Nonterrestrial artifacts hard to pin down

(PhysOrg.com) -- Two Pioneer probes left our solar system carrying plaques about humankind, and two Voyager probes will soon join them to gather information about places far out in our galaxy. We can and will send more autonomous probes into outer space, but why have we never found evidence of other civilizations doing the same? A pair of postdoctoral researchers at Penn State, approaching the problem mathematically, shows that we have not looked in enough places to ensure that no extraterrestrial artifacts exist in our solar system.

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