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hteenth and nineteenth centuries； American and Russian explorers； traders； and settlers raced west and east across two vast continents to the Pacific。 This zest to explore and exploit； however thoughtless its agents may have been； has clear survival value。 It is not restricted to any one nation or ethnic group。 It is an endowment that all members of the human species hold in mon。
Since we first emerged； a few million years ago in East Africa； we have meandered our way around the planet。 There are now people on every continent and the remotest islands； from pole to pole； from Mount Everest to the Dead Sea； on the ocean bottoms and even； occasionally； in residence 200 miles up—humans； like the gods of old； living in the sky。
These days there seems to be nowhere left to explore； at least on the land area of the Earth。 Victims of their very success the explorers now pretty much stay home。
Vast migrations of people—some voluntary； most not— have shaped the human condition。 More of us flee from war； oppression； and famine today than at any other time in human history。 As the Earth's climate changes in the ing decade。 there are likely to be far greater numbers of environmental refugees。 Better places will always call to us。 Tides of people will continue to ebb and flow across the planet。 But the lands we run to now have already been settled。 Other people； often unsympathetic to our plight； are there before us。
LATE IN THE NINETEENTH CENTURY； Leib Gruber was growing up 111 Central Europe； in an obscure town in the immense； polyglot； ancient Austro…Hungarian Empire。 His father sold fish when he could。 But times were often hard。 As a young man； the only honest employment Leib could find was carrying people across the nearby river Bug。 The customer； male or female； would mount Leib's back； in his prized boots； the tools of his trade； he would wade out in a shallow stretch of the river and deliver his passenger to the opposite bank。 Sometimes the water reached his waist。 There were no bridges here； no ferryboats。 Horses might have served the purpose； but they had other uses。 That left Leib and a few other young men like him。 They had no other uses。 No other work was available。 They would lounge about the riverbank； calling out their prices； boasting to potential customers about the superiority of their drayage。 They hired themselves out like four…footed animals。 My grandfather was a beast of burden
I don't think that in all his young manhood Leib had ventured more than a hundred kilometers from his little hometown of Sassow。 But then； in 1904； he suddenly ran away to the New World to avoid a murder rap； according to one family legend。 He left his young wife behind。 How different from his tiny back…water hamlet the great German port cities must have seemed； how vast the ocean； how strange the lofty skyscrapers and endless hub…bub of his new land。 We know nothing of his crossing； but have found the ship's manifest for the journey undertaken later by his wife； Chaiya joining Leib after he had saved enough to bring her over。 She traveled in the cheapest class on the Batavia； a vessel of Hamburg registry。 There's something heartbreakingly terse about the document： Can she read or write？ No。 Can she speak English？ No。 How much money does she have？ I can imagine her vulnerability and her shame as she replies； 〃One dollar。〃
She disembarked in New York； was reunited with Leib； lived just long enough to give birth to my mother and her sister； and then died from 〃plications〃 of childbirth。 In those few years in America； her name had sometimes been anglicized to Clara。 A quarter century later； my mother named her own firstborn； a son； after the mother she never knew。
OUR DISTANT ANCESTORS； watching the stars； noted five that did more than rise and set in stolid procession； as the so…called 〃fixed〃 stars did。 These five had a curious and plex motion。 Over the months they seemed to wander slowly among the stars。 Sometimes they did loops。 Today we call them planets； the Greek word for wanderers。 It was； I imagine； a peculiarity our ancestors could relate to。
We know now that the planets are not stars； but other worlds； gravitationally lashed to the Sun。 Just as the exploration of the Earth was being pleted； we began to recognize it as one world among an uncounted multitude of others； circling the Sun or orbiting the other stars that make up the Milky Way galaxy。 Our planet and our solar system are surrounded by a new world ocean the depths of space。 It is no more impassable than the last。
Maybe it's a little early。 Maybe the time is not quite yet。 But those other worlds—promising untold opportunities—beckon。
In the last few decades； the United States and the former Soviet Union have acplished something stunning and historic—the close…up examination of all those points of light； from Mercury to Saturn； that moved our ancestors to wonder and to science。 Since the advent of successful interplanetary flight in 1962； our machines have flown by； orbited； or landed on more than seventy new worlds。 We have wandered among the wanderers。 We have found vast volcanic eminences that dwarf the highest mountain on Earth； ancient river valleys on two planets enigmatically one too cold and the other too hot for running water； a giant planet with an interior of liquid metallic hydrogen into which a thousand Earths would fit； whole moons that have melted； a cloud…covered place with an atmosphere of corrosive raids； where even the high plateaus are above the melting point of lead ancient surfaces on which a faithful record of the violent formation of the Solar System is engraved； refugee ice worlds from the transplutonian depths； exquisitely patterned ring systems； marking the subtle harmonies of gravity； and a world surrounded by clouds of plex organic molecules like those that m the earliest history of our planet led to the origin of life。 Silently； they orbit the Sun； waiting。
We have uncovered wonders undreamt by our ancestors who first speculated on the nature of those wandering lights in the night sky。 We have probed the origins of our planet and ourselves。 By discovering what else is possible； by ing face to face with alternative fates of worlds more or less like our own； we have begun to better understand the Earth。 Every one of these worlds is lovely and instructive。 But； so far as we know； they are also； every one of them； desolate and barren。 Out there； there are no 〃better places。〃 So far； at least。
During the Viking robotic mission； beginning in July 1976； in a certain sense I spent a year on Mars。 I examined the boulders and sand dunes； the sky red even at high noon； the ancient river valleys； the soaring volcanic mountains； the fierce wind erosion； the laminated polar terrain； the two dark potato…shaped moons。 But there was no life—not a cricket or a blade of grass； or even； so far as we can tell for sure； a microbe。 These worlds have not been graced； as ours has； by life。 Life is a parative rarity。 You can survey dozens of worlds and find that on only one of them does life arise and evolve and persist。
Having in all their lives till then crossed nothing wider than a layer； Leib and Chaiya graduated to crossing oceans。 They had one great advantage： On the other side of the waters there would be…invested with outlandish customs； it is true—other human beings speaking their language and sharing at least some of their values； even people to whom they were closely related。
In our time we've crossed the Solar System and sent four ships to the stars。 Neptune lies a million times farther from Earth than New York City is from the banks of the Bug。 But there are no distant relatives； no humans； and apparently no life waiting for us on those other worlds。 No letters conveyed by recent émigrés help us to understand the new land— only digital data transmitted at the speed of light by unfeeling； precise robot emissaries。 They tell us that these new worlds are not much like home。 But we continue to search for inhabitants。 We can't help it。 Life looks for life。
No one on Earth； not the richest among us； can afford the passage； so we can't pick up and leave for Mars or Titan on a whim； or because we're bored； or out of work； or drafted into the army； or oppressed； or because； justly or unjustly； we've been accused of a crime。 There does not seem to be sufficient short…term profit to motivate private industry。 If we humans ever go to these worlds； then； it will be because a nation or a consortium of them believes it to be to its advantage—or to the advantage of the human species。 Just now； there are a great many matters pressing in on us that pete for the money it takes to send people to other worlds。
That's what this book is about： other worlds； what awaits us on them； what they tell us about ourselves； and—given the urgent problems our species now faces—whether it makes sense to go。 Should we solve those problems first？ Or are they a reason for going？
This book is； in many ways； optimistic about the human prospect。 The earliest chapters may at first sight seem to revel overmuch in our imperfections。 But they lay an essential spiritual and logical foundation for the development of my argument。
I have tried to p