun­ dary between the sunlit and dark sides), and the 400-meter­ high mountain beyond the terminator (in the Giotto images) jutting up into the sunlight. It seems that this primordial dirty snowball may actually be an agglomeration of many smaller snowballs stuck together. Because Halley's Comet has been stored in the distant Oort cloud - far from the Sun's heat - for most of its lifetime, the nucleus has never been melted, never bringing all those smaller snow­ balls together into a single, well-consolidated body. The surface of the nucleus is divided into active and inactive areas, characterized by either bright dust jets driven by sublimating ices, or a dark crust of presumably car­ bonaceous materials. This tells us that the surface is not entirely primitive, and that it has evolved due to the comet's repeated passages close to the Sun. What we don't know is whether the pattern of active and inactive areas is constant or is constantly changing. Perhaps the most interesting feature is the apparent "crater" on the surface in the Giotto image. At highest reso­ lution, we can see that it is not a traditional impact crater. This matches our expectation, since Halley is a very small target and not likely to have many craters. Most probably, this feature is a dormant active area, the site of a previous outburst from ices buried beneath the carbonaceous crust. And as it rotates farther into the sunlight, it might again burst forth with a massive jet of dust and gas. Halley's Comet has given up some of its secrets in the Giotto and Vega images. But, as in all previous spacecraft missions, we have had some questions answered, and found many new questions to ask. Only future missions to comets, both rendezvous and sample return, will answer those questions. - PAUL WEISSMAN, Jet Propulsion Laboratory, Pasadena THIS IS A DISTURBED PLANETESIMAL - A SURVIVOR from days of planet formation. It is a planetesimal in this sense: It is one of the small bodies that formed in the solar nebula, and from which the planets were built. It is like an asteroid, except that it formed farther from the Sun than the main asteroid belt, and hence contained ice as well as rocky material. At its distance from the Sun, the rocky component had two sub­ components: ordinary silicate minerals similar to those that formed Earth and the Moon, and very black, carbon-rich minerals, similar to soot, spread among the ice and sili­ cates, making the whole assemblage very black. In the last few years, we have found that this black carbonaceous component, which also contains organic molecules (but not materials as advanced as living organisms!), is the main My NUCLEUS - BASED HALF ON FACTS, HALF ON coloring component of the outer solar system interplanetary imagination -looks like this: It is a very dark, irregularly bodies, which are all very dark. The black carbonaceous shaped object, with dimensions of 16 x 8 x 8 kilometers. stuff and the ices are native primarily to the outer solar sys­ As it was first revealed by the Vega spacecraft, its shape is tem, which was cold when the planets formed. close to that of an avocado, but more irregular. The surface It is disturbed in three senses: First, its original orbit was is probably covered by two types of material. The most disturbed when it passed close to one of the newly forming abundant - covering maybe two-thirds of the nucleus - giant planets, which kicked it to the outermost solar system is thick, impervious to gas and can be heated to 300-400 (the so-called Oort Cloud), just as the Voyager spacecraft degrees Kelvin (27-127 degrees Celsius) when the comet is were kicked outward when they passed close to Jupiter. as close to the Sun as our Earth. This surface is criss­ Second, it was once again gravitationally disturbed during crossed by cracks which are the sources of the jets. its long, Oort-c1oud orbits around the Sun, causing it to fall There is a big linear feature around the middle of the toward the Sun into the inner solar system, on the orbit we avocado, uniting many smaller jet sources. This was the see today. Third, it is physically disturbed. For perhaps only most important jet source during the Vega 2 encounter. the one hundredth or five hundredth time in its existence, There is limited but observable jet activity on the dark side. it is passing closer to the Sun than Earth, where solar heat The emitted material (IO tons per second) demonstrates a is sublimating the ices in the surface layers. As the ice tums cooler core temperature. Water vapor measurements from to gas and expands, it blows off particles of the carbona­ the Kuiper Airbome Observatory revealed a 30-40 degrees ceous and silicate dust, along with giant complex Kelvin ( - 243 to - 233 Celsius) source temperature. molecules and simpler molecules of gas. These materials The composition is similar to that of the solar system as form the tail and coma, and were sampled at close range a whole. The comet is surrounded by more very light - by Giotto and the Vegas. lighter than cigarette smoke - particles than were antici­ The object also is a victim of astronomical historical pated. Many of these dust particles are composed of only baggage: a Victorian conceptual dichotomy between carbon, hydrogen, oxygen and nitrogen (CHON particles). comets and asteroids, as if they were completely separate The density of the nucleus is very light, about 0.1-0.4 grams species. Probably we will have to recognize a range - or per cubic centimeter. The structure is very fluffy. spectrum - of planetesimals, from the rocky types left The nucleus' rotation period is about 53-54 hours, but stranded in the inner solar system and called asteroids, the rotation axis precesses in such a way that the Sun through moderately icy types, to very icy types that were illuminates a different part of the surface at each revolution. stored in the Oort cloud and later brought in to visit us, This causes the changing coma brightness seen from the called comets. This icy planetesimal is one of the latter ground. - KAROLY SZEGO, Central Research Institute for group. - WILLIAM K. HARTMANN, Planetary Science 12 Physics, Hungarian Academy of Sciences, Budapest Institute, Tucson, Arizona As A RESULT OF THE SPACE MISSIONS TO HALLEY'S Comet in 1986, the nucleus was clearly identified as an irregularly shaped object with dimensions of 16 (± 1) x 8.5 (± 1) x 8.5 (± 1) kilometers. Comparison of Vega 1 and Vega 2 images leads to estimation of the rotational period at 53 hours, but really the rotational movement could be more complicated, with some precession and nutation. The direction of rotation coincides with the direction of its orbit (prograde rotation) . The nucleus' low density (about 0.1-0.4 grams per cubic centimeter) implies a friable material. Its substance is a mixture of frozen gases (H20, CO2, CO, HCHO and proba­ bly some others) and stony meteoric material. There is also evidence of hydrocarbons. The nucleus' surface is covered with a thin, porous, refractory mantle with low heat conductivity. Being black, this mantle reflects extremely little light - less than four percent of the Sun's radiation. During the encounters, the surface temperature on the illuminated side reached 360 degrees Kelvin (87 degrees Celsius). Under the crust, the ice is about 200 degrees Kelvin (-73 degrees Celsius). As "WHAT IS !T?" THAT IS THE QUESTION THAT NOT a result of sublimation, the volatile ices penetrate pores in only laymen, but also experts would like an answer to. And the mantle, which then cracks and breaks. In the course of I don't think we've got the answer yet. this process, dust particles of different sizes are caught, The cometary nucleus may very well be a conglomera­ accelerated and blown away with the gas streams. Gas and tion of ices and dust, a la Fred Whipple. It is clear, mainly dust are emitted from the dayside of the nucleus. The from the surface temperature estimates, that what we see gas production was about 30 tons per second (or 1030 visually is mostly the dust - presumably in the form of a molecules per second) during the encounters. Dust crust blanketing the volatile ices underneath. The regions production was several times lower. from which the gas and the entrained dust emanate (which The distribution of the dust is not even - there are many correspond to less than 10 percent of the surface, predom­ narrow jets. The jets' sources are probably born when the inantly in the sunward section) are probably associated mantle cracks and breaks. The dust outside the nucleus' with cracks or fissures in this thin crust. surface is optically thin, with the exception of certain dust Alternately, the surface may not be uniformly covered by jets that are dense enough to absorb significant amounts of such a blanket of dust, due to the irregular shape and/or light. Brightness differences on the nucleus' surface were initial inhomogeneity of the gas-dust mix. clearly seen during the encounters, and some of them may Everything that is presently being said about the internal be connected with its topography. - G. AVANESOV, structure of this object is sheer speculation. V TARNOPOLSKY, Institute for Space Research, Soviet - D. A50KA MENDIS, University of California at San Diego Academy of Sciences, Moscow 13 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Halley's was Here! • • • • • • • • • • • • • • • • • • • • • • • • • • • •

