n 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 the Earth。 A world 10 kilometers across could be transported from Saturn； say； to Mars through nuclear burning of the hydrogen in an icy et 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 ets； 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 modities resupplied from Earth； but in time we'd manufacture more and more of them ourselves。 We'd bee 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 bee 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 bee 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 fortable 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 bees 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； positions； 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 all clear that our knowledge is suffi