On the night of August 15, 1977, a radio telescope in Ohio recorded an unusually strong, narrow radio signal. A few days later, a volunteer astronomer looking over the printouts circled a short string of characters and wrote a single word beside it: "Wow!" Nearly fifty years on, nobody has heard the signal again, and its origin is still unresolved. This article explains what was really recorded, the main explanations, and what the search for extraterrestrial intelligence (SETI) has and has not found.
The instrument was the Big Ear radio telescope at Ohio State University, which spent years scanning the sky as part of one of the first long-running SETI programs. Big Ear did not point at a single target. It stayed fixed while Earth's rotation carried each patch of sky across its view, so any given source stayed in the beam for only about 72 seconds.
The Wow! signal lasted for that entire window. Its strength rose and then fell in the way expected from a fixed source drifting through the beam, which is one reason it was taken seriously. The signal was narrow in frequency and lay close to 1420 MHz, the radio frequency at which neutral hydrogen naturally emits.
Big Ear's computer printed the signal strength as a sequence of characters, one for each 12-second measurement. Digits stood for signal-to-noise ratios of 1 to 9, and letters continued the scale upward. The sequence 6EQUJ5 therefore rises from a modest level to a peak, labelled U, of roughly 30 times the background noise, and then falls again. The astronomer who spotted it, Jerry Ehman, circled it and added his famous comment. It is a code for signal strength, not a decoded message.
Several natural and human-made explanations have been proposed. None is universally accepted.
The comet idea. In 2016, Antonio Paris proposed that a comet and the hydrogen cloud around it could have produced the signal, pointing to comet 266P/Christensen, which was not known in 1977. Other astronomers were unconvinced, noting that comets are not known to emit at that strength and that the comet's position needs to be checked against the beam. If you want to know how comets differ from asteroids, see our guide to the difference between asteroids, comets and meteors.
A natural maser flare. In 2024, a team led by Abel Méndez proposed that a cloud of hydrogen may have been briefly boosted into a natural, maser-like burst, perhaps by a flare from a magnetar. Searching archived data from the Arecibo Observatory, they reported several weaker, similar narrowband signals near the same frequency. This remains a hypothesis. It is hard to test because the original event has never repeated.
Interference. Radio telescopes pick up human-made signals from satellites, aircraft and ground transmitters. A signal found at one frequency in one beam is exactly the kind of thing interference can imitate, although the Wow! signal showed the smooth rise and fall of a source on the sky.
An artificial signal. Without a repeat, there is no evidence for it, and nothing in the data shows that anyone was sending a message.
SETI searches for evidence of technology, usually radio waves or laser pulses. The modern field is often dated to 1959, when physicists Giuseppe Cocconi and Philip Morrison argued in the journal Nature that microwaves near the hydrogen frequency were a sensible place to listen. In 1960 Frank Drake ran Project Ozma at Green Bank, West Virginia, the first modern radio search, aimed at two nearby Sun-like stars. Drake later wrote the equation described in our article on the Drake equation.
The hydrogen line is attractive because hydrogen is the most common element in the universe. Researchers look for narrowband signals, because natural sources usually spread their energy over a wide range of frequencies.
Large programs such as Breakthrough Listen, launched in 2015, have recorded enormous amounts of data from nearby stars and many other targets. So far, no confirmed technosignature has been announced. Some candidates have attracted attention and then faded. A well-known example is BLC1, a narrowband signal seen in the direction of Proxima Centauri in 2019. A 2021 analysis published in Nature Astronomy concluded that it was most likely human-made interference.
That string of non-detections is informative but not conclusive. The searches have covered only a small part of the possible frequencies, directions and times. The silence is part of what drives the question in our article on the Fermi paradox.
Was the Wow! signal a message? There is no evidence of that. It was a single burst without any detectable content.
Could it come back? Possibly, but astronomers have looked repeatedly and have not seen it. A natural one-off event would not be expected to return on a schedule.
Does it have anything to do with Apophis? No. Apophis is an asteroid whose approach in 2029 is understood well. Learn more in our Apophis guide.
For primary information, see: