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Radio Days 35 extensive, detailed studies of radio signals from the sky. By the early 1950s, astronomers were actively observing such features as hydrogen clouds that outlined our galaxy’s spiral arms.'° Radio astronomy in the United States received public support with the founding of the National Radio Astronomy Observatory at Green Bank, West Virginia. Other nations built their own radio observatories, the most famous being the giant dish at Ta dna Da 1.4 at Jodrell Bank in England. Spotting the potential of these systems, physicists Philip Morrison and Giuseppe Cocconi began discussing the feasibility of interstellar commu- nication. Cocconi calculated that the 250-foot radio telescope at Jodrell Bank could detect Earth-like radio signals from the nearest star. In a paper published in Nature in 1959, he and Morrison pointed out that such instru- ments made it feasible to communicate with other civilizations—and to search for their signals. They concluded their paper with a now classic statement: “The probability of success is difficult to estimate; but if we never search, the chance of success is zero.””"! Astronomer Frank Drake realized independently that searching for interstellar radio signals might allow us to detect technologically advanced extraterrestrials. Drake launched the modern radio search with his Project Ozma in 1960, using a radio telescope at Green Bank. His brief observa- tion of two stars detected a false positive coming from a secret military facility, but no evidence of extraterrestrial technology. Drake persisted in his quest, becoming a major figure in the scientific search.” By 1962, scientific attitudes toward extraterrestrial intelligence were shifting from slightly amused neglect to more open-minded inquiry. As so often happens in science, observed astronomer Robert Rood and physicist James Trefil, the transition occurred because someone pointed out that technological developments had brought within reach a goal previously thought to be unattainable.'* A new, controversial scientific field was emerging—a search for electro- magnetic evidence of alien technologies, rather than for living beings. For radio searches, that meant detecting beacons or broadcasts, intercepting beamed transmissions, or eavesdropping on local communications. A Dissenting Voice In a 1960 paper that did not attract much attention at the time, radio astronomer Ronald Bracewell presented a different model of interstel- lar contact. He proposed that, instead of searching for radio signals from many light-years away, more advanced civilizations would send out robotic probes to the most interesting stars. Those machines could report back to their launching civilizations and could use radio to contact any technological civilization they found."* Bracewell later made an important point about this model: We should focus on those techno- logical civilizations that can reach us.