OUR WORLDS is available in paperback for $19.95 at the C.U.P. web site . It contains personalized essays by other planetary scientists: Steve Squyres, Ellen Stofan, Carle Pieters, Paul Weissman, John Spencer, Jonathan Lunine, and Bill McKinnon. The preliminary version of Clark Chapman's essay is presented below.
Editor's Intro.: Clark Chapman. Deep thinker. Old hand. Charmer. Wizard. These are the words that come to mind for Clark Chapman. Born [CRC note: actually not...I was born in California] and reared in New York (as are a surprising number of the authors in this book), Clark was educated at Harvard and MIT. After completing his PhD, he moved to Tucson and dedicated his career to the study of the belt of debris collectively called the asteroids which orbits in the emptiness between Mars and Jupiter. A scientific life dedicated to the study of asteroids might seem very narrowly focused but, as we learn below, beyond being endlessly fascinating in their own right, asteroids retain important clues to the birth of the solar system, to impact catastrophes on the Earth, and to the way in which objects "weather" in space. In 1996, Clark and his wife, LYnda, moved to Colorado, where Clark now works, plays, and enjoys his mountain home, dubbed "Rancho Europa."
Our lives are filled with many fascinating things. How did I come to love the myriad of small worlds known as asteroids? First, let me try to help you visualize what these little objects are like, which is not easy. After all, extraterrestrial environments seem truly alien to beings evolved on an immense, apparently flat landscape. Notwithstanding the failure of Christopher Columbus's ships to fall off the sea's "edge", no amount of book learning compels us to truly feel that our world is round. Perhaps, on a tiny round world, as the French writer Antoine de Saint-Exupery depicted asteroids in his fable "The Little Prince," we could actually see the horizon falling away just before us and finally know what it's like to live on a sphere. But the laws of physics tell us otherwise. Many years ago, I heard M.I.T. physicist Phil Morrison explain that the nature of matter itself dictates that the Earth must be round, but that rocky asteroids need not be round at all. And -- except for a little guy named Dactyl, who we'll meet much later -- they aren't!
As a kid, I toiled for weeks with my crayons to transform my father's desk-top globe into a giant, flat map, drawn on the white sides of dozens of cardboards from his laundered shirts. Immersed in my project, it was just a small leap of faith to picture myself on the surfaces of the planets, for instance as painted by Chesley Bonestell in Life magazine. I also marvelled at an occasional comet, hovering over the elms in the smoggy skies of the industrial eastern city that was my home. I was especially amazed at the rings of Saturn, revealed through the cardboard-tubed 2-inch telescope my father assembled. The rings were flat, solid objects, so far as I could see.
How much more difficult it is to visualize a swarm of individual small worlds! That, it turns out, is the reality of Saturn's rings -- the Voyager spacecraft's radio experiment showed that much of the bulk of the rings is in house-sized objects. And, despite the representations in sci-fi films and "Star Trek," the asteroid belt is even more difficult to envisage than the rings. The belt is really a torus, located beyond the orbit of Mars, containing hundreds of worlds larger than Rhode Island. It contains perhaps a couple thousand bodies each having a volume exceeding the volume of water in all of the Great Lakes combined. Yet, if like the "Little Prince," you could venture to the middle of the main asteroid belt, you would barely see a few other asteroids -- and they would be among the faintest stars against the black sky of space. Far from ducking hurtling projectiles as in movies, a visitor to the asteroids would be struck by the wondrous emptiness of interplanetary space.
My childhood reading managed, somehow, to miss Saint-Exupery's sweet tale, which might have triggered in me an early fascination with the mysterious environment of life on a tiny world. Instead, notwithstanding a college classmate's project of photographing an asteroid's motions amid the stars, my developing scientific interests increasingly centered on the atmospheres and solid surfaces of the disparate planets. It would be many years before I would turn, by chance, to the asteroids.
I became particularly interested in the craters of the Moon (which I could see through my backyard telescope), and the question of whether they were volcanoes or impact craters. (They never looked like pointy-shaped volcanoes to me, but then I didn't know at the time the remarkable variety of volcanic profiles.) After graduating from high school in upstate New York, I got a summer job in Arizona where I was assigned to measure lunar craters, for a catalog, using the best telescopic photographs of the Moon. The work was in the fledgling Lunar and Planetary Laboratory of the University of Arizona, which the indomitable Dutch astronomer Gerard Kuiper had just established after relocating from the University of Chicago.
While still a college student in Cambridge, Massachusetts, I published my first professional paper, about lunar craters, in the "Journal of Geophysical Research" (which I was, much later, to edit). When Mariner 4 flew past Mars in 1964, returning a few grainy, light-struck pictures, I marvelled at the immense circular ramparts on the surprisingly cratered landscape of our supposedly canal-crossed neighboring world. Naturally, I wanted to extend my studies of lunar craters to Mars. For that college project, I was fortunate to collaborate with two of the three greatest planetary scientists of my lifetime, Jim Pollack and Carl Sagan. (The third was Gene Shoemaker, to whom I'll return. All three are now, sadly, deceased.) Sagan, the famous popularizer of astronomy, was then formulating his vision of Mars as a wind-blown, desiccated world, inhospitable to the life he so fervently wished were there. Thus, along with Pollack (who applied his talents and insights to the broadest range of planetary research topics of anyone of his generation), Sagan was more interested in how the dust was covering up the Martian craters than in the craters themselves.
Gene Shoemaker, on the other hand, developed the basic concept that craters result from the impact of extraterrestrial projectiles, which literally explode when they strike planetary surfaces at hypervelocities of several to tens of kilometers per second. Shoemaker was an extroverted geologist with infectious enthusiasm and a laugh that could be heard a block away. Having demonstrated, in the 1950's, that Meteor Crater in Arizona has the same geological structure as nuclear explosion craters in the Nevada desert, Shoemaker -- rock hammer in hand -- would lead hundreds of tours down into "his" crater over the ensuing four decades. By taking them into Meteor Crater and across the volcanic landscapes of northern Arizona, Shoemaker trained the Apollo astronauts and a generation of planetary geologists in the techniques of field geology. For Shoemaker, the association between asteroids and craters was second-nature. It would take me, and most planetary scientists, much longer to appreciate the profound connection between asteroidal space debris and the evolution of planetary surfaces...and of any life evolved within planetary ecospheres.
