osphere 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 munities 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 paratively 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。* 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。
* If it had been the other way； then we and everything else in this part of the Universe would be made of anti…matter。 We would； of course； call it matter—and the idea of worlds and life made of that other kind of material。 the stuff with the electrical charges reversed； we'd consider wildly speculative。
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 ing 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 ets around the inner Solar System taking a main…belt asteroid； for example； and inserting it into orbit around