planets remain paratively bare of atmosphere。 We are reconstructing the histories of worlds—how mainly ices and organics collected together in the chilly outskirts of the early Solar System； and mainly rock and metal in the inner regions warmed by the young Sun。 We have begun to recognize the dominant role of early collisions in knocking worlds over； gouging huge craters and basins in their surfaces and interiors； spinning them up； making and obliterating moons； creating rings； carrying； it may be； whole oceans down from the skies； and then depositing a veneer of organic matter as the neat finishing touch in the creation of worlds。 We are beginning to apply this knowledge to other systems。
In the next few decades we have a real chance of examining the layout and something of the position of many other mature planetary systems around nearby stars。 We will begin to know which aspects of our system are the rule and which the exception。 What is more mon—planets like Jupiter； planets like Neptune； or planets like Earth？ Or do all other systems have Jupiters and Neptunes and Earths？ What other categories of worlds are there； currently unknown to us？ Are all solar systems embedded in a vast spherical cloud of ets？ Most stars in the sky are not solitary suns like our own； but double or multiple systems in which the stars are in mutual orbit。 Are there planets in such systems？ If so； what are they like？ If； as we now think； planetary systems are a routine consequence of the origin of suns； have they followed very different evolutionary paths elsewhere？ What do elderly planetary systems； billions of years more evolved than ours； look like？ In the next few centuries our knowledge of other systems will bee increasingly prehensive。 We will begin to know which to visit； which to seed； and which to settle。
Imagine we could accelerate continuously at 1 g—what we're fortable with on good old terra firma—to the midpoint of our voyage； and decelerate continuously at 1 g until we arrive at our destination。 It would then take a day to get to Mars； a week and a half to Pluto； a year to the Oort Cloud； and a few years to the nearest stars。
Even a modest extrapolation of our recent advances in transportation suggests that in only a few centuries we will be able to travel close to the speed of light。 Perhaps this is hopelessly optimistic。 Perhaps it will really take millennia or more。 But unless we destroy ourselves first we will be inventing new technologies as strange to us as Voyager might be to our hunter…gatherer ancestors。 Even today we can think of ways—clumsy； ruinously expensive； inefficient to be sure—of constructing a starship that approaches light speed。 In time； the designs will bee more elegant； more affordable； more efficient。 The day will e when we overe the necessity of jumping from et to et。 We will begin to soar through the light…years and； as St。 Augustine said of the gods of the ancient Greeks and Romans； colonize the sky。
Such descendants may be tens or hundreds of generations removed from anyone who ever lived on the surface of a planet。 Their cultures will be different； their technologies far advanced； their languages changed； their association with machine intelligence much more intimate； perhaps their very appearance markedly altered from that of their nearly mythical ancestors who first tentatively set forth in the late twentieth century into the sea of space。 But they will be human； at least in large part； they will be practitioners of high technology； they will have historical records。 Despite Augustine's judgment on Lot's wife； that 〃no one who is being saved should long for what he is leaving；〃 they will not wholly forget the Earth。
But we're not nearly ready； you may be thinking。 As Voltaire put it in his Memnon； 〃our little terraqueous globe is the madhouse of those hundred thousand millions* of worlds。〃 We； who cannot even put our own planetary home in order； riven with rivalries and hatreds； despoiling our environment； murdering one another through irritation and inattention as well as on deadly purpose； and moreover a species that until only recently was convinced that the Universe was made for its sole benefit—are we to venture out into space； move worlds； reengineer planets； spread to neighboring star systems？
* A value that nicely approximates modern estimates of the number of planets orbiting stars in the Milky Way Galaxy。
I do not imagine that it is precisely we； with our present customs and social conventions； who will be out there。 If we continue to accumulate only power and not wisdom； we will surely destroy ourselves。 Our very existence in that distant time requires that we will have changed our institutions and ourselves。 How can I dare to guess about humans in the far future？ It is； I think； only a matter of natural selection。 If we bee even slightly more violent； shortsighted； ignorant； and selfish than we are now； almost certainly we will have no future。
If you're young； it's just possible that we will be taking our first steps on near…Earth asteroids and Mars during your lifetime。 To spread out to the moons of the Jovian planets and the Kuiper et Belt will take many generations more。 The Oort Cloud will require much longer still。 By the time we're ready to settle even the nearest other planetary systems； we will have changed。 The simple passage of so many generations will have changed us。 The different circumstances we will be living under will have changed us。 Prostheses and genetic engineering will have changed us。 Necessity will have changed us。 We're an adaptable species。
