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In the final weeks of 1960, two young radio enthusiasts were monitoring transmissions from a home-built listening station on a rooftop in Turin, Italy, when the brothers detected something extraordinary: a faint signal that bore the hallmarks of one originating from space. That wasn't the only surprising thing about the alleged signal:, the transmission carried a very familiar message: it was Morse code for the international distress signal SOS.

As the youths listened, the signal seemed to grow weaker. This raised a troubling possibility: could it be that the signal's source was moving farther away from Earth ?…