ry parts of their solar systems。
Second： Spend a moment thinking about brown dwarfs； hypothetical very low temperature stars； considerably more massive than Jupiter； but considerably less massive than the Sun。 Nobody knows if brown dwarfs exist。 Some experts； using nearer stars as gravitational lenses to detect the presence of more distant ones； claim to have found evidence of brown dwarfs。 From the tiny fraction of the whole sky that has so far been observed by this technique； an enormous number of brown dwarfs is inferred。 Others disagree。 In the 1950s； it was suggested by the astronomer Harlow Shapley of Harvard that brown dwarfs—he called them 〃Lilliputian stars〃—were inhabited。 He pictured their surfaces as warm as a June day in Cambridge； with lots of area。 They would be stars that humans could survive on and explore。
Third： The physicists B。 J。 Carr and Stephen Hawking of Cambridge University have shown that fluctuations in the density of matter in the earliest stages of the Universe could have generated a wide variety of small black holes。 Primordial black holes—if they exist—must decay by emitting radiation to space； a consequence of the laws of quantum mechanics。 The less massive the black hole； the faster it dissipates。 Any primordial black hole in the final stages of decay today would have to weigh about as much as a mountain。 All the smaller ones are gone。 Since the abundance—to say nothing of the existence—of primordial black holes depends on what happened in the earliest moments after the Big Bang； no one can be sure that there are any to be found； we certainly can't be sure that any lie nearby。 Not very restrictive upper limits on their abundance have been set by the failure so far to find short gamma ray pulses； a ponent of the Hawking radiation。
In a separate study； G。 E。 Brown of Caltech and the pioneering nuclear physicist Hans Bethe of Cornell suggest that about a billion non…primordial black holes are strewn through the Galaxy； generated in the evolution of stars。 If so； the nearest may be only 10 or 20 lightyears away。
If there are black holes within reach—whether they're as massive as mountains or as stars—we will have amazing physics to study firsthand； as well as a formidable new source of energy。 By no means do I claim that brown dwarfs or primordial black holes are likely within a few light…years； or anywhere。 But as we enter interstellar space； it is inevitable that we will stumble upon whole new categories of wonders and delights； some with transforming practical applications。
I do not know where my train of argument ends。 As more time passes； attractive new denizens of the cosmic zoo will draw us farther outward； and increasingly improbable and deadly catastrophes must e to pass。 The probabilities are cumulative。 But； as time goes on； technological species will also accrue greater and greater powers； far surpassing any we can imagine today。 Perhaps； if we are very skillful （lucky； I think； won't be enough）； we will ultimately spread far from home； sailing through the starry archipelagos of the vast Milky Way Galaxy。 If we e upon anyone else—or； more likely； if they e upon us—we will harmoniously interact。 Since other spacefaring civilizations are likely to be much more advanced than we； quarrelsome humans in interstellar space are unlikely to last long。
Eventually； our future may be as Voltaire； of all people； imagined：
Sometimes by the help of a sunbeam； and sometimes by the convenience of a et； ＇they＇ glided from sphere to sphere； as a bird hops from bough to bough。 In a very little time ＇they＇ posted through the Milky Way 。 。 。
We are； even now； discovering vast numbers of gas and dust disks around young stars—the very structures out of which； in our solar system four and a half billion years ago； the Earth and the other planets formed。 We're beginning to understand how fine dust grains slowly grow into worlds； how big Earthlike planets accrete and then quickly capture hydrogen and helium to bee the hidden cores of gas giants； and how small terrestrial 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 m