cent of patients survive only if our disease has a greater than 5 percent chance of killing us。 Mere 40…to…1 odds on our species surviving another 12 years Would be； if valid； a cause for supreme concern。 If Gott is right； not only may we never be out among the stars； there's a fair chance we may not be around long enough even to make the first footfall on another planet。
To me； this argument has a strange； vaporish quality。 Knowing nothing about our species except how old it is； we make numerical estimates； claimed to be highly reliable； about its future prospects。 How？ We go with the winners。 Those who have been around are likely to stay around。 Newers tend to disappear。 The only assumption is the quite plausible one that there is nothing special about the moment at which we inquire into the matter。 So why is the argument unsatisfying？ Is it just that we are appalled by its implications？
Something like the Principle of Mediocrity must have very broad applicability。 But we are not so ignorant as to imagine that everything is mediocre。 There is something special about our time—not just the temporal chauvinism that those who reside in any epoch doubtless feel； but something； as outlined above； clearly unique and strictly relevant to our species' future chances： This is the first time that （a） our exponentiating technology has reached the precipice of self…destruction； but also the first time that （b） we can postpone or avoid destruction by going somewhere else； somewhere off the Earth。
These two clusters of capabilities； （a） and （b）； make our time extraordinary in directly contradictory ways—which both （a） strengthen and （b） weaken Gott's argument。 I don't know how to predict whether the new destructive technologies will hasten； more than the new spaceflight technologies will delay； human extinction。 But since never before have we contrived the means of annihilating ourselves； and never before have w e developed the technology for settling other worlds； I think a pelling case can be made that our time is extraordinary precisely in the context of Gott's argument。 If this is true； it significantly increases the margin of error in such estimates of future longevity。 The worst is worse； and the best better： Our short…term prospects are even bleaker and—if we can survive the short…term—our long…term chances even brighter than Gott calculates。
But the former is no more cause for despair than the latter is for placency。 Nothing forces us to be passive observers； clucking in dismay as our destiny inexorably works itself out。 If we cannot quite seize fate by the neck； perhaps we can misdirect it； or mollify it； or escape it。
Of course we must keep our planet habitable—not on a leisurely timescale of centuries or millennia； but urgently； on a timescale of decades or even years。 This will involve changes in government； in industry； in ethics； in economics； and in religion。 We've never done such a thing before； certainly not on a global scale。 It may be too difficult for us。 Dangerous technologies may be too widespread。 Corruption may be too pervasive。 Too many leaders may be focused on the short term rather than the long。 There may be too many quarreling ethnic groups； nation…states； and ideologies for the right kind of global change to be instituted。 We may be too foolish to perceive even what the real dangers are； or that much of what we hear about them is determined by those with a vested interest in minimizing fundamental change。
However； we humans also have a history of making long…lasting social change that nearly everyone thought impossible。 Since our earliest days； we've worked not just for our own advantage but for our children and our grandchildren。 My grandparents and parents did so for me。 We have often； despite our diversity； despite endemic hatreds； pulled together to face a mon enemy。 We seem； these days； much more willing to recognize the dangers before us than we were even a decade ago。 The newly recognized dangers threaten all of us equally。 No one can say how it will turn out down here。
THE MOON WAS WHERE the tree of immortality grew in ancient Chinese myth。 The tree of longevity if not of immortality； it seems； indeed grows on other worlds。 If we were up there among the planets； if there were self…sufficient human munities oil many worlds； our species would be insulated from catastrophe。 The depletion of the ultraviolet…absorbing shield on one world would； if anything； be a warning to take special care of the shield on another。 A cataclysmic impact on one world would likely leave all the others untouched。 The more of us beyond the Earth； the greater the diversity of worlds we inhabit； the more varied the planetary engineering； the greater the range of societal standards and values—then the safer the human species will be。
If you grow up living underground in a world with a hundredth of an Earth gravity and black skies through the portals； you have a very different set of perceptions； interests； prejudices； and predispositions than someone who lives on the surface of the home planet。 Likewise if you live on the surface of Mars in the throes of terraforming； or Venus； or Titan。 This strategy—breaking up into many smaller self…propagating groups； each with somewhat different strengths and concerns； but all marked by local pride—has been widely employed in the evolution of life on Earth； and by our own ancestors in particular。 It may； in fact； be key to understanding why we humans are the way we are。* This is the second of the missing justifications for a permanent human presence in space： to improve our chances of surviving； not just the catastrophes we can foresee； but also the ones we cannot。 Gott also argues that establishing human munities on other worlds may offer us our best chance of beating the odds。
