In 1977, a radio telescope in Ohio picked up a 72-second signal that matched every predicted characteristic of an extraterrestrial transmission. It was never heard again. Here's where the science stands now.
The Big Ear radio telescope at Ohio State University was running a SETI survey on August 15, 1977, when it detected a narrowband signal at 1420.4556 MHz — the hydrogen line, the frequency that SETI researchers had long predicted an intelligent civilization might use to communicate, precisely because hydrogen is the most abundant element in the universe and any technologically advanced species would know it.
The signal was so striking that astronomer Jerry Ehman, reviewing the computer printout days later, circled it and wrote "Wow!" in the margin. The name stuck.
What made the Wow! Signal remarkable wasn't just its frequency. It was its intensity — 30 times stronger than the background noise — and its duration, which matched exactly what you'd expect from a signal originating from a fixed point in space as the Earth rotated and the telescope's beam swept past it. It came from the direction of the constellation Sagittarius. It had the characteristics of a signal that had traveled a very long way.
And then it was gone. Dozens of subsequent observations of the same region of sky, using the same and more powerful equipment, have detected nothing. The Wow! Signal has never been repeated.
The Spaxob archives have covered various proposed explanations over the years, and the landscape has shifted. For a long time, the leading mundane explanation was a hydrogen cloud — a natural source of hydrogen emission that could produce a signal at the right frequency. But the Wow! Signal's intensity and the way it swept through the telescope beam are difficult to explain with a natural source. Natural hydrogen emission is broadband; the Wow! Signal was narrowband.
A 2017 paper proposed that the signal came from a comet — specifically, comets 266P/Christensen and P/2008 Y2 (Gibbs), which were in the right region of sky in 1977. The hypothesis was that hydrogen gas surrounding the comets could have produced the signal. Subsequent observations of these comets at the same frequency found no such emission. The comet hypothesis is largely considered ruled out.
The signal analysis tools available in 1977 were limited by the standards of what we have now. The Big Ear telescope couldn't record the signal in real time — it was printed on paper, which is why Ehman's review came days later. Modern SETI surveys use digital backends that can record and analyze signals in real time, across much wider frequency ranges, with much greater sensitivity.
The Allen Telescope Array, the Parkes Observatory, and the Breakthrough Listen initiative have all conducted targeted searches of the Wow! Signal's origin region. Nothing matching the original signal has been detected. This is either because the signal was a one-time event — a transmission that happened to be aimed at Earth at that moment and hasn't been repeated — or because it was something we don't yet understand.
The transmission theory community has its own interpretations, some more rigorous than others. What's interesting from a scientific standpoint is that the Wow! Signal remains genuinely unexplained. It's not that we've looked at it and concluded it was natural. It's that we've looked at it and can't explain it with any natural mechanism we know of.
That's a different thing from saying it was extraterrestrial. The absence of a natural explanation isn't evidence of an artificial one. But it does mean the question is still open.
The data interpretation challenge here is fundamental: we have 72 seconds of data from a single observation, recorded on paper, from equipment that no longer exists. We can't go back and look at it differently. We can only look at the same data with new frameworks and see if anything changes.
So far, nothing has changed. The Wow! Signal is still the most compelling candidate for an extraterrestrial signal ever detected. It's also still a single data point. In science, a single data point isn't a conclusion. It's a question.
The question is still open.