It lasted 72 seconds, came from the direction of Sagittarius, near a frequency where the universe itself is quiet — and it has never been heard again. The Wow! signal is the most famous unexplained detection in the search for extraterrestrial intelligence, not because it proves anything, but because, nearly fifty years later, nobody has been able to explain it away.
A volunteer, a printout, and a word in red ink
The signal was recorded on 15 August 1977 by the Big Ear radio telescope, operated by Ohio State University near the Perkins Observatory in Delaware, Ohio, then being used for the search for extraterrestrial intelligence (SETI). It scanned the sky continuously, and its data were processed by an IBM 1130 computer and printed out on paper in long columns of characters.
Volunteer astronomer Jerry R. Ehman was reviewing the printouts by hand a few days later when he found something extraordinary in the data from 15 August. In one frequency channel, the signal strength rose, peaked, and fell in a smooth curve over six readings. He circled the string of characters that recorded it — “6EQUJ5” — and wrote “Wow!” in the margin. The name stuck.
“6EQUJ5” is not a message, and it was not sent by anyone — the most common misunderstanding. Each character records the signal’s intensity during a ten-second interval, measured against the background noise. Digits stood for lower intensities and letters for higher ones, so the sequence means the signal climbed from about six times the background, to fourteen, twenty-six, and thirty times the background at its peak, then fell back through nineteen and five. The characters are a measurement, not a code.
Why the signal looked so interesting
Several features made the detection stand out. First, it was narrowband — confined to a thin slice of radio frequency, less than 10 kilohertz wide. Natural radio sources in space tend to broadcast across a broad range of frequencies. A narrow signal is the kind of thing a transmitter produces, which is why SETI searches look for them.
Second, it sat close to 1420 MHz, the frequency at which neutral hydrogen — the most common element in the universe — emits radio waves. In 1977, Ehman gave the frequency as 1420.4556 MHz. Astronomers had long suggested this “hydrogen line” as a natural meeting place for interstellar communication, and international agreements protect it from terrestrial transmissions, making an Earth-based source less likely, though not impossible.
Third, the signal lasted the full 72 seconds Big Ear could observe any one point in the sky, rising and falling exactly as a steady, distant source would while drifting through the beam. It appeared in only one of Big Ear’s two receiver beams, consistent with a far-away source rather than nearby interference, and peaked at about thirty times the background noise.
The telescope’s design also meant the signal could have come from either of two possible positions in Sagittarius, near the Chi Sagittarii star group and north-west of the globular cluster M55. There is no single star to point at.
The search that found nothing
If the signal had been a genuine transmission, the obvious next step was to listen again. Astronomers did — many times, with Big Ear and with far more sensitive instruments. The signal never recurred. The Ohio SETI survey continued until 1983 without another Wow!-like event, and follow-ups since, including with the Allen Telescope Array, have all come up empty.
That silence is the signal’s central problem. A single detection, however striking, cannot be confirmed the way science normally demands, because there is no second observation to check it against. The original printout survives, preserved by the Ohio History Connection, but the sky has not repeated itself.
Ehman himself was cautious, at one point suggesting an Earth-sourced transmission reflected off space debris, then stepping back from that idea. The man who found the signal never claimed it was proof of aliens.
The explanations that have been offered
No explanation, terrestrial or otherwise, has been confirmed. Several have been proposed, and each is worth describing with its status attached.
Terrestrial interference and satellites. Checks at the time found no publicly known Earth satellite near the signal’s position, and the frequency is protected from terrestrial use. That rules out the most obvious local sources, but not every possible one — an unknown transmitter or unusual reflection remains conceivable, if unproven.
Comets. In 2017, astronomer Antonio Paris proposed that the signal came from the hydrogen cloud around a comet, naming two comets, 266P/Christensen and P/2008 Y2 (Gibbs), that he said had been in that part of the sky in August 1977. Robert Dixon, director of the Ohio State Radio Observatory, and colleagues rebutted this, arguing that the comets had not been inside Big Ear’s beam on 15 August 1977, and that strong hydrogen emission from comets was not a known phenomenon. The comet explanation remains disputed, not accepted.
A natural astrophysical burst. In 2024, a team led by Abel Méndez of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo proposed that a flare from a magnetar — a highly magnetised neutron star — could have struck a cold cloud of neutral hydrogen in space, causing a brief burst of radio waves at the hydrogen line. It is a hypothesis based on modelling, not a confirmed identification, and other astronomers have noted that it still lacks the detailed evidence needed to explain the signal’s exact behaviour. It remains unproven.
An extraterrestrial transmission. This is the explanation the public reaches for first, and it cannot be ruled out — but it cannot be confirmed either. A single, unrepeated signal, with two possible sky positions and no message content, is not evidence of intelligence. It is evidence of something unusual that happened once.
What the evidence says
- Documented: On 15 August 1977, Ohio State University’s Big Ear telescope recorded a strong, narrowband radio signal near 1420 MHz from the direction of Sagittarius. It lasted the full 72-second observing window, peaked at about thirty times the background noise, and was found days later by volunteer astronomer Jerry R. Ehman, who circled the intensity reading “6EQUJ5” and wrote “Wow!” beside it.
- Documented: The string “6EQUJ5” records signal intensity over time; it is not a message. The original printout is preserved by the Ohio History Connection.
- Documented: Despite numerous follow-up searches with more sensitive instruments, the signal has never been detected again.
- Disputed: The 2017 comet hypothesis — that hydrogen around comets 266P/Christensen and P/2008 Y2 (Gibbs) produced the signal. The Ohio State team rebutted it, arguing the comets were not in the telescope’s beam in 1977.
- Unproven: The 2024 magnetar-and-hydrogen-cloud hypothesis, and the idea that the signal was an extraterrestrial transmission. Neither has been confirmed.
- Claimed, not proven: That any particular star or object in Sagittarius was the source. Big Ear’s design leaves two possible sky positions, and neither has yielded a repeat signal.
What remains unknown
What produced the Wow! signal? After nearly fifty years, the honest answer is still that nobody knows. It may have been a rare natural event, an unusual reflection, or something no one has yet thought of. It may have been a transmission from elsewhere. Until it repeats — or until a natural explanation is demonstrated rather than proposed — it will remain the most famous unanswered question in the search for life beyond Earth.