of interlopers has never been smaller than it is today。 We can then calculate how many craters there should be on the Moon。 The number we figure turns out to be much less than the number we see on the Moon's ravaged highlands。 The unexpected profusion of craters on the Moon speaks to us of an earlier epoch when the Solar System was in wild turmoil； churning with worlds on collision trajectories。 This makes good sense； because they formed from the aggregation of much smaller worldlets—which themselves had grown out of interstellar dust。 Four billion years ago； the lunar impacts were hundreds of times more frequent than they are today； and 4。5 billion years ago； when the planets were still inplete； collisions happened perhaps a billion times more often than in our becalmed epoch。
The chaos may have been relieved by much more flamboyant ring systems than grace the planets today。 If they had small moons in that time； the Earth； Mars； and the other small planets may also have been adorned with rings。
The most satisfactory explanation of the origin of our own Moon； based on its chemistry （as revealed by samples returned from the Apollo missions）； is that it was formed almost 4。5 billion years ago； when a world the size of Mars struck the Earth。 Much of our planet's rocky mantle was reduced to dust and hot gas and blasted into space。 Some of the debris； in o：…bit around the Earth； then gradually reaccumulated—atom by atom； boulder by boulder。 If that unknown impacting world had been only a little larger； the result would have been the obliteration of the Earth。 Perhaps there once were other worlds in our Solar System—perhaps even worlds on which life was stirring—hit by some demon worldlet； utterly demolished； and of which today we have not even an intimation。
The emerging picture of the early Solar System does not resemble a stately progression of events designed to form the Earth。 Instead； it looks as if our planet was made； and survived； by mere lucky chance；* amid unbelievable violence。 Our world does not seem to have been sculpted by a master craftsman。 Here too； there is no hint of a Universe made for us。
* If it had not； perhaps there would today be another planet； a little nearer to or farther from the Sun； on which other； quite different beings would be trying to reconstruct their origins。
THE DWINDLING SUPPLY of worldlets is today variously labeled： asteroids； ets； 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。 ets； on the other hand； sometimes e and visit us； as Halley's et did most recently in 1910 and 1986。 ets 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 et's orbit； generating a debris trail around the Sun。
Every time a bit of etary 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 ets 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 etary debris。 We may then witness a meteor shower； or even a meteor storm—the skies ablaze with the body parts of a et。 For example； the Perseid meteors； seen on or about August 12 of each year； originate in a dying et 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 etdom and asteroidhood。 Some small moons going around the planets are probably captured asteroids or ets； 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 et 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 et Shoemaker…Levy 9 （this is the ninth time that these collaborators have together discovered a periodic et）。
But calling these objects a et is confusing。 There was a horde of them； probably the fragmented remains of a single； hitherto undiscovered； et。 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 et 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 et 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 et was then determined to high precision。 Between July 16 and 22； 1994； all the etary 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 etary 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 ets without a trace。 Other astronomers thought there would at least be bright fireballs and giant plumes as the etary fragments plunged into the atmosphere。 Still others suggested that the dense cloud of fine particles acpanying the fragments of et Shoemaker…Levy 9 into Jupiter would disrupt the magnetosphere of Jupiter or form a new ring。
A et 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