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133 out in our solar system might be only a temporary solution. Yoji Kondo warned that even the most distant bodies eventually will become unsuit- able for our species as the Sun turns into a white dwarf star. Rocket pioneer Robert Goddard, writing in 1918, described an inter- stellar journey to find new worlds for humans fleeing a dying Sun. This would meet the criterion of the Project Cyclops report, that only the most extreme crisis would justify mass interstellar travel.* It is difficult to construct a plausible scenario whereby an intelligent species develops and retains for centuries an interest in interstellar communication together with the technology to engage in it, and yet does not begin interstel- lar travel. The classic SETI paradigm rests on an assumption that technological civilizations do not expand beyond their home systems. That may not be a sound assumption. If some other civilizations have much greater techno- logical capabilities and much greater resources than we do, interstellar vehicles carrying intelligent beings may be more easily within their reach. Andrew Clark and David Clark addressed this by inventing a category of extraterrestrials they called IMETI, or extraterrestrial intelligence capable of interstellar mobility. In their view, it is just as possible that there are significant numbers of IMET1as it is that there are significant numbers of radio-transmitting ETI.’ Darling projected that an intelligent technological species will be space- aring and star-colonizing within a period that is completely insignificant on a geological timescale.** The primary question might be one of motiva- ion: Why would an advanced civilization choose to launch inhabited inter- stellar spacecraft? During the 1970s and 1980s, several physical scientists assumed that population growth would be a driving force for expansion. Rood and Trefil hought that every civilization must face its own Malthusian dilemma. Only hose who solve it by expansion are likely to try to establish contact with other races; any aliens we come across are likely to have already expanded into space.*’ Other scientists proposed models of migration and coloniza- ion, with widely varying estimates of how rapidly alien civilizations might expand through the Galaxy. Morrison questioned the Malthusian view that human society or its counterpart will grow indefinitely and be pressed to dwell among the stars. Today, these models are being forgotten as population growth com- mands less of the intellectual world’s attention. Alien Colonizers Alien Colonizers —John Barrow and Frank Tipler, 1986%°