The Planetary Report Different from Most Magazines Reporting Galileo Imaged Tyre on Apri L 4, 1997 from a Distance of 703,776 Kilometers (434,430 Mi Les)

On the Cover: Volume XVII Tyre, an ancient impact basin more than 140 ki lomete rs Table of (90 miles) across, grabs our attention in 'one of ihe highest Number 5 resolution pictures of Europa yet obtained by Galileo. In this September/October 1997 false-color image, the bright bl ue, east-west ridges crossi ng Contents the region are much you nger than the crater, illustrating how the moon maintains its youthful appearance. Older, darker surfaces are covered over with clean, relatively coarse grained ice that is extruded along the fractures in Europa's Features frigid crust. Coarse-grained ice, wh ich forms at warm er A Brief History of the Moon temperatures, may be a sign of liquid water below. 4 One thing that makes The Planetary Report different from most magazines reporting Galileo imaged Tyre on Apri l 4, 1997 from a distance of 703,776 kilometers (434,430 mi les). on scientific discovery is that we ask the people actually making the discoveries to write their own stories, In this article, William K. Hartmann recounts the leading theory of the Moon's Image: NASAlJPL formation- based largely on his research with Donald R. Davis. As you will see in the paint ings that accompany this article, Bill is also a talented artist.

12 The Color of Europa FroIO If one world has stolen the spotlight of Galileo's mission to Jupiter, it's the icy moon Europa. Voyager gave us hints that this Moon-sized satellite might harbor an ocean of water The beneath its shielding crust, and the continuing stream of data from Galileo makes it seem like lier than ever that Earth may not be the only world in our solar system with an ocean. On its Editor way to Jupiter, Galileo made two passes by Earth, turning instruments on its home world and providing scientists with "ground truth" to compare with their findings in the Jupiter system. Here's a brieflook at some of that comparative data. s the editor of a bimonthly publi cation, I put together each issue A 14 On to Eros! kriowing that there is no way we can With Mathilde in its wake, the Near-Earth Asteroid Rendezvous spacecraft is bow compete in timeliness with daily news on its way to Eros. As scientists prepare for the rendezvous, it is possible that amateur astron papers and weekly magazines. Our read omers could help the professionals gather the most complete information possible about ers sometimes have to wait two or more the target asteroid. Clark Chapman gives some hints on observing this tiny nearby object. months before they read space mission results in The Planetary Report. At no 18 Carl Sagan Menllorial Station time have I felt the pressure of this more When Mars '96 crashed to Earth, we felt a particularly keen sense of loss at the Plan keenly than now, as results pour in from etary Society: the spacecraft had been carrying a list of names of all our members (as of October the Near-Earth Astero'id Rendezvous 1993). But the Society has friends- very good friends- who surprised us with a special an (NEAR) at Mathilde and from Pathfinder, nouncement as Pathfinder landed on Mars. The Pathfinder team honored the Society in another with its rover Sojourner, on Mars. And way when they named the landed craft after our co-founder and first president, Carl Sagan. Galileo continues to funnel data back from its continuing mission around Jupiter. We were already in production with this Departn1ents issue when NEAR passed Mathilde, and as I write this, Pathfinder and Sojourner 3 Menllbers' Dialogue are still exploring Mars. We have dropped in Mathilde images as a giant Factino in 16 Basics of Spaceflight place of the usual Questions & Answers column. On the back cover, we've inserted 19 Ne\Ns & Revie\Ns an image from Pathfinder. In place of timeliness, we have the lux 20 Factino ury of waiting until things calm down and scientists have a chance to think about 22 World Watch mission results. After the dailies and weeklies have lost interest, we continue 23 Society Ne\Ns to report on planetary missions and give our readers a fuller, more considered account of scientific discovery. In up com Contact Us ll1g issues look for detailed reports from Mars, Mathilde, and the Jupiter system. Mailing Add ress: The Planetary Society, 65 North Catalina Ave nue, Pasadena, CA 91106-2301 We hope they'll be worth the wait. General Calls: 626 -793 -5100 Sales Calls Only: 626-793-1675 - Charlene M. Anderson E-mail: tps@marsplanetaryorg World Wide Web Home Page: http://planetarY.org

The Planetary Report (ISSN 073&-3680) is published bimonthly at the editorial offices of the Plant::tary Society, 65 North Catalina Avenue, Pasadena, CA 91106-2301,626-793-5100. It is available to members of the Planetary Soclet~. Ann.ual dues I~ the US are $25 (US dollars); in Cana~a. $35 (Canadia~dollars). Dues in other countries are $45 (US dollars). Printed In USA. Third-class postage at Pasadena, California, and at an additional mailing office. Canada Post Agreement Number 87424. A Publication of Editor. CHARLENE M. ANDERSON Technical Editor, JAMES D. BURKE Assistant Editor, DONNA ESCANDON STEVENS Copy Editor, KARL STULL THE-PL.A~T1RY SOCIETY Pr:oduction Editor, W ILLIAM MCGOVERN Art Director, BARBARA S. SMITH E'ditoriaf Assistant, JENNIFER VAUGHN o .~ --e- cP -B- 0 Viewpoints expressed in columns or editorials are those of the authors and do not necessarily represent positions of the Planetary Societ y, ils officers, or advisors. © 1997 by the Planetary Society. o Co.. founder CARL SAGAN Members' 1934· 1996 Board of Directors

BRUCE MURRAY Dialogue President Professor of PiafTetary Science and Geology, California Institute of Technology

LAUREL L. WILKENING Vice President Cnam:ellor, UniverSity of California, Irvine

LOUIS D. FRIEDMAN Executive Director

NORMAN R. AUGUSTINE Eugene ShoelTlaker It was accompanied by the photos sary to our spirit as the air we President and CEO, I feel a great loss with the recent of the planets and a nice little breathe. Lockheed Martin Corporation ANN DRUYAN tragic death of Gene Shoemaker. sticker with Saturn on it, and ~ROSANNA SHAPIRO, author and producer DONALD J. KUTYNA It was from watching Gene was followed shortly by a copy Cranford, New Jersey former Commander, US Space Command

explaining the origins of lunar of The Planetary Report, and I JOSEPH RYAN Executive Vice President and rilles on TV during the Apollo began to feel my money was not Science and Genemf Counsel, Marriott International missions of the 1970s that I first completely wasted. the Public ROALD Z. SAGDEEV former Director, caught the planetary science bug. Institute for Space Research. Today I came back from day In the May/June 1997 issue of Russian Academy of Sciences

Since then, his works have pushed three of Planet fest '97. The awe The Planetary Report, Clark STEVEN SPIELBERG me on toward formal study. No I feel is beyond my ability to Chapman addresses a critical director and producer KATHRYN D. SULLIVAN doubt there are hundreds or express in words. I have never shortfall in the scientific com President and CEO, Ohio's Center of Science and Industry thousands like 1p.e who have been to anything so well man munity. The inability to capture and former astronaut

been inspired by Gene. My only aged, so well thought out and the imagination and spirit of the Advisory Council

