The Wow! Signal
Back in 1977, Jerry Ehman was scanning radio waves from deep space as a volunteer for SETI, the Search for Extraterrestrial Intelligence. At one point, his measurements spiked with an uncanny signal that lasted for 72 seconds. It seems to have come from within the Sagittarius constellation, which lives by the star Tau Sagittarii, a mere 120 light-years away. Ehman wrote the words “Wow!” on the original printout of the signal, and it’s been known by that appropriate exclamation ever since. So what's so wow-worthy? As National Geographic notes, “the signal that was received was at precisely the right frequency that wouldn’t be interpreted as noise, and wouldn’t be intercepted along its journey. In other words, if we were going to send a signal out into the universe to try to communicate with an alien race, that’s exactly the frequency we would use.” Since then, despite much effort, the signal has never been heard again. Ow!