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.[36] 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 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.