regret is that I never got the orchestrated, nor have I felt so public with the wonderment of JOHN M. LOGSDON Chairman opportunity to meet him. proud to be able to call myself a real science was overcome mag Director, Space Policy Institute. The Planetary Society is in member of the organizing body. nificently by Dr. Sagan. To con George Washington University DIANE ACKERMAN the best position to find ways of I wish I could express, face to tinue his legacy, those of us in poet and author BUZZ ALDRIN honouring so great a scientist. face, my heartfelt thanks to every the Planetary Society need to Apollo 11 astronaut One idea would be to approach member of the staff, and all the take personal initiative to rescue RICH ARD BERENDZEN the right people at NASA to see volunteers, for all their hard work American society from its recent educator and astrophysicist JACQUES BLAMONT if the next Mars station can be and the dedication that was evi disturbing, and growing, fascina Chief Scientist, Centre National d'Etudes Spatia/es, France named for him after its successful dent in the quality of the event. tion with pseudo-science. RAY BRADBURY arrival. It seems appropriate to The membership fee is embar The answer is there on the page poet and author ARTHUR C. CLARKE have all future Mars landers rassingly small and I am further facing Chapman's article (see author named in the memory of plane embarrassed that I ever felt it "Mars Link Evolves into Red CDRNELIS DE JAGER Professor of Space Research. tary scientists from whatever might not be worth the money. Planet Connection" in Society The Astronomica/lnstitute at Utrecht. the Nett/errands nation. The naming of the Carl Thank you one and all. News). Get the public on your FRANK DR AKE President, SET/Institute; Sagan Memorial Station should ~DEAN JOHNSON, side while they are young! Every Professor of Astronomy and AstrophYSics, just be the first. Ventura, California Society member should go to University of California. Santa Cruz GEDRGIY GOLITSYN ~GARY EVANS, their nearest elementary or mid Institute of Atmospheric PhySics, RUSSian Academy of Sciences London, England Men or Machines? dle school and give one of these STEPHEN JAY GOULD In the July/August issue of The Alexander Agassiz Professor of Zoology. educational packages to every Harvard University Planetfest Praise Planetary Report, Norman teacher there. The $30 cost isn't SHIRLEY M. HUFSTEDLER I have just been humbled. I do Horowitz opposed the idea of much, but if a school has not educator and jurist GARRY E. HUNT not know how the Planetary human missions to the planets. budgeted for it, a teacher would space scientist. United Kingdom Society first came by my address He stated that exploratory robots have to use his or her very limited SERGEI KA PI TSA 'Institute for Physical Problems, to solicit my membership, nor could do all that needed to be discretionary funds to get the RUSSian Academy of Sciences HANS MARK do I know why I did not toss the done. Not quite. Only human materials. Don't wait for the The University of Texas at Austin

letter into the trash as I do most voyages can satisfy our insatiable teachers and school systems to MARVIN MINSKY Toshiba Professor of of the unsolicited mail I receive, curiosity. Only human voyages come to us, take it to them from Media Arts and Sciences, Massachusetts Institute of Technology but something about the very can bring about the joy and free the member level. In addition, PHILIP MORRISON name of the group piqued my Institute Professor, dom of the indomitable human the Society should undertake a Massachusetts Institute of Technology

interest and caused me to open spirit, the exhilaration of having program to provide competitive PAUL NEWMAN the envelope. I read the letter and set foot on truly foreign soil and scholarships for elementary and actor JUN NISHIMURA it fed my curiosity even more, so living to tell the tale. Human middle school children to pro former Director General, Institute of Space and Astronautical SCience, Japan T decided the cost of membership beings have always been explor grams like Space Camp. SALLY RIOE was minimal and I could afford ~ROBERT L. BYERS, Professor of Physics, ers, seeking to experience first University of California, San Diego. to lose the money if it was indeed hand what is unknown. Jacksonville, Florida and former astronaut HARRISON H. SCHMID a sham. I did recognize Carl Let us not forget our Asimov: former US Senator, New Mexico. Sagan's name from the Cosmos to allow machines to be substi Please send your letters to and Apollo 17 astronaut DONNA L SHIRLEY series, which helped me believe tutes is to allow the human spirit Members' Dialogue, Manager, Mars Program. Jet Propulsion Laboratory that I might not be throwing my to shrivel and die. Yes, human The Planetary Society, s. ROSS TAYLOR Emeritus Professor, money away. voyages are dangerous, extremely 65 North Catalina Ave. , Australian National University, Canberra Some weeks later I received expensive, and may not always be Pasadena, CA 911 06-2301 my membership card in the mail. called for, but they are as neces- or e-mail : [email protected]. 3

e planetary scientists often fInd ourselves asked by journalists, "What positive results came from the Apollo lunar missions?" For those of us who watched the epic lunar voyages live on TV, it is a shock to realize that some of the earnest young reporters who ask the question were not even born at the time of the landings. In my own answers, I stress some thing that is an underlying principle of the Planetary Society and an ideal of its co-founder, Carl Sagan: when we speak of our environment, it is no longer just the meadow next door or even the global ecosystem, but a larger cosmip environment. We can no longer speak of the history of our own planet without speaking of the history of its sister world, the Moon, because the two are linked. We live in a double-world system, moving with in the larger system of planets. Before Apollo, geologists understood little of the Moon's history, and essentially only the last 10 to 20 percent of the history of Earth- the portion revealed clearly by the fossil record. Before Apollo, geologists discounted aster oid/comet impacts as a signifIcant process in Earth's history. The Apollo missions revealed the story of the Moon and, by doing so, fIlled in many chapters missing from the fust half of our planet's story. Apollo set the stage for awareness that we live in a larger cosmic environment. We can divide the history of the Moon into rough intervals, admitting that interval boundaries are not hard and fast: some processes overlapped or tapered off gradually; for clarity, we round off numbers here and there.

Origin o~ the Moon: 4.5 Billion Years Ago The Moon formed 4.5 billion years ago, probably from debris blown out of the newborn Earth by the most colossal impact in the history of the inner solar system. This hypothesis, derived in part from Apollo data, explains several mysteries that had puzzled scientists. For example, the double-world system of EarthIMoon, with a satellite one-fourth the size of its planet, is unique or nearly unique among planets. Pluto/Charon is a similarly double system, but Pluto is (continued on page 6) ------~

(continued from page 5) horizontal compared to the "north-south" smaller than our Mooll and its system is in axis of other planets in the solar system. some ways more like a large double asteroid. Though Safronov's work was little known Either way, the impact hypothesis, by giving in the West, my colleague Donald R. Davis statistical reasons to expect that a moon and I studied it and went on to make calcu forming collision would happen to only one lations of relative growth rates ofplanetesi or two planets out of eight or nine, explains mals. We presented a paper at a meeting in why our Earth/Moon pair is unusual. 1974 proposing that there had been bodies A second pre-Apollo mystery explained in Earth's zone large enough to blast out by the giant-impact hypothesis was that the enough mantle to make the Moon. Moon has very little iron compared to Earth's As a young scientist giving the paper, bulk composition; the Moon has a mean I was apprehensive when the famous re density and rocky composition that resembles searcher A. G. W. Cameron raised his hand not the Earth as a whole but rather the mate after my talk. Would he make some point rial of Earth's rocky mantle. Suppose the that would demolish our ideas? Instead, he impact happened after Earth's core fOlmed. said that he and William Ward had been Then an impact blew rocky mantle material, working on the same idea but coming at it from both Earth and the impactor, into a from considerations of angular momentum, debris swarm around Earth. According to the or total spin rates in the system. A giant giant-impact hypothesis, the Moon formed impact could solve certain puzzles about from this debris, which explains why it is the angular momentum of the Earth/Moon made of mantle-like material. system. Combining models of the total A third mystery: the Earth is rich in water angular momentum brought into the system and other volatiles, but the Moon is not. This by a glancing blow, Cameron and Ward difference is explained by the giant-impact called for an even larger impactor than we model: the ejecta were heated to extreme had- they suggested that Earth had been hit temperatures, so water and other volatile by something about the size of Mars! materials escaped from the debris into space Davis and I published our work in 1975, as gases. and Cameron and Ward published the next Lunar rocks revealed that the lunar mate year. However, the ideas received little atten rial has exactly the same oxygen-isotope tion because other researchers- who had composition as Earth (that is, the different been taught to abhor catastrophes as a mode oxygen isotopes exist in the same relative of explanation in geophysics-felt that the proportions in both bodies), whereas rocks giant impactor had literally been conjured from other parts of the solar system (Mars up out of the blue. Other theories should be rocks and meteorites of different types) explored, they said, before this "ad hoc" have different oxygen-isotope compositions. explanation could be permitted. Years later, The oxygen-isotope data thus ruled out researchers became more comfortable with earlier theories that the Moon formed else the idea that catastrophic impacts helped where and then was captured whole into an shape the solar system. Big impacts could orbit around the Earth. The oxygen data not be dismissed as "ad hoc" just because suggest that the Earth and the impactor had they were random. The giant-impact model similar oxygen-isotope chemistry and that emerged as the dominant hypothesis in a the impactor that hit infant Earth was a plan 1984 conference on the origin of the Moon. etesimal that had formed at about Earth's A new consensus has developed since distance from the Sun. then: you can't form planets without a hier archy of impacts. Small impacts, trillions Aggregat:ion of' Consensus of them, act to produce generally similar Is a giant impact plausible? A personal note properties of planets, such as circular orbits, may be in order to explain the answer. Dur prograde rotations (in the "normal" direction), ing the 1960s the Russian scientist Victor and low axial tilts. The few large impacts, Safronov, studying the idea that planets depending on their timing and the collision formed by aggregation from countless angles, impose "finishing touches" on the small planetesimals, inferred that many planets: quirky individual personalities like planetesimals had grown in each planetary Uranus' high axial tilt and Earth's unusual zone. A cosmic "survival of the biggest" satellite. rule operated, with the big ones sweeping up the smaller ones to make single planets Forming t:he Highland Crust:: in each zone. Safronov suggested that the 4.4 Billion Years Ago axial tilts of planets might be the result of The aggregation of the Moon from the debris this process; for example, a collision with cloud around the Earth involved so much a large planetesimal might have caused the infalling debris that the Moon's surface was 6 odd tilt of Uranus, whose axis is nearly (continued on page 9)

