Delaware, Ohio — On August 15, 1977, astronomer Jerry Ehman was reviewing printed data from Ohio State University's Big Ear radio telescope when he spotted something that made him circle the numbers in red pen and scrawl "Wow!" in the margin. Four decades and five years later, that signal remains the strongest single unexplained candidate in the history of SETI—and we still don't know what sent it.
The Wow! Signal—technically designated Chi Aquarii, using the nomenclature of the beam that detected it—was a narrowband radio emission at 1420 megahertz, the hydrogen line frequency, received for roughly 72 seconds on that August night. The signal's intensity was roughly 30 times the background noise of the sky, a signal-to-noise ratio that would command attention in any era.
"What made Wow! different was its profile," explained Robert Gray, an engineer and SETI researcher who has spent decades investigating the signal. "It arrived at one beam of the antenna array, stayed for a full rotation, and vanished. The Big Ear observed each part of the sky twice—once as the Earth rotated through that region, once as the sky itself rotated. Wow! showed up in the first pass but not the second. If it had repeated, we might have answered the question forty-five years ago."
The Peculiar Profile
The Big Ear was not a steerable telescope. Instead, it operated as a fixed parabolic reflector with a movable flat mirror that allowed it to sweep a fixed declination across the sky. Data was recorded as a continuous paper chart—a long roll that unfurled over hours of observation, creating a visual log of the radio sky.
The Wow! Signal appears on that chart as a clear bump in the noise, localized to a 20-minute window as the Big Ear's scan carried it past the signal source. The signal's intensity rose above threshold, persisted, and fell back into background. No modulation. No obvious pulsation. No hallmark of any known astrophysical object.
What made it resonate through decades of SETI literature was its specificity: a narrowband emission at the hydrogen line frequency, precisely the wavelength that physicists Carl Sagan and Frank Drake had identified as the universe's most logical "meeting place" for interstellar communication. A technological civilization seeking to be heard would transmit at 1420 megahertz. The odds of receiving a natural narrowband signal at that exact frequency seemed negligibly small.
Yet the non-repetition gnawed at researchers. SETI luminaries like Sagan himself urged caution. A genuine signal would likely repeat, allowing confirmation via independent observers. The Wow! Signal's one-time appearance suggested either instrumental artifact or something far more ephemeral.
Competing Hypotheses: The Case for and Against
The leading natural explanation, for decades, was a pulsar—a rotating neutron star whose beam sweeps across Earth once per rotation. But pulsars have telltale signatures: periodic pulses, measurable dispersion (frequency-dependent arrival time delays), and broad radio emission spectra. Wow! showed none of these. In 2020, Paris and colleagues published analysis arguing that if Wow! had been a pulsar, the Big Ear would have detected periodicity. The absence of periodicity argued against the pulsar hypothesis.
More recently, researchers have revived the cometary hypothesis: the possibility that Wow! was a bright signal from a comet or meteor ionizing Earth's upper atmosphere. Comets emit hydroxyl (OH) lines and other molecular emissions when heated by solar radiation. A particularly active comet passing through Earth's sphere of influence might produce an intense, narrowband signal detectable by radio telescopes—and would explain the non-repetition. Comets don't orbit the same spot twice.
"The cometary explanation is elegant," Gray noted, "but it requires a cometary event we haven't independently documented in historical records. No major comet was reported near the declination of Wow! in 1977."
Other researchers have proposed terrestrial interference—satellite transmissions, military radar, or ground-based transmitters reflecting off the ionosphere. The issue is that the Big Ear was shielded from direct ground transmissions and positioned to minimize such interference. The signal arrived from a direction that required either an object in near space or an extragalactic source.
The Unresolved Question
Forty-five years of follow-up observations—conducted by dozens of teams using increasingly sensitive equipment—have failed to detect the source again. The signal lies in the direction of the constellation Aquarius, between the stars Chi Aquarii and Zeta Aquarii, in a region of the sky that has been repeatedly observed by modern SETI facilities.
Silence.
"If Wow! was an artificial transmitter from an alien civilization, there are possibilities we don't like to contemplate," said Ehman in a recent interview. "Either they were broadcasting for a limited time and have since ceased. Or they were broadcasting a specific message toward Earth and have moved their beam elsewhere. Or the signal was never aimed at Earth at all—we simply received a stray lobe of their transmission."
The Wow! Signal occupies a unique place in SETI. It is not a repeating FRB or pulsar, both of which have natural explanations. It is not a false alarm or instrumental noise. It is, quite precisely, an anomaly: a signal that arrived once, from the cosmos, at the frequency most theoretically likely for intentional communication, and then vanished.
Science demands reproducibility. The Wow! Signal offers none. And yet it persists in our imagination precisely because of that absence—a moment when the universe seemed to be calling, and we lacked the means to answer back.
If you've observed something unusual in radio data, contact the SETI Institute or your local astronomy club.