Have you ever seen an alien? Scientists haven’t, but that hasn’t stopped them from speculating that imperialist extraterrestrials could be on the way.

With the exception of one inconclusive blip in 1977, we haven’t detected signs of alien intelligence. Having scoured our solar system with probes and turned up empty, extraterrestrial (ET) signal searchers now survey the galaxy on the radio portion of the electromagnetic spectrum, where photons can traverse interstellar distances and arrive on Earth unscathed.

The most prominent and well-funded organized effort to search radio bands is led by the SETI (Search for Extraterrestrial Intelligence) Institute, a private nonprofit foundation that borrows time on radio telescopes or scans the skies using their own arrays.