I followed the work of Shoemaker since my early work on the lunar crater catalog. He, along with my summer job big-boss Gerard Kuiper, studied the remarkable photographs of small lunar craters taken by Ranger 7 seconds before it, historically, crashed onto the Moon on July 31, 1964. Shoemaker's early, insightful analysis continues to inform my 1990's research concerning the Galileo spacecraft's photographs of the surfaces of the Galilean satellites of Jupiter. In spring of 1968, on the day that Lyndon Johnson told the nation on television that he would not run for a second term (which I watched in the Caltech student union), I had my first one-on-one meeting with Gene Shoemaker. He saw me in his role as Chairman of Caltech's Division of Geological Sciences. I had travelled to Pasadena to visit Caltech so I could decide whether to continue my graduate studies at M.I.T. or instead move to Caltech.
I chose to stay at M.I.T. But it hardly mattered, for my thesis advisor, Tom McCord, had just gotten his degree from Caltech. His connections with Caltech meant that I spent much of my graduate school career in Pasadena and at the nearby Mt. Wilson Observatory. McCord, a short, ruddy-complexioned fellow who had embarked on a scientific career after years as a truck driver, quickly established a small research empire, occupying parts of two buildings at M.I.T. One day, he called me into his office and offered a thesis topic to me: I would, he suggested, use his new 24-filter spectrophotometer to measure the colors of asteroids. McCord had an optimistic conviction that astronomers could identify different rocky minerals and ices on the surfaces of distant planets; he hoped and expected that simple color measurements would reveal the tell-tale absorption bands. Other graduate students had been assigned Mars, the Moon, and the rings of Saturn. I would do the asteroids, he suggested. Asteroids? Not planets? O.K.: I'd follow a new path. It wouldn't have seemed strange to Gene Shoemaker.
Shoemaker, disqualified for medical reasons from achieving his boyhood dream of going to the Moon, led the Apollo geology investigations from back on Earth. The astronauts were his surrogates on the Moon. The lunar program was cut short after Apollo 17. Already disillusioned with NASA politics, Shoemaker began to use other approaches (besides journeying to the Moon) to learn about why craters exist. A Princeton-pedigreed geologist with two decades of practical experience, Shoemaker made a radical decision: he decided to become an astronomer. For him, it was obvious that an understanding of the cratering history of the solar system required knowledge about comets and asteroids -- the objects that make the craters. So he began, along with Eleanor "Glo" Helin, an observational program at Mt. Palomar, north of San Diego, to discover more about those Near Earth Asteroids (NEA's) that have wandered in from the asteroid belt to where the Earth orbits the Sun. NEA's are the only asteroids that can crash into our planet. During the several million years that they last in the inner solar system, some of them eventually find themselves intersecting the orbit of the Earth at the unlucky moment that the Earth (or the Moon) is right there. A crater results.
Meanwhile, during 1970 and 1971, I dutifully used Tom McCord's 24-filter spectrophotometer to measure the spectra of sunlight reflected from several dozen main- belt asteroids. The question was whether we could learn what the asteroids were made of and how they were related to various types of meteorites -- the stones that fall from the skies. Although young astronomers are normally beset with cloudy skies and equipment failures that protract their graduate-school careers practically into middle age, I was greeted -- night-after-night -- with clear skies over my observing posts at Mt. Wilson and at Arizona's Kitt Peak National Observatory.
In the course of writing my Ph.D. thesis, I concentrated on massaging all of my data and trying to understand what they implied about the composition of asteroids. It wasn't as simple as McCord had surmised -- absorption bands were few and weak and didn't lead to a straightforward identification of the minerals. Almost as a sidelight, I devoted one chapter of my thesis to cratering of asteroid surfaces. Would they, indeed, be cratered? Despite the tiny gravities on asteroids, could they nevertheless retain regoliths (a term coined by Shoemaker to refer to "soils" on the Moon generated by the repeated crushing and reworking by meteoroid impacts)? Dusty regoliths might affect the colors of asteroid surfaces, modifying the pure mineral spectra. As I thought about asteroidal cratering, it began to slowly dawn on me that there was a direct connection between the asteroids, their own collisional interactions, and the "holes" on planetary surfaces -- the lunar and Martian craters that I had been measuring during the 1960's.
Shoemaker's collaborator Helin, a hefty blonde geologist, bore the nickname "Glo," aptly reflecting her generous, outgoing nature. At various scientific conferences during the mid-70's, Glo tried in her almost messianic way to enthuse me about NEA's. It wasn't that I disagreed that they were interesting and important. I would, much later, devote much of my scientific research energies to NEA's, but -- back in the mid-70's -- I still didn't quite get it. I continued to pursue my studies of the mineralogy of chiefly main-belt asteroids and the frustrating question of exactly what mixture of minerals was implied by the various absorption bands in their spectra. I didn't discriminate against NEA's: I observed whatever near-Earth asteroid or "dead" comet nucleus might be available in the skies, and I even obtained some of the best spectra of the large NEA, 433 Eros. Eros, the target of a 1999 visit by the Near Earth Asteroid Rendezvous (NEAR) mission, is about 35 kilometers long. Should it crash into the Earth, which it very well could do sometime in the next few million years, its impact would dwarf even the dinosaur-killing event 65 million years ago. Fortunately, Eros can't harm us during the next few centuries!
