uous moons now orbiting the giant planets will one day blossom forth into vast and lovely rings。
These ideas are supported by the appearance of a number of satellites in the Solar System。 Phobos； the inner moon of Mars； has a large crater named Stickney； Mimas； an inner moon of Saturn； has a big one named Herschel。 These craters—like those on our own Moon and； indeed； throughout the Solar System—are produced by collisions。 An interloper smashes into a bigger world and makes an immense explosion at the point of impact； A bowl…shaped crater is excavated； and the smaller impacting object is destroyed。 If the interlopers that dug out the Stickney and Herschel craters had been only a little larger； they would have had enough energy to blow Phobos and Mimas to bits。 These moons barely escaped the cosmic wrecking ball。 Many others did not。
Every time a world is smashed into； there's one less interloper—something like a demolition derby on the scale of the Solar System； a war of attrition。 The very fact that many such collisions have occurred means that the rogue worldlets have been largely used up。 Those on circular trajectories around the Sun； those that don't intersect the orbits of other worlds； will be unlikely to smash into a planet。 Those on highly elliptical trajectories； those that cross the orbits of other planets； Will sooner or later collide or； by a near miss； be gravitationally ejected from the Solar System。
The planets almost certainly accumulated from worldlets which in turn had condensed out of a great flat cloud of gas and dust Surrounding the Sun—the sort of cloud that can now be seen around young nearby stars。 So； in the early history of the solar System before collisions cleaned things up； there should have been many more worldlets than we see today。
Indeed； there is clear evidence for this in our own backyard： If we count up the interloper worldlets in our neighborhood in space； we can estimate how often they'll hit the Moon。 Let us make the very modest assumption that the population 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 unkn