The Wow! Signal
UnsolvedSummary
On the night of 15 August 1977, Ohio State University's Big Ear radio telescope recorded an intense, narrowband radio signal close to the hydrogen line, from the direction of Sagittarius. An astronomer reviewing the printout days later circled the reading and wrote a single word beside it. Nothing like it has been detected from that point in the sky since, despite decades of searching. A natural mechanism was proposed in 2024 that fits the data well; it is not yet confirmed.
What is documented
The detection. Big Ear, a transit telescope operated by Ohio State University, was running a survey for the university's SETI programme, which ran from 1973 to 1995 and remains the longest-running such programme in history. On 15 August 1977 it recorded a strong narrowband signal at a frequency near 1420 MHz, the emission line of neutral hydrogen. The signal reached roughly 30 times the background noise level around it.
The discovery. The telescope's output was printed as columns of characters representing signal intensity. Astronomer Jerry R. Ehman, reviewing the printout some days after the observation, found an anomalous sequence: 6EQUJ5. He circled it and wrote "Wow!" in the margin. The name stuck to the signal.
What "6EQUJ5" is. It is an intensity encoding, not a message. Each character records how far the received power at that moment exceeded the background, using digits and then letters as the scale climbs. The sequence describes a source rising to a peak and falling away. It carries no content, and nothing in it has ever been "decoded," because there is nothing in it to decode.
The 72 seconds. The signal was present in the telescope's beam for about 72 seconds. This is the key technical fact of the case, and it is the one most often misread. Big Ear was a fixed instrument: it did not track, and the sky drifted past it. Any fixed celestial source would remain in its beam for approximately 72 seconds. The duration therefore tells us how long Big Ear was looking at that patch of sky, not how long the source was transmitting. The signal's rise and fall matched the beam's sensitivity profile exactly, which is what a genuine sky source should do and what local interference generally does not.
The position problem. Big Ear used two feed horns. Because of how the data were processed, it was not possible to establish with certainty which horn had recorded the signal. This left two candidate right ascension values roughly three minutes apart. The declination was approximately -27° 03'. The direction lies in Sagittarius, northwest of the globular cluster M55.
The non-repetition. The signal was never detected again. Follow-up searches began within years of the detection and have continued, including work with far more sensitive instruments than Big Ear. Nothing from that position has repeated.
Leading explanations
Stimulated hydrogen emission (leading, unconfirmed). In August 2024, a team from the Planetary Habitability Laboratory at the University of Puerto Rico, led by Abel Méndez, published a preprint proposing a natural mechanism. The team had been examining archival Arecibo Observatory data and found signals that resembled the Wow! signal in every respect except brightness. These corresponded to the locations of cold clouds of neutral hydrogen.
Their hypothesis: a sudden, intense burst of radiation from a source behind such a cloud, for example a magnetar flare or a soft gamma repeater, stimulated maser-like emission from the hydrogen in the cloud, causing it to brighten sharply at 1420 MHz for a short period. The alignment required is rare, which would explain why the event has never repeated.
Two things are worth noting about the standing of this hypothesis. It is a preprint, and it explains the signal's character rather than proving its cause; the specific source has not been identified. And it has drawn scepticism: some researchers doubt the case is closed. Méndez himself had spent years assuming the signal was an instrumental artefact before the Arecibo data changed his view.
A follow-up paper, Arecibo Wow! II (2025), reanalysed the original Ohio SETI data with modern methods and revised the signal's parameters, reporting a higher peak flux density, a position roughly seven arcminutes off the accepted right ascension, and a different frequency. The revision matters: the signal's basic properties are still being refined nearly fifty years on.
Comets (dismissed). In 2017, Antonio Paris proposed that hydrogen clouds surrounding the comets 266P/Christensen and 335P/Gibbs were the source. The hypothesis was rejected by astronomers, including members of the original Big Ear team, on several grounds: the comets were not in the beam at the relevant time; comets do not emit strongly at the frequencies involved; and the hypothesis offers no account of why the signal appeared in one feed horn and not the other.
Terrestrial interference (not supported). Earth-based interference reflected off space debris or a satellite has been raised repeatedly. It struggles with the observation that the signal's intensity profile tracked the telescope's beam pattern precisely, which is the signature of a source on the sky rather than a local one.
Extraterrestrial transmission (untested, and untestable on current evidence). The signal's narrow bandwidth and its proximity to the hydrogen line, long argued to be the frequency an interstellar broadcaster would choose, are the reasons it has held attention for five decades. But a single non-repeating detection cannot establish an artificial origin, and the registry records this as a hypothesis that the evidence permits rather than one the evidence supports.
What the popular version gets wrong
"The signal lasted 72 seconds, then stopped." No. The signal was in Big Ear's beam for 72 seconds. The telescope then stopped looking at that piece of sky. The source may have been transmitting for a moment, or for a century. The record contains no information on the point.
"The Wow! signal has never been decoded." There is nothing to decode. 6EQUJ5 records signal strength. Presenting it as an undeciphered message is the single most common error in coverage of this case.
"Scientists found it was aliens" or "scientists proved it wasn't aliens." Neither. The 2024 hypothesis proposes a natural mechanism that fits the data; it has not been confirmed, and the source has not been located.
"Big Ear picked up an alien broadcast and NASA covered it up." Big Ear was a university instrument. Ehman published, the printout is public, and the case has been openly worked on for five decades, which is precisely why it remains famous.
Current status
Unsolved. The registry does not classify the Wow! signal as explained. A well-argued natural mechanism now exists and is the leading candidate; it remains an unconfirmed preprint hypothesis, the emitting source has not been identified, and the signal's own measured properties were being revised as recently as 2025.
What is settled: something real was detected, from the sky, once. What is not settled: what it was.
Sources
- Ehman, J. R. Big Ear Radio Observatory printout and observation records, 15 August 1977.
- Méndez, A. et al. (2024). "Arecibo Wow! I: An Astrophysical Explanation for the Wow! Signal." arXiv:2408.08513 (preprint).
- Méndez, A. et al. (2025). "Arecibo Wow! II: Revised Properties of the Wow! Signal from Archival Ohio SETI Data."
- Scientific American (2024). "The Wow! Signal SETI Mystery Might at Last Be Solved."
- Smithsonian Magazine (August 2024). Coverage of the Arecibo Wow! hypothesis.
- Paris, A. (2017). "Hydrogen Clouds from Comets 266P/Christensen and P/2008 Y2 (Gibbs) are Candidates for the Source of the 1977 Wow! Signal." arXiv:1706.04642, and subsequent rebuttals from the Big Ear team.
- North American AstroPhysical Observatory (NAAPO), Big Ear programme records.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.