Following my 1972 Ph.D., I continued to measure asteroid spectra, collaborating with McCord and others in his group. My frigid nights in cold domes on Mt. Wilson, Kitt Peak, Anderson Mesa (near Flagstaff), and Hawaii's Mauna Kea gradually evolved into pleasanter nights in observatory "warm-rooms" as observational astronomy evolved into an ever more automated endeavor. Finally, the telescopes and instruments could be left alone in the cold. Meanwhile, NEA searches remained a largely unfunded scientific backwater. The few observers trying to find NEA's operated on shoestring budgets. Geologist-turned- astronomer Gene Shoemaker, later accompanied by his wife, Carolyn, and by amateur- turned-pro David Levy, continued to observe into the 1990's in the obsolete, unheated dome of Mt. Palomar's Schmidt telescope -- searching for Earth-approaching asteroids, and comets beyond.
It was June 28th 1997, twenty-nine years after I first met Gene Shoemaker. At a patio restaurant overlooking the lake in downtown Columbia, Maryland, I was having a comfortable lunch with Gene and Carolyn. With me were David Morrison and Alan Harris, two prominent asteroid researchers, and Carolyn Porco -- leader of the imaging team of NASA's Cassini mission, which was to be launched toward Saturn a few months later. The five of them were the guests to an "encounter" -- one of those periods of frenzied scientific and media activities when a spacecraft flies past a body, returning a flood of new data. Encounter activities, this time, were centered in Laurel, Maryland, at the Applied Physics Laboratory of Johns Hopkins University, the organization responsible for designing, assembling, and operating the NEAR (Near Earth Asteroid Rendezvous) mission. I am one of the select group of scientists involved in overseeing NEAR's camera and spectrometer.
We were basking in the glow of the first incredible pictures returned the previous afternoon from the Near Earth Asteroid Rendezvous (NEAR) spacecraft of the huge black behemoth called Mathilde. The enormous, gaping craters that dominated the form of this dark asteroid astonished us all. Mathilde looked like a giant piece of blackened Swiss cheese. Yet Mathilde is but a way-station for NEAR before reaching its main goal: NEAR will orbit the NEA Eros in 1999, the culmination of a goal I first heard Gene Shoemaker advocate fifteen years earlier. The six of us reflected on our many interactions during the previous decades that had finally led to this initial success in the first-ever dedicated spacecraft mission to an asteroid. The conversation turned to reminiscing about Gene's long association with Caltech -- beginning as an undergraduate student and culminating only a few years earlier with his eventual retirement as a faculty member. Carolyn Porco, a black-haired New Yorker with traces of attitude and accent from the Big Apple, fondly recalled her grad student days in the department with Gene, where her interests settled onto a different swarm of particles (Saturn's rings). I recounted my day with Gene and Lyndon's farewell.
When we returned to the Applied Physics Laboratory, people were abuzz over NEAR's possible discovery, soon to be discounted, of a satellite in the vicinity of Mathilde. It was my job to somehow shield our savvy guests from the "secret" discovery until the science team could confirm that it was for real. Our team leader, a strong-minded Czech-Canadian astronomer named Joe Veverka, who I first met in college, was determined not to repeat the faux pas of another colleague, Lyle Broadfoot, a quarter- century earlier. Broadfoot, nearly incoherent from a sleepless night working on brand-new data from Mariner 10, made headlines worldwide by "discovering" a small moon of the planet Mercury. He named it after his dog. A day later, Broadfoot realized to his great embarrassment that the supposed satellite was, in fact, a well-known distant star far beyond Mercury. Veverka was adamant that history would not repeat itself. He ushered CNN reporters out the door.
I escorted Gene and the other visitors to a remote conference room, and engaged them in a passionate scientific discussion about the recent pictures of the surface of Jupiter's moon, Europa, obtained by the Galileo spacecraft. At issue was whether Europa is a very active world, with a thin ice layer atop an ocean, conceivably teeming with aqueous life. Gene, a half-year before, had advocated the idea that Europa's surface was a billion years old -- "young" in comparison with the ancient surface of the Moon, but probably frozen solid, at least to considerable depths where it would be capable of harboring nothing more than ancient frozen fish if it ever had been an abode of life in our solar system. The latest Galileo photos showed almost no impact craters at all on Europa, I told him. Always willing to adapt to new data, Gene was happy to reconsider Europa as an extraordinarily geologically active body.
Then, in mid-afternoon, the Shoemakers departed for the airport. They had a trip to Nova Scotia, where they would meet with David Levy, and then they were off for their annual resuscitation, and crater explorations, in the outback of Australia. They waved goodbye. It was the last time I saw Gene. He would never return from Down Under. In an accident, seemingly as unlikely as a globally destructive asteroid impact on Earth -- a topic Gene brought into our consciousness during the 1980's -- his vehicle encountered head-on, at high speed, another car on one of the rare curves along a desolate track in the Outback.
Let me tell you of my studies of asteroids, from the days of my post-graduate studies with Tom McCord's instrument, to the memorable weekend of NEAR's Mathilde encounter. In that quarter century, asteroids finally gained respect. They had long been considered the dregs of the solar system. Astronomers had called them the "vermin of the skies" because their trailed images messed up pictures of more glorious celestial objects. Now asteroids are the chief targets of numerous ongoing and prospective spacecraft missions, ranging from a Japanese endeavor to land an asteroid sample in the Utah desert to a would-be first-ever private enterprise deep-space mission, being promoted by Colorado entrepreneur Jim Benson. Asteroids have also become popular icons of the potential catastrophic end of our world -- stars of Hollywood blockbusters about the greatest imaginable disasters. Some researchers now think of asteroids as playing one of the most exalted roles in the cosmos: asteroids (and comets) could very well be the driving force of biological evolution on Earth, and conceivably everywhere else in the universe, as well.
