In our usual jittery, two-steps-forward-one-step-back mode, We are moving toward unification anyway. There are powerful influences deriving from transportation and communications technologies, the interdependent world economy, and the global environmental crisis. The impact hazard merely hastens the pace.
Eventually, cautiously, scrupulously careful to attempt nothing with asteroids that could inadvertently cause a catastrophe on Earth, I imagine we will begin to learn how to change the orbits of little nonmetallic worlds, smaller than 100 meters across. We begin with smaller explosions and slowly work our way up. We gain experience in changing the orbits of various asteroids and comets of different compositions and strengths. We try to determine which ones can be pushed around and which cannot. By the twenty-second century, perhaps, we move small worlds around the Solar System, using (see next chapter) not nuclear explosions but nuclear fusion engines or their equivalents. We insert small asteroids made of precious and industrial metals into Earth orbit. Gradually develop a defensive technology to deflect a large asteroid or comet that might in the foreseeable future hit the Earth, while, with meticulous care, we build layers of safeguards against misuse.
Since the danger of misusing deflection technology seems so much greater than the danger of an imminent impact, we can afford to wait, take precautions, rebuild political institutions—for decades certainly, probably centuries. If we play our cards right and are not unlucky, we can pace what we do up there by what progress we're making down here. The two are in any case deeply connected.
The asteroid hazard forces our hand. Eventually, we must establish a formidable human presence throughout the inner Solar System. On an issue of this importance I do not think we will be content with purely robotic means of mitigation. To do so safely we must make changes in our political and international systems. While much about our future is cloudy, this conclusion seems a little more robust, and independent of the vagaries of human institutions.
In the long term, even if we were not the descendants of professional wanderers, even if we were not inspired by exploratory passions, some of us would still have to leave the Earth—simply to ensure the survival of all of us. And once we're out there, we'll need bases, infrastructures. It would not be very long before some of us were living in artificial habitats and on other worlds. This is the first of two mussing arguments, omitted in our discussion of missions to Mars, for a permanent human presence in space.
OTHER PLANETARY SYSTEMS must face their own impact hazards—because small primordial worlds, of which asteroids and comets are remnants, are the stuff out of which planets form there as well. After the planets are made, many of these planetesimals are left over. The average time between civilization-threatening impacts on Earth is perhaps 200,000 years, twenty times the age of our civilization. Very different waiting times may pertain to extraterrestrial civilizations, if they exist, depending on such factors as the physical and chemical characteristics of the planet and its biosphere, the biological and social nature of the civilization, and of course the collision rate itself. Planets with higher atmospheric pressures will be protected against somewhat larger 1mpactors, although the pressure cannot be much greater before greenhouse warming and other consequences make life improbable. If the gravity is much less than on Earth, impactors will make less energetic collisions and the hazard will be reduced—although it cannot be reduced very much before the atmosphere escapes to space.
The impact rate in other planetary systems is uncertain. Our system contains two major populations of small bodies that feed potential impactors into Earth-crossing orbits. Both the existence of the source populations and the mechanisms that maintain the collision rate depend on how worlds are distributed. For example, our Oort Cloud seems to have been populated by gravitational ejections of icy worldlets from the vicinity of Uranus and Neptune. If there are no planets that play the role of Uranus and Neptune in systems otherwise like our own, their Oort Clouds may be much more thinly populated. Stars in open and globular stellar clusters, stars in double or multiple systems, stars closer to the center of the Galaxy, stars experiencing more frequent encounters with Giant Molecular Clouds in interstellar space, may all experience higher impact fluxes at their terrestrial planets. The cometary flux might be hundreds or thousands of times more at the Earth had the planet Jupiter never formed—according to a calculation by George Wetherill of the Carnegie Institution of Washington. In systems without Jupiter-like planets, the gravitational shield against comets is down, and civilization-threatening impacts much more frequent.
To a certain extent, increased fluxes of interplanetary objects might increase the rate of evolution, as the mammals that flourished and diversified after the Cretaceous-Tertiary collision wiped out the dinosaurs. But there must be a point of diminishing returns: Clearly, some flux is too high for the continuance of any civilization.
One consequence of this train of argument is that, even if civilizations commonly arise on planets throughout the Galaxy, few of them will be both long-lived and non-technological. Since hazards from asteroids and comets must apply to inhabited planets all over the Galaxy, if there are such, intelligent beings everywhere will have to unify their home worlds politically, leave their planets, and move small nearby worlds around. Their eventual choice, as ours, is spaceflight or extinction.
CHAPTER 19: REMAKING THE PLANETS
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,1 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 anti-hydrogen 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.1 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 one-particle-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 ar
e 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.