The Detection at Big Ear Observatory
On the night of August 15, 1977, the Big Ear radio telescope at Ohio State University recorded a signal that would become the most famous event in SETI history. The telescope was scanning the sky as part of a SETI survey when it picked up an extraordinarily strong narrowband radio signal at 1420.4556 MHz, very close to the hydrogen line frequency that scientists had long considered the most logical channel for interstellar communication.
Volunteer astronomer Jerry Ehman was reviewing the computer printout of the previous night's observations several days later when he noticed the anomalous signal. The data showed a sequence of characters "6EQUJ5" representing a signal that rose and fell in intensity over 72 seconds, exactly matching the pattern expected from a point source moving through the telescope's beam as Earth rotated. Astonished by the strength and characteristics of the signal, Ehman circled it in red pen and wrote "Wow!" in the margin, giving the detection its enduring name.
Analyzing the Signal Characteristics
The Wow! signal had several properties that made it an exceptional SETI candidate. It was observed at approximately 1420 MHz, the emission frequency of neutral hydrogen, which is significant because this frequency is universally recognized and would be known to any scientifically literate civilization. The signal was narrowband, less than 10 kHz wide, which is characteristic of an artificial transmission rather than natural astrophysical sources that typically emit across broad frequency ranges.
At its peak, the signal was approximately 30 times stronger than the background noise, making it by far the most powerful signal ever detected in a SETI survey. The signal's 72-second duration precisely matched the time it would take a celestial source to pass through Big Ear's fixed beam, ruling out local terrestrial interference which would not show this characteristic rise and fall pattern. The signal appeared to originate from the direction of the constellation Sagittarius, roughly in the direction of the Chi Sagittarii star group.
Attempts to Relocate the Signal
Despite more than 100 follow-up observations of the same region of sky using Big Ear and other radio telescopes, the Wow! signal has never been detected again. Robert Gray conducted extensive searches using the META array at Oak Ridge Observatory and later the Very Large Array in New Mexico, targeting the signal's location with far greater sensitivity than the original detection, but found nothing. The failure to relocate the signal remains its most puzzling and frustrating aspect.
The non-repeatability does not necessarily rule out an extraterrestrial origin. A deliberate alien transmission might be a brief, one-time broadcast or a rotating beacon that sweeps past Earth only occasionally. The original Big Ear telescope had two feed horns that observed the same patch of sky about three minutes apart, but the signal appeared in only one of them, adding another layer of mystery. If the signal were truly from a distant source, it should have appeared in both horns unless it was transmitted for less than three minutes or was somehow transient in nature.
Proposed Explanations Natural and Alien
Several natural and terrestrial explanations have been proposed for the Wow! signal. In 2017, astronomer Antonio Paris suggested that hydrogen clouds surrounding two comets, 266P/Christensen and 335P/Gibbs, which were near the signal's position in 1977, could have produced the emission. However, this hypothesis was widely criticized by other astronomers who noted that comets do not produce signals of the observed intensity and narrowband character. Satellite transmissions and aircraft reflections have also been considered but fail to match the signal's specific characteristics.
The extraterrestrial hypothesis remains viable precisely because no natural explanation has been widely accepted. If the signal was produced by an intelligent civilization, it could have been an intentional beacon, a leaked transmission, or even an interstellar communication not directed at Earth. After nearly five decades, the Wow! signal remains an open question in SETI science. It stands as a tantalizing reminder that a confirmed detection could come at any time, and as a powerful motivator for continued investment in radio SETI observations with increasingly sensitive instruments.
Frequently asked questions
Where was the Wow! signal detected?
At the Big Ear radio telescope of Ohio State University, located in Delaware, Ohio, which was then running a SETI survey. The telescope was dismantled in 1998.
When did the Wow! signal happen?
At 10:16 p.m. Eastern time on August 15, 1977. Astronomer Jerry Ehman found it days later while reviewing the computer printout and circled the sequence "6EQUJ5," writing "Wow!" in the margin.
What is 1420 MHz?
The frequency of the hydrogen line, the radio emission of neutral hydrogen at a wavelength of 21 centimeters. SETI searches watch that band because any technological civilization would know it, and the Wow! signal was recorded right beside it, at about 1420.456 MHz.
Has the Wow! signal been explained?
Not conclusively. A 2017 proposal that it was hydrogen from passing comets was rejected by the original team. A 2024 analysis led by Abel Méndez suggested a rare natural event, a maser-like flare from a cold hydrogen cloud, as a possibility. The signal was never detected again despite repeated searches, which is the strongest argument against a deliberate transmission.