We have seen that collisions with asteroids and comets have played an important role in Earth history. On the long time scales of geology, impacts have repeatedly produced ecological catastrophes and influenced the course of biological evolution. We, as mammals, owe our dominant position on this planet in part to the impact 65 millon years ago that eliminated the dinosaurs, our chief rivals among the large land animals. In this chapter we move from geology to history, to see the evidence that the Earth may still be subject to a cosmic bombardment. This search begins with the records of recorded history and their prehistorical antecedents in mythology and oral tradition. We look at the ideas of Immanuel Velikovsky and Victor Clube, but conclude that their interpretations of history are ambiguous and suspect. We are on more solid ground as we turn to the detailed reconstruction of the 15-megaton 1908 impact near the Tunguska River in Siberia, which leveled a thousand square miles of forest. New calculations reveal that the impactor was a stony asteroid about 60 meters in diameter. Other contemporary impact events include the 1947 Sikote-Aline and 1965 Revelstoke meteorite showers, the 1972 fireball over the Teton Mountains that skipped out of the atmosphere, and recently declassified military surveillance data on upper atmospheric explosions, which are recorded several times each month. Yes, the Earth is still a cosmic target!
Shoemaker-Levy 9 was not the astronomers' comet; it was the people's comet. It was the comet that had the entire population of the world watching as it hit Jupiter and wondering, "what if it had hit here instead?" -- Comet discoverer David Levy, in Three Minutes to Impact (February 1997)
No astronomer had expected to witness an interplanetary collision; prior to 1993 such events were judged too rare. Interplanetary space is immense and very empty. Imagine compressing the inner solar system to fit inside Houston's Astrodome. A ping pong ball slowly floats over the stands. That's Jupiter. Beyond second base, at the edge of the outfield, is a brilliant basketball‑sized sphere ‑‑ the Sun. A bluish B.B. is behind home plate. That is our Earth, to scale. The larger asteroids are but a few hundred motes of dust, hovering over the seats. Comets, like Hale-Bopp, which make beautiful apparitions in our skies, are little more than isolated molecules. In such a vast emptiness, it is difficult to believe that worlds can collide. Yet during the near eternity that our solar system has existed, the circulating comet‑molecules occasionally try to occupy those same few cubic millimeters as our B.B. ‑‑ a microscopic pock‑mark is then made on the surface of the B.B., and Earth's ecosystem is changed forever.
Since Jupiter is a bigger target than Earth, it is struck much more frequently by cosmic projectiles -- but still very rarely by human standards. The remarkable events of July 1994, when about twenty fragments of Comet Shoemaker‑Levy 9 crashed into the giant planet, may have represented the largest jovian collision since Rome fell. How lucky we were to see it in our lifetimes! How fortunate that telescopic instruments had been devised and spacecraft launched, so that we could study this exceptional event. We have recounted how the repetition of such rare impacts over the aeons has gradually cratered the planets, shaped the surface of our own world, and affected the evolution of life. But to actually watch an impact...what a treat!
Gene Shoemaker was among the scientists who had only dreamed of witnessing a major impact. In the early 1970s, he had retrained himself as an astronomer in order to discover near-Earth asteroids and comets. For two decades, Gene and his wife Carolyn regularly drove south from their home in Flagstaff, Arizona, through the Prescott Mountains, across the barren Sonoran and Mohave deserts to Palomar Mountain, northeast of San Diego. There stands the dome of the 200‑inch telescope, for years the world's largest. Gene first began to use Palomar's much smaller 18‑inch wide-field telescope to survey asteroids in the early 1970's, as we told in Chapter 2. Until he Carolyn said good‑bye to Palomar in 1995, they conducted a multi‑faceted search program. While Carolyn racked up comet discoveries (she has found more comets than any other woman in history), Gene concentrated on finding Earth‑approaching asteroids.