It will not be we who reach Alpha Centauri and the other nearby stars。 It will be a species very like us； but with more of our strengths and fewer of our weaknesses； a species returned to circumstances more like those for which it was originally evolved； more confident； farseeing； capable； and prudent—the sorts of beings we would want to represent us in a Universe that； for all we know； is filled with species much older； much more powerful； and very different。
The vast distances that separate the stars are providential。 Beings and worlds are quarantined from one another。 The quarantine is lifted only for those with sufficient self…knowledge and judgment to have safely traveled from star to star。
ON IMMENSE TIMESCALES； in hundreds of millions to billions of years； the centers of galaxies explode。 We see； scattered across deep space； galaxies with 〃active nuclei；〃 quasars； galaxies distorted by collisions； their spiral arms disrupted； star systems blasted with radiation or gobbled up by black holes—and we gather that on such timescales even interstellar space； even galaxies may not be safe。
There is a halo of dark matter surrounding the Milky Way； extending perhaps halfway to the distance of the next spiral galaxy （M31 in the constellation Andromeda； which also contains hundreds of billions of stars）。 We do not know what this dark matter is； or how it is arranged—but some* of it may be in worlds untethered to individual stars。 If so； our descendants of the remote future will have an opportunity； over unimaginable intervals of time； to bee established in intergalactic space； and to tiptoe to other galaxies。
* Most of it may be in 〃nonbaryonic〃 matter； not made of our familiar protons and neutrons； and not anti…matter either。 Over 90 percent of the mass of the Universe seems to be in this dark； quintessential； deeply mysterious stuff wholly unknown on Earth。 Perhaps we will one day not only understand it； but also find a use for it。
But on the timescale for populating our galaxy； if not long before； we must ask： How immutable is this longing for safety that drives us outward？ Will we one day feel content with the time our species has had and our successes； and willingly exit the cosmic stage？ Millions of years from now—probably much sooner—we will have made ourselves into something else。 Even if we do nothing intentionally； the natural process of mutation and selection will have worked our extinction or evolved us into some other species on just such a timescale （if we may judge by other mammals）。 Over the typical lifetime of a mammalian species； even if we were able to travel close to the speed of light and were dedicated to nothing else； we could not； I think； explore even a representative fraction of the Milky Way Galaxy。 There's just too much of it。 And beyond are a hundred billion galaxies more。 Will our present motivations remain unchanged over geological； much less cosmological； timescales—when we ourselves have been transfigured？ In such remote epochs； we may discover outlets for our ambitions far grander and more worthy than merely populating an unlimited number of worlds。
Perhaps； some scientists have imagined； we will one day create new forms of life； link minds； colonize stars； reconfigure galaxies； or prevent； in a nearby volume of space； the expansion of the Universe。 In a 1993 article in the journal Nuclear Physics； the physicist Andrei Linde—conceivably； in a playful mood—suggests that laboratory experiments （it would have to be quite a laboratory） to create separate； closed…off； expanding universes might ultimately be possible。 〃However；〃 he writes to me； 〃I myself do not know whether ＇this suggestion＇ is simply a joke or something else。〃 In such a list of projects for the far future； we will have no difficulty in recognizing a continuing human ambition to arrogate powers once considered godlike—or； in that other more encouraging metaphor； to plete the Creation。
FOR MANY PAGES NOW； we have left the realm of plausible conjecture for the heady intoxication of nearly unconstrained speculation。 It is time to return to our own age。
My grandfather； born before radio waves were even a laboratory curiosity； almost lived to see the first artificial satellite beeping down at us from space。 There are people who were born before there was such a thing as an airplane； and who in old age saw four ships launched to the stars。 For all our failings； despite our limitations and fallibilities； we humans are capable of greatness。 This is true of our science and some areas of our technology； of our art； music； literature； altruism； and passion； and even； on rare occasion； of our statecraft。 What new wonders undreamt of in our time will we have wrought in another generation？ And another？ How far will our nomadic species have wandered by the end of the next century？ And the next millennium？
Two billion years ago our ancestors were microbes； a halfbillion years ago； fish； a hundred million years ago； something like mice； ten million years ago； arboreal apes； and a million years ago； proto…humans puzzling out the taming of fire。 Our evolutionary lineage is marked by mastery of change。 In our time； the pace is quickening。
When we first venture to a near…Earth asteroid； we will have entered a habitat that may engage our species forever。 The first voyage of men and women to Mars is the key step in transforming us into a multiplanet species。 These events are as momentous as the colonization of the land by our amphibian ancestors and the descent from the trees by our primate ancestors。
Fish with rudimentary lungs and fins slightly adapted for walking must have died in great numbers be