Why the most famous ‘alien signal’ in history has never been picked up again

Of all the signals ever picked up in the decades-long search for extraterrestrial intelligence, one remains more famous — and more unexplained — than any other. The “Wow!” signal, detected for just 72 seconds on the night of August 15, 1977 by a radio telescope in rural Ohio, has never been recorded again despite nearly 50 years of follow-up observations by multiple radio observatories.

The signal arrived 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. When astronomer Jerry Ehman reviewed the computer printout several days later, he circled the sequence “6EQUJ5” and wrote a single word in the margin that gave the signal its name: “Wow!”

What made the detection so remarkable was how closely it matched what SETI researchers expected a distant artificial transmission might look like. It 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 extremely narrow radio frequency unlike most naturally occurring cosmic radio emissions, and lasted for the telescope’s entire 72-second observing window before disappearing completely.

More recently, the Breakthrough Listen initiative pointed some of the world’s most powerful radio telescopes, including the Green Bank Telescope and Allen Telescope Array, toward candidate stars near the signal’s estimated origin, but found no convincing technosignature. Scientists have proposed a range of explanations, from human-made interference and instrumental errors to flaring hydrogen clouds struck by radiation from objects like magnetars, but none has been confirmed — leaving the “Wow!” signal one of astronomy’s most enduring unsolved puzzles.