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172 The implications of expansionist models are much different, Claudius Gros pointed out, if we drop the assumption that the colonies of expanding civilizations automatically inherit all their characteristics. An expanding civilization’s motives might change before it reaches us.** What other reasons might motivate interstellar migration? Groups may flee their home systems for religious, ideological, or political reasons. For those migrants, the jump need be made only once in the lifetime of their home star—unless a centralizing power pursued them. A civilization that felt threatened by another technological species might spread out to reduce the risk of annihilation. However, that would only work if the escapees could assure that they would not be found in their new locations. There may be other reasons for sending interstellar missions: scientific curiosity; precautionary observation of emerging intelligence; preemption of a potential threat from another technological civilization. None of these requires the sending of inhabited spacecraft; well-equipped probes could do the job. Finney invoked a Galilean premise. We cannot determine by logical argument, equation, or debate how far and wide our descendants will spread. We can know the answer only by seeing what actually happens. The same principle would apply to expansion by extraterrestrial civilizations. In the human case, the classic agenda for manned spaceflight is driven by the particular layout of our solar system—first the Moon, then Mars.* Other star-planet systems may be quite different. Would another civiliza- tion engage in spaceflight if there were no other detectable planet in the system? What if the home world were the only known body with a solid surface on which to land? At the other extreme, consider the model proposed by Dole and Asimov: twin planets, revolving around a common center of gravity. With another planet close and large, with clouds, oceans, and continents clearly visible, would the urge to reach the companion hasten technological progress and eventual expansion? Could any egocentric philosophy of the universe develop?** Mind-Stretcher. Where would we be today if Venus were as many had imagined it, a warmer but still habitable sister to the Earth? What if Mars were the size of our planet, able to hold a denser atmosphere and main- tain warmer temperatures on its surface? There may be other physical barriers to expansion. One of the major risks of human interplanetary flight is the occasional solar flare intense Why Don’t We See Them? Cosmic Geography