THE DWINDLING SUPPLY of worldlets is today variously labeled: asteroids, comets, small moons. But these are arbitrary categories—real worldlets are able to breach these human-made partitions. Some asteroids (the word means "starlike," which they certainly are not) are rocky, others metallic, still others rich in organic matter. None is bigger than 1,000 kilometers across. They are found mainly in a belt between the orbits of Mars and Jupiter. Astronomers once thought the "main-belt" asteroids were the remains of a demolished world, but, as I've been describing, another idea is now more fashionable: The Solar System was once filled with asteroid-like worlds, some of which went into building the planets. Only in the asteroid belt, near Jupiter, did the gravitational tides of this most massive planet prevent the nearby debris from coalescing into a new world. The asteroids, instead of representing a world that once was, seem to be the building blocks of a world destined never to be.
Down to kilometer size, there may be several million asteroids, but, in the enormous volume of interplanetary space, even that's still far too few to cause any serious hazard to spacecraft on their way to the outer Solar System. The first main-belt asteroids, Gaspra and Ida, were photographed, in 1991 and 1993 respectively, by the Galileo spacecraft on its tortuous journey to Jupiter.
Main-belt asteroids mostly stay at home. To investigate them. we must go and visit them, as Galileo did. Comets, on the other hand, sometimes come and visit us, as Halley's Comet did most recently in 1910 and 1986. Comets are made mainly of ice, plus smaller amounts of rocky and organic material. When heated, the ice vaporizes, forming the long and lovely tails blown outward by the solar wind and the pressure of sunlight. After many passages by the Sun, the ice is all evaporated, sometimes leaving a dead rocky and organic world. Sometimes the remaining particles, the ice that held them together now gone, spread out in the comet's orbit, generating a debris trail around the Sun.
Every time a bit of cometary fluff the size of a grain of sand enters the Earth's atmosphere at high speed, it burns up, producing a momentary trail of light that Earthbound observers call a sporadic meteor or "shooting star." Some disintegrating comets have orbits that cross the Earth's. So every year, the Earth, on its steady circumnavigation of the Sun, also plunges through belts of orbiting cometary debris. We may then witness a meteor shower, or even a meteor storm—the skies ablaze with the body parts of a comet. For example, the Perseid meteors, seen on or about August 12 of each year, originate in a dying comet called Swift-Tuttle. But the beauty of a meteor shower should not deceive us: There is a continuum that connects these shimmering visitors to our night skies with the destruction of worlds.
A few asteroids now and then give off little puffs of gas or even form a temporary tail, suggesting that they are in transition between cometdom and asteroidhood. Some small moons going around the planets are probably captured asteroids or comets; the moons of Mars and the outer satellites of Jupiter may be in this category.
Gravity smooths down everything that sticks out too far. But only in large bodies is the gravity enough to make mountains and other projections collapse of their own weight, rounding the world. And, indeed, when we observe their shapes, almost always we find that small worldlets are lumpy, irregular, potato-shaped.
THERE ARE ASTRONOMERS whose idea of a good time is to stay up till dawn on a cold, moonless night taking pictures of the sky—the same sky they photographed the year before . . . and the year before that. If they got it right last time, you might well ask, why are they doing it again? The answer is: The sky changes. In any given year there might be worldlets wholly unknown, never seen before, that approach the Earth and are spied by these dedicated observers.
On March 25, 1993, a group of asteroid and comet hunters, looking at the photographic harvest from an intermittently cloudy night at Mount Palomar in California, discovered a faint elongated smudge on their films. It was near a very bright object in the sky, the planet Jupiter. Carolyn and Eugene Shoemaker and David Levy then asked other observers to take a look. The smudge turned out to be something astonishing: some twenty small, bright objects orbiting Jupiter, one behind the other, like pearls on a string. Collectively they are called Comet Shoemaker-Levy 9 (this is the ninth time that these collaborators have together discovered a periodic comet).
But calling these objects a comet is confusing. There was a horde of them, probably the fragmented remains of a single, hitherto undiscovered, comet. It silently orbited the Sun for 4 billion years before passing too close to Jupiter and being captured, a few decades ago, by the gravity of the Solar System's largest planet. On July 7, 1992, it was torn apart by Jupiter's gravitational tides.