On top of an office file cabinet, I have a pile of reports about two feet high. With publication dates from the late 60's to the late 80's, each one presents the recommenda- tions of some committee or task group about a prospective asteroid spacecraft mission. Except for NEAR, none were ever funded and flown. These committees were assembled most often by NASA, but also by the U.S. National Academy of Sciences, the French and Japanese space agencies, and others. Some reports have plain grey covers, others sport portraits of the Little Prince, and one fancifully depicts a spacecraft orbiting through a hole that pierces a small asteroid. For decades, the accumulating reports gathered dust and all the plans were for nought. But now we are in a new era with missions galore. How did we get here?
My own research -- and that of my colleagues -- on asteroid reflectance spectra may unfortunately have delayed the exploration of asteroids by spacecraft. It's a story about the hubris of scientists, who are often optimistic about how soon they and their particular techniques can solve crucial problems. As it turns out, there's nothing like going out there to explore a distant world to find out what it's like. Astronomers who study distant stars and galaxies have no choice -- our technology will never get us to the stars in our lifetimes. But for asteroids, which look like distant stars through a telescope (hence the word "asteroid," or star-like object), there was another choice. Implementation of the spacecraft alternative was delayed because of competition and expense...but also because Tom McCord, I, and our colleagues were too successful at promoting our likely success at assaying asteroid mineralogy from afar.
Early this century, asteroids were used by astronomers as standards to calibrate their data as though they were all colorless, grey reflectors of sunlight. But colorimetric studies of some asteroids during the 1950's and 1960's had begun to show that different bodies had different colors. My own spectra went farther and revealed a wide variety in the actual spectral features in their reflected sunlight: minerals absorb and transmit light of various wavelengths (colors) very differently from each other, and -- evidently -- asteroids differ widely in the suites of minerals of which they are composed. See one asteroid and you most assuredly have not seen them all!
The more that I, and several other young asteroid researchers, observed these minor planets with a variety of different techniques -- mid-infrared radiometry, polarization, and light-curve photometry, which reveals the various shapes and spin rates of asteroids -- the more we realized that each of the thousands of known asteroids has its own personality. 324 Bamberga is unexpectedly large, round, and black; and it spins languidly about its axis of rotation, more slowly than the Earth turns. 349 Dembowska is highly irregular in shape, spins around 5 times in a single Earth day, and it has a surface made up largely of the bright green mineral, olivine. 16 Psyche spins even faster than Dembowska, yet has a featureless reflection spectrum like that of pure steel. Radar echoes bounced off Psyche have confirmed that Psyche is the largest known hunk of pure metal in the solar system, 250 kilometers in diameter!
One of the obvious questions for asteroid researchers to address was the connection between asteroids and the rocky fragments, called meteorites, which plummet through the Earth's atmosphere as fiery "shooting stars", lodge in farmers' fields, and wind up in the mineral displays of science museums. Meteorites were once considered to be "thunderstones" -- some kind of congealed atmospheric residues formed by lightning bolts. Then, about two centuries ago, several meteorite showers struck in Europe and scientists began to appreciate that these alien, often metal-rich rocks actually come from interplanetary space. Coincidentally, about the same time, the first asteroids were discovered. Putting two-and-two together, a general theory developed that a planet between Mars and Jupiter had exploded, creating the asteroid belt; meteorites were taken to be smaller fragments of the erstwhile planet.
That seemingly plausible hypothesis has not stood the test of time. No large planet ever existed or exploded in the asteroidal region. Instead, asteroids are now known to be the residue of small primordial objects called planetesimals, congealed from the original solar nebula of dust and gas from which the Sun and planets eventually formed. Planetesimals between Mars and Jupiter never managed to form into a planet, most probably because of the gravitational effects of massive Jupiter. Instead of bumping into each other slowly and growing into a planet, the extra velocities imparted by Jupiter resulted in asteroids colliding with each other at many kilometers per second, breaking each other into smaller pieces, resulting in the torus of fragments we see today.
Indeed meteorites are pieces of asteroids, although when I began my observational program back in graduate school, it was very unclear how that could be true. For one thing, nobody could imagine how rocks could be extracted from the asteroid belt, far beyond Mars, so that they could strike the Earth. If you hit a rock hard enough to send it into an Earth-crossing orbit, you would vaporize it. As a start to solving the mystery, I would determine if asteroids were made of the same minerals as the meteorites. Using Tom McCord's spectral reflectance techniques, I would try to determine if certain types of meteorites had the same reflectance spectra as some asteroids. If so, those asteroids could be parent bodies for the meteorites. Then we could move on to the question of just what physical process/es could deliver meteorites from those particular asteroids.
I must have seemed pretty confident at the time, for the eminent University of Chicago meteoriticist Ed Anders argued that I and my colleagues would soon be successful in linking the meteorites to specific asteroids. Hence, he declared, it would be premature to send a spacecraft mission to an asteroid, for we might simply return a piece of it to Earth only to find that we had merely one more meteorite of an already known kind. NASA was only too happy to send its spacecraft elsewhere. During the succeeding decades, however, asteroid researchers have been confounded with puzzles in trying to link meteorites to specific asteroids, even though the extraction and delivery mechanism was eventually solved in the 1980's when it was realized that the dynamics of chaos could gently and efficiently put asteroidal fragments into Earth-crossing orbits.
Like Lewis and Clark exploring northwest America, or Darwin investigating the Galapagos, we early asteroid researchers could do little more than observe, classify, and tabulate. From the mid-70's to the mid-80's, such taxonomy was the main activity in asteroid research. By 1974, more than 100 asteroids had been observed with some combination of the spectral, radiometric, and polarimetric techniques. Together, such observations tell not only about an asteroid's color, the shape of its spectrum, and the presence of distinctive absorption bands, but also about how bright or dark the surfaces are. By the mid-70's, it seemed as though most asteroids were made of moderately reflective rocks rich in pyroxene and olivine, which are the basic minerals (along with quartz, feldspar, and some others) of which the Earth's crust is made. A few asteroids were apparently very different: their surfaces are coal-black in color. Or, so it seemed, until a special collaboration revealed a rather different picture of the asteroids.