(continued from page 6) rock ages, revealed that the impact rate l strongly heated by the impacts. This heat 4 billion years ago was hundreds of times ing kept the surface molten, at least during the rate today. In fact, it is hard to trace the late stages of the Moon's growth, the rate farther back than 4 billion years fonning a global ocean of magma. In such because the cratering was so intense that a magma, low-density crystals float to the it obliterated older surfaces. top like ice slush on water. The magma This early bombardment fits beautifully ocean cooled and solidified into a low with our understanding that planets grew density crust that grew in thickness. This by sweeping up the interplanetary debris. explains why the ancient highlands of the Isotopic studies show that Earth grew to Moon-the initial crustal surfaces-are most of its present size 4.5 billion years ago, composed mostly of rocks fonned from within a short interval of about 50 million crystals of a common low-density silicate years. To accumulate an Earth-mass ofma mineral called feldspar. Feldspar is also a terial in 50 million years requires roughly major rock-fonning mineral on Earth, but a billion times the present impact rate. In not as concentrated at the surface. Rocks other words, the initial impact rate dropped fonned from feldspar aggregates are from around a billion times today's rate to called anorthosites, and astronauts brought a value a few hundred times today's rate by back many anorthosites, which seem to 4 billion years ago. This decline is roughly make up most ofthe lighter-colored regions consistent with the calculated rates for of the Moon. planets to sweep up interplanetary flotsam By perhaps 4.45 billion years ago the and jetsam during their statistical dance of anorthosite was fonning, though it was chance among the terrestrial planets. In continually blasted, pulverized, and even short, the Moon's ancient, cratered surface remelted locally by the rain of impactors shows the final stages of the collision pro and flooded by local eruptions of fresh cesses that gave rise to the whole planetary lava. Despite all this havoc, lunar soils system. contain telltale rock chips dating back This realization has affected our under beyond 4.4 billion years. standing of Earth and other planets. Earth itself was affected by the intense early Recession of' the Moon: bombardment, a fact not known until the 4.4 Billion Years Ago Apollo expeditions. For example, giant At the time of its fonnation, the Moon basin-fonning impacts may have thinned was much closer to Earth than it is today, the crust in some regions and piled up probably only a few Earth-radii away continent-like masses of crustal ejecta in (perhaps 24,000 kilometers, or 15,000 other regions. Such a process has been miles, out from Earth's center, instead of suggested as the origin of Mars' striking the present-day 380,000 kilometers, or asymmetry, with an old, cratered crust in 240,000 miles). Tidal forces between the one hemisphere and massive volcanic Earth and Moon caused it to move out plains in the other. The intense impact ward, away from its parent. Tidal forces, rate may also have made Earth's initial which arise from gravitational interaction climate so unstable and the surface so between a planet and its satellite, are very precarious that the establishment of stable much stronger when a planet and moon are life fonns could have been delayed. . close to each other. As a result, the reces Everyone has heard of the impact that sion of the Moon was fast at the beginning wiped out the dinosaurs and 75 percent and slow later. By the first few hundred ofthen-existing species 65 million years million years, the Moon had moved out ago; with an impact rate 10,000 times the to an orbit over half of its present distance present value, primordial impacts compa from Earth. The proof of all this is that the rable to that one were happening every Moon is still receding from Earth, and the few thousand years. rate of recession has been accurately meas ured, using radar reflectors left on the Was There a Catastrophe lunar surface by Apollo astronauts and 4 Billion Years Ago? Soviet lunakhod rovers. When astronauts were sent to the Moon, they were supposed to look for "genesis Intense BOlllbardlllent: rocks," rocks that might date back to the 4.4 to 4 Billion Years Ago origin ofthe EarthIMoon system. However, Impacts are crucial in understanding the there seemed to be a cutoff at about 4 bil infancy of the whole Earth/Moon system. lion years ago. The geochemists who mea Counts of the numbers of craters at Apollo sured the first Apollo samples, especially landing sites, which had different measured a team involving Gerald Wasserburg and 9 others at the California Institute of Technology, theorized of Hawaii researchers B. Ray Hawke, Jeff Bell, and that a temporary spurt in the impact rate 4 billion years others. Additional evidence for very ancient basalt flows l ago destroyed most ofthe earlier surface and most earli comes from Galileo multispectral images of the Moon's er rocks. This hypothetical burst of cratering came to be far side, where traces of basaltic material were found in called the terminal cataclysm-implying a sudden event ancient, heavily cratered basins. at the end of planet formation. Various theories were Lava patches that formed after 4 billion years ago spun as to the source of this burst of impactors. were better preserved because the cratering rate had On the other hand, the cutoff in rock ages around declined. They appear as the dark-colored, broad lava 4 billion years ago might be explained not by a cata plains of the Moon. Dated around 3.8 to 3 billion years clysm but simply by a steep decline in the impact rate. ago, they make up the features variously known as the In this scenario, any rocks formed in the surface layers "man in the Moon" or the "rabbit in the Moon," as iden 4.2 billion years ago would have been exposed to intense tified by diverse cultures down through the ages. These cratering and pulverized quickly, while similar rocks lava plains, mistaken for seas by the first telescopic formed 3.8 billion years ago would have experienced observers, were given the Latin name for seas, maria impact rates so much lower that they survived to the (MAH-ree-a; singular mare, pronounced MAH-ray). present. These two ideas-steady decrease versus a sudden Las1: 6asps and New Beginnings spike in the cratering rate 4 billion years ago- continue By 3 billion years ago, the Moon looked much as it to be debated today. Houston researcher Graham Ryder does today. Probably a few local lava flows erupted here points to the absence of impact melt-glasses from the and there until about 2 billion years ago. Half the history ancient period to support the terminal cataclysm. Impact of the EarthIMoon system was over as the Moon became melt-glasses are formed from rock melted and sprayed dormant. On the blue Earth, high in the sky over the out from impact sites and rapidly cooled into glassy lunar plains, the landscapes were barren, but multi shards and droplets. Does the paucity of these glasses cellular life was organizing itself in the seas. The Moon's older than 4 billion years mean the impact rate was history was nearly over, whereas the exciting story of lower before the proposed cataclysm or just that the pri life on Earth was still in its opening chapters. mordial glass fragments were pulverized by the intense Early telescopic observers proclaimed that "nothing cratering? A terminal cataclysm, if it happened, would ever happens on the Moon," although sporadic, uncon have strongly affected geological and biological devel firmed gas emissions have been reported. Something opments on Earth during the same epoch. important-at least in our human scheme of things finally happened in 1969, when a spindly spacecraft Forming 1:he Lava Plains- allowed the first humans to leap across the void from 4 1:0 3 Billion Years Ago the parent world to the daughter world. The sparsely While cratering proceeded from the outside, geological cratered basaltic plains were the sites chosen for the structuring of the Moon continued from the inside. first two lunar landings, because they afforded relatively Like most worlds, the Moon followed a law promulgat flat terrain-hence the famous landing site of Mare ed by workers as diverse as Isaac Newton in the 1600s, Tranquillitatis, or the Sea of Tranquillity. Georges Buffon in the 1700s, and Percival Lowell in Tranquility persists on the Moon, but we may soon the 1800s: small planets tend to cool offfaster than big return, not only to develop our capability of voyaging planets, all other things being equal. By the same princi the cosmic ocean but to tie up some loose ends in our ples, small baked potatoes cool faster than big ones. The understanding. One project for future explorers would subsurface regions of the Moon retained their heat only be to measure the dates of, say, 5,000 impact craters for the first one or two billion years, and then volcanic near a lunar base. Such a count would allow us to plot activity on the Moon tapered off. a chart of the bombardment rate versus time, showing During the period of volcanic activity, pockets of detail or structure in the curve at a resolution of a few molten material existed in the Moon at depths of a few million years. This level of precision might allow us to hundred kilometers, just as Earth today has regions of establish once and for all whether there was a catastro molten material below the surface. Whenever large im phe 4 billion years ago. We could also determine pactors fractured the surface layers, these deeper lavas whether the impactor that wiped out the dinosaurs was gained access to the surface and erupted into giant pools a single, random event or part of a wave of impactors that filled the available depressions. The types of rocks that repeats every 30 million years or so, as some have produced were lavas darker colored than the light crustal proposed. Making this determination would be a anorthosite rocks. Generally they are in the rock family worthy goal for a return to the Moon- a goal that known as basaltic lavas and are similar to basaltic lavas would unite planetary geologists, dynamicists, and that still erupt from many volcanoes on Earth today. biologists in a broad effort to fill in more pages in the Basaltic lava patches that formed before 4 billion years biography of our double planet and its hesitant march ago were rapidly obliterated, or at least buried under toward intelligent life. ejecta debris, by the violent impact cratering. Evidence of ancient basalts is revealed in certain dark-halo craters, William K. Hartmann is a Senior Scientist at the Planetary which are the result of impacts that punched through the Science Institute and an accomplished space artist. His light-colored debris and ejected dark-basaltic debris, as first novel, Mars Underground, about Mars exploration, 10 shown in recent spectroscopic observations by University was published this year.

