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340 The first key factor that would make the difference between controllable and noncontrollable consequences of contact is interstellar flight. Not whether it is possible—as it almost certainly would be for a species more technologically advanced than our own—but whether any civilization actu- ally chooses to do it, whether any society is sufficiently motivated to explore, colonize, or take other action at interstellar distances. We can refine the issue further by eliminating the requirement that interstellar vehicles carry biological organisms. Intelligent machines could do the job. The second key factor is the choice of actions made by the civilizations in contact—the extraterrestrials, and ourselves. Given such possibilities, one policy question rises above all others: Should we call attention to ourselves? Should we reply to an alien signal, whether or not it is addressed to us? In the absence of a detection, should we send targeted, high-powered signals in an Active SETI program? Or should we simply leave unchanged what we are doing now, thoughtlessly radiating our normal radar, television, and radio signals out into the cosmos? We cannot assume that extraterrestrials have detected us or that they will. Many other intelligent beings may believe that their civilizations are unique, as many humans believe their own is. If alien intelligences do search for others, those searches may not be sustained. If sentient aliens are searching, they may be doing so with technologies that are not optimized for the wavelengths that we emit most strongly. Our normal signals may be below their detection threshold. Our highly detect- able phase may be brief. Unless we receive a transmission clearly aimed a us, we won’t know if another civilization is aware of us or not. What if we find alien technology in our solar system? There is no point in remaining silent, Bracewell commented, if a probe has already reached our solar system and has reported our presence (emphasis added).'° The second condition is crucial, as an alien spacecraft might have ceased func- tioning long ago. If the probe showed no signs of activity, we could no assume that it had informed its sending civilization of our existence. We still might have a choice about calling attention to ourselves. The underlying issue is whether we think that the effects of contact will be positive or negative. Advocates of Active SETI implicitly assume tha alien civilizations are benign, or that distance will insulate us from direc contact if they are not. Those assumptions remain unproven. Some Conclusions Drawn The Key Factors The Prime Question