ALook at Halleymania BY LOUIS D. FRIEDMAN ) ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• e ••••••••

ALLEYMANIA WAS EPITOMIZED BY live long enough to see it again. I heard cruises and tours to exotic places. Yet Owen Ryan: "We are very proud to stories set in Australia, China, Russia, enough were found to satisfy the demands Hbe in a position to create some Scandinavia, the Philippines, Indonesia, of people who wanted an excuse to travel meaning to that [Halley's Cometl ... allow­ South America and across the United to Tahiti and other distant sites. ing people to buy a symbol of their faith States. Many recollections were faulty­ Comet-watchers traveled almost every­ in the future." Mr. Ryan is a salesman, wrong season, time of day, year - or simply where to the South, where the viewing was marketing T-shirts, knapsacks, pins, shares confused. (There was at least one other best: the Amazon, Macchu Picchu, the in the comet and other commemorative bright comet in 1910, and several others in nether parts of Australia, New Zealand, the knick-knacks. Hucksterism was only one the early 20th centul)'.) But it didn't matter. South Atlantic, the South Pacific, the Black aspect of Halleymania - perhaps the With its 76-year period, Halley's Comet Sea, southern China. Astronauts, astrono­ most lucrative and obvious, but not the was seen as a link between generations. mers, writers, teachers, engineers, archae­ most notable or widespread. We can look A lot of Halleymania looked to the future, ologists and other friends of the comet pro­ with skepticism at Ryan's search for mean­ as a mirror image of the interest in the past vided "expert" commental)' on tours, and ing, but all those affected by Halleymania­ apparitions. Evel)'Where mothers and helped people find the comet in the sky. teachers, amateur astronomers, scientists, fathers took their children out into the cold A friend on a geology field trip in South engineers, writers, filmmakers, impre­ night, hoisted them on their shoulders, America was camped out in a remote area sarios, merchants and occasional doom­ pointed out the comet, and asked them to at least a half-day's drive from the nearest sayers - had their own reasons for anti­ remember, in 2061, that they had shared village. One night around 2:00 a.m., while cipating the comet. that moment in 1986. driving alone on an unpaved mountain Curiosity was probably the most wide­ The European Space Agency and The road, he rounded a corner and nearly ran spread feeling about Halley's Comet. We PlanetaI)' Society collected children's im­ into a crowd of about 50 people excitedly had all heard stories of its magnificent pressions of the comet to form an archive pointing to the sky. As he slowly com­ 1910 apparition and looked forward to for the next apparition. Legions of students prehended the Spanish for "There it is!," sharing the wonder and excitement our visited planetariums, museums, observato­ he realized that he had bumped into a bit grandparents felt. Millions read articles, ries and any place they could find informa­ of local Halleymania. watched television, attended lectures, tion about the comet. bought memorabilia and looked for the I was privileged to witness one very spe­ Tough Viewing comet in the night sky. Tens of thousands cial visit. Roald Sagdeev, head of the Soviet Just finding the comet in the night sky was of amateur astronomers took this chance Institute for Space Research and an Advisor the chief disappointment and frustration to make serious contributions to the Inter­ to The PlanetaI)' Society, invited a group of for many victims of Halleymania. I am not national Halley Watch (see the July/August Soviet children and students from the an astronomer and, having been born in 1985 Planetary Report). Thousands of pro­ Anglo-American School in Moscow to tour New York City and now living in Los fessional astronomers threw themselves the institute during the Vega encounters Angeles, I can say with little exaggeration into cometary studies. Hundreds of scien­ with Halley's Comet. The tour was held in that I've hardly seen anything in the night tists and engineers took part in spacecraft memory of NASA's Teacher-in-Space, S. sky. I am a theoretician; I know the stars, missions to the comet, and vicariously vis­ Christa McAuliffe, and in this remarkable planets and comets are there, I can calcu­ ited this famous celestial guest. event, nationalities became unimportant. late where they should be, but I rarely look The students heard talks by Academician up. For me, Halleymania was an excuse to Cometary Curiosity Sagdeev, Carl Sagan and others working learn some astronomy. The cometary curiosity was dramatically on the cornetaI)' encounter. The scientists I found out what the Pleiades are, I emphasized to me by two invitations to spoke while live pictures of the comet made out constellations, I saw planets lecture at the Braille Institute in Los from the spacecraft played on one video move across the sky. I discovered that Angeles. My audiences were all blind­ screen, and a tribute to Christa McAuliffe most people are like me: They don't know some remembered seeing Halley's Comet was displayed on the other. how to find things in the sky. On one tour, in 1910, and some had never seen the PlanetaI)' Society members sent us news some people came without their glasses night sky - yet all wanted to know what clippings about special planetarium shows and complained that they couldn't see the the comet looked like. Once, while describ- around the world. Many reported record comet. On the Planetary Society cruise, we ,/ ing why comets are important to under­ attendance and sales. Guest speakers on had a terrible time convincing people that standing the planets, I was interrupted by cornetaI)' matters were in high demand. the deck lights had to be turned off; we a 90-year-old who had been blind for 30 Some of my friends on the International couldn't see the comet from a brightly lit years, but who told me, in detail, how the Halley Watch gave four or five talks per area. comet looked 76 years ago. week. Members of nearly every Kiwanis Despite all advance warnings, many I heard such stories from Halley "two­ Club, Boy or Girl Scout troop, retirement people were distraught not to see a long, timers" evel)'Where, and found them fas­ center, convention and astronomy club bright tail. Because it was at its brightest cinating. People were intensely interested wanted someone to speak to them about on the opposite side of the Sun from Earth, in these accounts of the comet from 1910. the comet. Community and extension col­ the comet appeared dim in this apparition. At The Planetary Society, we received leges offered courses on the comet. De­ Most people I went out with - whether at many letters from elderly people who re- mand so outweighed supply of speakers home in the mountains near Los Angeles 14 membered Halley's Comet, and hoped to that some scientists turned down fantastic or on comet-watching tours - needed about 30 to 60 minutes to find the comet sible, I have not heard complain. dinary