In the mid-70's, I was working at an off-campus research institute in Tucson, near the University of Arizona. Ben Zellner, a stolid southerner, was using Arizona telescopes to painstakingly measure the polarization of light reflected from some of the brighter asteroids. David Morrison, a lanky Harvard-educated astronomer on sabbatical leave in Tucson from the University of Hawaii, observed the infrared heat radiated by asteroids. The three of us compared our data sets and realized that we could classify most asteroids into just two types. The brighter asteroids we termed "S-types," using S as a mnemonic for "silicaceous," meaning composed of the silicate minerals pyroxene and olivine, whose weakened absorption bands were the most prominent features of S-type reflectance spectra. The black asteroids we termed "C-types," using C as a mnemonic for "carbonaceous," since almost all black substances on the Earth (or black meteorites that fall from the skies) are black because of the presence of carbon.
Ten asteroids in our sample were neither fish nor fowl. In our taxonomy, they were unclassifiable, so we called them U's. Naturally, as more asteroids were observed in later years, these early U's became the archetypes for new, less populous classes of asteroids given the letters V, M, E, R, A, and D. Still other distinctive types have been found subsequently, and a veritable alphabet soup of taxonomic types has grown from our meager C,S beginnings.
In our 1975 paper, published in Icarus (the international journal of solar system studies, then edited by Carl Sagan), Morrison, Zellner, and I went beyond simple taxonomy to try to understand the population of asteroids. Appearances can be deceiving, and we wanted to understand all the asteroids, not just those that were easiest for us to observe. We needed to take account of, and correct for, the biases imposed by our observational circumstances. We realized that the C-type asteroids, being very black and also commonly located in the outer part of the asteroid belt, would be underrepresented in our sample. Compared with a more reflective S-type asteroid in the inner belt, a C-type of the same size would be more dimly illuminated by the faraway Sun, would reflect less light because of its inherently black color, and would be further dimmed by its great distance from Earth. So we would probably miss it, while measuring lots of smaller, closer S-types. We corrected for all of these factors and realized that, despite C- types being only about a third of our sample, the asteroid belt must be overwhelmingly populated by C-types.
Our Icarus paper was one of the most widely cited planetary science papers of the decade and helped to make the study of asteroid physical properties a significant subdiscipline of planetary astronomy. By 1975, asteroids had gained enough respect that Scientific American commissioned me to write an article about asteroids. A few years later, the National Academy of Sciences started taking seriously the small "dregs" of solar system accretion -- the asteroids and comets -- and began to draft recommendations about what NASA should do about exploring them. I was invited to join the Academy's planetary advisory committee as the committee began work on its small bodies report. It would turn out to be yet one more report that gathered dust. But work on the report led me into a new direction in asteroid research.
Looking back, 1980 was a seminal year in melding the study of asteroids with the role of impacts on the Earth and in the solar system. During the 1970's, while I had been observing asteroid physical properties from chilly mountain tops, other observers (especially Gene Shoemaker, Glo Helin, and University of Arizona astronomer Tom Gehrels) were systematically searching for Earth-approaching asteroids. As of 1971, less than a dozen non-cometary objects were known to pass inside the Earth's orbit. By 1979, the number of known Earth-approachers had tripled. By correcting for the great incompleteness of the combined surveys due to inadequate sky coverage and observational biases against faint objects, Shoemaker estimated that the total number of Earth-crossers larger than 1 km across is about 1,300. After two more decades of searching, hundreds have now been found, and refined estimates of the total population have risen slightly, to about 2,000.
Also during the 1970's, NASA's spacecraft were venturing farther afield. Beginning with the surprise of Mariner 4's first pictures of Mars, the "golden age" of planetary exploration was showing that every solid surface in the solar system is being bombarded by comets and asteroids, leaving crater-scarred surfaces. Mariner 10 flew past Mercury in the early 70's and found that it was heavily cratered, just like the Moon and Mars. In March 1979, the first Voyager spacecraft flew past Jupiter, revealing the cratered surfaces of its two largest moons, Ganymede and Callisto. (It also was obvious why several worlds were virtually crater-free: for instance, Voyager snapped pictures of furious volcanic eruptions on Jupiter's inner moon, Io, that certainly would quickly cover up any impact crater that was formed on its surface. For the same reason, the Earth's surface retains only the most recent, tiny fraction of impact craters ever formed.)
The intimate connections between craters and small cosmic bodies that had driven Gene Shoemaker through his career gradually became abundantly clear to all planetary scientists, and to NASA. In a summer 1980 meeting in Martha's Vineyard, an elite group of NASA advisors suggested that the space agency look into the question of possible danger from Near-Earth Asteroids. That same year, the seminal paper was published in Science magazine by Nobel laureate Luis Alvarez and his colleagues, who argued that impact of a 10-km-diameter asteroid was the probable cause of the last great mass extinction of species in the Earth's fossil record, 65 million years ago. They had found that a thin layer of rock, enriched in metals (like iridium) that are rare in the crust of the Earth, coincided exactly to the level where fossils of the Cretaceous era suddenly give way to Tertiary fossils (the so-called K/T boundary). Most asteroids and meteorites contain abundant metals; if the iridium-rich layer were distributed worldwide, as was soon verified from measurements of K/T boundary layer rocks acquired from around the world, then a 10-kilometer-diameter asteroid was implicated, according to Alvarez et al.
Although it took another decade of research before gaining general acceptance in the scientific community (and a few doubters remain even today), Alvarez's idea was immediately plausible to planetary scientists familiar with the numbers of comets and asteroids in the inner solar system. One needs to use only simple arithmetic, involving the area of target-Earth and the numbers of Earth-crossing asteroids, to calculate that Earth must be struck by a kilometer-sized object once every 100,000 years, or so. Objects ten kilometers across are almost 1000 times rarer than kilometer-sized objects, so a body the size estimated by Alvarez should strike the Earth every 50 to 100 million years. The last one evidently hit 65 million years ago: it fits! In fact, it's inevitable that catastrophic impacts have affected Earth's ecosystem in the past...and will do so in the future.
