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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