worldwide efforts to study the for the first time. It took me longer. But Media Coverage comet. A New Jersey publication called after I learned a little about the sky, I could Television around the world featured HaI­ the "Halley Watch" was apparently a one­ find it in two minutes. Once the comet man effort. ley's Comet. The Planetary ~ociety ~as was found, most reactions were, at be~t, given a videotape of a SOVIet specIal, A Japanese Halley's Comet society subdued. And in many large groups, a SIZ­ featuring their Vega spacecraft. It was replete claimed a membership of 100,000. In Bn­ able fraction seemed content to give up tain, several societies were organized for with computer graphics, scenes fr~m and never see it! 1910, scientists at work and Halleymama. the occasion, including a Halley's Comet But there were a few great moments. I Coinciding with the Giotto encounter, the Society of 5,000 founding members ~epre­ spent nearly. an hour with a 90-y~ar-old French aired "The Night of the Comet," an senting only "distinguished people In ~ll woman, trying to teach her to use bmoc~­ II-hour live production of mostly rock walks of life." The British didn't qUIte lars and where to look for the comet. FI­ claim ownership of the comet, but you music, celebrating the comet and ~h~ nally she exclaimed, "My God, it looks just could tell that, since it was named for an opening of La Villette, a new Paris exhl~l­ like it did in 191O!" My most thrilling mem­ tion hall. Responding to enormous publIc Englishman, they felt that way. . . ory is of seeing the tail extended through interest, each Japanese network ran a My hope that Halleymania would InspIre the Milky Way. For several people on !he comet special. Asahi Broadcasting based new education about astronomy and Society's cruise, the best part was seeIng their show on the Planetary Society special space science was largely fulfilled. Based the changing position of the comet <.m produced by John Wilhelm and WETA-TV. on anecdotal reports, most teachers used successive nights, and realizing t~at ItS Our Emmy-nominated special, "Comet their students' interest in the comet to motion through the solar system IS very Halley - Here It Comes Again," shared build up their science curricula. Despite different from other objects'. (A beautiful the Public Broadcasting System airwaves some people's disappointment In the demonstration of this can be seen in the with a Nova show on the comet. The comet's appearance, many people, includ­ lMAX movie "Sacred Site," made near British and most European networks each ing me, learned to use a telescope. Te~e­ Ayers Rock in central Australia during the produced at least one major. sh?w and scope sales were the greatest. commerCIal Halley apparition.) provided special coverage of VIewIng con­ manifestation of Halleymama, although Many people now describe the comet­ ditions. The Canadian Broadcasting Corpo­ many people, organizations and publica­ tions (including The Planetary Report) watching as a bust. But I've noticed . th~t ration produced major television and r~dio description comes from those w~o dldn t specials about the comet. In the Umted warned that, for general viewing, the really try to see it. Those who clImbed a States, ABC-TV broadcast a series of comet comet would best be seen with binocu­ mountain, hiked through the desert, sought briefs featuring Society President Carl lars. There was some flim-flam and most out some remote rural area, took an ocean Sagan. Through Society arrangements, they who bought telescopes were disappointe.d. cruise, or simply went as far south as pos- provided live coverage of the Vega encoun­ Scientists suffered their own HalleymanIa: ters from Moscow and the Giallo encounter international conferences, scores of publica­ from Darmstadt in the Federal Republic of tions, trips to everywhere, payloads in space Germany. Local stations provided varied and innumerable toasts to Edmund Halley Halley programming, ranging from spots and his comet. In 1981, after drinking such on weather reports to major news features. a champagne toast at 8:00 a.m. on a Sund~y In discussing news reports, people like morning in Washington with a group of mIs­ to recount major errors. A colleague in En­ guided but well-placed people who had in­ gland sent me a notice of "Wally's ~omet fluenced the new administration to gIve up Discovered." My favorite mistake ran In the on a NASA comet mission and "invest" in a prestigious International Herald Tribune on privately funded effort, I vowed to forego a!1 the morning after a Vega encounter. toasts to the comet. I kept that vow untIl Alongside a well-written article on the September 1986 at the Heidelberg confer­ encounter, they ran a 1910 photograph of ence summarizing results from Vega, the comet, with a caption explaining that Giotto and the International Halley Watch. it was what Vega saw. Other byproducts of Halleymania, Best Legacy ., The spacecraft missions were the pIeces created to satisfY enormous public curiosity, de resistance of the comet expenence. were telephone hotlines. There wer~ at These extraordinary international efforts least 25 in various countries. Phone lInes were the best legacy of Halleymania. Only at planetariums and other institutions were the enormous drawing power of the comet so busy that many had to set up recorded enabled people like Roald Sagdeev in the information messages. Many astronomy clubs, museums, colleges and even th.e Jet Soviet Union, Jacques Blamont and Roger Bonnet in Europe, and many others, to Propulsion Laboratory set up local lI~es. break down institutional and political bar­ The Planetary Society-ABC-TV InformatIon riers to join together exploring the comet. line was an 800 number available across Twelve years ago Carl Sagan wrote, the United States. (Unfortunately, as in­ terest waned in March 1986, ABC tired of "Centuries hence, when the political prob­ lems of today will seem as important as the comet and closed the line.) People in those of the Thirty Years' War appear now the International Halley Watch set up lines to us this generation will be remembered around the world; Arizona State University as first to take steps off its home planet ran an innovative computer network that th~ recorded worldwide reports on observa­ into the solar system." In 1986 we made humanity's first visit to Halley's Comet; that tions almost as soon as they were made. will be a legacy to the next generatIOn. Comet newsletters - sometimes ob­ For all of us who participated in Halley­ scure - sprang up to provide information. mania, it's enough. The Planetary Society helped distribute "The International Halley Watch Amateur Louis Friedman is the founder of the Inter­ Observer's Bulletin" to amateur astronom­ national Halley Watch and is Executive Di­ ers who hoped to contribute to the extraor- rector of The Planetary Society. 15 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • The Next Step ...... • • • • • • • • • • • • • • • • • • • • • • • • • • • In• Exploring J • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