The nights are long and cold at Palomar, and the daytime work of developing and searching the films is tedious. The Shoemakers' funding has been minimal, so they engaged volunteer assistants, who donated their time to the cause. During the 1990's, their prime assistant was amateur astronomer David Levy. Levy, who grew up in Montreal and earned a degree in literature, was always interested in astronomy. As a young man, he moved to Arizona to pursue his hobby seriously under the clear skies of the desert. That is where he developed his "telescope farm," which at one time comprised about 50 telescopes, ranging from toys made from toilet paper cardboard tubes to substantial instruments he still uses nightly to sweep the skies for comets.
Years before Levy joined forces with the Shoemakers, he had discovered more than half a dozen comets with his own eyes and had made a name for himself throughout the world of amateur ‑‑ and professional ‑‑ astronomy. Levy was just the right person to become part of the Shoemaker‑Levy media event in 1994, because he had already honed his skills as a public lecturer. Often supplemented by taped music, Levy translates the arcane world of astronomy with soft‑spoken words that touch a layperson's sense of wonder.
In March 1993, a major spring rainstorm was approaching the California coast, as Gene, Carolyn, David, and a fourth assistant began yet another observing run on Palomar Mountain. A canopy of wispy cirrus clouds had spread over the whole southwest, and the forecast was dismal for the next several nights. Clouds are the usual bane of astronomers, forcing them to close up their dome, play a little pool, and finally head for bed if the skies aren't clear by 3 a.m.
Comet Shoemaker‑Levy 9 might never have been discovered, certainly not at Palomar, were it not for a fluke accident. The team had found, to their chagrin, that someone had opened their box of specially prepared films, and much of their supply had been light‑struck. They had another box of good film, but it is expensive (and the Shoemakers had no NASA funding at all in 1993), so they would not have wasted good film on a cirrusy night. David Levy suggested, however, that they might as well try some film from the middle of the light‑struck supply, where only the edges had been affected. It was on one of these films, the next day, that sharp‑eyed Carolyn found the unique "squashed comet" that was to bear their names.
wonder that she recognized the elongated photographic smudge as a comet at all, for it did not look like any of the other 30‑or‑so comets she had discovered, nor any other comet ever witnessed by human‑kind. Indeed, other observers had captured the object on their films and failed to recognize it for what it was. But the three observers soon concluded that it was not a fixed object (like a galaxy) in the night‑time sky. For confirmation they phoned a colleague observing with Tom Gehrels' Spacewatch Telescope on Kitt Peak several hundred miles to the east, where the clouds had not yet thickened.
Astronomers around the world learned of the discovery within days through a bulletin service run by the International Astronomical Union. Within a week, photographs taken by larger telescopes revealed an extraordinary "string of pearls": about 20 separate little comets, whose images practically touched each other in telescopic photos but which were actually spread across hundreds of thousands of kilometers of space. A handful of other comets had been seen to split into a few pieces, but S-L 9 was exceptional indeed. Its appearance alone would have made it one of the more famous comets of recent times, but there was much, much more to come.
Soon, enough measurements had been made to specify S-L 9's motion through the heavens. The comet was close to Jupiter's position in the sky, and astronomers saw that it shared the giant planet's motion against the background stars. In other words, S-L 9 was accompanying Jupiter in the giant planet's orbit around the Sun, in a long slow orbit around the planet. (This is not wholly unprecedented ‑‑ a few other comets have been seen in temporary orbits around Jupiter.) It wasn't long before S-L 9's path could be pinpointed accurately enough to project its motion into the future, and backwards in time, as well.
Such orbit predictions are the job of Brian Marsden, who runs the International Astronomical Union bulletin service. Marsden is a bouncy sandy-haired man who retains his strong British accent even after decades of living in Massachusetts. Using the position observations reported by many astronomers from around the world, Marsden used his computer programs to run the comet backwards in time, hoping to find some clue about its remarkable string-of-pearls configuration. He found that, about a year beforehand, S-L 9 had passed very near Jupiter, just a sixth of Jupiter's diameter above its cloud tops. At that distance, Jupiter's gravity pulling on the nearer side of the comet more strongly than on the far side must have exceeded the comet's self‑gravity holding it together. A solid, cohesive body might well have zoomed by Jupiter unaffected, but S-L 9 was evidently more loosely held together, like a gravel pile. Unable to resist the mighty planet's forces, the comet was literally torn to shreds. With further analysis, the motions of each of the dozen brightest fragments of S-L 9 could be individually projected back to a single date -- July 7, 1992 -- where they converged to the same spot, the point of disruption next to Jupiter. Incontrovertibly, this once‑whole comet, which had orbited Jupiter since 1929, had accidentally ventured too close and been torn asunder.