THE 2F u

alileo has seen two worlds that have ice with a distinc groove lanes-but that was over and done with long ago. Gtive blue tinge- a subtle coloring associated with the Europa is clearly different, however, and the clue is its dis _ ebb and flow of an ocean underneath. Finding a simi tinctive hue. larity like this in widely separate parts of the solar system Now compare Figure 2 (below), which is a Galileo image commands attention, especially since variety is the lesson of Antarctica from the spacecraft's second Earth flyby in we learned so well from Voyager imaging, which ranged in December 1992, made from the same false-color filter unexpected wonders from the yellow burst of a volcano on combination as in Figure 1. The cyan (blue-green) color of 10 to the black sprays of geysers on Triton. Galileo, too, the Ross Ice Shelf is due to coarse-grained sea ice, contrast has amassed a heterogeneous gallery of planetary images. ing against the white clouds and continental snow. Sea-ice Along its winding path to the Jupiter system, the spacecraft encountered Venus, the Earth and Moon, and asteroids Gaspra and Ida. It was at Earth that Galileo got its first glimpse of blue ice. Arriving at Jupiter in December 1995, Galileo swung into orbit around the giant planet for repeated encounters with the large Jovian moons. From left to right, Figure 1 (above) shows Europa, Ganymede, and a slice of Callisto. These composite images were made by combining images taken through violet, green, and infrared filters. Given this combination of filters, these are not the colors you would see with the naked eye. We use false-color images and enhance contrast to get a better look at the details in a distant surface. White in these images indicates fine-grained, fragmented ice, as seen in recent craters such as Pwyll on Europa, Osiris on Ganymede, and Burr on Callisto (black arrows). Looking around these comparatively young features, you'll notice that older terrains on Europa have a bluish cast, much bluer than terrains on Ganymede and Callisto, even allowing for darken ing of their surfaces by non-ice contaminants. These color dif ferences reflect the varied geological histories of the moons. Callisto's life has been relatively dull apart from a gradually diminishing bombardment by impactors; Ganymede experi enced an episode of tectonism, probably associated with core 12 formation, that disrupted its surface and produced its bright Figure 1, left: Europa, Ganymede, and Callisto (left to right) have been imaged repeatedly by the Galileo spacecraft, which has served us as a latter-day Audubon of the Jupiter system. False color imaging imparts a characteristic blue to ice on Europa, a shade not seen on the other Jovian moons. Figure 2, bottom: Sea ice underlying Antarctica's Ross Ice Shelf emerges as a blue-green hue in this false-color image from Galileo's Earth flyby in December 1992. The striking resemblance to the blue ice in similarly processed images of Europa adds to evidence that Earth may not be the solar system's only watery world.

Images: NASAlJPL and Lunar and Planetary Laboratory, University of Arizona

crystals grow much larger than snow crystals because sea relatively coarse-grained ice extruded along fractures in ice forms at higher temperatures, being in contact with the Europa's frigid crust. warm ocean below. By analogy with Antarctica, scientists Impacting comets and asteroids, along with the intense believe that the blue in the older regions of Europa probably radiation from Jupiter's magnetosphere, tend to disrupt indicates resurfacing by ice crystallized from a warm liquid Europa'S surface over time, fragmenting the ice crystals to form a relatively coarse-grained surface, similar to the and making Europa appear less blue and more like its sister blue sea-ice seen on Earth. The bright blue ridges seen in the satellites, which we believe are geologically dead. How Tyre region (see front cover) illustrate this process at work, ever, it appears Europa is shaped also by heat from within, showing how older, darker surfaces are replaced with clean, probably generated within a warm ocean not far below the surface. It seems internal heat keeps pace with the destructive processes of radiation and im pact cratering to maintain the blue tinge of a world that is geo logically active, perhaps with an ocean. The possibility of a water ocean, first suggested in previ ous Galileo imaging of Europa, made headlines earlier this year. Water is a precursor oflife, when combined with carbon based molecules and energy to drive chemical reactions. Having learned from Voyager the lesson of variety, we may be on the verge of an even greater discov ery, coming more clearly into view through the eyes of Galileo.