And that was the theme of a workshop that NASA mounted in summer 1981, in response to the challenge from its advisory council. Gene Shoemaker was asked to chair the meeting, held in Snowmass, Colorado. Along with others coming off the National Academy of Sciences' study of small bodies (our report was published in 1980, I was invited to participate in this "Spacewatch Workshop." I was assigned to a sub-panel of half-a-dozen scientists who examined, from the limited data then available, what environmental and social consequences might result from the impact of asteroids and comets of various sizes. We couldn't second-guess the work of the Alvarez group, who were represented at the workshop, about the effects of an extremely rare 10 km object. But we immediately realized that far more frequent impacts, with far smaller megatonnage explosive yields, might have dire consequences, as well.
Sitting in our break-out room at the Snowmass resort, we realized that impact of an asteroid "only" 2 km across probably would be sufficient to change the climate, destroy agriculture worldwide, and risk the end of civilization as we know it. Our species and most others would survive, but millions or billions of people might die, and we might well be cast into a new Dark Ages. We estimated that there was a 1-in-10,000 chance of such a catastrophe happening in our lifetimes. Though small, such chances are greater than dangers that governmental agencies (like the Environmental Protection Agency) and individuals (like frequent air travellers) take very seriously. (Your chances of dying in the immediate aftermath of a cosmic impact are much greater than of winning big in a state lottery during your lifetime!) Worse, our panel feared, a much smaller impacting asteroid might be misinterpreted as a nuclear attack, triggering an escalating nuclear war. Remember, we were in the midst of the Cold War then!
Sobered by the discussions, I left the Snowmass workshop convinced that the impact hazard needed further study, and that the military leaders and opinion makers around the globe needed to know more about rare cosmic impacts. I toiled on drafts of the Shoemaker report, but it never saw the light of day. Some suspected that it was squelched because of Cold War sensitivities. So far as I can tell, however, Gene Shoemaker -- who always had ten times as many things to do as time to do it -- got busy with other responsibilities and NASA officials failed to keep the pressure on him to finish it up. Nevertheless, I felt it was a story that needed to be told and, when David Morrison and I decided to work on a popular trade book on astronomical catastrophes, I had my chance. The concluding chapter of our book "Cosmic Catastrophes," published in 1989, summarizes the Snowmass workshop and its conclusions about the cosmic impact hazard.
Although our book was marketed badly and sold poorly, it was read by some people who count -- for instance, by Congressional staffers associated with the space caucus. During the subsequent decade, public awareness of impacting asteroids has gone from nothing to truly bizarre levels: bad TV mini-series now deal with astronomers and Emergency Management officials responding to crashing rocks; commercials show fiery objects descending from the skies; Federal Aviation officials compare airline hazards with the risks of falling comets; and blockbuster movies about deadly asteroids are produced by Hollywood studios. The spectacular crash of Comet Shoemaker-Levy 9 into the planet Jupiter in 1994 had much to do with reinforcing the perceived reality of the danger from the skies...but that is another story. Meanwhile, although a few modest telescopic searches for Earth-approaching asteroids continue, essentially no NASA funds have been made available to research the impact hazard.
NASA funding notwithstanding, an intellectual revolution is emerging about the role of asteroids in the cosmos. As astronomers search for planetary systems around other stars, they are finding "disks" around some stars. Although most stars are too far away to tell if they have planets let alone smaller bodies, it is likely that some of these disks are composed of swarms of asteroids, comets, and other space debris grinding itself into interplanetary dust. If Earth-like planets exist around such stars, any emerging life would have to deal with the continual rain of asteroids and comets onto their surfaces. It is now believed that evolution of life on our own planet was delayed until the last huge ocean- evaporating, sterilizing impacts had ceased 4 billion years ago. There remain occasional K/T-like impacts to this epoch, which drastically rearrange ecological niches, producing abrupt changes in evolution of species in between long periods of stasis, akin to the punctuated equilibrium model of evolution touted in popular essays by the Harvard evolutionist (and baseball fan) Stephen Jay Gould. University of Chicago paleontologist David Raup has suggested that all of the great mass extinctions might eventually be explained as the result of cosmic impact.
Given the existence of huge comets and asteroids still remaining in the solar system today, it is just a matter of chance that one of them -- like the asteroid Eros or Comet Hale-Bopp -- failed to strike during the 65 million years it has taken mammals (and ourselves!) to evolve from the ashes of the dinosaurs. We may very soon have the technology to colonize other planets and render ourselves immune from an ecocatstrophe on the planet of our birth. Already, we have the telescopes to discover, and the rockets and bombs necessary to divert, any incoming asteroid. The probability of ubiquitous asteroid bombardment throughout other planetary systems in the universe just might explain why we have failed, so far, to detect or make contact with other intelligent civilizations, despite radio telescopic searches. Maybe Earth is unusually well protected from bombardment (Jupiter has been hypothesized to shield us from what would be even greater cometary bombardment) or maybe our evolution proceeded just fast enough to give us the chance to protect ourselves before something like Eros hits.
Through a series of quirks, I've managed to be sitting in the "front seat" during the first three encounters of asteroids by spacecraft. I'm among a lucky few people who got to see, as soon as the data hit the ground, the first close-up images of the asteroids Gaspra, Ida, and Mathilde. And I'm looking forward to the fourth: Earth- approacher Eros. How did this happen?
