REMAKING THE PLANETS -Pale Blue Dot_ A Vision of the Human Future in Space
Who could deny that man could somehow also make the heavens, could he only obtain the instruments and the heavenly material?
MARSILIO FICINO, “THE SOUL OF MAN” (CA. 1474)
In the midst of the Second World War, a young American writer named Jack Williamson envisioned a populated Solar System. In the twenty-second century, he imagined, Venus would be settled by China, Japan, and Indonesia; Mars by Germany; and the moons of Jupiter by Russia. Those who spoke English, the language in which Williamson was writing, were confined to the asteroids-and of course the Earth.
The story, published in Astounding Science Fiction in July 1942, was called “Collision Orbit” and written under the pseudonym Will Stewart. Its plot hinged on the imminent collision of an uninhabited asteroid with a colonized one, and the search for a means of altering the trajectories of small worlds. Although no one on Earth was endangered, this may have been the first appearance, apart from newspaper comic strips, of asteroid collisions as a threat to humans. (Comets impacting the Earth had been a staple peril.)
The environments of Mars and Venus were poorly understood in the early 1940s; it was conceivable that humans could live there without elaborate life-support systems. But the asteroids were another matter. It was well known, even then, that asteroids were small, dry, airless worlds. If they were to be inhabited, especially by large numbers of people, these little worlds would somehow have to be fixed.
In “Collision Orbit,” Williamson portrays a group of “spatial engineers,” able to render such barren outposts clement. Coining a word, Williamson called the process of metamorphosis into an Earth-like world “terraforming.” He knew that the low gravity on an asteroid means that any atmosphere generated or transported there would quickly escape to space. So his key terraforming technology was “paragravity,” an artificial gravity that would hold a dense atmosphere.
As nearly as we can tell today, paragravity is a physical impossibility. But we can imagine domed, transparent habitats on the surfaces of asteroids, as suggested by Konstantin Tsiolkovsky, or communities established in the insides of asteroids, as outlined in the 1920s by the British scientist J. D. Bernal. Because asteroids are small and their gravities low, even massive subsurface construction might be comparatively easy. If a tunnel were dug clean through, you could jump in at one end and emerge some 45 minutes later at the other, oscillating up and down along the toll diameter of this world indefinitely. Inside the right kind of asteroid, a carbonaceous one, you can find materials for manufacturing stone, metal, and plastic construction and plentiful water all you might need to build a subsurface closed ecological system, an underground garden. Implementation would require a significant step beyond what we have today, but unlike “paragravity” nothing in such a scheme seems impossible. All the elements can be found in contemporary technology. If there were sufficient reason, a fair number of us could be living on (or in asteroids by the twenty-second century.
They would of course need a source of power, not just to sustain themselves, but, as Bernal suggested, to move their asteroidal homes around. (It does not seem so big a step from explosive alteration of asteroid orbits to a more gentle means of propulsion a century or two later.) If an oxygen atmosphere were generated from chemically bound water, then organics could be burned to generate power, just as fossil fuels are burned on the Earth today. Solar power could be considered, although for the main-belt asteroids the intensity of sunlight is only about 10 percent what it is on Earth. Still, we could imagine vast fields of solar panels covering the surfaces of inhabited asteroids and converting sunlight into electricity. Photovoltaic technology is routinely used in Earth-orbiting spacecraft, and is in increasing use on the surface of the Earth today. But while that might be enough to warm and light the homes of these descendants, it does not seem adequate to change asteroid orbits.
For that, Williamson proposed using anti-matter. Antimatter is just like ordinary matter, with one significant difference. Consider hydrogen: An ordinary hydrogen atom consists of a positively charged proton on the inside and a negatively charged electron on the outside. An atom of anti-hydrogen consists of a negatively charged proton on the inside and a positively charged electron (also called a positron) on the outside. The protons, whatever the sign of their charges, have the same mass; and the electrons, whatever the sign of their charges, have the same mass. Particles with opposite charges attract. A hydrogen atom and an antihydrogen atom are both stable, because in both cases the positive and negative electrical charges precisely balance.
