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137 Mysterious Probes We cannot assume that we will detect alien machines by their signals; probes might be designed not to initiate contact until they were detected. Or they might be uninterested in contact with humans, incapable of making contact in a way that humans understand, or actively avoiding contact. The best chance for successful contact, Burke-Ward thought, may be to develop a distinctive communications system that the probe can interpret as unequivocal proof that humans know that the probe exists. What would be the purpose of an interstellar probe? Burke-Ward advanced three possibilities: data gathering probes that would collect information and transmit that to their home civilization; direct action probes that might be designed to intervene in the affairs of the system visited; sentient entities. However, a probe may not be here to study humans or the Earth. It may not consider humans to be intelligent—or it may have a profoundly different purpose.” An interstellar probe would be obvious if it were a self-reproducing machine, Barrow and Tipler claimed; it would construct an artifact in our solar system. This object would be so noticeable that it could not possibly be overlooked.” As we do not see such an artifact, this perspective sup- ports Tipler’s view that we are unique. Habitats that migrated to our solar system in the past could be drawing on its asteroidal or cometary resources. As our asteroid belt is an excellent source of raw materials, Papagiannis thought that it would be the best place to search for alien space colonies, which might reradiate in the infrared.” To date, searches have not detected any. What if they do not wish to be seen? Matloff and Martin speculated that alien world ships could exist silently in our solar system, masquerading as asteroids or comets.” Moravec suggested another reason for invisibility: Alien machines could be very small. Robots might miniaturize their operations until they are working with matter on a scale much finer than we can see. A wave of reorganization could pass through a solar system without leaving its native inhabitants any the wiser.”* We still would be looking for evidence of alien technology, not the aliens themselves. The two categories might merge in the form of a highly intel- ligent machine capable of autonomous operations, an artificial sentient being that might contact us on its own initiative.” Detecting evidence of any nonhuman technology within our solar system, or anywhere in the vast spaces between the stars, would tell us that at least one civilization had achieved interstellar flight and is able to send robotic spacecraft or inhabited vehicles toward us. Contact could be direct. Searching for Artifacts