Paul Geissler is a Senior Re search Associate at the Lunar and Planetary Laboratory in Tucson, Arizona. 13 ASA's new, smaller planetary spacecraft have started the object at visible, infrared, and radio wavelengths. Radar returning dividends. Mars Pathfinder' s landing and signals were bounced off Eros as well. Now NEAR will study Sojourner's rovings across the surface of Mars brought it up close for a full year. And you can come along for the ride. Nunprecedented public attention to the Red Planet, after two decades of neglect. What is next on the horizon? How can NEHn Is HEHDED []un WHY YOU become involved? The most fun way to get involved would be to watch the little By getting to know Eros! The "erotic" adventure I have in NEAR spacecraft when it sails close by in January. Unfortu mind has little to do with the god oflove, however. Asteroids, nately, that happens roughly over Baghdad in daytime. Perhaps especially those discovered in the nineteenth century, were of well-equipped astronomers in Hawaii and the western United ten named after gods and goddesses. In 1898, G. Witt of Berlin States can watch NEAR approach a few hours earlier .. . but found the object we know as 433 Eros, one ofthe so-called most of us will just have to use our mind's eye. Amor asteroids that orbits far inside the asteroid belt, crossing There will be times when you can study Eros yourself- if the orbit of Mars and- in fact-coming quite close to Earth. It you are an amateur astronomer or if you can lay hands on a is not (at least not yet!) an Apollo, a class of Earth-orbit-crossers. telescope and learn the tricks of backyard astronomy. The Eros is among the largest Earth-approaching asteroids, and NEAR camera-spectrometer science team welcomes interest that is why it is the chief target of the Near-Earth Asteroid ing observations of Eros by amateur astronomers and will add Rendezvous mission (NEAR). The Volkswagen Beetle-sized their data to the existing database in support of the mission. spacecraft had a remarkably successful encounter with the Asteroid orbits aren't so nicely circular and don't hug the larger, black, main-belt asteroid 253 Mathilde on June 27 (for ecliptic as closely as planetary orbits (the ecliptic is an imag encounter details, see page 20). Next January 23, NEAR will ined plane from the center of the Sun passing through the cen fly just 500 kilometers (300 miles) above the Earth's surface ter of the Earth). The good news is that Eros' elongated orbit to gain a final trajectory change from our gravity so that the periodically brings it in from well beyond the orbit of Mars to spacecraft can nuzzle up to Eros a year later. good visibility from Earth. During 1999, NEAR will maneuver around the asteroid, The bad news for northern observers is that at closest using sensitive instruments to map its surface, measure its approach next April, the inclination, or tilt, of Eros' orbit sends chemistry, and generally investigate Eros like no asteroid has it far south of the ecliptic. It will be doing an epicycle (an ever been investigated before. apparent looping motion as seen against the background of Back in 1975, Eros came very close to the Earth (within 22.5 stars) along the fringes of the Milky Way in the constellations million kilometers, or 14.0 million miles), the closest pass since Lupus and Centaurus. Observers in the US, unless you live in the asteroid was discovered, and astronomers responded with southern Texas or Florida, will have difficulty spotting Eros an observing campaign. Telescopes around the world scrutinized low in the southern sky from March through May 1998.

14 BV [LHnH n. [HHPmHN

But there are times of good visibility, especially if you use Internet, for the asteroid will be lost in the Sun's glare until a telescope 6 inches or more in diameter. You can start look about November 1999. ing for Eros in the morning sky. As October begins, Eros is As the NEAR mission draws to a close, early morning in Gemini; it moves through Leo in November and is near observers may again find a faint, 14th magnitude Eros shift the bright star Spica as 1998 begins, brightening from 13th ing amid the stars south of Spica. magnitude to 12th. Eros will brighten to 11th magnitude Finding a faint asteroid in a telescope is tricky but reward (100 times fainter than the unaided eye can see) when it is ing. You need good coordinates for Eros (interpolated at far south in Lupus. hourly intervals) and a good star map, showing stars at least as faint as Eros. Eros will be the one that is "out-of-place" SPHcn:nHfT mEETS HSTEnl1l11 and that visibly moves from hour to hour. You will then Next summer, Eros climbs higher in the sky- and is visible want to monitor Eros' brightness as it spins, with its broader during more convenient evening hours-while it fades to 14th and narrower faces alternately reflecting more or less sun magnitude (fainter than Pluto!), as the Earth's faster orbital light. Eros typically varies about a magnitude in brightness motion propels us ever farther away from Eros. During autumn (compare it with nearby stars), showing two maxima and 1998, it will take a larger telescope (8- or lO-inch aperture) two minima during its 5 hour, 16 minute "day." and sharp eyes to follow the small asteroid's path through For star charts showing Eros' path through the stars, check Scorpio, Sagittarius, and then into the evening twilight. out the NEAR project's Web site, where non-telescope-owners In January 1999, NEAR reaches the vicinity of Eros. A too can follow NEAR's progress. There are links to NEAR's series of rocket bums slows the spacecraft down until it recent pictures of Mathilde, as well as an educator's guide hovers about 500 kilometers (300 miles) away from Eros. that introduces the spacecraft, its instruments, its trajectory, The spacecraft instruments will map the global properties of and some of the issues that NEAR was designed to investigate. the oblong object. Engineers at NEAR's home base (The During most of 1999, with Eros behind the Sun and new Johns Hopkins University Applied Physics Laboratory in pictures and data being posted almost daily, NEAR's Web Laurel, Maryland) will gain confident knowledge about Eros' site will become the place to be. shape and gravity field. Then they will lower NEAR to orbit just 35 kilometers (22 miles) from the center of Eros, which Clark R. Chapman is a member of the NEAR camera is indeed close, given that Eros is 35 kilometers long (at its spectrometer science team. He is also the regular "News elongated end, the surface sticks out only half that far from & Reviews " columnist for The Planetary Report. the center). The sharpest pictures and best compositional measurements will be made from the close-in orbit. Through , Visit the NEAR home pageo~' the World Wide Web at: these maneuvers, NEAR fans will have to watch over the http://sd-www.jhuapl.edu/NEAR!

15 ------_._--...._- _._ ...... _ ------,

Basics 0" Space-flight:

by DaV'e Doody

he acronym ATLO is used so frequently through- CDS support equipment to monitor and operate the CDS. out space flight operations it might as well be a An Attitude and Articulation Control Subsystem (AACS) valid English word. ATLO stands for Assembly, engineer uses a different set of hardware and software for Test, and Launch Operations- the period of gestation and the AACS support equipment. birth for a robotic spacecraft. In this installment of Basics, At times the spacecraft power is turned off to allow we'll look in on Cassini's ATLO, since it's typical of mechanical engineers (the "mechanics") to attach this flight ATLO for many spacecraft now plying the depths of the computer or that science instrument. They perfOlID electri solar system. cal measurements with the power still turned off to be sure all the connections were properly made. When power Aml.fD :n O:wTeJ'tdi.e~1lI7 comes back on, the new components are put through their A spacecraft's conception in the minds and computers of paces. Component developers send commands to the newly planners, designers, and scientists covers a lot of ground. installed equipment, load it with any operating software it It begins with studies of the concept for a mission and needs, and scrutinize all the telemetry that comes back includes planning and obtaining funds, which in the case "down" to them. They have to check out all the commands of NASA's interplanetary robots come from the congres their piece of the spacecraft is designed to recognize, and sional budget. The spacecraft's design proceeds until metal they have to make sure the telemetry it returns truly reflects is cut and bent and electronic circuits and mechanisms are its operational states-typically, both hardware and soft fabricated and individually tested. This whole process may ware states. take the better part of a decade for the larger missions, such At certain points during ATLO, the spacecraft is treated as Galileo and Cassini. Mars Pathfinder, one of the faster just as though it were flying through interplanetary space. development, lower-cost Discovery missions, took only The spacecraft's radio transmitter and receiver are connect about four years from startup to its bouncing, spectacular ed to a special antenna, and the Deep Space Network (DSN) stop on Mars. talks to the spacecraft using a station near the grounded ATLO ends after launch, and a spacecraft then enters its spacecraft. The DSN uses these opportunities to measure cruise phase, freefalling toward its destination and giving the minute amount of time the spacecraft takes to receive its ground operations crew some valuable flying lessons. and re-transmit signals-a key piece of information when Encounter is the next phase, when all the science observa DSN people use the ranging technique (see the July! August tions are carried out. Most interplanetary missions then en 1995 Planetary Report) to precisely measure the space ter an extended-mission phase, when the last bits of data craft's location in the solar system. are squeezed from the spacecraft. The extended-mission phase is a good time to take some risks in search of new C:8"-ssIn:. -.:n fPEaJ'ffi:c:dlar data, as was done with Magellan's aerobraking adventures Early in 1997, eight years after the Cassini program's start (see the March/April 1994 Planetary Report) and will be up, a thermal-engineering model of the Huygens probe done in Galileo's repeat encounter with 10, deep within the was attached to Cassini at the Jet Propulsion Laboratory dangerous Jovian radiation belts. (JPL), just as the real Huygens probe, bound for Titan's A TLO is the phase when a spacecraft takes recognizable atmosphere, will be attached prior to launch. Then the shape. Early in thi-s period the spacecraft's computers assembled spacecraft was carried slowly up the hill at JPL come alive, and they communicate almost constantly with on a truck, flanked by engineers and technicians on foot. the people working on the craft. Most of the time this com Atop the hill, the spacecraft was subjected to the most munication, which includes telemetry and command (see torturous tests of all. During this phase of ATLO, affection the NovemberlDecember 1995 Planetary Report), passes ately called "shake and bake," the spacecraft received through support equipment operated by engineers who spe vibrations that simulate the rough, noisy ride it will have cialize in particular spacecraft components. A Command atop its Titan IV -B launch vehicle. 16 and Data Subsystem (CDS) engineer, for example, uses Then the spacecraft was placed in a huge vacuum chamber-85 feet tall and over 25 feet wide, the Solar Thermal Vacuum chamber gave Cassini a taste of space. The inner walls of The Chamber were lined with panels containing liquid nitrogen. A hot simulated Sun shone down from a large mirror above. The thermal engineers tested all the extremes of heating and cooling the spacecraft would have to endure for its 12-year primary mission. One cold, rainy night as I stood outside The Chamber, a tanker truck pumped liquid helium to a special and very cold target plate inside. Only a handful of degrees above absolute zero, this was as cold as it gets. The target plate simulated the extremely cold, dark space in which some of Cassini's infrared-sensitive instruments would have to work. I realized how balmy the rain was by comparison. Having passed shake-and-bake with flying colors, the CassinilHuygens pair was trucked back down the hill to the clean room. With all the scientific instruments and other components installed, we carried out a "science end-to-end test." Science teams around the world, without leaving their home institutions, used their local computer worksta tions to develop sequences of commands to operate their instruments. This is how the science teams will operate when Cassini is in flight. Once their individual command sequences were completed and checked, the commands were sent to JPL and assembled into one file of command data. The DSN established a two-way radio communications link with the live spacecraft, still in its "hangar." Finally, the Ace (see the November/December 1996 Planetary Report) radioed the command data aboard Cassini, check ing to make sure every bit arrived intact. Over the hours while the command sequence clocked away, all the results flowed back through the DSN as telemetry and out to the scientists worldwide. Once the scientists analyzed the data from their instruments, it was clear that Cassini, Huygens, and the ground-based operations system were alive and very well! f#i:rn:a'l S-'e-.P$.O 'L

he Pathfinder landing on Mars will continue as a may remember "Visions of Mars," the compact disc (CD) source of special joy and pride to Planetary Society containing works of science fiction that flew with the ill ~members, because we are there! Planetary Society fated Russian mission Mars '96. Accompanying the CD names made it to the Red Planet in two surprising ways. was a "documentation chip" that included the names of the First, the memory of Carl Sagan, our late president and 100,000 members of the Planetary Society at the time the co-founder, was honored in the naming of the Pathfinder CD was prepared. This documentation chip was included in lander station. NASA Administrator Dan Goldin announced MAPEX- the microelectronics and photonics experiment at Planetfest '97 that the spacecraft on Mars would be devised by the Jet Propulsion Laboratory (JPL) as a passive known as the Carl Sagan Memorial Station, acknowledg recorder of incoming radiation at the Martian surface. The ing Carl's lifelong devotion to the exploration of Mars, Society enabled inclusion of MAP EX on Mars '96 with the including scientific participation and public education in idea that when the first human explorers came to Mars, it every US Mars mission from the Mariners to Mars Global would be there to provide information about the radiation Surveyor. This gesture is deeply appreciated by Carl's environment on the planet. family and by all of us at the Planetary Society. The Carl JPL's Microdevices Laboratory made several MAPEX Sagan Memorial Station follows a precedent set in the spares-on the chance they might be useful for future naming of the Viking 1 lander, called the Thomas A. Mutch missions. Memorial Station after the NASA Associate Administrator It came as a wonderful surprise when we learned, shortly who led the imaging team on that mission. before the July 4 landing, that the Mars Pathfinder Project Team, under the direction of Manager Anthony J. Spear, Ral1 af Sncist'l Ilcz.rn..es had put MAPEX aboard their spacecraft. Our members' SLLlT.v-L1rE!5 LCl55 at In.cur5 '2Q names and names of project personnel and others associated Members ofthe Planetary Society (including Carl Sagan) with the mission are all there together on a little chip made it to Mars in another way-aboard Pathfinder. You inscribed by electron-beam lithography. To read these names on Mars would require a scanning electron microscope, not to mention an intelligent life form on site to do the reading. That may happen someday, but for now I am sure our members agree: it's nice to be there.

A CCS55 to. t.h.e.. Ad~ Getting everybody involved-at least those who care about the future-has been a goal of the Planetary Society since our founding in 1980. Carl Sagan's initiative in developing the Pioneer plaque and Voyager Record, with their messages on behalf of all humanity, pointed the way to the naming of individuals for the "Visions of Mars" CD. The name ofthe microrover Sojourner is an example in the same spirit from the Pathfinder mission. Valerie Ambroise of Bridgeport, Connecticut entered the winning name in an international student contest co-sponsored by the Jet Propulsion Labora tory and the Planetary Society. Looking ahead, we have gathered more than half a million signatures in cooperation with the Cassini Project for delivery to Saturn. And we will continue to offer opportu nities for expressions of human confidence in the future, because we hope, as Carl Sagan hoped, that humans from all nations will someday venture to Mars and throughout the solar system.

As Mars Pathfinder was being aSSBmbled for launch, a microdot containing the names of a/l Planetary Society members as of Louis D. Friedman is Executive Director ofthe Planetary 18 October 1993 was attached to the spacecraft. Photo: JPUNASA Society. nI News and