It was the comet's disruption that made it visible to the Palomar observers. Wide‑field telescopes, like the one used by the Shoemakers, can search large patches of sky at once (as big as the bowl of the Big Dipper), but they have the drawback of being able to do so only for stars (and comets) that are relatively bright. The limit of the 18‑inch Palomar telescope isn't so shabby, around ten-thousand times fainter than the faintest star a person can see in a dark, moonless sky. But as a tidy, modest sized object perhaps a mile across, far‑away S-L 9 would have been simply too faint to see with such a search telescope. Indeed, subsequent perusal of photographic archives, even though we now know precisely which parts of which photographs to search, has turned up no trace of the comet in the years before break‑up. However, the disruption of S‑L 9 made it much brighter. Not only the twenty pieces of the comet train, but innumerable smaller pieces, ranging down to clouds of dust particles, all caught the sunlight and reflected it toward Earth, producing an apparent "smudge" that was bright enough for Carolyn Shoemaker to recognize on the light‑fogged film.
The discovery of Comet S-L 9 also solved a mystery that had puzzled planetary geologists ever since the Voyager flybys of Jupiter in 1979. During those historic exploratory sweeps through the jovian system, the spacecraft cameras had photographed about a dozen long "crater chains" on Jupiter's outer big moon Callisto, and another two on the next inner moon, Ganymede. No one had ever figured out a satisfactory explanation for these strings of small craters precisely aligned, marching across the topography of Callisto and Ganymede. Then came the photographs of S-L 9, and a clever young planetary geologist, Paul Schenk, put the story together. He suggested that other comets had passed this way before and been pulled apart into a string of pearls, just like S-L 9. Of the numerous comets that had been disrupted over past aeons, a few would have accidently intersected one of the large jovian moons during the days following disruption; the fragments, colliding with machine-gun-like regularity across the moon's surface, would have replicated on the landscape the string-of-pearls arrangement of S-L 9 in the heavens. And indeed, the number of craters and their spacing and alignment perfectly match the calculations for S-L 9. This comet was not unique, but just the most recent example of something that had been happening throughout solar system history.
Well over 99.9% of such broken-up comets would not have hit one of Jupiter's moons immediately. Most of them must have missed, and subsequently some of them encountered Jupiter like S-L 9, while others were dispersed though the inner solar system -- eventually, perhaps, to collide with a planet like Earth. Thus if we can understand S-L 9's break-up and subsequent evolution, it would help us to understand not only what happens when a disrupted comet strikes Jupiter (S-L 9's fate) but also how comet fragments are formed that threaten us here on Earth.
Interesting as it was to project S-L 9's motion backwards in time to learn why it was now in twenty pieces, Brian Marsden's responsibility was to turn his attention to predicting where the comet would be in the future. It was about a month after the initial discovery of S-L 9 when he came to the startling realization that the comet train would actually crash directly into Jupiter a little more than a year later, in July 1994.
Following the 1992 disruption, the broken comet sailed almost directly away from Jupiter's southern hemisphere. Like a baseball thrown up overhead, S-L 9 was about to turn the corner in July 1993 and fall straight back down toward Jupiter. It would make a series of impacts as each fragment stuck in turn, one following another over a period of a week from July 16 to 21, 1994. Marsden issued his prediction, with characteristic British understatement, in one of his regular announcements: "[Our] initial estimate is that more than half the nuclear train could collide with Jupiter -- over an interval approaching three days . . . [although] it must be emphasized that a collision [of the entire train of fragments] with Jupiter is not assured . . ."