BY MARCIA NEUGEBAUER

, 'COMETS APPEAR TO BE and most of the refractory (high-meIting­ missions at the same time that they are Rosetta Stones" declared a point) minerals remain unidentified. Fur­ analyzing their data from Halley's Comet. recent headline in Science thermore, we know very little about the magazine. Most . scientists studying the physical structure of the dust grains. New Comet studies data on Halley's Comet gathered by space­ Example: Both Comets Halley and The next generation of missions to comets craft and ground-based instruments would Giacobini-Zinner (visited in 1985 by the will involve rendezvous and sample-return agree with this assessment. We now be­ International Cometary Explorer, see missions. In a rendezvous mission, the tra­ lieve, even more firmly than we did before the May/June 1985 and May/June 1986 jectory of the spacecraft is exactly matched the Halley experience, that comets hold Planetary Reports) were surrounded by to the orbit of the comet, so the spacecraft the key to understanding the origin of our huge regions of energetic (fast-moving) doesn't simply fly by its target. The comet solar system - they are the most primitive particles in a plasma, that is, a gas whose and the spacecraft Will travel about the Sun material left from the interstellar cloud that atoms are largely ionized. Since these dis­ together indefinitely. This is the principal gave birth to the solar system. coveries, plasma theorists have proposed objective of the Comet Rendezvous Asteroid A question naturally arises: "What next?" a mechanism, called "second-order Fermi Flyby miSSion, called CRAF for short. This Will thorough analyses of the new data acceleration," for accelerating cometary past October, NASA picked the scientific reveal most of what comets have to tell us ions to the energies observed. Still they experiments to be flown on CRAF. These about our solar system's origin? Will we must test this and other hypotheses about instruments, combined With the rendezvous then understand the astrophysical processes the behavior of cometary plasmas under a trajectory, will substantially advance our Within interstellar clouds? Probably not­ greater variety of conditions in the solar understanding of comets. the observations of Halley's Comet not only wind (the supersonic plasma flow emitted According to current plans, CRAF Will be left some important questions unanswered, by the Sun) and over a much greater range launched in early 1993 by a Titan lV-Centaur but raised many new ones as well. of cometary activity levels before these G launch vehicle. After a gravitational as­ Example: The Giotto images of the hypotheses can be applied to other astro­ sist from a swingby near Venus, followed by comet's nucleus showed that gas and dust physical plasmas. another from Earth, CRAF will head for the spewed out in jets from only a few regions Thus, it's not surprising that comet sci­ comet named Tempel 2. En route to Tempel while the rest of the body was covered entists are actively planning future comet 2, CRAF Will pass through the asteroid belt with a dark crust. What is this dark c;:rust? Is it made of dust particles that have fallen back to the surface, or is it a porous, pumice-like material from which the vol­ atile (easily evaporated) material has boiled off? And how do the active regions differ from the rest of the comet? Example: The Vega, Giotto and ground­ based data all showed that much of the cometary dust was made of organic (carbon-based) material. For instance, we found grains made of carbon, hydrogen, oxygen and nitrogen (CHON particles). How were these elements combined into /' molecules? Does the organic material in­ clude amino acids or other biogenically important molecules that might tell us a great deal about the origin of life on Earth and about the universality of life in the cosmos? Because we observed only the gases and dust out in the comet's coma, where the ejected materials have already been broken up, we did not detect the "parent molecules" from which the coma materials originated. Many of these parent 16 molecules making up the cometary ices and will fly close to a large asteroid named the mass of the comet's nucleus in a series sured with an accuracy of 10 percent or 46 Hestia. It will take pictures of Hestia with of slow flybys, the spacecraft can be put better. From such data, we can learn about details as small as 100 meters, measure its in a leisurely orbit around the comet, mak­ how solid bodies, such as comets, accumu­ mass and density and map it in the infrared ing a complete circuit in tens of days. It lated during the formation of the solar sys­ to determine the distribution of minerals on can hover over points of interest or carry tem and about how they have "weathered" its surface. out other maneuvers in the comet's weak since then. CRAF would then move on to rendezvous gravity. A Visual/infrared spectrometer will map with Tempel 2 in October 1996, when the CRAF's cameras will map the entire sur­ the minerals and ices on the surface, while comet is at its farthest point from the Sun, face, resolving details smaller than one an infrared radiometer maps the surface and so at its least active. After measuring meter. The comet's density will be mea- temperature. Comet modelers can use these data to learn about the tempera­ ture and pressure in the regions where comets formed - perhaps an interstellar cloud or the fringes of the solar system. We will also learn how the comet absorbs sunlight, and how the Sun's energy gener­ ates the coma and tail that we see from Earth. We will use the first maps of the comet's topography and temperature to decide where to land the penetrator experiment. The penetrator will carry an array of instru­ ments with a week's worth of batteries to study the comet's crust and the material beneath it. Accelerometers will determine the depth of penetration and the strength of the surface material and the layers beneath it. In the penetrator's tip, buried about a meter below the surface, will be a gamma­ ray spectrometer to measure the relative abundances of carbon and all important heavier elements. A set of temperature probes will measure the rate at which heat works its way from the surface down into the nucleus. 17 A small sample of material taken near comet at different distances from the their velocities. Sudden bursts of dust from the bottom of the penetrator's hole will be Sun and what chemical reactions occur in the comet could damage the instruments, sealed in a container and heated in a con­ the coma. This instrument will also con­ therefore this instrument will be used to trolled way. Pressure and temperature mea­ centrate and analyze heavy organic warn other instruments so their protective surements taken during the heating will tell molecules. covers can be closed. us more about the nature of the ices: At A battery of instruments will focus on The Federal Republic of Germany will what temperature do they change from ice solid grains in the coma. One will gently supply one of the dust experiments and to liquid, and how much heat does this knock off ions from dust grains and then the propulsion subsystem needed to ren­ require? Does the heating set off any analyze the ions with a mass spectrometer dezvous with Comet Tempel 2. chemical reactions? How much gas comes that can measure abundances of chem- Future Plans NASA is developing a new generation of planetary spacecraft, named Mariner Mark II, for CRAF and other missions to study main­ belt asteroids and the outer planets. The Mariner Mark Irs will be easy to reconfigure from one mission to the next. They will use several new technologies to increase reliability and to decrease costs. Perhaps the ultimate comet mission would bring back to Earth a well-preserved, still frozen, deep core sample of a comet for analysis with instruments too large and complex to send into space. Both NASA and the European Space Agency (ESA) have such a Comet Nucleus Sample Return mis­ sion in their long-range plans. The return of a pristine cometary sample is one of four major program elements, called corner­ stone missions, identified by ESA NASA's Solar System Exploration Committee has similarly recommended a sample-return mission as part of its Core Program of planetary exploration. A joint NASA-ESA science working group is planning such a mission. The data from CRAF will be enor­ mously valuable to the mission design. It would be extremely difficult and expen­ sive to return a well-preserved nucleus sam­ ple with the launch vehicles now available, so both ESA and NASA's Goddard Space Flight Center are studying easier missions to retrieve samples of cometary dust and gas. These simpler and less expensive missions would fly through a comet's coma at many kilometers per second. Dur­ ing its flyby, the spacecraft would expose a sample-collection surface, seal it up and return to Earth. However, the sample would be very small and its properties would be altered by high-speed impact with the collector. Although potentially important for under­ standing the composition of cometary out of the ice at each temperature? At sev­ ical isotopes as well as atoms and mole­ dust, these missions would not address eral stages during the heating, the gases cules. The differences from one grain to most problems concerning comets' nuclei. will pass through a gas chromatograph to another may tell us about the histories of Neither NASA nor ESA now plans to under­ identify what molecules are present. These the different materials making up the solar take this type of mission. data will tell us about the conditions that system. Thus, the hopes of the cometary science formed the