Anti-matter is not some hypothetical construct from the perfervid musings of science fiction writers or theoretical physicists. Anti-matter exists. Physicists make it in nuclear accelerators; it can be found in high-energy cosmic rays. So why don’t we hear more about it? Why has no one held up a lump of antimatter for our inspection? Because matter and anti-matter, when brought into contact, violently annihilate each other, disappearing in an intense burst of gamma rays. We cannot tell whether something is made of matter or anti-matter just by looking at it. The spectroscopic properties of, for example, hydrogen and anti-hydrogen are identical.
Albert Einstein’s answer to the question of why we see only matter and not anti-matter was, “Matter won” by which he meant that in our sector of the Universe at least, after almost all the matter and anti- matter interacted and annihilated each other long ago, there was some of what we call ordinary matter left over. As far as we can tell today, from gamma ray astronomy and other means, the Universe is made almost entirely of matter. The reason for this engages the deepest cosmological issues, which need not detain us here. But if there was only a oneparticle-in-a-billion difference in the preponderance of matter over anti- matter at the beginning, even this would be enough to explain the Universe we see today.
Williamson imagined that humans in the twenty-second century would move asteroids around by the controlled mutual annihilation of matter and anti-matter. All the resulting gamma rays, if collimated, would make a potent rocket exhaust. The anti-matter would be available in the main asteroid belt (between the orbits of Mars and Jupiter), because this was his explanation for the existence of the asteroid belt. In the remote past, he proposed, an intruder anti-matter worldlet arrived in the Solar System from the depths of space, impacted, and annihilated what was then an Earthlike planet, fifth from the Sun. The fragments of this mighty collision are the asteroids, and some of them are still made of anti-matter. Harness an anti-asteroid Williamson recognized that this might be tricky and you can move worlds around at will.
At the time, Williamson’s ideas were futuristic, but far from foolish. Some of “Collision Orbit” can be considered visionary. Today, however, we have good reason to believe that there are no significant amounts of anti-matter in the Solar System, and that the asteroid belt, far from being a fragmented terrestrial planet, is an enormous array of small bodies prevented (by the gravitational tides of Jupiter) from forming an Earthlike world.
However, we do generate (very) small amounts of antimatter in nuclear accelerators today, and we will probably be able to manufacture much larger amounts by the twenty-second century. Because it is so efficient converting all of the matter into energy, E = MC2, with 100 percent efficiency perhaps anti-matter engines will be a practical technology by then, vindicating Williamson Failing that, what energy sources can we realistically expect to be available, to reconfigure asteroids, to light them warm them, and move them around?
The Sun shines by jamming protons together and turning them into helium nuclei. Energy is released in the process, although with less than 1 percent the efficiency of the annihilation of matter and anti-matter. But even proton-proton reactions are far beyond anything we can realistically imagine for ourselves in the near future. The required temperatures are much too high. Instead) of jamming protons together, though, we might use heavier kinds of hydrogen. We already do so in thermonuclear weapons. Deuterium is a proton bound by nuclear forces to a neutron; tritium is a proton bound by nuclear forces to two neutrons. It seems likely that in another century we will have practical power schemes that involve the controlled fusion of deuterium and tritium, and of deuterium and helium. Deuterium and tritium are present as minor constituents in water (on Earth and other worlds). The kind of helium needed for fusion, 3He (two protons and a neutron make up its nucleus), has been implanted over billions of years by the solar wind in the surfaces of the asteroids. These processes are not nearly as efficient as the proton-proton reactions in the Sun, but they could provide enough power to run a small city for a year from a lode of ice only a few meters in size.
Fusion reactors seem to be coming along too slowly to play a major role in solving, or even significantly mitigating, global warming. But by the twenty-second century, they ought to be widely available. With fusion rocket engines, it will be possible to more asteroids and comets around the inner Solar System taking a main-belt asteroid, for example, and inserting it into orbit around the Earth. A world 10 kilometers across could be transported from Saturn, say, to Mars through nuclear burning of the hydrogen in an icy comet a kilometer across. (Again, I’m assuming a time of much greater political stability and safety.)
PUT ASIDE FOR THE MOMENT any qualms you might have about the ethics of rearranging worlds, or our ability to do so without catastrophic consequences. Digging out the insides of worldlets, reconfiguring them for human habitation, and moving them from one place in the Solar System to another seems to be within our grasp in another century or two. Perhaps by then we will have adequate international safeguards as well. But what about transforming the surface environments not of asteroids or comets, but of planets? Could we live on Mars?
