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t still won't be entirely safe。 In the long run； the Sun may generate stupendous X…ray and ultraviolet outbursts； the Solar System will enter one of the vast interstellar clouds lurking nearby and the planets will darken and cool； a shower of deadly ets will e roaring out of the Oort Cloud threatening civilizations on many adjacent worlds； we will recognize that a nearby star is about to bee a supernova。 In the really long run； the Sun—on its way to being a red giant star—will get bigger and brighter； the Earth will begin to lose its air and water to space； the soil will char； the oceans will evaporate and boil； the rocks will vaporize； and our planet may even be swallowed up into the interior of the Sun。
Far from being made for us； eventually the Solar System will bee too dangerous for us。 In the long run； putting all our eggs in a single stellar basket； no matter how reliable the Solar System has been lately； may be too risky。 In the long run； as Tsiolkovsky and Goddard long ago recognized； we need to leave the Solar System。
If that's true for us； you might very well ask； why isn't it true for others？ And if it is true for others； why aren't they here？ There are many possible answers； including the contention that they have e here—although the evidence for that is pitifully slim。 Or there may be no one else out there； because they destroy themselves； with almost no exceptions； before they achieve interstellar flight； or because in a galaxy of 400 billion suns ours is the first technical civilization。
A more likely explanation； I think； issues from the simple fact that space is vast and the stars are far apart。 Even if there were civilizations much older and more advanced than we—expanding out from their home worlds； reworking new worlds； and then continuing onward to other stars—they would be unlikely； according to calculations performed by William I。 Newman of UCLA and me； to be here。 Yet。 And because the speed of light is finite； the TV and radar news that a technical civilization has arisen on some planet of the Sun has not reached them。 Yet。
Should optimistic estimates prevail and one in every million stars shelters a nearby technological civilization； and if as well they're randomly strewn through the Milky Way—were these provisos to hold—then the nearest one； we recall； would be a few hundred light…years distant： at the closest； maybe 100 lightyears； more likely a thousand light…years…and； of course； perhaps nowhere； no matter how far。 Suppose the nearest civilization on a planet of another star is； say； 200 light…years away。 Then； some 150 years from now they'll begin to receive our feeble post…World War II television and radar emission。 What will they make of it？ With each passing year the signal will get louder； more interesting； perhaps more alarming。 Eventually； they may respond： by returning a radio message； or by visiting。 In either case； the response will likely be limited by the finite value of the speed of light。 With these wildly uncertain numbers； the answer to our unintentional mid…century call into the depths of space will not arrive until around the year 2350。 If they're farther away； of course； it will take longer； and if much farther away； much longer。 The interesting possibility arises that our first receipt of a message from an alien civilization； a message intended for us （not just an all…points bulletin）； will occur in a time when we are well situated on many worlds in our solar system and preparing to move on。
With or without such a message； though； we will have reason to continue outward； seeking other solar systems。 Or—even safer in this unpredictable and violent sector of the Galaxy—to sequester some of us in self…sufficient habitations in interstellar space； far from the dangers constituted by the stars。 Such a future would； I think； naturally evolve； by slow increments； even without any grand goal of interstellar travel：
For safety； some munities may wish to sever their ties with the rest of humanity—uninfluenced by other societies； other ethical codes； other technological imperatives。 In a time when ets and asteroids are being routinely repositioned； we will be able to populate a small world and then cut it loose。 In successive generations； as this world sped outward； the Earth would fade from bright star to pale dot to invisibility； the Sun would appear dimmer； until it was no more than a vaguely yellow point of light； lost among thousands of others。 The travelers would approach interstellar night。 Some such munities may be content with occasional radio and laser traffic with the old home worlds。 Others； confident of the superiority of their own survival chances and wary of contamination； may try to disappear。 Perhaps all contact with them will ultimately be lost； their very existence forgotten。
Even the resources of a sizable asteroid or et are finite； though； and eventually more resources must be sought elsewhere—especially water； needed for drink； for a breathable oxygen atmosphere； and for hydrogen to power fusion reactors。 So in the long run these munities must migrate from world to world； with no lasting loyalty to any。 We might call it 〃pioneering；〃 or 〃homesteading。〃 A less sympathetic observer might describe it as sucking dry the resources of little world after little world。 But there are a trillion little worlds in the Oort et Cloud。
Living in small numbers on a modest stepmother world far from the Sun； we will know that every scrap of food and every drop of water is dependent on the smooth operation of a
farsighted technology； but these conditions are not radically unlike those to which we are already accustomed。 Digging resources out of the ground and stalking passing resources seem oddly familiar； like a forgotten memory of childhood： It is； with a few significant changes； the strategy of our hunter…gatherer ancestors。 For 99。9 percent of the tenure of humans on Earth； we lived such a life。 Judging from some of the last surviving hunter…gatherers just before they were engulfed by the present global civilization； we may have been relatively happy。 It's the kind of life that forged us。 So after a brief； only partially successful sedentary exp