comet's ices arid about the A miniature scanning electron micro­ community are tied to CRAF. NASA is sup­ physical and chemical processes in proto­ scope will take pictures of the structure porting the necessary planning and de­ stellar clouds. and arrangement of mineral phases in velopment activities. But the spacecraft As the comet and the spacecraft ap­ individual grains. Another instrument and its instruments cannot be built until proach the Sun, the comet's activity will will use x-ray fluorescence to measure the project has been officially approved. increase. CRAF's instruments will measure the elemental composition of dust sam­ Congress must approve funds for CRAF in the dust, gas and plasma coming off the ples. The same samples will also be early 1988 if we are to rendezvous with a comet. The spacecraft will explore the heated, and the gases which escape at dif­ comet before the end of this century. coma and move tens of thousands of ki­ ferent temperatures will pass through a set lometers down the comet's tail to collect of gas chromatograph columns to search Marcia Neugebauer is a senior research dust samples and to map the ever-changing for different molecules, including heavy scientist at the Jet Propulsion Laboratory interaction between the comet's gases and organics. in Pasadena. She is Project Scientist for the solar wind. The mass spectrometer Still another dust detector will monitor CRAF and a member of the Giotto ion 18 will tell us what gases emerge from the the impact rate of dust grains and measure mass spectrometer team. t has been well over a year since that terrible morning when the space shuttle Challenger disintegrated i~ ~e I Florida sky. Still, the American space program remaIns m disarray as we grope toward regaining our stairway to the Ne~eviews heavens. The question confronting us is whether we can learn from the history of our mistakes, or must we repeat our errors time and time again. It's easy to see where we went wrong. ~e es~ential problem with the shuttle program was aptly explamed Just before the Challenger accident by James Van Allen in a Scientific American article which I reviewed in this column a year ago. An even more by Clark R. Chapman cogent history of the shuttle's beginnings, by JOh.n Lodgson, appeared last spring in Science and was also reVIewed here. Members of The Planetary Society have all been keenly aware of NASA's misguided priorities, since planetary exploration has "taken it on the chin" more than almost any other part of the engine if the space plane is to work. "The technical leap" is space program. (That trend continues with President Reagan's Fallows' third stage; remember the required breakthroughs for latest budget proposal for fiscal year 1988: Solar system explo­ the shuttle's main engines and its heat-resistant tiles? Next ration is one of the few NASA programs slated for a further de­ comes "the unpleasant surprise" stage, the 2.8 person-days cline.) As we face an uncertain future in space, these history per tile for installation and the "unexpected" repair or replace­ lessons bear repeating because of the disquieting signs that ment of all three of Challenger's main engines after its maiden they remain unlearned. flight. The astronauts need not have died in vain. An eloquent re­ The last stage, of course, is "collapse of the house of cards." capitulation of the shuttle's early and mo~t recent hist~ry h~s Not only were the hopes and dreams of America's schoolchil­ been written by James Fallows. He examines the President s dren sunk beneath the Atlantic, along with the bodies of seven latest space proposal - the "Orient Express" space plane­ astronauts and fragments of one-quarter of our main launch and finds that it looks depressingly familiar. His essay in the fleet, but "major scientific, commercial and military programs December 18th issue of The New York Review of Books reviews are delayed at least two years," Fallows writes. As I write these three works, including the report of the Rogers Commission words, only two months after Fallows' essay appeared, further on the Challenger accident. delays seem certain. Many NASA-watchers have noted parallels between develop­ Will NASA Ever Visit a Comet? ment of the shuttle and the early phases of the space station. NASA's shortsightedness continues to delay progress on space Fallows says little about the station - NASA's prime goal after science and planetary exploration. (See, for example, Mitch we get flying again - instead he emphasizes the less familiar, Waldrop's report "A Crisis in Space Research" in the January 23 would-be space plane. Once again, military goals threaten to issue of Science.) In its November 1986 issue, Astronomy become paramount as the budget-starved space agency turns magazine's cover story asked ''Why Can't We Explore a CornetT' to the Department of Defense for most of the bucks. NASA Donald Frederick Robertson explains how NASA had planned, would pay only about 20 percent. until a year ago, to explore Comet Wild 2. He laments that the But then, according to Stephen Korthals-A1tes, whose book mysterious celestial visitor will swing back out of the inner The Aerospace Plane: Technological Feasibility and Policy solar system unexplored by the delayed Comet Rendezvous Implications Fallows reviews, the Air Force esti~ate of $3 bil­ Asteroid Flyby (CRAF) mission (see pages 16-18). lion in development costs for the space plane IS less than 20 Robertson quotes NASA about the "probable" new start for percent of the likely costs of over $17 billion. Korthals-A1t~s, a CRAF in 1988, which would send CRAF instead to Comet Tem­ young engineer and cost analyst at the Massachusetts Institute pel 2. But NASA backtracks almost faster than Astronomy can of Technology, presents his own readable, nicely illustrated roll off the presses. Despite frequent promises over the past synopsis as the cover story of the January issue of Technology year by NASA officials, the President's 1988 budget proposal Review, MIT's popular science magazine. does not have a new start for CRAF. The next launch opportu­ "The technological challenges have been downplayed, the nity wouldn't get CRAF to a comet until after the turn of the development costs are grossly underestimated, and the utility century! Some pundits believe that NASA's Solar System Explo­ of the aircraft is vastly exaggerated," writes Korthals-A1tes. The ration Division may cancel all outer planet exploration (in­ initial project concept, since President Reagan first .proposed cluding suggested missions to Saturn's moon Titan, comets it in his State of the Union address a year ago, reminds us of and asteroids) if Congress cannot be persuaded to reverse the the space station, and of the shuttle before that. administration's shortsightedness. James Fallows has outlined a five-stage process typical of NASA officials themselves recognize that the space agency huge military procurements. First, there is .the "Vegem~tic is losing its capacity for leadership in space, not only in the promise" ("It slices, it dices!" and does practically everythmg international arena but even among the executive branch agen­ under the Sun.) The space plane would "combine the v~I?t~ge cies in Washington. Craig Covault, writing in the February 2nd point of a reconnaissance satellite with the maneuver~blhty of issue of the aerospace trade journal Aviation Week & Space an SR-71 (a high-altitude, high-speed aircraft), dehver the Technology, documents some recent he~vy-handed actions by ordnance of a bomber with the speed of an ICBM, launch pay­ the Departments of Defense, Transporatlon and the Treasury; loads into orbit with the ease of a DC-9, and whisk civilian various federal agenCies seem to be trying to commandeer passengers across oceans in a few hours. NASA's responsibility. NASA's plans for launch vehicles are As Korthals-A1tes notes, "perhaps the most important lesson being compromised, and recent international negotiations con­ of the shuttle is that a space vehicle designed to perform many cerning the space station broke down due to Department of functions is optimal for none." We, who were promised that a Defense interference. low-earth-orbit transfer vehicle (the shuttle) would be ideal for In an editorial in the same issue entitled "NASA Under launching planetary missions, can bitterly acknowledge Fallows' Seige," Aviation Week calls for renewed independence by first stage. He reminds us that it was originally touted that the NASA. It is encouraging that NASA officials are now vocally shuttle would carry four vehicles per year to planetary explora­ seeking to re-establish the space agency's preeminence in for­ tion. He concludes that "so far it has carried none." mulating and implementing United States space policy. How­ Stage 2 is "the rosy prospect." The numerous flights per year ever if NASA is to succeed in recovering from the Challenger would so reduce commercial launch costs that space would trag~dy, there must emerge a more lofty and long-range goal. in triumph on Wall Street. Korthals-A1tes quote~ a French Aero­ space than the military-oriented goals that are now threatening spatiale official, still smarting from the finanCial falh!re~ of the to dominate our country's approach to space. To establish such Concorde, that "our American friends are daydreaming about visionary goals and give NASA the go-ahead to pl!rsue. them the viability of the "Orient Express." requires leadership from the very top of the admmlstratlon. The Technology Review article also docu~ents enormo~s technical hurdles that must be overcome With the scram]et Clark R. Chapman is beginning work on two new popular books about space. 19 ------

