For 72 seconds in August 1977, an Ohio radio telescope picked up a signal thirty times stronger than the background hum of space, sitting almost exactly on the frequency scientists had guessed an alien civilization would choose — and it has never been heard again

The most famous document in the search for alien life is a strip of computer printout with six characters circled in red biro.

Those characters are 6EQUJ5. A volunteer astronomer named Jerry Ehman found them a few days after 15 August 1977, working through a pile of paper from the Ohio State University Radio Observatory, an instrument so vast and so ungainly that everybody called it the Big Ear. He ringed the sequence and scrawled one word beside it: Wow! The name has outlived the telescope.

What the six characters actually record

People occasionally assume 6EQUJ5 was the message. It was the volume knob.

Each character logged one ten-second sample of signal strength, expressed as how far the reading sat above the surrounding background noise. A blank space meant almost nothing was happening. Digits ran from one to nine. Past nine the printer had no digits left and switched to letters, so A stood for ten, B for eleven, and on up the alphabet. In Ehman’s own explanation of the code, U marks a reading thirty standard deviations above the hum.

The shape of the run matters more than the peak. The numbers climb, top out, then fall away over 72 seconds, which is exactly how a fixed point in the sky should behave as the Earth swings a telescope past it.

Why the frequency raised eyebrows

Eighteen years before the detection, two Cornell physicists worked out where to listen. Giuseppe Cocconi and Philip Morrison argued in a two-page 1959 paper in Nature that any civilisation hoping to be noticed would transmit near 1420 megahertz. Neutral hydrogen radiates at that frequency, hydrogen is the commonest stuff in the universe, and anyone with radio astronomy would already have receivers trained on the spot.

The Wow! signal landed in a single 10 kilohertz channel right beside it, inside a stretch of spectrum that international agreement reserves for astronomy and forbids to terrestrial transmitters. Narrow, too. Ehman’s twenty-year review of the event puts the whole thing in essentially one column of the printout, which is the signature of a transmitter rather than the broad smear most natural sources produce.

A telescope that could not look twice

Why only 72 seconds? Big Ear was a meridian transit instrument, adjustable for height above the horizon and otherwise stuck, and Earth’s rotation did the scanning. That bought 72 seconds per target, after which the data went to a printer, the printout went on a stack, and the stack waited for a volunteer with an afternoon free.

By the time anyone knew something odd had happened, the sky had moved on by days.

Then there is the horn problem. Big Ear listened through two feed horns aimed at slightly offset patches of sky. The recording method makes it impossible to say which horn caught the signal, so the event has two candidate positions in Sagittarius instead of one. A steady source should also have shown up in the second horn about three minutes later. It never did. Observatory director John Kraus laid out the whole tangle in an unpublished paper he mailed to Carl Sagan in 1994, now held in the National Radio Astronomy Observatory archives.

Dozens of attempts, no repeat

“We should have seen it again when we looked for it 50 times,” Ehman told the Cleveland Plain Dealer in 1994, already unconvinced by his own discovery. He had swung Big Ear back at those same coordinates for weeks right after the detection and found nothing. Others kept at it long after the paper printouts stopped. Robert Gray and Kevin Marvel pointed the Very Large Array at the locale, an array considerably more sensitive than Big Ear, and reported a null result in the Astrophysical Journal in 2001. In 1999, Gray worked with Simon Ellingsen on six fourteen-hour watches using the University of Tasmania’s 26-metre dish at Mount Pleasant, later published as a hunt for periodic emissions at the Wow locale that also came up empty.

In 2022 a Breakthrough Listen team ran the first coordinated hunt using the Green Bank Telescope and the Allen Telescope Array together, and wrote up, in Research Notes of the AAS, that no technosignature candidates turned up.

What the 2025 reanalysis changed

Abel Méndez and colleagues at the University of Puerto Rico at Arecibo went back to the paperwork. Digging out decades of unpublished Ohio SETI observations, their 2025 preprint narrows the candidate position and revises the peak brightness upward, past 250 janskys. It also shifts the frequency to 1420.726 megahertz, further from the hydrogen rest frequency than earlier estimates allowed, which points to a galactic source moving fast. Their preferred culprit is a cold hydrogen cloud flaring briefly like a maser, not interference from Earth.

That paper has not yet cleared peer review, and one preprint is one preprint. It tightens the field without closing it.

Forty thousand other blips

Big Ear came down in 1998, when developers bought the land to extend a golf course. The archive survived.

Méndez, with former observatory staff Robert Dixon and Russell Childers, catalogued the Ohio SETI legacy in June 2026. Three decades of hydrogen-line surveys covered roughly 70 per cent of the radio sky and logged more than 40,000 transient narrowband events. Nobody has ever properly examined most of them. Wow! became famous because a volunteer happened to be reading that page.

Which raises an awkward thought about the other pages. Whatever produced the loudest 72 seconds in SETI history may well have produced quieter cousins, sitting in that archive, logged and ignored, waiting for somebody with a decent script and no particular hurry.