One word scribbled in a margin turned a 1977 radio blip into astronomy’s biggest mystery

Some of the biggest mysteries in science start with a small gesture. In this case, it was astronomer Jerry Ehman circling a string of characters — “6EQUJ5” — on a computer printout and writing one word in the margin: “Wow!”

The signal Ehman circled had been picked up several days earlier, on the night of August 15, 1977, by a radio telescope in rural Ohio. It lasted just 72 seconds, arriving from the direction of the constellation Sagittarius, close to the 1,420 MHz hydrogen line — a frequency long considered one of the most likely channels an intelligent civilisation might use to communicate across interstellar distances. Nearly five decades later, scientists are still debating what produced it.

What made the detection so remarkable wasn’t simply its strength, but how closely it matched what researchers participating in the Search for Extraterrestrial Intelligence (SETI) expected a distant artificial transmission might look like. The signal gradually increased and decreased in intensity exactly as a fixed celestial source would while passing through the Big Ear radio telescope’s field of view, occupied an unusually narrow radio frequency, and lasted the telescope’s entire 72-second observing window before disappearing completely — and it has never been detected again despite decades of follow-up observations.

Scientists have proposed numerous explanations over the years, ranging from human-made interference and instrumental errors to hydrogen clouds surrounding comets, rare astrophysical events, and the possibility of an extraterrestrial transmission. None has been confirmed. Because the signal occurred only once and could never be reproduced, researchers have never gathered the additional evidence needed to identify its true origin.