The Wow! signal was a strong radio signal with a narrow range of frequencies detected on August 15, 1977, by the Big Ear radio telescope at Ohio State University in the United States. This telescope was used to search for signs of life beyond Earth. The signal appeared to come from the direction of the constellation Sagittarius and showed characteristics that scientists expected from a signal of extraterrestrial origin.
Astronomer Jerry R. Ehman found the signal a few days later while reviewing data from the telescope. On the printed record, he circled the number "6EQUJ5," which showed the signal's strength, and wrote "Wow!" next to it. This is how the signal got its name.
The signal lasted for the full 72-second time window during which the Big Ear telescope could observe it. Many searches and ideas have been considered, such as reflections from space debris, interstellar scintillation, and hydrogen clouds from comets. However, the signal has not been seen again, and no explanation has been confirmed. Some scientists think it might have come from an extraterrestrial source, but its one-time appearance and lack of repetition make this idea uncertain. The Wow! signal has led to focused searches, discussions about rare space events, and mentions in popular culture.
Background
In 1959, scientists Philip Morrison and Giuseppe Cocconi from Cornell University suggested that advanced alien civilizations might use radio signals to communicate. They believed these signals could be sent at a frequency of 1420 megahertz, which is naturally produced by hydrogen, the most common element in the universe. Since hydrogen is found everywhere, scientists think this frequency might be understood by all advanced civilizations.
In 1973, Ohio State University assigned its radio telescope, called "Big Ear," to search for signs of extraterrestrial intelligence (SETI). This was the longest-running SETI program at the time. The telescope was located near Perkins Observatory on the campus of Ohio Wesleyan University in Delaware, Ohio.
By 1977, a man named Ehman worked as a helper at the SETI project. His job was to examine data collected by an IBM 1130 computer and printed on paper. While reviewing data from August 15 at 22:16 EST (23:16 EDT, 03:16 UTC), Ehman noticed unusual numbers showing signal strength and frequency. These results surprised him and his coworkers. The observatory’s director later described the event in detail in technical reports.
Signal measurement
The string 6EQUJ5 is often misunderstood as a hidden message in a radio signal. In reality, it represents changes in the signal's strength over time, measured using a specific system for the experiment. The signal itself was a steady wave without changes in pattern. 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 using a method called signal-to-noise ratio. This compares the signal to background noise, which was averaged over the previous few minutes. The signal was recorded for 10 seconds, then processed by a computer in 2 seconds. Each frequency channel’s result was printed as one letter or number, showing the average signal strength over 10 seconds minus the baseline. This value was expressed as a number without units, based on how many times the signal’s standard deviation it was above the baseline.
On this scale, a space character means the signal was between 0 and 1 standard deviation above the baseline. Numbers 1 through 9 represent signal strengths from 1 to 9 standard deviations above the baseline. Intensities of 10 or more were shown with letters: "A" meant 10 to 11 standard deviations, "B" meant 11 to 12, and so on. The highest measured value of the Wow! signal was "U," meaning it was between 30 and 31 standard deviations above the background noise.
John Kraus, the observatory director, reported the signal’s frequency as 1420.3556 MHz in a 1994 summary for Carl Sagan. However, Ehman reported a different value in 1998: 1420.4556 MHz, with a possible error of 0.005 MHz. This frequency is 50 kHz higher than the hydrogen line frequency of 1420.4058 MHz, which would mean the source was moving toward Earth at about 10 km/s (6.2 mi/s) if the shift was due to blue-shift.
Ehman explained the difference in his paper. The telescope’s radio receiver used a local oscillator set to 1450.4056 MHz. However, a mistake in the order form caused the university to receive an oscillator set to 1450.5056 MHz, which was 0.1 MHz higher than intended. The software used in the experiment was adjusted for this error. When Ehman calculated the Wow! signal’s frequency, he included this correction.
The Wow! signal had a bandwidth less than 10 kHz. This is considered narrowband because its fractional bandwidth was very small (about 0.001%). However, compared to some astrophysical sources (like masers with 1 kHz bandwidth) or modern SETI searches (with 1 Hz resolution), the 10 kHz bandwidth is not extremely narrow. The Big Ear telescope had a receiver that measured 50 channels, each 10 kHz wide. Each channel’s data was printed as a column of letters and numbers on a computer printout. The Wow! signal was concentrated in one column.
