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173 enough to damage or kill humans aboard spacecraft.*” An alien star that generated more frequent and more powerful flares might discourage all attempts to journey beyond the home planet. Location might matter in a grander sense. Some regions of the Galaxy may be more likely to produce technological civilizations than others. If some do expand, they will not do so equally in all directions; as David Stephenson pointed out, a uniform exponential model of interstellar expan- sion would only be followed if all systems were equally interesting and distributed uniformly in space. The central region of the Milky Way is an inviting target for our searches because of its higher density of stars. Cade proposed that superior civiliza- tions, hungry for energy, would be more likely to migrate toward the center, where the average separation between the stars is less. MacGowan and Ordway came to an even more forceful conclusion: When “executive automata” reach a certain level of development, they must be engaged in a general migration toward preferred galactic locations, probably in the galactic core (emphasis added). In our Sun’s own rather sparse region of the Galaxy, only very slow star-hopping colonization would ever be possi- ble by biological life. There are counterarguments. Life and intelligence in the galactic center may suffer more frequently from astrophysical extinction; the same may be true for the bulk of the Galaxy’s spiral arms. According to one theory, the most likely place to find old civilizations is near the outer edge of the Galaxy’s spiral structure, where there are fewer threats.** How likely is it that each phase of an expansion would take place? Like the “bottlenecks” that limit the number of communicating civilizations, there may be filters that determine the probability that an intelligent species would engage in, or sustain, interstellar exploration or expansion. The first filter would be ascending from the planetary surface to orbit around the home world; the second would be journeys to other bodies in that star— planet system. The third would be migration beyond the home star’s family, which could lead to the colonization of interstellar space rather than to settlements on other planets. Each filter might reduce the probability tha extraterrestrials would explore and colonize other solar systems—includ- ing ours. Even for a civilization capable of interstellar flight, distance would be a factor. Journeys covering a few light-years may be considered feasible, bui a gap of hundreds of light-years might be seen as a serious obstacle. Each spacefaring civilization may have a different concept of its accessible uni- verse at different times in its history. Even if some parts of the Galaxy are colonized, others may not be. I technological civilizations do come into contact, Bracewell proposed, i may be because of accidental proximity due to random spacing.*” We migh be in one of the voids. Cosmic Geography