by an effort of will alone； your limbs hanging limply。 Off you'd soar。
I know many people have had similar dreams。 Maybe most people。 Maybe everyone。 Perhaps it goes back 10 million years or more； when our ancestors were gracefully flinging themselves from branch to branch in the primeval forest。 A wish to soar like the birds motivated many of the pioneers of flight； including Leonardo da Vinci and the Wright brothers。 Maybe that's part of the appeal of spaceflight； too。
In orbit about any world； or in interplanetary flight； you are literally weightless。 You can propel yourself to the spacecraft ceiling with a slight push off the floor。 You can go tumbling through the air down the long axis of the spacecraft。 Humans experience weightlessness as joy； this has been reported by almost every astronaut and cosmonaut。 But because spacecraft are still so small； and because space 〃walks〃 have been done with extreme caution； no human has yet enjoyed this wonder and glory： propelling yourself by an almost imperceptible push； with no machinery driving you； untethered； high up into the sky； into the blackness of interplanetary space。 You bee a living satellite of the Earth； or a human planet of the Sun。
Planetary exploration satisfies our inclination for great enterprises and wanderings and quests that has been with us since our days as hunters and gatherers on the East African savannahs a million years ago。 By chance—it is possible； I say； to imagine many skeins of historical causality in which this would not have transpired—in our age we are able to begin again。
Exploring other worlds employs precisely the same qualities of daring； planning； cooperative enterprise； and valor that mark the finest in the military tradition。 Never mind the night launch of an Apollo spacecraft bound for another world。 That makes the conclusion foregone。 Witness mere F…14s taking off from adjacent flight decks； gracefully canting left and right； afterburners flaming； and there's something that sweeps you away—or at least it does me。 And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling。 It simply speaks to another part of me。 It doesn't want recriminations or politics。 It just wants to fly。
〃I 。 。 。 had ambition not only to go farther than anyone had done before；〃 wrote Captain James Cook； the eighteenth…century explorer of the Pacific； 〃but as far as it was possible for man to go。〃 Two centuries later； Yuri Romanenko； on returning to Earth after what was then the longest space flight in history； said 〃The Cosmos is a magnet 。 。 。 Once you've been there； all you can think of is how to get back。〃
Even Jean…Jacques Rousseau； no enthusiast of technology； felt it：
The stars are far above us； we need preliminary instruction； instruments and machines； which are like so many immense ladders enabling us to approach them and bring them within our grasp。
〃The future possibilities of space…travel；〃 wrote the philosopher Bertrand Russell in 1959； which are now left mainly to unfounded 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