Revievvs by Clark R. Chapman

niversity of Arizona astronomer took over his responsibilities for solar other to address what might be done Tom Gehrels has hosted many system exploration- from the smallest should a dangerous projectile be found. U scientific conferences over theoretical research project to the largest Rahe skillfully negotiated the tricky the years, often connected with plans for deep-space missions, like Galileo and politics of the impact hazard. Vice books in the seminal Space Science Se Cassini. liirgen Rahe especially nurtured President Quayle had been ridiculed for ries about the solar system. Following an the new Discovery series of smaller expressing concern about asteroids, early conference, Gehrels would invite missions; some say he was almost single Edward Teller (father of the H-bomb participants to join him on a hike up to handedly responsible for making the and the Star Wars program) wanted to Seven Falls in the Catalina Mountains, Near-Earth Asteroid Rendezvous (NEAR) blow up asteroids near the Earth, and and it was during one of these hikes that mission a reality. Regrettably, Rahe few people could relate to the combina I met and chatted with an impeccably died only a week before NEAR achieved tion of extremely high consequences dressed European astronomer, walking its first scientific triumph, returning and extremely low probability that up the trail in street shoes. Clearly, he pictures of the unexpectedly giant defines this hazard. It took a man with was not prepared for a trek into the cratered asteroid Mathilde. Officials at Rahe's diplomacy and quiet fortitude wilderness, but he nevertheless walked The Johns Hopkins University Applied to handle these complex issues. the several miles up the dusty, desert Physics Laboratory, which built and Most people succumb to cancer, heart trail. This was Jiirgen Rahe, the comet operates NEAR, paid tribute to Rahe on attacks, automobile accidents, and simi scientist from Bamberg, Germany. encounter day. lar causes. Almost nobody dies from Later, Rahe became responsible for A week later, Mars Pathfinder cap cosmic impacts, and yet such events do NASA's planetary astronomy research tured the front pages- and hearts-of happen, as the sorrow of Rahe's untimely programs, and, finally, he led NASA's America and the world, as it bounced death attests. When a freakish impact on whole Solar System Exploration Program. onto Mars and its miniature rover ex a global scale happens, almost everyone In June, unexpectedly, Rahe was again plored nearby rocks, the first successful might die. We live with capricious unprepared for a bout with nature: a vio return to Mars since Viking, two decades Nature. And liirgen Rahe labored to help lent thunderstorm and microburst blew ago. The coordination of programs like us to deal with that reality. I don't know over trees in suburban Washington, DC, Mars Pathfinder was eased by Rahe's how many people are killed annually by near Rahe' s home, one of which crashed oversight, and it is again too bad that falling trees, but it is a rare occurrence. onto his automobile, killing him instantly. he did not live to witness the rebirth of We must accept the fact that freakish popular excitement in NASA's explo events are part of the fabric of our exis AlTlid Bureaucracy ration of the planets. tence. Rahe is survived by his wife, Jiirgen Rahe brought scientific integrity, Hazel, and his daughter Isabell, near . excellence, and diplomacy to a federal The IlTIportance of the whose horse stable he was driving when agency that already stands above most IlTIprobable he died. government bureaucracies. I first worked Jiirgen Rahe, more than any other NASA Rahe was a co-investigator of the with Rahe in connection with the Inter official, paid heed to the newly appreci Giotto mission to Comet Halley long national Astronomical Union's Commis ated hazard of impacts by asteroids and before assuming his management re sion 15, which coordinates comet and comets. When Congress mandated, in sponsibilities for the NEAR and Rosetta asteroid research programs. Later, I 1990, that NASA evaluate the impact missions to asteroids and comets. But worked with him when he reorganized hazard, Rahe was assigned to get the someone got out to the solar system's NASA's Planetary Astronomy research studies accomplished. In order to estab tiny worlds before Jiirgen Rahe-Saint programs. In that capacity, he skillfully lish the scientific context of the hazard, Exupery's Little Prince. In the penulti combined an objectivity in distributing with full international participation mate chapter of the famous tale, the the declining research funds, while still (appropriate for a hazard that knows no little prince departed for good: "He managing to care for the individual needs national boundaries), Rahe asked me to remained motionless for an instant. He of each researcher supported by the organize-with the Planetary Society's did not cry out. He fell as gently as a program. He found pathways through the assistance- the 1991 First International tree falls." bureaucratic labyrinth to do what was Conference on Near-Earth Objects. And right for science. he appointed two committees to investi Clark R. Chapman is an Institute Scien When Wesley Huntress was put in gate the matter-one to consider how tist in the Boulder, Colorado office of charge of all science in NASA, Rahe we might detect possible impactors, the the Southwest Research Institute. 19 r-----· --- ~_~~~-----______.,._ -..,

-no: Flyby Yields First Closeups of Mathilde

er a flawless encounter with asteroid 253 Mathilde, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft ~ sent back images revealing an intensely dark body that has survived a barrage of world-shaking impacts. The NEAR .images of Mathilde provide our first detailed look at one of the C-type asteroids, which scientists believe to be leftover hunks of the primordial material from which planets formed. If so, the pictures you see here represent pristine remains from the early solar system. The June 27 encounter lasted only 25 minutes, returning 534 images from the spacecraft's multispectral camera. Lighting con ditions were a challenge, as NEAR intercepted Mathilde at twice Earth's distance from the Sun, with only 3 percent of available sunlight reflected from the target surface. Mathilde is about twice as dark as charcoal. With perfect timing and no unanticipated problems, the encounter proved successful for the NEAR mission team, headed by Robert W. Farquhar of The Johns Hopkins University Applied Physics Laboratory. In less modest terms, the flyby was a slam dunk. Mathilde is thoroughly cratered, like the Galileo-encountered asteroids Gaspra and Ida. However, NEAR scientists were surprised to find several craters on Mathilde that must have come close to blowing apart this tiny world, which has a mean diameter of only 52 kilometers (33 miles). One particularly deep scoop, penetrating an estimated 10 kilometers (6 miles), allows a cutaway view into Mathilde's interior. Joseph Ververka of Cornell University, head of the NEAR imaging science team, notes that slopes on this and other craters show no signs of differentiated rock types. Apparently Mathilde is made of the same dark material through and through. This observation is consistent with C-type asteroids being made of pre-planetary, carbon-rich material- rock so primitive it precedes the geologic processes that form the rocks we recognize on Earth. The bulk density of Mathilde is remarkably low, according to preliminary analysis of the radio science experiment, which mea sured the slight tug from the asteroid's gravity on the trajectory of the spacecraft. Density gives a clue to Mathilde's origin, explains Donald K. Yeomans of the Jet Propulsion Laboratory. Low density suggests Mathilde may have accumulated as a loose conglomera tion of material, a kind of rubble pile. The heavy cratering may be responsible for at least part of the loose-conglomeration structure of Mathilde-if that is indeed the con'ect model. A solid, monolithic structure would probably have shattered as a result of the hits taken by Mathilde. If Mathilde is a At closest approach (1,200 kilometers, rubble pile type of object, the shock of large impacts would be more or 700 miles), the NEAR spacecraft could easily absorbed without busting up the asteroid completely. As an resolve craters smaller than 500 meters across, Looking closely along certain analogy, if you hit a brick with a bullet, it shatters. A sand pile crater rims, you can see sections that suffering the same hit will only form a crater. are curiously straight, perhaps indicating Scientists continue to study the Mathilde images as NEAR heads that faults or fractures have influenced on to its primary target, 433 Eros. crater formation. Raised crater rims - from The Johns Hopkins University Applied Physics suggest ejecta flew only a short distance 20 LaboratorylNASA before returning to Mathilde's surface. Asteroid 253 Mathilde images from the NEAR spacecraft constitute the first science return from NASA's "faster, cheaper, better" Discovery program. At first glance, scientists are intrigued by two characteris tics revealed in these images-the uniformity of Mathilde's rock and the asteroid's history of drastic impacts. From this perspective, Mathilde measures 59 by 47 kilometers (36 by 29 miles), and the shadowed indentation at center goes to an estimated depth of 10 kilometers (6 miles). The wedge-shaped shadOW at lower right defines the edge of another substantial crater, as does the diagonal aUhe top left. The mountain-shaped feature just left of the diagonal may mark the edge of a fourth large crater. Battered Mathilde is shown in this image mosaic at a distance of 2,400 kilometers (1,500 miles) with resolution ofdetails as small > as 380 meters. World

