Pale Blue Dot - Carl Sagan [119]
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 anti-matter 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 move 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