The Wow! Signal

On August 15, 1977, at 11:16 PM Eastern, a computer at the Big Ear radio telescope in Delaware, Ohio printed out a row of characters that read 6EQUJ5. Each character was the intensity, in standard deviations above background, of the signal in a one-minute slice. The peak — U — was 30 sigma. The signal lasted 72 seconds. The frequency was 1420 MHz, the neutral-hydrogen line, the frequency SETI's founding papers had argued was the most rational place to listen for a deliberate broadcast. Astronomer Jerry Ehman, reviewing the printout days later, circled the row in red pen and wrote 'Wow!' in the margin. The annotation gave the case its name.

The signal has never been heard again.

What Happened

Big Ear was a transit-mode radio telescope: a fixed paraboloid that observed the sky as the Earth rotated through it. It had two adjacent receiving horns, offset by about three minutes of beam-drift time. A real point source would normally be logged twice on the same transit — once in each horn — as the source crossed the telescope's field. The 6EQUJ5 detection appeared in one horn only. The second detection never came.

Everything else about the signal had the SETI signature: narrowband (less than 10 kHz), at the canonical hydrogen frequency, far above background, with the exact 72-second envelope a fixed beam would record for a celestial point source drifting through. The direction was roughly toward the constellation Sagittarius — the Tau Sagittarii region, a part of the sky containing many distant stars but no obvious nearby candidate emitter. The Big Ear team logged the detection, flagged it as anomalous, and quietly returned to routine operation. Ehman's annotation was internal; the broader scientific community did not learn about the signal until later.

Follow-up campaigns have been steady. Robert H. Gray, an independent investigator who spent decades on the case, conducted searches in 1987, 1995, and 1996 using the META II array and Arecibo. The Allen Telescope Array searched in the 2000s. A 2017 Very Large Array campaign by Gray and Kunal Mooley, published in The Astronomical Journal, is the most recent peer-reviewed null result. Breakthrough Listen has also reanalyzed the coordinates. Across forty-eight years and dramatic improvements in instrumental sensitivity, nothing matching the original signal has been detected.

The Comet Hypothesis

The most-discussed natural-cause hypothesis is the Paris 2017 paper proposing that hydrogen clouds around two comets — 266/P Christensen and P/2008 Y2 (Gibbs) — happened to be in the Big Ear beam at the relevant time. Hydrogen-emission lines from cometary comae are real and well-characterized at 1420 MHz. Paris argued the geometry worked.

The response from the radio-astronomy community has been polite skepticism. The hydrogen emission from a comet's hydrogen coma is diffuse and weak. Producing a 30-sigma signal would require either a much denser hydrogen distribution than typical cometary models predict, or a configuration of the comae and the telescope geometry that has not been independently demonstrated. Robert Gray published a counter-analysis. The comet hypothesis is documented in the record but is not the consensus explanation.

What Resolution Would Look Like

The brand voice rule for space-anomalies is careful with alien framing. The Wow! Signal is not evidence of extraterrestrial intelligence. It is evidence that on a specific August evening in 1977, a real instrument recorded a real narrowband signal at the SETI-relevant frequency, with the duration a point source crossing the beam would produce, that has not been reproduced under decades of attentive follow-up. The honest description is: we have an unrepeated anomalous narrowband detection at 1420 MHz from a direction in Sagittarius. That is what the record says.

Resolution means either (a) a repeat detection — at the same coordinates, narrowband, near the hydrogen line, with intensity in the same range — or (b) a natural-cause mechanism that quantitatively reproduces the 30-sigma intensity, the 72-second envelope, the single-horn detection, and the lack of any subsequent reappearance. Neither has happened. The case is, by any reasonable standard, the strongest unresolved candidate technosignature in the modern SETI archive — strong enough that the original printout is preserved, the coordinates are re-searched every few years, and Ehman's marginalia remains the most-quoted handwritten note in the field.