If we wanted to set up housekeeping on Mars, it’s easy to see that, in principle at least, we could do it: There’s abundant sunlight. There’s plentiful water in the rocks and in underground and polar ice. The atmosphere is mostly carbon dioxide. It seems likely that in self-contained habitats-perhaps domed enclosures we could grow crops, manufacture oxygen from water, recycle wastes.
At first we’d be dependent on commodities resupplied from Earth, but in time we’d manufacture more and more of them ourselves. We’d become increasingly self-sufficient. The domed enclosures, even if made of ordinary glass, would let in the visible sunlight and screen out the Sun’s ultraviolet rays. With oxygen masks and protective garments but nothing as bulky and cumbersome as a spacesuit we could leave these enclosures to go exploring, or to build another domed village and farms.
It seems very evocative of the American pioneering experience, but with at least one major difference: In the early stages, large subsidies are essential. The technology required is too expensive for some poor family, like my grandparents a century ago, to pay their own passage to Mars. The early Martian pioneers will be sent by governments and will have highly specialized skills. But in a generation or two, when children and grandchildren are born there and especially when self-sufficiency is within reach that will begin to change. Youngsters born on Mars will be given specialized training in the technology essential for survival in this new environment. The settlers will become less heroic and less exceptional. The full range of human strengths and deficiencies will begin to assert themselves. Gradually, precisely because of the difficulty of getting from Earth to Mars, a unique Martian culture will begin to emerge distinct aspirations and fears tied to the environment they live in, distinct technologies, distinct social problems, distinct solutions and, as has occurred in every similar circumstance throughout human history, a gradual sense of cultural and political estrangement from the mother world.
Great ships will arrive carrying essential technology from Earth, new families of settlers, scarce resources. It is hard to know, on the basis of our limited knowledge of Mars, whether they will go home empty or whether they will carry with them something found only on Mars, something considered very valuable on Earth. Initially much of the scientific investigation of samples of the Martian surface will be done on Earth. But in time the scientific study of Mars (and its moons Phobos and Deimos) will be done from Mars.
Eventually as has happened with virtually every other form of human transportation interplanetary travel will become accessible to people of ordinary means: to scientists pursuing their own research projects, to settlers fed up with Earth, even to venturesome tourists. And of course there will be explorers.
If the time ever came when it was possible to make the Martian environment much more Earth-like so protective garments, oxygen masks, and domed farmlands and cities could be dispensed with the attraction and accessibility of Mars would be increased many-fold. The same, of course, would be true for any other world which could be engineered so that humans could live there without elaborate contrivances to keep the planetary environment out. We would feel much more comfortable in our adopted home if an intact dome or spacesuit weren’t all that stood between us and death. (But perhaps I exaggerate the dangers. People who live in the Netherlands seem at least as well adjusted and carefree as other inhabitants of Northern Europe; vet their dikes are all that stand between them and the sea.
Recognizing the speculative nature of the question and the limitations in our knowledge, is it nevertheless possible to envision terraforming the planets?
We need look no further than our own world to see that humans are now able to alter planetary environments in a profound way. Depletion of the ozone layer, global warming from an increased greenhouse effect, and global cooling from nuclear war are all ways in which present technology can significantly alter the environment of our world and in each case as an inadvertent consequence of doing something else. If we had intended to alter our planetary environment, we would be fully able to generate still greater change. As our technology becomes more powerful, we will be able to work still more profound changes.
But just as (in parallel parking) it’s easier to get out of a parking place than into one, it’s easier to destroy a planetary environment than to move it into a narrowly prescribed range of temperatures, pressures, compositions, and so on. We already know of a multitude of desolate and uninhabitable worlds, and with very narrow margins only one green and clement one. This is a major conclusion from early in the era of spacecraft exploration of the Solar System. In altering the Earth, or any world with an atmosphere, we must be very careful about positive feedbacks, where we nudge an environment a little bit and it takes of on its own a little cooling leading to runaway glaciation, as may have happened on Mars, or a little warming to a runaway greenhouse effect, as happened on Venus. It is not at allclear that our knowledge is sufficient to this purpose.
