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Alien Probes 129 Interstellar radio communication has significant limitations. As Brace- well pointed out in 1960, a search for radio signals directed at our planet would have been fruitless throughout most of the billions of years of the Earth’s existence. It is more logical to assume that superior civilizations would send automated messengers to orbit each candidate star and await the possible awakening of a civilization on one of its planets, using radio signals to attract the attention of any indigenous intelligent beings. Such a probe might repeat back to Earth exactly what it received; to us, its signals would have the appearance of long-delay echoes. (Some have suggested that long delayed radio echoes detected in the past might originate from such a machine, although no such probe has been found.™*) These ideas provoked resistance from those most attached to the remote detection scenario; Morrison described them as tendentious. “It’s rather amusing,” Bracewell commented, “to see the way they jump on both the probe idea and the idea of uniqueness with very strong, very fierce feelings.”* Others were not so dismissive. Tipler believed that robot probes would achieve the aims of an exploring civilization much better than radio signals; they can contact civilizations that are not listening or those that do not have radio technology, even intelligent but pretechnical societies. Such probes also could be used to explore uninhabited systems.*° Our own future interstellar machines might be, in part, Bracewell probes, designed to detect and communicate with other intelligences as well as to search for new biospheres. If a civilization at our level of technological development already is projecting such probes, older and more advanced civilizations might have launched them long ago. Mind-Stretcher. If other advanced technological societies conclude that probes are the most efficient means of exploration, those machines might be the most widely distributed artifacts of intelligence. We are far more likely to meet the technological creations of extraterrestrials than the aliens themselves. Searching for signals and sending out probes are not mutually exclusive strategies. An alien civilization that detected our emissions might send a probe to look us over. Remote detection could lead to direct contact. Engineer Christopher Rose and astrophysicist Gregory Wright proposed another scenario. If speedy delivery is not required, other civilizations might send out artifacts packed with information that were intented to be found—a vastly upgraded version of the Pioneer plaques. Astronomer Woodruff Sullivan, while defending the search for electromagnetic signals, admitted that some attention should be paid to the possibility of finding an information-drenched object sent by an extremely advanced civilization interested only in one-way communication.” Finding such an object, or any form of alien spacecraft, might tell us far more