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The famous 72-second cosmic radio signal that was never heard again

On August 15, 1977, Ohio State University's Big Ear radio telescope picked up an intense narrowband radio signal lasting 72 seconds from the direction of Sagittarius. Astronomer Jerry Ehman was so stunned by how closely it matched the expected signature of an interstellar broadcast that he circled '6EQUJ5' on the printout and scribbled 'Wow!'. Despite decades of searching, the signal has never repeated.

The Mechanical Sweep of the Big Ear

The Ohio State University Radio Observatory, popularly known as Big Ear, was situated on the grounds of the Perkins Observatory in Delaware, Ohio. Designed by Dr. John Kraus and constructed across the 1950s and 1960s, Big Ear was not a steerable dish that could pivot toward arbitrary points in the sky. Instead, it was an enormous transit telescope roughly the size of three football fields. It relied on a large flat reflector directing radio waves toward a curved parabolic reflector, which then focused the incoming energy into feed horns resting on the ground. Because the telescope remained fixed along the meridian, it depended on the rotation of the Earth to sweep its receptive beams across the celestial sphere.

On the night of August 15, 1977, Big Ear was conducting routine survey work for its ongoing Search for Extraterrestrial Intelligence (SETI) program, an effort initiated under Kraus and software director Bob Dixon. As the Earth turned eastward, celestial objects drifted across the antenna's reception pattern at a steady, predictable rate. Big Ear’s beam had an antenna pattern with a half-power width of roughly eight arcminutes. At the declination being observed, any celestial source stationary against the distant background stars would take precisely 72 seconds to sweep through the telescope's field of view.

During those 72 seconds, an extraterrestrial radio source would not appear instantly at full power. Instead, the geometry of Big Ear’s antenna beam meant a point source would register as a gradual rise and fall in intensity. It would require 36 seconds for the source to move from the outer boundary to the center of the beam, reaching its peak intensity, followed by 36 seconds of symmetrical decline as the source departed. This 72-second bell-shaped profile was the unmistakable physical signature of an object originating far beyond Earth's atmosphere.

Translating the Code: What '6EQUJ5' Actually Meant

Data collection at Big Ear was managed by an IBM 1130 computing system. Because digital storage in the late 1970s was extraordinarily constrained, the computer could not retain raw audio waveforms or high-density time-series data. Instead, the incoming analog signals were split into 50 discrete channels, each 10 kilohertz wide. Every 12 seconds, the computer sampled the signal strength in each channel: ten seconds of active integration followed by two seconds of internal processing. To display this information economically, Bob Dixon designed a character-mapping scheme printed onto continuous-feed paper.

The system tracked signal strength as an integer multiple of the baseline background noise, known as the signal-to-noise ratio. A blank space on the page represented baseline noise between zero and one standard deviation. The digits 1 through 9 represented signal levels from one to nine standard deviations above the baseline. When signal strength surpassed nine standard deviations, the software switched to the Latin alphabet: 'A' denoted a strength between 10 and 11, 'B' between 11 and 12, and so forth. The scale climbed all the way to 'Z', which would have represented an off-scale value between 35 and 36.

A few days after the August 15 recording, astronomer Jerry R. Ehman, a volunteer researcher on the project, was reviewing the thick stack of computer printouts. In channel two, at approximately 22:16 local time, he encountered the alphanumeric sequence '6EQUJ5'. The signal had surged from a '6' (six standard deviations above noise) to 'E' (14), then 'Q' (26), peaking at 'U'—an astonishing intensity between 30 and 31 standard deviations above the cosmic background. It then fell symmetrically to 'J' (19) and '5' (5) before disappearing into background noise. Stunned by the flawless match to the expected 72-second antenna transit curve, Ehman circled the characters in red pen and penned the exclamation 'Wow!' in the margin.

The famous 72-second cosmic radio signal that was never heard again

The Mechanical Sweep of the Big Ear

Connects to

Translating the Code: What '6EQUJ5' Actually Meant

The Hydrogen Line and the Search for Artificial Beacons

The Dual-Horn Mystery and Celestial Coordinates

Investigating Earthly Interference and Natural Candidates

The Vanishing Beacon and the Scintillation Theory

Key takeaways

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