by Louis D. Friedman

WASHINGTON --"I am writing to inform Science and Transportation, Washington, cosmos (USSR); Roger Bonnet, European you that the President's Fiscal Year 1988 DC 20510 Space Agency; M. Oda, Institute of Space budget reflects a delay in the launch of the Senator Ernest Hollings, Chairman and Astronautical Sciences (Japan); and Mars Observer mission from the present -- NASA authorizing subcommittee, Burt Edelson, NASA. International Halley launch date of 1990 to 1992." Senator Don Riegel, Chairman Watch leaders Ray Newburn and Jiirgen This sentence opened a letter from Rahe joined them. --Senate Appropriations Committee, NASA Administrator James Fletcher to the In meeting the group, the Pope said: Washington, DC 20510 four congressional committees responsi­ "I wish to commend this splendid initia­ Senator John Stennis, Chairman ble for the space program. It was written tive which brings you together and which -- NASA appropriations subcommittee, on January 2, three months after a previ­ seeks to foster ever more effective interna­ Senator William Proxmire, Chairman ous NASA attempt to delay the mission tional cooperation in the space sciences. had failed -- due, in part, to strong action -- House Committee on Science and It is indeed a kind of celebration of scien­ by Planetary Society members. On January Technology, Washington, DC 20515 tific cooperation, a celebration which can 5, the new budget was released: Representative Robert Roe, Chairman offer hope to men and women of science, as well as to all people of good will, as -- House Committee on Appropriations, Space shuttle-$1,229 million (up 22 they seek to identify those areas of know­ Washington DC 20515 percent) ledge and concern which unite the human Representative Jamie Whitten, Chairman Space transportation operations - $1 ,885 family rather than divide it. The participa­ -- NASA appropriations subcommittee, million (up 2 percent) tion of the Vatican Observatory serves to il­ Representative Edward Boland, Chairman Space communications - $949 million lustrate the desire of the Church to encour­ (up 10 percent) age these worthy endeavors and to con­ Construction of facilities - $196 million WASHINGTON -- NASA's launch schedule (up 18 percent) tribute, as far as possible, to the realization for major scientific missions currently stands of the noble goal of harmonious human Civil service manpower - $1,598 million as follows: (up 9 percent) coexistence, in the achieving of which sci­ Space station - $767 million (up 82 per­ • Hubble Space Telescope to Earth orbit, ence can play an active and vital part .... cent) November 1988 on the space shuttle "There is yet another way, one that we Space transportation research and de­ • Magellan to Venus orbit, April 1989 on commemorate today, namely, that collab­ velopment - $569 million (up 15 percent) the space shuttle oration in a scientific endeavor which Aeronautics and space technology - $691 • Galileo to Jupiter, November 1989 on transcends all national boundaries and re­ million (up 17 percent) the space shuttle with the Inertial Upper quires knowledge and dedication to sci­ Tracking and data - $18 million (up 6 Stage, with possible postponement to ence and technology by men and women percent) 1991 if Ulysses is launched in 1989. of many nations, races and creeds. Last Commercial programs - $54 million (up • Ulysses European mission to solar orbit, week, in commemorating the Fiftieth An­ 32 percent) scheduled for October 1990 on the niversary of the Pontifical Academy of Sci­ Safety and reliability - $16 million (up space shuttle with the Inertial Upper ence, I spoke of the great esteem which 77 percent) Stage, but could move up to November the Church has for scientists, not only for Physics and astronomy - $567 million 1989. (The congressional Appropria­ their intellectual prowess, but also for their (up 3 percent) moral character, their intellectual honesty Life sciences - $74 million (up 3 per­ tions Committees have recommended delay to 1991 with launch by a Titan! and objectivity, their self-disciplined search cent) for truth, their desire to serve mankind and Space applications - $559 million (up 3 Centaur.) percent) • Mars Observer to Mars orbit, August their respect for the mysteries of the uni­ verse which they explore .. .. PLANETARY EXPLORATION - $307 mil­ 1990 (however, NASA is seeking a two lion (down 14 percent) year delay, saying there is no room on "I hope and pray that all of the scientists the shuttle manifest, largely because of and engineers in your space agencies will TOTAL - $9,481 million (up 12 percent) military payloads). continue to work together in your explo­ ration and thus merit to be called peace­ Space shuttle flights are scheduled to re­ Every area -- except planetary explora­ makers, in addition to your other worthy tion --received a major increase. In the sume on February 18, 1988. If, as seems titles .... increasingly likely, this date slips, the planetary exploration budget, the Mars Ob­ "I also wish you to know how much I server was delayed two years, and the above-mentioned launch dates are all un­ appreciate your field of science, and how Comet Rendezvous and Asteroid Flyby likely. The first flight will launch a tracking much I admire the contribution that you (CRAF) was put on hold. No new Mars in­ and data relay satellite. This satellite will are making to it. Your science opens up to provide a crucial communications link for itiative was proposed. And there was no man so many of the wonders of the uni­ these future space missions. mention of space at all in the President's verse and leads him in a new and deeper State of the Union address. way to be aware of its greatness. Your sci­ Congress has begun consideration of ROME-On November 7, 1986, Pope entific research and discoveries are likewise the proposed budget. Extensive question­ John Paul II met with the leaders of the capable of becoming effective instruments ing about the NASA program is expected Inter-Agency Consulting Group (lACG) to for a more profound understanding of and public interest in planetary explora­ receive a report about the Halley's Comet man, for whose well-being the whole ad­ tion could force reconsideration of these encounters of 1986. The world's four lead­ venture of science is conducted ... ." priorities. The congressional committees ing space agencies had formed lACG to acting on the NASA budget are: coordinate the mission activities. The Louis Friedman is the Executive Director of 20 --Senate Committee on Commerce, agency leaders were: Roald Sagdeev, Inter- The Planetary Society. ------~

