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founded fantasy； could be more soberly treated without ceasing to be interesting and could show to even the most adventurous of the young that a world without war need not be a world without adventurous and hazardous glory。* To this kind of contest there is no limit。 Each victory is only a prelude to another； and no boundaries can be set to rational hope。
* Russell's phrase is noteworthy： 〃adventurous and hazardous glory。〃 Even if we could make human spaceflight risk…free…and of course we cannot…it might be counterproductive。 The hazard is an inseparable ponent of the glory。
In the long run； these—more than any of the 〃practical〃 justifications considered earlier—may be the reasons we will go to Mars and other worlds。 In the meantime； the most important step we can take toward Mars is to make significant progress on Earth。 Even modest improvements in the social； economic； and political problems that our global civilization now faces could release enormous resources； both material and human； for other goals。
There's plenty of housework to be done here on Earth； and our mitment to it must be steadfast。 But we're the kind of species that needs a frontier—for fundamental biological reasons。
Every time humanity stretches itself and turns a new corner； it receives a jolt of productive vitality that can carry it for centuries。
There's a new world next door。 And we know how to get there。
CHAPTER 17 ROUTINE INTERPLANETARY VIOLENCE
It is a law of nature that Earth and all other bodies should remain in their proper places and be moved from them only by violence。
—ARISTOTLE （384…322 B。C。）； PHYSICS
There was something funny about Saturn。 When； in 1610； Galileo used the world's first astronomical telescope to view the planet—then the most distant world known—he found two appendages； one on either side。 He likened them to 〃handles。〃 Other astronomers called them 〃ears。〃 The Cosmos holds many wonders； but a planet with jug ears is dismaying。 Galileo went to his grave with this bizarre matter unresolved。
As the years passed； observers found the ears 。 。 。 Well； waxing and waning。 Eventually； it became clear that what Galileo had discovered was an extremely thin ring that surrounds Saturn at its equator but touches it nowhere。 In some years； because of the changing orbital positions of Earth and Saturn； the ring had been seen edge…on and； because of its thinness； it seemed to disappear。 In other years； it had been viewed more face…oil； and the 〃ears〃 grew bigger。 But what does it mean that there's a ring around Saturn？ A thin； flat； solid plate with a hole cut out for the planet to fit into？ Where does that e from？
This line of inquiry will shortly take us to world…shattering collisions； to two quite different perils for our species； and to a reason—beyond those already described—that we must； for our very survival； be out there among the planets。
We now know that the rings （emphatically plural） of Saturn are a vast horde of tiny ice worlds； each on its separate orbit； each bound to Saturn by the giant planet's gravity。 In size； these worldlets range from particles of fine dust to houses。 None is big enough to photograph even from close flybys。 Spaced out in an exquisite set of fine concentric circles； something like the grooves on a phonograph record （which in reality make； of course； a spiral）； the rings were first revealed in their true majesty by the two Voyager spacecraft in their 1980/81 flybys。 In our century； the Art Deco rings of Saturn have bee an icon of the future。
At a scientific meeting in the late 1960s； I was asked to summarize the outstanding problems in planetary science。 One； I suggested； was the question of why； of all the planets； only Saturn had rings。 This； Voyager discovered； is a nonquestion。 All four giant planets in our Solar System— Jupiter； Saturn； Uranus； and Neptune—in fact have rings。 But no one knew it then。
Each ring system has distinctive features。 Jupiter's is tenuous and made mainly of dark； very small particles。 The bright rings of Saturn are posed mainly of frozen water； there are thousands of separate rings here； some twisted； with strange； dusky； spoke…like markings forming and dissipating。 The dark rings of Uranus seem to be posed of elemental carbon and organic molecules—something like charcoal or chimney soot； Uranus has nine main rings； a few of which sometime seem to 〃breathe；〃 expanding and contracting。 Neptune's rings are the most tenuous of all； varying so much in thickness that； when detected from Earth； they appear only as arcs and inplete circles。 A number of rings seem to be maintained by the gravitational tugs of two shepherd moons； one a little nearer and the other a little farther from the planet than the ring。 Each ring system displays its own； appropriately unearthly； beauty。
How do rings form？ One possibility is tides： If an errant world passes close to a planet； the interloper's near side is gravitationally pulled toward the planet more than its far side； if it es close enough； if its internal cohesion is low enough； it can be literally torn to pieces。 Occasionally we see this happening to ets as they pass too close to Jupiter； or the Sun。 Another possibility； emerging from the Voyager reconnaissance of the outer Solar System； is this： Rings are made when worlds collide and moons are smashed to smithereens。 Both mechanisms may have played a role。
The space between the planets is traversed by an odd collection of rogue worldlets； each in orbit about the Sun。 A few are as big as a county or even a state； many more have surface areas like those of a village or a town。 More little ones are found than big ones； and they range in size down to particles of dust。 Some of them travel on long； stretched…out elliptical paths； which make them periodically cross the orbit of one or more planets。
Occasionally； unluckily； there's a world in the way。 The collision can shatter and pulverize both the interloper and the moon that's hit （or at least the region around ground zero）。 The resulting debris—ejected from the moon but not so fast…moving as to escape from the planet's gravity—may form； for a time； a new ring。 It's made of whatever the colliding bodies were made of； but usually more of the target moon than the rogue impactor。 If the colliding worlds are icy； the net result will be rings of ice particles； if they're made of organic molecules； the result will be rings of organic particles （which will slowly be processed by radiation into carbon）。 All the mass in the rings of Saturn is no more than 'would result from the plete impact pulverization of a single icy moon。 The disintegration of small moons can likewise account for the ring systems of the three other giant planets。
Unless it's very close to its planet； a shattered moon gradually reaccumulates （or at least a fair fraction of it does）。 The pieces； big and small； still in approximately the same orbit as the moon was before the impact； fall together helter…skelter。 What used to be a piece of the core is now at the surface； and vice versa。 The resulting hodgepodge surfaces might seem very odd。 Miranda； one of the moons of Uranus； looks disconcertingly jumbled and may have had such an origin。
The American planetary geologist Eugene Shoemaker proposes that many moons in the outer Solar System have been annihilated and reformed—not just once but several times each over the 4。5 billion years since the Sun and the planets condensed out of interstellar gas and dust。 The picture emerging from the Voyager reconnaissance of the outer Solar System is of worlds whose placid and lonely vigils are spasmodically troubled by interlopers from space； of world…shattering collisions； and of moons re…forming from debris； reconstituting themselves like phoenixes from their own ashes。
But a moon that lives very close to a planet cannot re…form if it is pulverized—the gravitational tides of the nearby planet prevent it。 The resulting debris； once spread out into a ring system； might be very long…lived—at least by the standard of a human lifetime。 Perhaps many of the small； inconspicuous 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