The Wow! Signal: The 72-Second Transmission SETI Still Can't Explain

Six Characters Changed Radio Astronomy Forever

On the night of August 15, 1977, the Big Ear radio telescope at Ohio State University detected a narrowband radio signal lasting exactly 72 seconds—the maximum possible duration given the telescope's fixed-dish design and Earth's rotation. The signal was so close to what scientists expected a transmission from an extraterrestrial technological civilization to look like that astronomer Jerry Ehman, reviewing the computer printout three days later, circled the sequence of characters and wrote "Wow!" in red ink beside it. That handwritten annotation gave the signal its permanent name. No confirmed detection of the same signal has occurred in the 48 years since.

The Wow! Signal remains the strongest candidate for an extraterrestrial radio signal in the history of SETI research—despite decades of follow-up observations using far more powerful equipment. Understanding why it was so remarkable requires understanding both the search methodology in use in 1977 and the specific characteristics of the signal itself.

The Big Ear Telescope and the Search for Extraterrestrial Intelligence

The Big Ear telescope was a flat-reflector design approximately the size of three football fields, built at Ohio Wesleyan University's Perkins Observatory and operated by Ohio State University. Unlike steerable dish telescopes, Big Ear could not point—it observed a fixed arc of sky and relied on Earth's rotation to sweep different portions of the sky through its field of view. Each point in the sky passed through Big Ear's beam in approximately 72 seconds, which is why the maximum possible signal duration was exactly 72 seconds: any genuine point source would appear and disappear within that window.

By 1977, Big Ear was participating in the Ohio SETI Program, scanning the sky and recording data on computer punch cards. The search focused on frequencies near 1,420.4056 MHz—the emission frequency of neutral hydrogen—for a specific reason: hydrogen is the most abundant element in the universe, and the 1420 MHz frequency is a universal constant that any technologically advanced civilization would know. In 1959, physicists Giuseppe Cocconi and Philip Morrison had proposed in the journal Nature that 1420 MHz was the most logical channel on which to broadcast or listen for interstellar signals. SETI researchers still call this region of the radio spectrum the "cosmic watering hole."

What the Signal Looked Like

The computer output was printed as alphanumeric characters: blank spaces represented background noise below detection threshold; numbers 1–9 represented increasing signal intensity; letters A through Z continued above 9 (with A = 10 times baseline, B = 11, etc.). The Wow! Signal sequence was: 6EQUJ5.

The peak intensity of U = 30 times above baseline noise was extraordinary. The signal rose and fell smoothly over the 72-second window—exactly the pattern expected for a point source moving through the telescope beam due to Earth's rotation. Its frequency was extremely close to 1420 MHz. It was narrowband, occupying less than 10 kHz of bandwidth, which is characteristic of a technologically generated signal rather than natural astrophysical phenomena (which tend to emit across broad frequency ranges).

Why It Could Not Be Easily Explained

Every natural astronomical source known to emit radio waves at that intensity would produce a broadband signal. Hydrogen clouds, pulsars, quasars, solar bursts—none emit the extreme narrowband signal at exactly the hydrogen frequency that would be expected from an artificial beacon. The signal also came from a direction in the constellation Sagittarius, away from the galactic plane—an unusual direction for known radio sources.

• The signal was narrowband (less than 10 kHz width)—natural sources are broadband
• It peaked at the hydrogen line frequency—the "expected" interstellar communication channel
• The intensity profile matched a point source moving through the fixed beam—not ground interference, which would persist across multiple sweeps
• It originated from deep space, not from Earth orbit (satellite reflections would have produced a different pattern)
• No known spacecraft was in that region of sky at that time

Failed Attempts to Re-Detect the Signal

The absence of confirmed re-detection is the most troubling aspect of the Wow! Signal. Big Ear itself made over 100 targeted follow-up observations of the same sky coordinates and never detected the signal again. Subsequently, the Very Large Array in New Mexico, the Arecibo Observatory in Puerto Rico, and multiple other facilities directed sensitivity toward the same coordinates at or near 1420 MHz—without success.

A 2016 announcement by astronomer Antonio Paris proposed that comets 266P/Christensen and P/2008 Y2 (Gibbs) passed near the coordinates of the Wow! Signal in August 1977 and might have produced a hydrogen cloud that generated the detection. Hydrogen clouds around comets can emit at 1420 MHz. The proposal was significant enough to be tested: Paris and colleagues observed comet 266P/Christensen in 2016–2017 and reported detecting signals near the hydrogen line. However, the community reception was skeptical—multiple researchers noted methodological concerns, and the cometary hydrogen explanation struggles to account for the narrowband nature and specific intensity profile of the original signal.

The Signal's Legacy in SETI Research

The Wow! Signal remains classified as unidentified—neither confirmed extraterrestrial nor definitively explained by any natural or terrestrial mechanism. It shaped SETI methodology for decades after its detection.

• It motivated the development of the META and BETA programs at Harvard, which used more sophisticated signal verification protocols requiring re-detection before announcement
• It established 1420 MHz and adjacent frequencies as high-priority search targets for subsequent SETI programs
• It defined the standard for "candidate signal" criteria used by modern SETI researchers
• Breakthrough Listen, launched in 2015 with $100 million in funding from Yuri Milner, includes the Wow! Signal coordinates among its priority observation targets

Big Ear's End

The Big Ear telescope that detected the Wow! Signal was demolished in 1998 to make way for a golf course expansion—a fact frequently cited as one of the more jarring footnotes in the history of science. The computer printout with Ehman's original "Wow!" annotation is preserved at Ohio State University. The mystery it represents has not diminished with time; if anything, the cumulative failure to explain it through decades of increasingly sensitive follow-up observation has made it more compelling. The Wow! Signal is 72 seconds of data. It is also 48 years of unanswered questions.