highest in the United States, UPCOMING SOCIETY thanks, in part, to these two HAWAII CALLS THE PLANETARY SOCIETY - ALOHA EVENTS members. You have the rare opportunity to attend a Planetary Society March 16-20 - Annual Lunar open forum with Sally Ride, Society Advisor, Astronaut and MARS CONTEST DEADLINE and Planetary Science Confer­ Special Assistant to the NASA Administrator. The Society's ence, in Houston Texas. The Board of Directors will also hold an open discussion of Society EXTENDED Planetary Society will sponsor a plans for the International Space Year, 1992 and the organiza­ public session on the evening of The fourth annual Mars Student tion of international membership activities. The occasion is March 16, with V. Barsukov and Contest deadline has been ex­ V. Moroz from the Soviet Union, the Pacific Nations Conference, organized by the University of tended to May 1, 1987; the win­ and Harold Masursky and Bruce Hawaii and the government of Japan, to help plan the Interna­ ner will be selected by May 30, Murray, Society Vice President, tional Space Year, which will be modeled on the International 1987. College and high school from the United States. Their Geophysical Year (IGy) of 1957. students are eligible to enter the topic will be planning the ex­ The Pacific Nations Conference will be held August 19-21, contest, which encourages stu­ ploration of Mars. 1987 in Kona, Hawaii and will be chaired by United States dent participation in research July 18-22 - Case for Mars III: Senator Spark Matsunaga CD-HI) and Japanese Minister of and development for future ex­ Strategies for Exploration, in Economic Planning Tetsuo Kondo. The conference itself will ploration of Mars. Boulder, Colorado. Public ses­ be limited to invited attendees, but The Planetary Society will This year's contest has two sions are planned. hold several public sessions for our members August 21-23. categories: In the first, students August 21-23 - The Pacific On the evening of the 21st, Dr. Ride will give the keynote will design an international Mars Nations Conference, Planetary So­ address at a Society session, followed by commentary from mission; in the second, students ciety sessions, in Kona, Hawaii. the Society directors. On the 22nd, the Board of Directors will will analyze the social, political and economic assets and liabil­ hold their open meeting. A field trip to the active Hawaiian ities of an international mission KUDOS TO . . . volcanos, a lecture on volcanos in the solar system and a to Mars. The winner in each cat­ Teinya Prusinski and Linda Low nighttime sky party will round out the weekend program. egory will receive $750.00 and an of Chicago, who spent many hours We will give more details of the conference and of a special all-expenses-paid trip to the Case planning and organizing a brunch Society travel package in the next issue of The Planetary Report. for Mars III conference to be held featuring astronaut Kathryn Sulli­ After March 1, you may also call our Information Lines, 818n93- in Boulder, Colorado in July, van, who spoke on "Challenges 4328 from east of the Mississippi, and 818n93-4294 from west 1987. Ahead: Our Future in Space." of the Mississippi. We hope that many of our members will For more information, write The enthusiasm of our Chicago be able to attend this exciting conference. to the Mars Institute at the Soci­ membership remains among the ety offices.

A CHALLENGE TO ALL MEMBERS "There remains, however, a lot of uncovered territory. I would like to challenge personally all of my fellow members to do as I did - do­ The Library Outreach Program - made possible by Nick Pavlica's nate subscriptions to your local libraries! gift to fund library subscriptions to The Planetary Report - has spon­ "Let us make The Planetary Report available to everyone. How taneously spawned a new membership program. Several members better to share the excitement and knowledge that The Planetary have given gift subscriptions to their local school and community Society offers. Then watch us grow!" libraries. One of those members, Regina Mitchell of Holbrook, Mas­ Ms. Mitchell gave subscriptions to Holbrook High School and the sachusetts has issued a challenge to all 100,000 members: Holbrook Public Library. If you accept her challenge, you can give "After reading in the July/August issue of Nick Pavlica's grant to subscriptions at our educational discount rate of $12.00 per year for have 1,000 libraries receive Planetary Report subscriptions, I was school and community libraries, and teachers who have the magazine thrilled - truly a fantastic idea! sent to their classrooms. Fill in and mail the coupon below.

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NEW LASER PRINTS! NEW BOOKS!

JUprrER (#322) On February 13, 1979, Voyager 1 imaged Jupiter's southern hemis­ phere, Passing in front of the planet are the two inner Galilean satellites, sulfurous 10 (on the left in front of the Great Red Spot) and icy Europa (right),

MARS (#323) NEW WORLDS (#137) Taken by the Viking Orbiter, this image captures the geological diversity of the This profusely illustrated book presents both martian landscape as the spacecraft looks historical and rnodern theories about the across the cratered surface to the cloud­ planets and the creation of the solar system, streaked horizon. Authors Heather Couper and Nigel Henbest examine each planet, and include the recent Voyager 2 results from Uranus.

THE CASE FOR MARS (not pictured) (#159) After numerousrequesls, this book has been reprinted and is again offered to our members. Drawn from papers presented at the 1981 Case for Mars Conference, cosponsored by SATURN The Planetary Society, this book covers mission strategy, life support, surface activities and (#333) other topics that will have to be addressed On November 16, 1980, four days before we set out for Mars, and 5.3 million kilometers after its encounter, Voyager 1 looked back at Saturn and observed the planet and its rings from this unique perspective, ,

NEW VIDEOTAPE!