At the time of the observation, the Big Ear telescope could only adjust its angle to look higher or lower in the sky. It relied on Earth’s rotation to scan the sky. Because of Earth’s rotation speed and the telescope’s size, it could observe any point in the sky for exactly 72 seconds. A continuous signal from space would be detected for 72 seconds, with intensity increasing for the first 36 seconds, peaking at the center of the observation window, and then decreasing as the telescope moved away. The Wow! signal followed this pattern exactly.
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, which are parts that receive signals from slightly different directions as Earth rotates. The signal was detected by one horn but not the other, and the way the data was processed makes 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, which is the angle above or below the celestial equator, was clearly determined to be as follows:
In galactic coordinates, the signal’s location is l = 11.7°, b = −18.9° for the positive horn, and l = 11.9°, b = −19.5° for the negative horn. Both positions are about 19° southeast of the galactic plane and about 24° or 25° east of the center of the Milky Way galaxy. This area of the sky is northwest of the globular cluster M55, in the constellation Sagittarius. It is roughly 2.5 degrees south of the fifth-magnitude star group Chi Sagittarii and about 3.5 degrees south of the ecliptic plane. The closest easily visible star in this region is Tau Sagittarii.
At first, no nearby Sun-like stars were known to be in the area covered by the telescope’s signal. However, in 2016, it was estimated that about six distant Sun-like stars might be within the telescope’s field of view in any direction. In 2022, a study published in the International Journal of Astrobiology identified three likely Sun-like stars within the area where the signal was detected. The best-studied star, 2MASS 19281982-2640123, is 1,800 light-years away and located only 132 light-years from a point where an intelligent civilization might be more likely to 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 this area may turn out to be similar to the Sun once more data is collected. In response to this discovery, 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. The search took place on May 21, 2022, lasting 1 hour at Green Bank, 35 minutes at 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 Earth's atmosphere—might explain the signal, but this does not rule out the possibility that the signal had an artificial origin. 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 sensitivity limit due to interstellar scintillation is very low. Other possible explanations include a rotating source that emits light 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 an Earth-sourced signal that simply got reflected off a piece of space debris." Later, he reconsidered his initial doubt after learning that a space-based reflector would need unrealistic conditions to produce the observed signal. The signal's frequency of 1420 MHz is part of a protected spectrum: a range reserved for astronomical research where Earth-based transmissions are not allowed. However, a 2010 study found that some Earth-based signals occasionally interfere with this range. In a 1997 paper, Ehman stated he was cautious about drawing major conclusions from limited data, acknowledging the possibility that the signal came from Earth-based sources, such as military equipment. In a 2019 interview, Ehman said, "I'm convinced that the Wow! signal certainly has the potential of being the first signal from extraterrestrial intelligence."
Douglas Vakoch, president of METI, told Die Welt that any signal detected by SETI must be confirmed through repeated observations. 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 a hydrogen cloud around two comets, 266P/Christensen and 335P/Gibbs, which were later found to be in the same region of the sky, might have caused the signal. This idea was rejected by astronomers, including members of the original Big Ear research team, because the comets were not in the correct position at the time of the signal. Additionally, comets do not emit strong signals at the relevant frequencies, and there is no explanation for why the signal would appear in one telescope beam but not the other.
Searches for recurrence of the signal
Ehman and other astronomers tried many times to find and understand the signal. The signal was expected to appear three minutes apart in each of the telescope's feed horns, but it did not happen. Ehman looked for the signal again using Big Ear in the months after it was first found, but he did not find it again.
In 1987 and 1989, Robert H. Gray looked for the signal using the META array at Oak Ridge Observatory, but he did not find it. In July 1995, H. Paul Shuch, the executive director of the SETI League, tested new signal detection software for a project called Argus. He used a 12-meter radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, to search the coordinates of the Wow! signal. He did not find any evidence of the signal.
In 1995 and 1996, Gray searched for the signal again using the Very Large Array, a telescope much more sensitive than Big Ear. 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 they did not find anything similar to the Wow! signal.
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 promotion related to one of its TV shows. The tweets used the hashtag "#ChasingUFOs." The promotion 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 sent the message toward one of the stars. It is not known how many tweets were successfully sent. None of the stars targeted in this effort are located in the area where the Wow! signal was originally detected. To help any possible extraterrestrial life recognize the message as intentional, scientists added a repeating pattern at the beginning of each message.
In popular culture
The signal appeared in the 2024 TV series 3 Body Problem. In this show, the signal is found in Inner Mongolia. The signal was also mentioned in the 1994 Season 2 premiere of The X-Files, called Little Green Men.
The English progressive rock band Muse's tenth album, The Wow! Signal, is set to release on June 26, 2026.