You can recognize that the inner part of such a comet would be pulled toward Jupiter a little more strongly than the outer part, because the inner part is closer to Jupiter than the outer part. The difference in pull is certainly small. Our feet are a little closer to the center of the Earth than our heads, but we are not in consequence torn to pieces by the Earth's gravity. For such tidal disruption to have occurred, the original comet must have been held together very weakly. Before fragmentation, it was, we think, a loosely consolidated mass of ice, rock, and organic matter, maybe 10 kilometers (about 6 miles) across.
The orbit of this disrupted comet was then determined to high precision. Between July 16 and 22, 1994, all the cometary fragments, one after another, collided with Jupiter. The biggest pieces seem to have been a few kilometers across. Their impacts with Jupiter were spectacular.
No one knew beforehand what these multiple impacts into the atmosphere and clouds of Jupiter would do. Perhaps the cometary fragments, surrounded by halos of dust, were much smaller than they seemed. Or perhaps they were not coherent bodies at all, but loosely consolidated—something like a heap of gravel with all the particles traveling through space together, in nearly identical orbits. If either of these possibilities were true Jupiter might swallow the comets without a trace. Other astronomers thought there would at least be bright fireballs and giant plumes as the cometary fragments plunged into the atmosphere. Still others suggested that the dense cloud of fine particles accompanying the fragments of Comet Shoemaker-Levy 9 into Jupiter would disrupt the magnetosphere of Jupiter or form a new ring.
A comet this size should impact Jupiter, it is calculated, only once every thousand years. It's the astronomical event not of one lifetime, but of a dozen. Nothing on this scale has occurred since the invention of the telescope. So in mid July 1994, in a beautifully coordinated international scientific effort, telescopes all over the Earth and in space turned towards Jupiter.
Astronomers had over a year to prepare. The trajectories of the fragments in their orbits around Jupiter were estimated. It was discovered that they would all hit Jupiter. Predictions of the timing were refined. Disappointingly, the calculations revealed that all impacts would occur on the night side of Jupiter, the side invisible from the Earth (although accessible to the Galileo and Voyager spacecraft in the outer Solar System). But, happily, all impacts would occur only a few minutes before the Jovian dawn, before the impact site would be carried by Jupiter's rotation into the line of sight from Earth.
The appointed moment for the impact of the first piece, Fragment A, came and went. There were no reports from ground-based telescopes. Planetary scientists stared with increasing gloom at a television monitor displaying the data transmitted to the Space Telescope Science Institute in Baltimore from the Hubble Space Telescope. There was nothing anomalous Shuttle astronauts took time off from the reproduction of fruit flies, fish, and newts to look at Jupiter through binoculars. They reported seeing nothing. The impact of the millennium was beginning to look very much like a fizzle.
Then there was a report from a ground-based optical telescope in La Palma in the Canary Islands, followed by announcements from a radiotelescope in Japan; from the European Southern Observatory in Chile; and from a University of Chicago instrument in the frigid wastelands of the South Pole. In Baltimore the young scientists crowding around the TV monitor—themselves monitored by the cameras of CNN—began to see something, and in exactly the right place on Jupiter. You could witness consternation turn into puzzlement, and then exultation. They cheered; they screamed; they jumped up and down. Smiles filled the room. They broke out the champagne. Here was a group of young American scientists—about a third of them, including the team leader, Heidi Hammel, women—and you could imagine youngsters all over the world thinking that it might be fun to be a scientist, that this might be a good daytime job, or even a means to spiritual fulfillment.
For many of the fragments, observers somewhere on Earth noticed the fireball rise so quickly and so high that it could be seen even though the impact site below it was still in Jovian darkness. Plumes ascended and then flattened into pancake-like forms. Spreading out from the point of impact we could see sound and gravity waves, and a patch of discoloration that for the largest fragments became as big as the Earth.
Slamming into Jupiter at 60 kilometers a second (130,000 miles an hour), the large fragments converted their kinetic energy partly into shock waves, partly into heat. The temperature in the fireball was estimated at thousands of degrees. Some of the fireballs and plumes were far brighter than all the rest of Jupiter put together.