As far as I know, the first suggestion in the scientific literature about terraforming the planets was made in a 1961 article I wrote about Venus. I was pretty sure then that Venus had a surface temperature well above the normal boiling point of water, produced by a carbon dioxide/water vapor greenhouse effect. I imagined seeding its high clouds with genetically engineered microorganisms that would take CO2, N2, and H2O out of the atmosphere and convert them into organic molecules. The more CO2 removed, the smaller the greenhouse effect and the cooler the surface. The microbes would be carried down through the atmosphere toward the ground, where they would be fried, so water vapor would be returned to the atmosphere; but the carbon from the CO2 would be converted irreversibly by the high temperatures into graphite or some other involatile form of carbon. Eventually, the temperatures would fall below the boiling point and the surface of Venus would become habitable, dotted with pools and lakes of warm water.
The idea was soon taken up by a number of science fiction authors in the continuing dance between science and science fiction in which the science stimulates the fiction, and the fiction stimulates a new generation of scientists, a process benefiting both genres. But as the next step in the dance, it is now clear that seeding Venus with special photosynthetic microorganisms will not work. Since 1961 we’ve discovered that the clouds of Venus are a concentrated solution of sulfuric acid, which makes the genetic engineering rather more challenging. But that in itself is not a fatal flaw. (There are microorganisms that live out their lives in concentrated solutions of sulfuric acid.) Here’s the fatal flaw: In 1961 I thought the atmospheric pressure at the surface of Venus N\-as a few “bars,” a few times the surface pressure on Earth. We now know it to be 90 bars, so that if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is an open question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO2, short-circuiting the process. At best, such a scheme can carry the terraforming of Venus only partway.
Let’s assume that by the early twenty-second century we have comparatively inexpensive heavy-lift vehicles, so we can carry large payloads to other worlds; abundant and powerful fusion reactors; and well-developed genetic engineering. All three assumptions are likely, given current trends. Could we terraform the planets? 1 James Pollack of NASA’s Ames Research Center and I surveyed this problem. Here’s a summary of what we found:
VENUS: Clearly the problem with Venus is its massive greenhouse effect. If we could reduce the greenhouse effect almost to zero, the climate might be balmy. But a 90-bar CO2 atmosphere is oppressively thick. Over every postage stamp-sized square inch of surface, the air weighs as much as six professional football players, piled one on top of another. Making all that go away will take some doing.
Imagine bombarding Venus with asteroids and comets. Each impact would blow away some of the atmosphere. To blow away almost all of it, though, would require using up more big asteroids and comets than there are at least in the planetary part of the Solar System. Even if that many potential impactors existed, even if we could make them all collide with Venus (this is the overkill approach to the impact hazard problem), think what we would have lost. Who knows what wonders, what practical knowledge they contain? We would also obliterate much of Venus’ gorgeous surface geology which we’ve just begun to understand, and which may teach us much about the Earth. This is an example of bruteforce terraforming. I suggest we want to steer entirely clear of such methods, even if someday we’ll be able to afford them (which I very much doubt). We want something more elegant, more subtle, more respectful of the environments of other worlds. A microbial approach has some of those virtues, but does not do the trick, as we’ve just seen.
We can imagine pulverizing a dark asteroid and spreading the powder through the upper atmosphere of Venus, or carrying such dust up from the surface. This would be the physical equivalent of nuclear winter or the Cretaceous-Tertiary post-impact climate. If the sunlight reaching the ground is sufficiently attenuated, the surface temperature must fall. But by its very nature, this option plunges Venus into deep gloom, with daytime light levels perhaps only as bright as on a moonlit night on Earth. The oppressive, crushing 90-bar atmosphere would remain untouched. Since the emplaced dust would sediment out every few years, the layer would have to be replenished in the same period of time. Perhaps such an approach would be acceptable for short exploratory missions, but the environment generated seems very stark for a self-sustaining human community on Venus.
We could use a giant artificial sunshade in orbit around Venus to cool the surface; but it would be enormously expensive, as well as having many of the deficiencies of the dust layer. However, if the temperatures could be lowered sufficiently, the CO2 in the atmosphere would rain out. There would be a transitional time of CO2 oceans on Venus. If those oceans could be covered over to prevent re-evaporation for example, with water oceans made by melting a large, icy moon transported from the outer Solar System then the CO2 might conceivably be sequestered away, and Venus converted into a water (or low-fizz seltzer) planet. Ways have also been suggested to convert the CO2
into carbonate rock.