Astronomy is the quintessential science of prediction. Over millennia, its practitioners had gained respect by predicting the times of sunrise and sunset, eclipses, and spectacular appearances of comets. In 1910, they even predicted that Earth would pass through the near‑vacuum of Comet Halley's tail. But never before had one solar system body been predicted to collide with another. One would no more expect to witness worlds colliding than one would expect to watch an Ice Age come. Astronomers around the globe began to make plans.
Not everyone initially recognized the importance of the prediction. But NASA scientists Kevin Zahnle and Mordacai Mark MacLow were almost too fast: they quickly modified their existing computer programs to predict the depth of penetration of the comet fragments and the dimensions of the expected explosion plumes, and they rushed a technical paper to the international scientific journal Nature within a few weeks of Marsden's announcement. Caught unprepared, the editors of Nature rejected the paper out of hand as too speculative and "not of interest to the readers of Nature." (Nature quickly realized it had missed the mark, so to speak, and they solicited an account of the impact prospects from one of us [Chapman] for a subsequent issue.) Although unpublished, the Zahnle-MacLow calculations quickly became the basis for planning observations to be made by the Galileo spacecraft (as we describe in detail below), and dog-eared copies of the Zahnle-MacLow manuscript circulated for months around the NASA Jet Propulsion Lab (JPL) and other research institutions.
The comet impact presented a unique chance for astronomers to plan ahead for a world‑wide campaign. A year's notice was just what was needed. There was time for funding agencies, like the National Science Foundation, to solicit proposals, evaluate them, and award funds so that scientists could design and build new instruments and mount observing campaigns. There was time to organize teams of observers to apply for time on telescopes around the world ‑‑ telescopes that are usually scheduled at least half‑a‑year in advance. There was time for theoreticians to construct computer programs designed to simulate a Jupiter comet crash, and to make predictions about observable phenomena. When a unique event is about to occur, one wants to have thought of every possible outcome in advance, so that nothing is overlooked.
In order to keep track of S-L 9, astronomers named the individual mini-comets for the letters of the alphabet. Fragment A would be the first to hit, on the evening of Saturday, July 16th, as seen from Europe (mid‑day in the U.S.); the show would conclude with W's demise the following Thursday evening, as seen from Pacific longitudes. Although individual fragments brightened and faded, there were always about 20 fragments to track, each with its own short tail, with the whole array immersed in a dusty cloud. The individual mini-comets were very small, however, and their sizes and their chemical composition remained matters of conjecture up to the time of impact.
The year's advance notice was critical for using the two most important observatories ‑‑ the Hubble Space Telescope (HST) and the Galileo spacecraft. Hubble, in orbit around the Earth, is not troubled by clouds or the blurring effect of the Earth's atmosphere. To be sure, its own optics were fuzzy, but astronauts fixed its vision seven months before the comet crash, enabling it to take the sharpest pictures of any telescope in the world. One year was actually short notice, but eventually xx [check] hours of observing time were scheduled on HST during comet‑crash week, providing the most detailed views of what transpired on Jupiter.
The other observatory requiring the full advance notice ‑‑ Galileo ‑‑ is a modest one in size, but it had a unique advantage. A NASA spacecraft, launched by the space shuttle Discovery in October 1989, Galileo was en route to Jupiter on the last leg of a six‑year journey. It was designed to drop a spinning probe into Jupiter's atmosphere, after which the rest of the spacecraft would go into orbit to study the giant planet's moons for two years. Although by July 1994 Galileo would be three times closer to Jupiter than all Earth-based observatories, that was not its chief advantage ‑‑ big Earth-based telescopes and HST could more than make up for their lack in proximity to Jupiter by their physical size. Galileo's advantage lay in the direction from which it could watch the crash: by great good fortune, it would see the pre‑dawn longitudes where all the fragments would hit. Marsden's calculations showed that S-L 9 was going to crash into Jupiter's back side, over the planet's horizon as seen from Earth. One of us (Chapman) had the dubious distinction of being the first to tell Gene Shoemaker of that sad fact. Nature had beaten all the odds by getting a comet to crash into Jupiter for us ‑‑ and then we lost the final flip‑of‑the‑coin about whether or not it would hit on the front side where we all could see it. Galileo alone was positioned to watch.