URANUS -I WILL SEE SUCH THINGS (VHS #450, BETA #451) ,URANUS William Herschel discovered Uranus In 1781, '1#337) but the planet remained a mysterious dot in astronomers' telescopes until, in January 1986, On its way to Neptune, Voyager 2 turned it was visited by Voyager 2. The spacecraft one las\ tirneto Uranus and irnaged the sun­ found a world tipped on its side, surrounded lit crescent of the planet. Methane in Uranus' with a shimmering glow of ultraviolet light, and atrnosphere absorbs red light, giving this orbited by strange and ancient moons. This enter­ planet its blue-green color. taining and informative videotape examines the mysterious and distant world, Uranus. ' • F • ." • '" Order 'Form ,

PRICE liN ORDER R PRICE (IN N~~:~ e Books us DOLLARS) NUMBER eCIoor eprod uctlons us DOLLARS) 105 Atlas of the Solar System by Patrick Moore 305 Apollo photograph of Earth - full disk (16" x 20" Laser Print) $ 8.00 and Garry Hunt. 464 pages. Soft Cover $20.00 310 Earthprint photograph of North America (8" x 10" Laser Print) $ 4.00 110 Comet by Carl Sagan and Ann Druyan. 398 pages. $20.00 315 Earthrise photograph of Earth from the Moon (16" x 20" Laser Print) $ 8.00 115 Cosmic Quest: Searching for Intell~ent Life Among the Stars by Margaret Poynter and Michael J. lein. 124 pages. $ 9.00 320 Halley Encounter - 2 pictures from Vega and Giotto missions. $ 2.50 120 Earth Watch by Charles Sheffield. 160 pages. $20.00 321 Uranus Encounter - 4 pictures from Uranus and its moons. $ 4.50 130 Mission to Mars by James Oberg. 221 pages. Soft Cover $ 6.00 322 Jupiter photograph of southern hemisphere (16" x 20") $ 8.00 135 Nemesis: The Death-Star and Other Theories of Mass Extinction by Donald Goldsmith. 166 pages. $14.00 323 Mars photograph of landscape frorn Viking Orbiter (16" x 20") $ 8.00 137 New Worlds by Heather Couper and Nigel Henbest. 144 pages. _ $11.50 325 Other Worlds (23" x 35" poster) -$ 7.00 140 Out of the Cradle: Exploring the Frontiers Beyond Earth 330 Planetfest '81 - Saturn and the F-ring (Two 23" x 35" posters) $ 5.00 by William K. Hartmann, Ron Miller and Pamela Lee. - 190 pages. Soft Cover $11.00 333 Saturn photograph of full view (16" x 20") $ 8.00 145 Pioneering the Space Frontier by the National Commission 335 Voyager 1 at Saturn (set of five posters) $16.00 on Space. 211 pages. $12.00 337 Uranus photograph of sunlit crescent (16" x 20") 150 Planetary Exploration through Year 2000: An Augmented Program -$ 8.00 Part two of a report by The Solar System Exploration Committee 340 "You Are Here" (23" x 29" poster) $ 5.00 of the NASA advisory council. 239 pages. $10.00 155 Rings - Discoveries from Galileo to Voyager -PRICE liN N~R~::R e 35mm Slide Sets us DOLLARS) by James Elliot and Richard Kerr. 209 pages. $16.00 159 The Case for Mars edited by Penelope J. Boston. 205 Chesley Bonestell's Vision of Space (40 sl ides with sound cassette) $15.00 314 pages. Soft Cover $18.00 1mB 210 Halley's Comet (20 slides with description) $10.00 160 The Case for Mars II edited by Christopher P. McKay. 700 pages. Soft Cover $26.00 215 The Solar System Close-Up, Part One (50 slides with booklet) $35.00 165 The Grand Tour: A Traveler's Guide to the Solar System 216 The Solar System Close-Up, Part Two (50 slides with booklet) $35.00 by Ron Miller and William K. Hartmann. 192 pages. $ 9.00 220 Viking 1 & 2 at Mars (40 slides with sound cassette) $15.00 170 The Mars One Crew Manual by Kerry Mark Joels. 156 pages. $10.00 225 Voyager 1 & 2 at Jupiter (40 slides with sound cassette) $15.00 175 The Mars Project by Senator Spark Matsunaga. 215 pages. $15.00 180 The Planets edited by Byron Preiss . 336 pages. $22.00 230 Voyager 1 Saturn Encounter (40 slides with sound cassette) $15.00 183 The Search For Extraterrestrial Intelligence: Listening For Life 231 Voyager 2 Saturn Encounter (40 slides with sound cassette) $15.00 In The Cosmos - by Thomas R. McDonough. 256 pages. $18.50 235 Voyager Mission to Uranus (20 slides with description) $ 7.00 185 The Surface of Mars by Michael Carr. 232 pages. $16.00 240 Worlds In Comparison (15 slides with booklet) $12.00 187 To Utopia and Back - The Search for Life in the Solar System by Norman H. Horowitz. 168 pages. $11.00 PRICE (IN N~~::R Other Items us DOLLARS) 190 Voyager: The Story of a Space Mission e by Margaret Poynter and Arthur C. Lane. 152 pages. $ 8.00 505 An Explorer's Guide to Mars (color map of Mars) $ 4.00 195 Voyage to Jupiter by David Morrison and Jane Samz. 199 pages. $11.00 510 Back issues of THE PLANETARY REPORT - Each volume contains six issues. 196 Voyages to Saturn by David Morrison. 227 pages. $14.00 (Vol. 1, #5 & 6; Vol. 2, #1 & 6; Vol. 3, #1 & 2 and' Vol. 4, #6 have been sold out.) Specify the issues you are ordering by volume and number. each $ 2.00 ORDER _ PRICE (IN NUMBER e VIdeotapes us DOLLARS) 515 Bookmark - blue with logo (6" x 2", 2 for $2 .00) $ 1.50

410 Comet Halley (60 min. videotape) $15.00 520 Calendar for 1987 $ 7.00 411 ~~¥A 530 "I Love Mars, That's Why I Joined The Planetary Society" 420 ~~¥A Mars, the Red Planet (30 min. videotape) $30.00 T-Shirt - burnt orange S M L XL $ 8.00 421 535 Mars Model by Don Dixon and Rick Sternbach $65.00 430 VHS The Voyager Missions to Jupiter and Saturn 431 BETA (28 min. videotape) $30.00 540 Men's T-Shirt - white with blue logo. S M L XL $ 9.00 440 Universe (30 min. videotape) $30.00 541 Women's T-Shirt - navy with white logo. S M L XL (sizes run small) $ 9.00 441 ~~¥A 545 Planetary Report Binders - blue with gold lettering. (2 for $16.00) $ 9.00 450 VHS Uranus - I Will See Such Things 451 BETA (29 min. videotape) $30.00 550 TPS Buttons - blue with logo (2 for $2.50) $ 1.50

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MAIL ORDER AND PAYMENT TO: THE PLANETARY SOCIETY, 65 N. CATALINA AVE., PASADENA, CA 91106 COMETARY ENCOUNTER - Because a thick coma surrounds a comet's nucleus as It nears the Sun, It is impossible for Earth-based ffllescopes to see Into the heart of a comet when It Is at Its most active. If we are to expand our under­ stllJJding of comets beyond the data gathered by the Vegas and Glotto, more spacecraft wHI have to visit comets. In this painting, a Giotto-Ilke craft approaches a comet spewing jets of dust.

Space artist Don Dixon is now at work on a series of paintings depicting the exploration of Mars. He recently created the covers for several science fiction books published by Ballantine, Wamer and Berkley Books.