Thus all proposals for terraforming Venus are still brute-force, inelegant, and absurdly expensive. The desired planetary metamorphosis may be beyond our reach for a very long time, even if we thought it was desirable and responsible. The Asian colonization of Venus that Jack Williamson imagined may have to be redirected somewhere else.
MARS: For Mars we have just the opposite problem. There’s not enough greenhouse effect. The planet is a frozen desert. But the fact that Mars seems to have had abundant rivers, lakes, and perhaps even oceans 4 billion years ago at a time when the Sun was less bright than it is today makes you wonder if there’s solve natural instability in the Martian climate, something on hair trigger that once released would all by itself return the planet to its ancient clement state. (Let’s note from the start that doing so would destroy Martian landforms that hold key data on the past especially the laminated polar terrain.)
As we know very well from Earth and Venus, carbon dioxide is a greenhouse gas. There are carbonate minerals found on Mars, and dry ice in one of the polar caps. They could be converted into CO2 gas. But to make enough of a greenhouse effect to generate comfortable temperatures on Mars would require the entire surface of the planet to be plowed up and processed to a depth of kilometers. Apart from the daunting obstacles in practical engineering that this represents fusion power or no fusion power and the inconvenience to whatever self-contained, closed ecological systems humans had already established on the planet it would also constitute the irresponsible destruction of a unique scientific resource and database, the Martian surface.
What about other greenhouse gases? Alternatively, we might take chlorofluorocarbons (CFCs or HCFCs) to Mars after manufacturing them on Earth. These are artificial substances that, so far as we know, are found nowhere else in the Solar System. We can certainly imagine manufacturing enough CFCs on Earth to warm Mars, because by accident in a few decades with present technology on Earth we’ve managed to synthesize enough to contribute to global warming on our planet. Transportation to Mars would be expensive, though: Even using Saturn V- or Energiya-class boosters, it would require at least a launch a day for a century. But perhaps they could be manufactured from fluorine-containing minerals on Mars.
There is, in addition, a serious drawback: On Mars as on Earth, abundant CFCs would prevent formation of an ozone layer. CFCs might bring Martian temperatures into a clement range, but guarantee that the solar ultraviolet hazard would remain extremely serious. Perhaps the solar ultraviolet light could be absorbed by an atmospheric layer of pulverized asteroidal or surface debris injected in carefully titrated amounts above the CFCs. But now we’re in the troubling circumstance of having to deal with propagating side effects, each of which requires its own large-scale technological solution.
A third possible greenhouse gas for warming Mars is ammonia (NH3). Only a little ammonia would be enough to warm the Martian surface to above the freezing point of water. In principle, this might be done by specially engineered microorganisms that would convert Martian atmospheric N2 to NH3 as some microbes do on Earth, but do it under Martian conditions. Or the same conversion might be done in special factories. Alternatively, the nitrogen required could be carried to Mars from elsewhere in the Solar System. (N2 is the principal constituent in the atmospheres of both Earth and Titan.) Ultraviolet light would convert ammonia back into N2 in about 30 years, so there would have to be a continuous resupply of NH3.
A judicious combination of CO2, CFC, and NH3 greenhouse effects on Mars looks as if it might be able to bring surface temperatures close enough to the freezing point of water for the second phase of Martian terraforming to begin temperatures rising due to the pressure of substantial water vapor in the air, widespread production of O2 by genetically engineered plants, and fine-tuning the surface environment. Microbes and larger plants and animals could be established on Mars before the overall environment was suitable for unprotected human settlers.
Terraforming Mars is plainly much easier than terraforming Venus. But it is still very expensive by present standards, and environmentally destructive. If there were sufficient justification, though, perhaps the terraforming of Mars could be under way by the twenty-second century.
THE MOONS OF JUPITER AND SATURN: Terraforming the satellites of the Jovian planets presents varying degrees of difficulty. Perhaps the easiest to contemplate is Titan. It already has an atmosphere, made mainly of N2
like the Earth’s, and is much closer to terrestrial atmospheric pressures than either Venus or Mars. Moreover, important greenhouse gases, such as NH3 and H20, are almost certainly frozen out on its surface. Manufacture of initial greenhouse gases that do not freeze out at present Titan temperatures plus direct warming of the surface by nuclear fusion could, it seems, be the key early steps to one day terraform Titan.