Like Hubble, Galileo is a complex facility, run by a team of hundreds of engineers. Although they were provided no more funds by NASA for the task, Galileo Project officials at JPL boldly decided to try observing the comet crash with only a year's advance warning. Normally, Galileo's commands were programmed two years in advance. Changing the interplanetary cruise operations for a comet watch campaign was an even more daunting task, however, for the spacecraft was seriously crippled by an earlier failure of its large, high-gain antenna. Without the high-gain antenna, Galileo was reduced to dribbling data back to Earth over a second, tiny antenna, designed for rudimentary communications only. After passing through the asteroid belt, Galileo would be so far from Earth that it could tell its story at the rate of only one character per second, slower than the transmission rate of a telegrapher sending Morse code. Since a picture is worth ten thousand words (actually more), Galileo could hardly send back a few dozen pictures at that slow rate. So JPL engineers were working on improving the effective data transmission rate, by designing data‑compression software, by building and using larger antennas on Earth to receive the weak signals, and in every way they could other than shaking loose the main antenna. By 1996, when Galileo began looping among Jupiter's moons, engineers managed to have enough fixes in place so that the cream of the data could be returned to Earth.
Through heroic efforts, some of the Galileo software under development was readied, much earlier than planned, in time for the comet crash. The observing sequences were changed to enable Galileo to take multiple exposures on the same frame. That saved the day. Otherwise Galileo's camera would have filled the spacecraft's tape recorder with only xx [check] frames, which might have missed recording any impacts at all. In June 1994, when instructions for the final observing sequence had to be transmitted to the spacecraft, the impact times were known only to within the nearest half hour. With multiple exposure capability, however, as many as 64 images of Jupiter could be packed into a single frame. Time‑lapse pictures could be taken for a whole hour around the predicted time of impact, and then later ‑‑ after Earth-based observers told Galileo engineers the precise time an impact actually occurred ‑‑ the recorded data bits could be dribbled back from just a single frame.
Astronomers planning to observe from mountain‑tops around the world also wanted the best predictions of the impact times, so that they could develop minute‑by‑minute plans to observe most efficiently. For Galileo, however, with its constrained capabilities, good predictions were not just desirable but absolutely essential for getting the proper data recorded on its tape recorder. Good predictions involved a worldwide campaign to measure the positions of the evolving comet fragments as they headed for their climatic impacts. Two JPL scientists, Donald Yeomans and Paul Chodas, put all of the data into a sophisticated computer program; every week or so during late spring of 1994, they posted on the Internet their revised predictions for each fragment. The Galileo Project was the prime beneficiary, but the Yeomans/Chodas predictions also shaped travel plans as astronomers scattered to observatories around the Earth.
For most astronomers in the Northern Hemisphere, Jupiter would be low in the southwestern sky after sunset, providing a couple hours of good viewing each night of comet crash week. It was a bit better in southern latitudes, but only two or three impacts could be witnessed from any particular longitude on Earth. Given the vagaries of weather, no single observatory could be counted on to cover an impact. Furthermore, there were many types of observations to make ‑‑ for example pictures as well as spectra at visible, infrared, and radio wavelengths ‑‑ which can hardly be obtained at once from any particular observatory. So nearly every telescope on the planet was scheduled for comet crash observations, and portable telescopes were shipped to remote locations to cover longitudes not well served by in‑the‑ground observatories.
One observing team ventured to Renunion Island, in the western Indian Ocean. Another trucked delicate instruments for days, at slow speeds, across pot‑holed roads into a mountain range in Baja California. Still others readied specialized equipment at every dome in the "telescope city" that has been built atop 14,000 foot Mauna Kea on the Big Island of Hawaii. A special team prepared to fly NASA's Kuiper Airborne Observatory to the Southern Pacific, where they would observe some of the comet impacts through a hatch in the side of the C‑141 aircraft from an altitude of 40,000 feet ‑‑ far above most of the Earth's murky atmosphere. In the bitter cold and round‑the‑clock darkness of Antarctic winter, a University of Chicago team set up operations at the South Pole; if the snow stopped blowing, they hoped to see every one of the fragment impacts.