What is the cause of the dark stains left after the impact? It might be stuff from the deep clouds of Jupiter-from the region to which ground-based observers cannot ordinarily see-that welled up and spread out. However, the fragments do not seem to have penetrated to such depths. Or the molecules responsible for the stains might have been in the cometary fragments in the first place. We know from the Vega 1 and 2 Soviet missions and the Giotto mission of the European Space Agency—both to Halley's Comet—that comets may be as much as a quarter composed of complex organic molecules. They are the reason that the nucleus of Halley's Comet is pitch black. If some of the cometary organics survived the impact events, they may have been responsible for the stain. Or, finally, the stain may be due to organic matter not delivered by the impacting cometary fragments, but synthesized by their shock waves from the atmosphere of Jupiter.
Impact of the fragments of Comet Shoemaker-Levy 9 with Jupiter was witnessed on seven continents. Even amateur astronomers with small telescopes could see the plumes and the subsequent discoloration of the Jovian clouds. Just as sporting events are covered at all angles by television cameras on the field and from a dirigible high overhead, six NASA spacecraft deployed throughout the Solar System, with different observational specialties, recorded this new wonder—the Hubble Space Telescope, the International Ultraviolet Explorer, and the Extreme Ultraviolet Explorer all in Earth orbit; Ulysses, taking time out from it investigation of the South Pole of the Sun; Galileo, on the way to its own rendezvous with Jupiter; and Voyager 2, far beyond Neptune on its way to the stars. As the data are accumulated and analyzed, our knowledge of comets, of Jupiter, and of the violent collisions of worlds should all be substantially improved.
For many scientists—but especially for Carolyn and Eugene Shoemaker and David Levy—there was something poignant about the cometary fragments, one after the other, making their death plunges into Jupiter. They had lived with this comet, in a manner of speaking, for 16 months, watched it split, the pieces, enshrouded by clouds of dust, playing hide-and-seek and spreading out in their orbits. In a limited way, each fragment had its own personality. Now they're all gone, ablated into molecules and atoms in the upper atmosphere of the Solar System's largest planet. In a way, we almost mourn them. But we're learning from their fiery deaths. It is perhaps some reassurance to know that there are a hundred trillion more of them in the Sun's vast treasure-house of worlds.
THERE ARE ABOUT 200 known asteroids whose paths take them near the Earth. They are called, appropriately enough, "near-Earth" asteroids. Their detailed appearance (like that of their main-belt cousins) immediately implies that they are the products of a violent collisional history. Many of them may be the shards and remnants of once-larger worldlets.
With a few exceptions, the near-Earth asteroids are only a few kilometers across or smaller, and take one to a few years to make a circuit around the Sun. About 20 percent of them, sooner or later, are bound to hit the Earth—with devastating consequences. (But in astronomy, "sooner or later" can encompass billions of years.) Cicero's assurance that "nothing of chance or hazard" is to be found in an absolutely ordered and regular heaven is a profound misperception. Even today, as Comet Shoemaker-Levy 9's encounter with Jupiter reminds us, there is routine interplanetary violence, although not on the scale that marked the early history of the Solar System.
Like main-belt asteroids, many near-Earth asteroids are rocky. A few are mainly metal, and it has been suggested that enormous rewards might attend moving such an asteroid into orbit around the Earth, and then systematically mining it—a mountain of high-grade ore a few hundred miles overhead. The value of platinum-group metals alone in a single such world has been estimated as many trillions of dollars—although the unit price would plummet spectacularly if such materials became widely available. Methods of extracting metals and minerals from appropriate asteroids are being studied, for example by John Lewis, a planetary scientist at the University of Arizona.
Some near-Earth asteroids are rich in organic matter, apparently preserved from the very earliest Solar System. Some have been found, by Steven Ostro of the Jet Propulsion Laboratory, to be double, two bodies in contact. Perhaps a larger world has broken in two as it passed through the strong gravitational tides of a planet like Jupiter; more interesting is the possibility that two worlds on similar orbits made a gentle overtaking collision and stuck. This process may have been key to the building of planets and the Earth. At least one asteroid (Ida, as viewed by Galileo) has its own small moon. We might guess that two asteroids in contact and two asteroids orbiting one another have related origins.
