cky or so prudent； perish。
Since； in the long run； every planetary society will be endangered by impacts from space； every surviving civilization is obliged to bee spacefaring—…not because of exploratory or romantic zeal； but for the most practical reason imaginable： staying alive。 And once you're out there in space for centuries and millennia； moving little worlds around and engineering planets； your species has been pried loose from its cradle。 If they exist； many other civilizations will eventually venture far from home。*
* Might a planetary civilization which has survived its adolescence wish to encourage others struggling with their emerging technologies？ Perhaps they would make special efforts to broadcast news of their existence； the triumphant announcement that it's possible to avoid self…annihilation。 Or would they at first be very cautious？ Having avoided catastrophes of their own making； perhaps they would fear giving away knowledge of their existence； lest some other； unknown； aggrandizing civilization out there in the dark is looking for Lebensraum or slavering to put down the potential petition。 That might be a reason for us to explore neighboring star systems； but discreetly。
Maybe they would be silent for another reason： because broadcasting the existence of an advanced civilization might encourage emerging civilizations to do less than their best efforts to safeguard their future— hoping instead that someone will e out of the dark and save them from themselves。
A MEANS HAS BEEN OFFERED of estimating how precarious our circumstances are—remarkably； without in any way addressing the nature of the hazards。 J。 Richard Gott III is an astrophysicist at Princeton University。 He asks us to adopt a generalized Copernican principle； something I've described elsewhere as the Principle of Mediocrity。 Chances are that we do not live in a truly extraordinary time。 Hardly anyone ever did。 The probability is high that we're born； live out our days； and die somewhere in the broad middle range of the lifetime of our species （or civilization； or nation）。 Almost certainly； Gott says； we do not live in first or last times。 So if your species is very young； it follows that it's unlikely to last long—because if it were to last long； you （and the rest of us alive today） would be extraordinary in living； proportionally speaking； so near the beginning。
What then is the projected longevity of our species？ Gott concludes； at the 97。5 percent confidence level； that there will be humans for no more than 8 million years。 That's his upper limit； about the same as the average lifetime of many mammalian species。 In that case； our technology neither harms nor helps。 But Gott's lower limit； with the same claimed reliability； is only 12 years。 He will not give you 40…to…1 odds that humans will still be around by the time babies now alive bee teenagers。 In everyday life we try very hard not to take risks so large； not to board airplanes； say； with 1 chance in 40 of crashing。 We will agree to surgery in which 95 percent 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