Well, to be a planetary scientist, you have to write proposals to NASA. There are several slots on the faculties of a few well-endowed or state-supported universities, but planetary science is not a core curriculum subject requiring numerous teachers, as compared with traditional sciences like physics, chemistry, and biology. So most planetary researchers live on salaries funded by NASA grants (the National Science Foundation, and other potential funding sources, mainly expect NASA to support planetary science). One can propose for funds to study some particular topic. Or one can propose to participate in a planetary mission. The available funds, and mission opportunities, are few so that competition is stiff and the chances of success for any given proposal are low. (Even as I write this, I've received yet another rejection notice, despite my colleagues' views that my proposal addressed one of the most important questions in planetary science and that I was the right person to do the research.) So one raises one's chances, in order to put food on the family table, by writing lots of proposals. The alternative, of course, is to turn one's talents toward more lucrative endeavors. But I, like most planetary researchers, love the field. And we owe it to the taxpayers, who have invested so heavily in our schooling and our experience, to continue trying to apply our hard-won expertise to the studies we were trained to do -- whatever short-term funding shortfalls exist.
So it was, in the mid-1970's, that I wrote a proposal to participate in the Galileo mission to Jupiter. Although Jupiter was not my first love, I had accumulated a credible list of publications about Jupiter and I could speculate about craters on Jupiter's moons. Apparently, I was one of three "youngsters" added to the Galileo imaging team as a risky experiment, along with a group of more experienced grey-beards. As someone now sporting a grey beard myself, I can testify that Galileo has been the project of a lifetime. It is still going on! Delays in developing the Shuttle...cancellation (due to funding cut-backs) of an energetic upper stage...the Challenger explosion...an extraordinarily long billiard-ball-like path through the solar system...until, eventually, Galileo reached Jupiter in late 1995, thirteen years late. That, too, is a story for another occasion.
However, Galileo's long traverse through the solar system, combined with recognition by NASA officials that asteroids were getting the short end of the stick (no approved missions through the early 90's), resulted in a piece of enormous luck. So that NASA could "check off" asteroids in its planetary exploration matrix, the Galileo Project was mandated to examine a couple of asteroids. It was an unfunded mandate: for no additional cost, Galileo managers were instructed to fire its thrusters in order to send it very close to two asteroids already along its path to Jupiter. Moreover, they would turn on the spacecraft's instruments and return data to Earth. Thus, by luck, the one spacecraft mission I, as an asteroid specialist, was then involved with -- a Jupiter mission -- would become the first to take pictures and spectra of asteroids!
I vividly recall the moment on the morning of November 9, 1991, sitting in a room in Jet Propulsion Laboratory's Multimission Image Processing Laboratory, when the first image of an oddly angular asteroid, named 951 Gaspra, showed up on the television screen. My colleague, Cornell geologist Peter Thomas, immediately began sculpting away pieces of a gridded sphere on his computer console to create a realistic three-dimensional representation of Gaspra's shape. Throughout the day, the pictures continued to be radioed back, ever so slowly, from the distant spacecraft, which had zipped past Gaspra two weeks before. The engineers had worked out an ingenious method of dribbling back a sampling of data from Galileo (whose main antenna was dysfunctional), rather than to wait a year for the spacecraft to return to Earth and disgorge its tape-load of data.
Gaspra certainly didn't look like any of the spherical worlds of the Little Prince. Nor did it exactly resemble a chip of a rock. Its shape, the most irregular of anything yet photographed in the solar system, was uniquely its own. To some of us, it looked almost as though two objects had joined into one. To others, Gaspra appeared to have had sections of its outer surface sheared off by glancing impacts. Immediately, something seemed odd to me about Gaspra -- its surface is peppered with numerous, small craters, but it lacks big craters, and the few moderate-sized craters it does have hardly cover its surface: it is not at all like the Moon, which is saturated by overlapping craters. To the degree that the numbers of craters of different sizes reflect the small-impactor population, we had our first glimpse into the numbers of small main-belt asteroids some tens of meters across.
Once all of the Gaspra data were back, following Galileo's Earth encounter in November 1992, I earnestly began to measure the craters on Gaspra, as I had measured lunar craters 30 years earlier at the beginning of my career. I wanted to do a comprehensive job. By the summer of 1994, hoping to finish my manuscript, I carried my Gaspra papers in my laptop computer bag to a scientific conference in Prague. But my bag was swiped by a pair of thieves in the train station. It took more than a year before I could re-do and finish the work; my paper finally saw the light of day in Icarus, along with all the results about Ida, in 1996.
Gaspra was but a foretaste compared with the wonders of Ida, encountered by Galileo on August 28, 1993. At closest approach, the much larger Ida filled up five separate frames of Galileo's camera. Galileo took pictures of Ida through color filters equivalent to those I had used back in 1975 when I observed Ida from Hawaii's Mauna Kea Observatory. As a result, pieces of a puzzle that had eluded us for a quarter century began to fall into place. Our color composite pictures showed that small, recent craters on Ida have different colors from most of Ida's surface. Apparently, the recent craters had dug into fresh bedrock revealing Ida's "true" colors. But after a spell of time, some process (perhaps the impacts of micrometeorites) was changing the spectral character of Ida's surface. No wonder it was difficult to directly match laboratory spectra of meteorites to the asteroid spectra I had struggled to obtain! Exposure of Ida's regolith to space seems to depress the apparent strength of the mineral bands and gives Ida an overall reddish tint - - just the traits that had seemed to invalidate a connection between the S-type asteroids (like Ida) and the most common meteorites, the so-called ordinary chondrites. Ordinary chondrites are believed to be heated-but-never-melted samples of the original material accreted from the solar nebula 4.5 billion years ago. Mysteriously, these common and important meteorites had not been previously linked to any spectral type represented in the main asteroid belt. Now that we see how "space weathering" processes can change mineral spectra of ordinary chondrites to look like S-type asteroids, I have renewed hope that we can connect other meteorite types with asteroid types.
