The Wow! signal was a powerful radio signal detected on August 15, 1977, by Ohio State University’s Big Ear radio telescope in the United States. This telescope was used to search for signs of intelligent life beyond Earth. The signal seemed to come from the direction of the constellation Sagittarius and showed signs that might suggest an extraterrestrial origin.
Astronomer Jerry R. Ehman discovered the unusual pattern in the data a few days later. On the computer printout, he circled the signal’s strength, labeled "6EQUJ5," and wrote the word "Wow!" next to it. This led to the signal being called the "Wow! signal."
The signal lasted for the full 72-second time window during which Big Ear could observe it. Many scientists have looked for the signal again and considered possible explanations, such as reflections from space debris, flickering light in space, and hydrogen clouds from comets. However, the signal has never been seen again, and no explanation from Earth or other sources has been confirmed. While some scientists think it might have been a message from an alien civilization, its single occurrence and lack of repetition make this idea uncertain. The Wow! signal has led to focused searches, scientific discussions about rare space events, and mentions in popular culture.
Background
In 1959, scientists Philip Morrison and Giuseppe Cocconi from Cornell University suggested that any advanced alien civilization trying to send radio messages might use the frequency of 1420 megahertz. This frequency is naturally produced by hydrogen, the most common element in the universe, and likely would be recognized by other advanced civilizations.
In 1973, after studying many radio signals from outside our galaxy, Ohio State University assigned its now-closed Radio Observatory, called "Big Ear," to search for signs of intelligent life beyond Earth. This was the longest-running project of its kind. The telescope was located near Perkins Observatory on the campus of Ohio Wesleyan University in Delaware, Ohio.
By 1977, a researcher named Ehman was helping with the SETI project. His task was to examine data printed on paper by an IBM 1130 computer. While reviewing data collected on August 15 at 22:16 EST (23:16 EDT, 03:16 UTC), Ehman noticed unusual patterns of signal strength and frequency that surprised him and his team. The observatory’s director later recorded details about this event in technical reports.
Signal measurement
The string 6EQUJ5 is often mistaken as a message in a radio signal, but it actually shows changes in the signal's strength over time, measured using a system specific to the experiment. The signal itself was a steady, unchanging wave, and any changes in the signal with a cycle shorter than 10 seconds or longer than 72 seconds would not have been noticed.
The signal strength was measured as the ratio of the signal to background noise, with the noise averaged over the previous few minutes. The signal was recorded for 10 seconds and then processed by a computer, which took 2 seconds. For each frequency channel, the computer printed a single letter or number to show the average signal strength over 10 seconds, compared to the baseline noise. This value was expressed as a multiple of the signal’s standard deviation, which is a way to measure how much the signal varied.
On this scale, a space character means the signal strength was between 0 and 1, or between the baseline and one standard deviation above it. Numbers 1 through 9 represented signal strengths from 1 to 9. Strengths of 10 or higher were shown with letters: "A" meant between 10 and 11, "B" between 11 and 12, and so on. The highest measured strength of the Wow! signal was "U," which meant the signal was between 30 and 31 standard deviations above the background noise.
John Kraus, the observatory director, reported a frequency of 1420.3556 MHz in a 1994 summary for Carl Sagan. However, in 1998, Ehman reported a frequency of 1420.4556 ± 0.005 MHz. This frequency is 50 ± 5 kHz above the hydrogen line value of 1420.4058 MHz, which is the frequency of hydrogen atoms without any shift due to motion. If the signal was caused by a source moving toward Earth, it would be traveling at about 10 km/s (6.2 mi/s).
Ehman explained the difference between his frequency and Kraus’s in his paper. The telescope’s radio receiver used a local oscillator set to 1450.4056 MHz. However, a mistake in the order form led to an oscillator with a frequency of 1450.5056 MHz, which was 0.1 MHz higher than intended. The software used in the experiment was adjusted to account for this error. When Ehman calculated the frequency of the Wow! signal, he included this correction.
The Wow! signal had a bandwidth of less than 10 kHz, which is considered narrowband because its range of frequencies was small compared to other signals, such as astrophysical masers (which have a bandwidth of about 1 kHz) or modern SETI searches (which have a resolution of about 1 Hz). The Big Ear telescope had a receiver that measured 50 channels, each 10 kHz wide. Each channel’s data was shown as a column of letters and numbers in the computer printout. The Wow! signal was concentrated in only one column.
At the time of the observation, the Big Ear telescope could only adjust its height above the horizon. It relied on Earth’s rotation to scan the sky. Because of Earth’s rotation and the telescope’s size, it could observe any point in the sky for exactly 72 seconds. A continuous signal from space would appear for 72 seconds, with its strength increasing for the first 36 seconds, peaking at the center of the observation window, and then decreasing as the telescope moved away. These patterns match the Wow! signal.
Celestial location
The exact position in the sky where the Wow! signal came from is unclear because of how the Big Ear telescope was built. The telescope had two feed horns, each receiving signals from slightly different directions as Earth rotated. The Wow! signal was detected in one horn but not the other, and the way the data was processed made it impossible to know which horn received the signal. This means there are two possible right ascension (RA) values for the signal’s location (shown below using two main reference systems):
The declination, however, was clearly determined to be as follows:
The galactic coordinates for the positive horn are l = 11.7°, b = −18.9°, and for the negative horn l = 11.9°, b = −19.5°. Both positions are about 19° toward the southeast of the galactic plane and about 24° or 25° east of the Galactic Center. The area of the sky in question is northwest of the globular cluster M55, in the constellation Sagittarius, roughly 2.5 degrees south of the fifth-magnitude star group Chi Sagittarii, and about 3.5 degrees south of the plane of the ecliptic. The closest easily visible star in this region is Tau Sagittarii.