IF THERE WERE A COMPELLING REASON for terraforming other worlds, this greatest of engineering projects might be feasible on the timescale we’ve been describing certainly for asteroids, possibly for Mars, Titan, and other moons of the outer planets, and probably not for Venus. Pollack and I recognized that there are those who feel a powerful attraction to the idea of rendering other worlds in the Solar System suitable for human habitation in establishing observatories, exploratory bases, communities, and homesteads there. Because of its pioneering history, this may be a particularly natural and attractive idea in the United States.
In any case, massive alteration of the environments of other worlds can be done competently and responsibly only when we have a much better understanding of those worlds than is available today. Advocates of terraforming must first become advocates of the long-term and thorough scientific exploration of other worlds.
Perhaps when we really understand the difficulties of terraforming, the costs or the environmental penalties will prove too steep, and we will lower our sights to domed or subsurface cities or other local, closed ecological systems, greatly improved versions of Biosphere II, on other worlds. Perhaps we will abandon the dream of converting the surfaces of other worlds to something approaching the Earth’s. Or perhaps there are much more elegant, cost effective, and environmentally responsible ways of terraforming that we have not yet imagined.
But if we are seriously to pursue the matter, certain questions ought to be asked: Given that any terraforming scheme entails a balance of benefits against costs, how certain must we be that key scientific information will not thereby be destroyed before proceeding? How much understanding of the world in question do we need before planetary engineering can be relied upon to produce the desired end state? Can we guarantee a long-term human commitment to maintain and replenish an engineered world, when human political institutions are so short lived? If a world is even conceivably inhabited perhaps only by microorganisms do humans have a right to alter it? What is our responsibility to preserve the worlds of the Solar System in their present wilderness states for future generations-who may contemplate uses that today we are too ignorant to foresee? These questions may perhaps be encapsulated into a final question: Can we, who have made such a mess of this world, be trusted with others?
It is just conceivable that some of the techniques that might eventually terraform other worlds might be applied to ameliorate the damage we have done to this one. Considering the relative urgencies, a useful indication of when the human species is ready to consider terraforming seriously is when we have put our own world right. We can consider it a test of the depth of our understanding and our commitment. The first step in engineering the Solar System is to guarantee the habitability of the Earth.
Then we’ll be ready to spread out to asteroids, comets, Mars, the moons of the outer Solar System, and beyond. Jack Williamson ‘s prediction that this will begin to come about by the twenty-second century may not be far off the mark.
THE NOTION OF OUR DESCENDANTS living and working on other worlds, and even moving some of them around for their convenience, seems the most extravagant science fiction. Be realistic, a voice inside my head counsels. But this is realistic. We’re on the cusp of the technology, near the midpoint between impossible and routine. It’s easy to be conflicted about it. If we don’t do something awful to ourselves in the interim, in another century terraforming may seem no more impossible than a human-tended space station does today.
I think the experience of living on other worlds is bound to change us. Our descendants, born and raised elsewhere, will naturally begin to owe primary loyalty to the worlds of their birth, whatever affection they retain for the Earth. Their physical needs, their methods of supplying those needs, their technologies, and their social structures will all have to be different.
A blade of grass is a commonplace on Earth; it would be a miracle on Mars. Our descendants on Mars will know the value of a patch of green. And if a blade of grass is priceless, what is the value of a human being? The American revolutionary Tom Paine, in describing his contemporaries, had thoughts along these lines:
The wants which necessarily accompany the cultivation of a wilderness produced among them a state of society which countries long harassed by the quarrels and intrigues of governments had neglected to cherish. In such a situation man becomes what he ought to be. He sees his species . . . as kindred.
Having seen at first hand a procession of barren and desolate worlds, it will be natural for our spacefaring descendants to cherish life. Having learned something from the tenure of our species on Earth, they may wish to apply those lessons to other worlds to spare generations to come the avoidable suffering that their ancestors were obliged to endure, and to draw upon our experience and our mistakes as we begin our open-ended evolution into space.
REMAKING THE PLANETS -Pale Blue Dot_ A Vision of the Human Future in Space