Ida continues to fascinate me. More convincingly than Gaspra, Ida's shape makes it look like the merger of two separate asteroids. Unlike Gaspra, it is covered with craters. Either it is much older than Gaspra, or else Gaspra is much stronger than Ida (e.g. made of metal) so that it is difficult for impacts to gouge out craters in Gaspra's surface. Also unlike Gaspra, Ida sports a small mile-wide companion -- a little spherical object orbiting around Ida, named Dactyl. It is unusually spherical, a fine place for The Little Prince, and a puzzle: after all, as Phil Morrison had explained, the natural shape for something so small is irregular. Dactyl's existence also raises the question of whether other asteroids have satellites, and -- if so -- how they come to be formed. For now, the Ida/Dactyl system is yet another case of each asteroid having a different personality.
By the early 90's, I had temporarily given up serving on NASA committees and I was getting disillusioned with the proposal-writing rat-race -- ever more proposals chasing ever smaller slices of a slowly dwindling pie. So I let some proposal opportunities pass by, including the Cassini mission to Saturn. But one day, at a June 1994 conference in Flagstaff, Arizona, University of Texas astronomer Anita Cochran mentioned to me that she was proposing for the Near Earth Asteroid Rendezvous mission, and she wondered about how my own proposal was going, due in a few days. Truthfully, I hadn't even thought about proposing, even though -- years earlier -- I had participated in the work of the NASA committee that had drafted the mission concept. (This, in fact, was the report with the spacecraft flying through a hole in the asteroid on its cover.)
I returned to my office and spent a single day dashing off a proposal for NEAR. I explained how I would help design the infrared experiment and interpret the spectral data, using my experience with spectral observations of asteroids. I didn't propose for NEAR's imaging team, which I guessed would be oversubscribed. A few months later, I was astonished to receive a phone call at home, telling me that my proposal was accepted. Moreover, I was told, the infrared-spectroscopy and imaging teams would be combined -- I'd have responsibilities for both investigations! Thus I became part of the Science Team of mankind's first dedicated mission to an asteroid. The proposal-writing process reminds me of my college days when it didn't matter whether I struggled over an assignment for weeks, or dashed off a paper during the half-hour before class began. My humanities teaching assistant instructor, Erich Segal (later to become the famous writer of "Love Story"), consistently graded my papers no higher than B- and no lower than C+. Despite all the toil and agony, NASA Proposal Review Panel results often end up looking like flips-of-a-coin.
NEAR is one of those fast-track missions in NASA Administrator Dan Goldin's cheaper/faster/better mould. The spacecraft and instruments were largely built before the new science team ever met. A little over a year later, we watched a Delta rocket propel the Volkswagen-sized spacecraft into the skies above Cape Canaveral. Like Galileo's investigations of Ida and Gaspra, the NEAR project availed itself of a chance opportunity: Mathilde was along NEAR's path toward Eros. So it was, a little over a year after launch, that I found myself (along with Gene and Carolyn Shoemaker, and our other guests) staring at images of another asteroid with its own unique personality: Mathilde. Even Gene Shoemaker, who had studied more craters than anyone else on Earth, had never seen craters so (comparatively) large! Of course, there are larger craters on much larger bodies and planets. But for modest-sized Mathilde, only 50 kilometers across, to sport not just one but several craters exceeding 25 kilometers in diameter was unbelievable. Mathilde's whole shape is an amalgamation of the ramparts of giant craters.
The NEAR Science Team struggled to understand how Mathilde's unique appearance might be a clue to another special trait, recently appreciated from analysis of telescopic photometry of this black, C-type asteroid: it hardly spins at all. Tumbling ever- so-slowly in space, Mathilde gradually turns around every two or three weeks, each time about a slightly different axis (there's no fixed north or south poles on Mathilde). Nobody could imagine why this might be so, so we hoped our pictures might tell us. But apparently not. And all of NEAR's other instruments, which might have provided different clues, were not yet turned on. After all, NEAR is a dedicated mission to the Earth- approacher Eros, not to faraway Mathilde, where the sunlight is so dim that NEAR's solar panels could power only a single instrument, the camera. Maybe detailed analysis of the flyby pictures will eventually yield clues to Mathilde's slow, tumbling spin...otherwise, Mathilde will hold her secrets, for it will be a long time before we're likely to get back to this serendipitously explored small world.
As NEAR sails on toward its rendezvous with Eros, I can only marvel at the variety of small worlds we have already seen among the asteroids. For me, asteroids benefit especially from exploration by spacecraft. From Earth, asteroids are mere "points-of-light." Astronomers struggle to milk every bit of information gleaned from photometry of these small remote worlds and still barely scratch the surface. As Ida, in particular, taught us, the average light from an asteroid hides essential details that require spatial resolution -- like the distinctive colors of small, recent craters. And no amount of theorizing or computer modelling can replace up-close exploration, just going out there to look, which is why we are so often surprised by pictures returned from deep space.
NEAR's mission director, Bob Farquhar, is a puckish, eccentric soul, whose talents at trajectory wizardry have only gradually gained overdue respect. His latest dream is to end NEAR's explorations in the year 2000 by dropping it directly and gently onto the surface of Eros. If this daring end-game succeeds, we may see pictures of small, individual rocks on Eros -- which would be an astonishing improvement over the football- field scale of resolution of asteroid images obtained to date. What a remarkable achievement it will be to gather comprehensive, detailed maps of what appears from Earth to be a just a slowly moving star, too faint even to be detected by a small telescope let alone by the naked eye. As we study Eros, let's remind ourselves of its very real potential, several million years from now, to transform the biosphere of our planet more dramatically than the K/T impactor did on the day the dinosaurs died. Perhaps, by that epoch, our species will be exploring asteroids around distant stars and may even have encountered other intelligent species lucky enough to have evolved and developed technology in time to escape the random cosmic bombardments that help shape the origin, evolution, and destiny of life throughout the universe.
Clark R. Chapman's Publications.
Clark R. Chapman's Home Page.