* Cf。 Shadows of Forgotten Ancestors： A Search for Who We Are； by Carl Sagan and Ann Druyan （New York： Random House； 1992）。
To take out this insurance policy is not very expensive； not on the scale on which we do things on Earth。 It would not even require doubling the space budgets of the present spacefaring nations （which； in all cases； are only a small fraction of the military budgets and many other voluntary expenditures that might be considered marginal or even frivolous）。 We could soon be setting humans down on near…Earth asteroids and establishing bases on Mars。 We know how to do it； even with present technology； in less than a human lifetime。 And the technologies will quickly improve。 We will get better at going into space。
A serious effort to send humans to other worlds is relatively so inexpensive on a per annum basis that it cannot seriously pete with urgent social agendas on Earth。 If we take this path； streams of images from other worlds will be pouring down on Earth at the speed of light。 Virtual reality will make the adventure accessible to millions of stay…on…Earths。 Vicarious participation will be much more real than at any earlier age of exploration and discovery。 And the more cultures and people it inspires and excites； the more likely it will happen。
But by what right； we might ask ourselves； do we inhabit； alter； and conquer other worlds？ If anyone else were living in the Solar System； this would be an important question。 If； though； there's no one else in this system but us； don't we have a right to settle it？
Of course； our exploration and homesteading should be enlightened by a respect for planetary environments and the scientific knowledge they hold。 This is simple prudence。 Of course； exploration and settlement ought to be done equitably and transnationally； by representatives of the entire human species。 Our past colonial history is not encouraging in these regards； but this time we are not motivated by gold or spices or slaves or a zeal to convert the heathen to the One True Faith； as were the European explorers of the fifteenth and sixteenth centuries。 Indeed； this is one of the chief reasons we're experiencing such intermittent progress； so many fits and starts in the manned space programs of all nations。
Despite all the provincialisms I plained about early in this book； here I find myself an unapologetic human chauvinist。 If there were other life in this solar system； it would be in imminent danger because the humans are ing。 In such a case； I might even be persuaded that safeguarding our species by settling certain other worlds is offset； in part at least； by the danger we would pose to everybody else。 But as nearly as we can tell； so far at least； there is no other life in this system； not one microbe。 There's only Earthlife。
In that case； on behalf of Earthlife； I urge that； with full knowledge of our limitations； we vastly increase our knowledge of the Solar System and then begin to settle other worlds。
These are the missing practical arguments： safeguarding 'the Earth from otherwise inevitable catastrophic impacts and hedging our bets on the many other threats； known and unknown； to the environment that sustains us。 Without these arguments； a pelling case for sending humans to Mars and elsewhere might be lacking。 But with them—and the buttressing arguments involving science； education； perspective； and hope—I think a strong case can be made。 If our long…term survival is at stake； we have a basic responsibility to our species venture to other worlds。
Sailors on a becalmed sea； we sense the stirring of a breeze。
CHAPTER 22 TIPTOEING THROUGH THE MILKY WAY
I swear by the shelters of the stars （a mighty oath； if you but knew it） 。 。 。
—THE QUR'AN； SURA 56 （7TH CENTURY）
The prospect of scaling heaven； of ascending to the sky； of altering other worlds to suit our purposes—no matter how well intentioned we may be—sets the warning flags flying： We remember the human inclination toward overweening pride； we recall our fallibility and misjudgments when presented with powerful new technologies。 We recollect the story of the Tower of Babel； a building 〃whose top may reach unto heaven；〃 and God's fear 。shout our species； that now 〃nothing will be restrained from them which they have imagined to do。〃
We e upon Psalm 15； which stakes a divine claim to other worlds： 〃＇T＇he heavens are the Lord's； but the Earth lath he given to the children of men。〃 Or Plato's retelling of the Greek analogue of Babel—the tale of Otys and Ephialtes。 They were mortals who 〃dared to scale heaven。〃 The gods were faced with a choice。 Should they kill the upstart humans 〃and annihilate ＇their＇ race with thunderbolts〃？ On the one Band； 〃this would be the end of the sacrifices and worship which mien offered〃 the gods and which gods craved。 〃But； on the other hand； the gods could not suffer ＇such＇ insolence to be unrestrained。''
If； in the long term； though； we have no alternative； if our choice really is many worlds or none； we are in need of other sorts of myths of encouragement。 They exist。 Many religions； from Hinduism to Gnostic Christianity to Mormon doctrine； teach that—as impious as it may sound—it is the goal of humans to bee gods。 Or consider a story in the Jewish Talmud left out of the Book of Genesis。 （It is in doubtful accord with the account of the apple； the Tree of Knowledge； the Fall； and the expulsion from Eden。） In the G