VVatch by Louis D. Friedn1an

The following column was first pub But these are technical details, and the Progress-Mir accident are doing lished on the Opinion page of the Los the space station is not about technolo just that. Angeles Times, the day before the Mars gy. It is about a vision-a vision ofhu Space is a difficult place-it is Pathfinder landing, and it cites the risk manity moving off its island Earth to risky. We've outgrown the "routine that we knew was inherent in that land new worlds and new achievements. Will access to space" nonsense promul ing. Risk is an important consideration the accident, the potential loss of life, gated by the NASA of the 1980s. in malang decisions. It must be weighed the delay to science experiments, and We don't send teachers or members against the potential gain. In an earlier the extra money needed for repair and of Congress or journalists to space. piece, entitled "Zero-risk, Zero-gain, " recovery cloud that vision? I heard a We shouldn't be promoting space J argued against those who opposed the congressman proclaiming that we tourism-it, too, is nonsense. (It's Cassini launch because ofrisk related to should stand down in our program until hard enough to book a tour to the the nuclear power sources on the space the Russians meet "American safety Antarctic or the Marianas Trench, far craft. The risk, analyzed exhaustively in standards." This is confusing since the more benign and easy to reach envi an environmental impact report, was Russians operate the only space station ronments.) A hundred reports remind very small. The risk ofanyone dyingfrom and have done so with a perfect safety us that someone will again die in Cassini was less than the risk ofyou, the record for 11 years. The US has not yet space- some accidents will occur. reader, being struck by lightning. But matched that achievement. Such postur Robotic missions to Mars have had the gain Font sending an international ing is not a challenge to the space sta their difficulties; there were failures in mission to Saturn and its moons was tion's design, but it is a challenge to the 1989,1993, and 1996. Still, we are on very large, a rich cascade ofn ew infor VISIOn. our way, with a national policy com mation about the worlds around us. The public- in this nation and others mitted to launches at every opportunity Ultimately all risk and gain tradeoffs - will have to decide whether to view - a real Mars program, leading to must be endorsed by the public, express the Mir incident and the inherent risk of robotic sample return and then human ing their will in the political process. space exploration with clear or clouded flights early in the next century. This vision. Ifthe risk, the cost, and the un robotic Mars program and the human Houston-Do We Really certainty are too great, then we will space station program converge in their Have a Problem? remain bound to Earth. But with prudent common goal of sending human ex What does the Progress-Mir collision risk, reasonable cost, and resilient plan plorers to Mars. This is in the public in mean to our future in space? First, let's ning, the public may support the contin terest. Indeed, it is the public's interest be thankful there was no loss of life uing quest to go farther and faster to in the questions of life in the universe, we should not forget the praise and ad explore new worlds. evolution of planets, and the origins of miration due our Russian colleagues for The space station itself cannot ful our physical and biological world that building such a resilient system and fill that quest. It leaves us still stuck fuels the space program. devising procedures that maintain safety in Earth orbit, doing experiments that If we want zero risk, we should stay even when the worst happens. As in the for the most part could be done better at home (and even that isn't so safe). If earlier fire incident, Mir has proved and cheaper with robots. But as an in we want to seek adventure, to explore remarkably robust. ternational venture, the space station and gain new knowledge of life in the The Russian space station has lasted can fulfill significant objectives: universe- past, present, and future 11 years in orbit-three years longer training humans to live in space and then we must accept risk. than it was designed for. It now carries cope with difficult engineering re If the recovery goes well, perhaps American crews and has enabled the quirements, defining the effects of the accident on Mir may be helpful. US to jump ahead five years in its hu long-duration flights on human The engineering lessons learned will man space program with long-duration physiology, and building the partner remind us both of the adventure and the flights and life science experiments. ships necessary for a trip to Mars, risk, preparing us to take even bolder The experience of working and coping the asteroids, or even a re-run to the steps into the future. in space, being gained on Mir, will cer Moon with human explorers. The tainly help the crews of the interna Mir mission, the construction of the Louis D. Friedman is Executive Director 22 tional space station. international space station, and even ofth e Planetary Society. Society NelNs

Thousands Celebrate with a copy of Comet by Carl Sagan and of Natural History-Hayden Planetarium Pathfinder at Planetfest Ann Druyan, with a special inscription in New York, Dr. Neil Tyson is the Over the July 4 weekend, the Planetary from Ms. Druyan. Friedman and Ocampo newest member of the Planetary Society's Society's Planetfest '97 welcomed thou led several discussions at the workshop, growing Board of Directors. A columnist sands of visitors to the Pasadena Con which focused on encouraging develop for Stardate magazine, Dr. Tyson has vention Center to witness first-hand the ing countries to participate in worldwide authored two space science books geared unfolding of Mars Pathfinder's landing efforts to increase near-Earth object toward nonscientists. Dr. Tyson's profes and the release of Sojourner. Members observations. sional study focuses on dwarf galaxies of the Jet Propulsion Laboratory (JPL) -Louis D. Friedman, and the "galactic bulge" at the center of Pathfinder mission team joined celebrants Executive Director the Milky Way. in the center's packed 1,800-seat Main -Charlene Anderson, Hall, offering updates, insights, and their Rover Roundup Director ofPublications own excitement as the weekend events Papers Published developed. Images came to the center Selected papers from the Planetary Soci Scholarship Created direct from mission operation headquar ety's International Conference on Mobile The Dr. W. Reid Thompson ofthe Class ters at JPL in Pasadena. Planetary Robots and Rover Roundup of 1969 Memorial Science Scholarship Exhibit hall displays, discussions (led will be published in the September/Octo has been established at Washington by some of the world's leading scientists ber issue of Space Technology, which is County High School, Springfield, Ken and engineers), book signings, and films dedicating a special section to the in tucky, where Reid graduated salutatorian were well attended by the more than ternational conference. Professor Klaus of his class. Thompson's life and success 8,000 people who joined in the event Schilling of Steinbeis Transferzentrum ful career in scientific research ended last each day. in Weingarten, Germany, which has de year after a long and courageous battle With the sponsorship of Sun Micro veloped rovers for the European Space with cancer. systems and NASA Integrated Network, Agency, selected 12 papers submitted by The scholarship will go to the student audio feeds for many symposium discus conference participants. The first confer with the highest grade point average of sions, as well as video feeds of maj or ence of its kind, the Rover Roundup took four or more high school science classes exhibits, were beamed to the world via place in Santa Monica, California in that include chemistry and physics or the Internet. Sun Microsystems will January 1997 and brought scientists and anatomy. The recipient must also have archive many Planetfest events for view engineers from around the world to been accepted at any four-year college ing at their site on the World Wide Web demonstrate prototype rover technology in the United States with an intended and at the Society's award-winning home for remote planetary exploration. major in natural or physical science or page. Watch for more information about -BM medicine. The Planetary Society has events at Planetfest in the November/ contributed $500 to the scholarship. December issue of The Planetary Report. Order Your Mars -Donna Stevens, Assistant Editor -Bill McGovern, Production Editor Symposium Proceedings Proceedings from the November 1996 Society Attends Washington, DC symposium Life on UN/ESA Meeting Mars, What Are the implications? More Nevvs Planetary Society Executive Director (cosponsored by the Planetary Society Louis D. Friedman and Advisory Council and George Washington University) are Mars Underground News: member Adriana Ocampo led the Plane now available to Society members. Orga A photo essay of Pathfinder'S tary Society delegation to the United nized by Society Advisory Council Chair success and the newest images of the Red Planet. Nations/European Space Agency's de John Logsdon, the symposium included veloping countries basic space science a wide range of discussion topics from Bioastronomy News: and past life on Mars; workshop in Tegucigalpa, Honduras science to religion. Complete proceed Pathfinder ice and ozone on Jupiter's moons. in June. The Planetary Society has co ings are available by sending $5 for sponsored these workshops for six years shipping and handling to "Symposium" The NEG News: CCD technology and the amateur as a United Nations nongovernmental at the Planetary Society address. astronomer. organization enlarging interest in plane - LDF For more information on the Soci tary exploration and the search for ety's special-interest newsletters, extraterrestrial life. Neil Tyson Joins Board call (626) 793-5100. Friedman presented Honduran Presi Princeton University astrophysicist and dentDr. Carlos Roberto Reina Idiaquez acting director of the American Museum 23 ~~-~----,---..----- ~ ~~------~---~ ------;"'~...... --o:-- --,

ere we see the rover Sojourner in action as it visits the rocks of Mars, The im age has been color-enhanced to make H the differences between the rocks and soi l more visible, Th e northeast face of the boulder Yog i (red arrow) is fresher than the rest of the rock, This may be due to eoli an (wi nd- driven) sco uring or simply because pi eces broke off, exposing a newer surface, The blue arrows point to Barnacle Bill (left) and Crad le, both typical of the area's small er, unweathere d rocks, As Sojourner turned to deploy the Alpha Proton X-Ra y Spectrometer, its wheels dug particularly deeply and exposed white material (see inset), which may underlie much of the site, The lander's rear ramp, from which Sojourner drove down to the planet's surface, is visib le at lower left. Image: JPUNASA

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