At first, no nearby Sun-like stars were known to be in the telescope’s pointing area. However, in any direction, the telescope’s design would cover about six distant Sun-like stars, as estimated in 2016. In 2022, a study in the International Journal of Astrobiology identified three likely Sun-like stars within the telescope’s coordinates. The best-characterized star, 2MASS 19281982-2640123, is 1,800 light-years away and only 132 light-years from Maccone’s estimated location where an intelligent civilization might exist. The other two candidates, 2MASS 19252173-2713537 and 2MASS 19282229-2702492, were not fully studied but are still likely to be Sun-like. Additionally, 14 other cataloged stars in the same area may be similar to the Sun once more data is available. In response, Breakthrough Listen conducted the first targeted search for the Wow! signal using the Green Bank Telescope and the Allen Telescope Array of the SETI Institute. Observations occurred on May 21, 2022, lasting 1 hour from Green Bank, 35 minutes from the Allen Telescope Array, and 9 minutes and 40 seconds simultaneously. No technosignature candidates were found.
Hypotheses on the signal's origin
Interstellar scintillation of a weaker continuous signal—similar to how stars appear to twinkle in the sky—might explain the observation, but this does not rule out the possibility that the signal came from an artificial source. The Very Large Array, a much more sensitive radio telescope, did not detect the signal. The chance that the Big Ear telescope could have detected a signal below the Very Large Array’s detection limit due to interstellar scintillation is very low. Other possible explanations include a rotating object that emits a signal like a lighthouse, a signal that changes frequency over time, or a single burst of energy.
In 1994, Ehman said, “We should have seen it again when we looked for it 50 times. Something suggests it was a signal from Earth that bounced off space debris.” Later, after further research showed that a space-based reflector would need unrealistic conditions to produce the signal, Ehman slightly changed his view. The signal’s frequency of 1420 MHz is part of a protected spectrum: a special frequency range reserved for astronomy research where Earth-based transmissions are not allowed. However, a 2010 study found that some Earth-based signals sometimes interfere with this range or are transmitted illegally within it. In a 1997 paper, Ehman said he was cautious about making big conclusions based on limited data, acknowledging the possibility that the signal could have come from a military or Earth-based source. In a 2019 interview, Ehman said, “I believe the Wow! signal could be the first sign of intelligence from another world.”
Douglas Vakoch, president of METI, told Die Welt that any potential signal detected by SETI must be confirmed by being observed again. Because the Wow! signal was not confirmed, it has limited credibility.
In August 2024, the Planetary Habitability Laboratory published a preprint stating that observations from 2020 at the Arecibo Observatory suggest the Wow! signal was likely caused by a rare astrophysical event. This event involved energy from a star heating a cold hydrogen cloud, causing it to suddenly become brighter.
In 2017, Antonio Paris, an astronomy professor, proposed that hydrogen clouds around two comets, 266P/Christensen and 335P/Gibbs, which were later found to be in the same area of the sky, might have caused the Wow! signal. Scientists, including members of the original Big Ear research team, rejected this idea. The comets were not in the correct position in the telescope’s view at the time, and comets do not emit strong signals at the involved frequencies. There is also no explanation for why the signal was detected in one telescope beam but not the other.
Searches for recurrence of the signal
Ehman and other scientists tried several times to find and study the signal. They expected the signal to appear three minutes apart in each of the telescope's feed horns, but this did not happen. Ehman did not find the signal again using the Big Ear telescope in the months after the first detection.
In 1987 and 1989, Robert H. Gray looked for the signal using the META array at Oak Ridge Observatory, but no signal was found. In July 1995, H. Paul Shuch, the SETI League executive director, tested signal detection software for Project Argus. He used a 12-meter radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, to scan the coordinates of the Wow! signal. No signal was detected during this test.
In 1995 and 1996, Gray searched again for the signal using the Very Large Array, which is much more sensitive than the Big Ear telescope. In 1999, Gray and Simon Ellingsen used the 26-meter radio telescope at the University of Tasmania's Mount Pleasant Radio Observatory to look for the signal. They made six observations lasting 14 hours each near the signal's location, but nothing similar to the Wow! signal was found.
Response
In 2012, on the 35th anniversary of the Wow! signal, the Arecibo Observatory sent a digital message toward three stars: Hipparcos 34511, 33277, and 43587. The message included about 10,000 tweets collected by the National Geographic Channel for a campaign promoting a TV show, using the hashtag "#ChasingUFOs." The campaign also included short videos with spoken messages from celebrities. Robert Kerr, the former director of the Arecibo Observatory, explained that the transmitter used to send the message overheated during the first transmission and only reached one of the target stars. It is unclear how many tweets were successfully sent. None of the stars targeted in this message are in the area where the Wow! signal was originally detected. To help any potential extraterrestrial recipients recognize the message as intentional, scientists added a repeating pattern to the beginning of each message.
In popular culture
The signal appeared in the 2024 television series 3 Body Problem. In this series, the signal is also shown to be detected in Inner Mongolia. The signal was also mentioned in the first episode of season 2 of The X-Files in 1994, titled Little Green Men.
The English rock band Muse's tenth album, The Wow! Signal, is planned to come out on June 26, 2026.