Theoreticians worked overtime trying to predict what would be seen. At Sandia National Laboratory, the world's most massively parallel computer had been put to work calculating a visual presentation of the physics of a typical impact. At a March scientific conference, scientists cautiously suggested that the debris plumes from some of the larger, later fragment impacts might rise high enough above Jupiter's back side to peek over its horizon, as seen from Earth. There was hope, after all, of seeing something of the explosion from observatories other than the well-placed Galileo spacecraft. Though hopes were raised, few astronomers counted on it.
More likely, they thought, the brilliant cometary meteors plunging through Jupiter's atmosphere at 60 kilometers per second would produce flashes so blinding ‑‑ though shielded from view from Earth's direction ‑‑ that Jupiter's moons would light up by reflection. Therefore plans were made to monitor the brightness of Jupiter's moons at the predicted times of impacts to see if their already sunlit surfaces would briefly brighten for the few seconds as a fragment scorched its way down into Jupiter's clouds. Then, according to theoretical modelers, each comet fragment would disintegrate, far below the clouds, and explode with the force of millions of megatons of TNT. A superheated bubble of jovian atmosphere would be expelled back up above the clouds, and erupt into space. This so‑called fireball was expected to appear 10 or 20 seconds after the meteor struck, perhaps making an even brighter reflection off Jupiter's moons.
During the half hour after each impact, Jupiter's fast axial rotation would carry the impact point around into sunlight and direct view from Earth. Then, and only then, could the power of modern astronomical instrumentation be focused on analyzing what had happened. Nobody knew for sure what to expect. Even after a year of study, astronomers disputed how big the comet fragments might be. Some, analyzing the break‑up mechanics, thought they might be only a few hundred meters across. Others, analyzing HST pictures, thought some fragments might exceed 4 kilometers across. If the HST team was correct, the series of impacts would add up to a yield equivalent of the K‑T impact on Earth 65 million years ago.
Surely, the optimists said, there would be some chemical changes to be seen by the world's most sensitive astronomical spectrographs. Surely, at least to Hubble's sharp view, there would be some changes in the visible clouds ‑‑ some new "spots" on Jupiter's belted face. A few observers even hoped to see waves spreading away from the target points, like ripples from a stone dropped in a pond. If jovian seismic waves could be detected, after Jupiter was "rung" like a bell by the impact explosions, they would provide invaluable knowledge about the interior of the solar system's largest planet.
Surely, if millions ‑‑ perhaps hundreds of millions ‑‑ of megatons of energy were deposited in Jupiter's atmosphere, all near the same latitude of about 45 degrees south, astronomers would see some perceptible changes during the ensuing half hour. For example, stratospheric hazes might form and spread, possibly forming a bright patch that even amateur astronomers could see during subsequent weeks. That was the optimistic view. Astronomers' articles and public statements began to capture the interest of the news media and the broader public. The Nature Company signed on David Levy to promote its telescopes and other astronomical paraphernalia as well as a new book Levy had written. Amateur astronomers practiced looking at Jupiter through their backyard scopes, so they would know its pre‑crash appearance in the off‑chance the comet crash changed it somehow. CNN, PBS, and ABC's Nightline made advance preparations for televised specials in the middle of comet crash week.
Other astronomers grew nervous, however. They had been burned by comets in the past. In the 1970's, a careless prediction was issued that a faint new comet would brighten into the "comet of the century." Comet Kohoutek, they said, would shine brightly enough to be seen in the daytime sky, yet it had proved to be a dud. Comet Halley itself had disappointed the public during its long‑awaited return in 1986. JPL comet expert Paul Weissman made world-wide headlines a few days before fragment A was to hit. Fearing that Shoemaker‑Levy 9's crash into Jupiter would prove to be another cometary dud, he predicted a "cosmic fizzle."