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135 The idea of colonization makes SETI true believers very nervous, observed journalist Gregg Easterbrook, because the apparent absence of settlements in our part of the Galaxy becomes another argument that aliens don’t exist. The dilemma would be eased if the destinations of alien colonizers were not planets. Civilizations that expand in self-contained, self-sufficient settlements may be acclimated to space, just as some forms of life became acclimated to land once they emerged from Earth’s oceans. An interstellar ark could be either a means of transportation to another biosphere, or a permanent traveling biosphere itself. David Stephenson speculated that the latter vessel might grow during the journey, drawing on material along the way, adapting to conditions wherever it went. There is another significant implication of inhabited spacecraft that do not require habitable planets as their destinations: They do not need Sun- like stars. Once intelligent beings have become accustomed to living in space colonies, proposed Rood and Trefil, it makes no difference where the colonies are.” We return to probability. Several analysts have given us mathematical models showing the average number of civilizations within each standard volume of space in our Galaxy, or the number of such civilizations per fixed number of stars. These calculations usually rest on an assumption that no civilization expands to other stars or to interstellar space. What if some do? This would change the demographics of technological intelligence. It may be easier to detect an expanded civilization than one that remains confined to its biosphere of origin. If colonization is indeed the galactic norm, Jill Tarter recognized, then our immediate search strategies might profitably concentrate closer to home; the nearest transmitter is likely to be far closer to us than is the nearest site of indigenous extraterrestrial intelligence. Intercolony communication may offer the best possibility for detecting signals. Many technological societies may accept being limited to their bio- spheres of origin. Those that expand outward into the Galaxy may be a small minority—but a very influential one. Migrating civilizations may be the ones that we are most likely to encounter. Electromagnetic signals might not be the only evidence of alien technologi- cal activities that we can detect. Recent years have seen growing interest in broadening our quest to include a Search for Extraterrestrial Artifacts (SETA). Scot Lloyd Stride of the Jet Propulsion Laboratory described two pre- dominant hypotheses as to how we can detect extraterrestrial intelligence: Searching for Artifacts Searching for Artifacts