On August 17, 1977, Jerry Ehman sat down with a stack of computer printouts.
He was a volunteer at the Big Ear Radio Observatory at Ohio State University in Delaware, Ohio, a largely volunteer-staffed operation that had been scanning the sky for radio signals from extraterrestrial intelligence since 1973 on a NASA grant of $100,000 per year. The work was routine: review the printouts produced by the telescope’s automated data collection system, look for anything unusual, file the results. The observatory’s computers were too primitive to process data in real time, so printouts accumulated over several days before anyone looked at them.
What Ehman found in that stack of papers was a sequence of characters: 6EQUJ5.
The sequence represents a signal that started at low intensity, rose rapidly to a peak, and then fell away again. The convention used by the Big Ear printout system encoded signal intensity as numbers from 1 to 9 and then letters starting from B for each order of magnitude above that. The peak character in the sequence, U, represents an intensity approximately 30 times above the background noise level. The pattern, a smooth rise and fall over the 72-second window during which a fixed sky position passed over the telescope’s field of view, was exactly the pattern that SETI astronomers had theorized an extraterrestrial transmission would produce as the rotating Earth carried the telescope beam across a distant transmitting source.
Ehman circled the sequence and wrote Wow! in red pen.
He then looked at the data for the following days and found nothing. The signal did not repeat. It had appeared in one of the 50 possible frequency channels being monitored, specifically the channel closest to 1420 MHz, the frequency of hydrogen emission that SETI theorists had identified as the most likely carrier frequency for an intelligent interstellar transmission because any technically sophisticated civilization capable of building a transmitter would know that hydrogen is the most abundant element in the universe and that 1420 MHz is therefore a logical meeting point for communication.
The specific frequency, the specific intensity profile, the specific narrowband character of the signal, and its appearance in the direction of the Sagittarius constellation all corresponded to what a genuine SETI detection would be expected to look like.
The Big Ear telescope was demolished by Ohio State University in 1998, five years after Congress cut its NASA funding. The original printout with Ehman’s handwritten Wow! is preserved. The signal has not been satisfactorily explained in the 47 years since he wrote that word in red ink.
What the Signal Was
The Wow! signal’s specific technical characteristics are documented in the SETI literature and deserve development because they are what distinguish it from the routine radio frequency interference that the Big Ear telescope regularly filtered out.
The signal appeared at 1420.356 MHz, within the hydrogen line frequency band whose significance for interstellar communication was proposed by Philip Morrison and Giuseppe Cocconi in their 1959 Nature paper establishing the theoretical foundation of the SETI field. The hydrogen line at 1420.405 MHz is protected from terrestrial radio transmission by international agreement specifically because it is recognized as scientifically important. A signal appearing at this frequency is not commercial or military radio interference.
The signal was narrowband, appearing in a single channel out of the 50 being monitored simultaneously. Natural astrophysical radio sources, including pulsars, quasars, and star-forming regions, typically emit across broad frequency ranges. A narrowband signal concentrated at a single frequency is not a natural astrophysical source’s expected signature. It is, however, the expected signature of a deliberately transmitted signal, which by engineering necessity would concentrate its power in a narrow frequency band to maximize range.
The signal showed no frequency drift. A signal from a moving source, such as a satellite, an aircraft, or a rotating planet, would show Doppler frequency shift as the source’s velocity relative to the receiver changed. The Wow! signal showed no such drift, which is consistent with a source at interstellar distance whose motion relative to Earth produces a negligible Doppler shift within the 72-second observation window.
The signal appeared in only one of the Big Ear telescope’s two feed horns, which were separated by approximately 3 minutes in their sweep of the sky. If the signal had been a local interference source, it would likely have appeared in both horns in rapid succession. The single-horn detection is consistent with a brief, non-repeating signal from a fixed sky position, and inconsistent with most local interference scenarios.
Taken together, these characteristics produced Ehman’s specific conclusion: the signal had the potential to be a sign of extraterrestrial intelligence. This is the assessment of the trained astronomer who first saw the data, and it remains the most honest available summary of what the signal’s characteristics indicate.
The Search That Followed
The Big Ear team’s response to the Wow! signal was systematic and extended. They returned to the same region of sky repeatedly over the following years, monitoring the position in the Sagittarius constellation where the signal had originated.
The telescope observed the Wow! source region continuously for one month following the detection. Nothing was found. A year later, another extended observation produced the same result. Over the Big Ear SETI program’s 24-year lifespan, which was the longest continuous SETI search in history until the program was defunded, the direction of the Wow! signal was monitored on multiple subsequent occasions.
The signal was never detected again.
Multiple other observatories have pointed instruments at the Wow! signal’s sky position following its documentation. The SETI Institute’s Allen Telescope Array in California, a dedicated SETI observatory whose computing power allows real-time signal processing and immediate follow-up, has monitored the region without finding a repetition.
Robert Gray and Simon Ellingsen conducted a specific targeted search of the Wow! signal region in 1999 and 2000 using the 26-meter radio telescope at the University of Tasmania’s Mount Pleasant Observatory. They found no evidence of a repeating signal at the Wow! frequency or at related frequencies. Their negative result was published in the Astrophysical Journal.
Whether the absence of repetition is evidence against the Wow! signal representing an extraterrestrial transmission depends on what assumptions are made about the nature of the transmission. A beacon deliberately designed to attract attention might transmit continuously, in which case the failure to re-detect it suggests the original detection was anomalous. A directed transmission aimed at Earth during a specific observation window, or a signal from a transmitter that was not designed as a permanent beacon, might appear only once from Earth’s perspective.
The single detection is the specific evidentiary ambiguity that has prevented the Wow! signal from being either confirmed as an extraterrestrial contact or definitively explained as something else.
The Antonio Paris Comet Hypothesis
The most recent mainstream explanation proposed for the Wow! signal is the comet hypothesis published by Antonio Paris of St. Petersburg College in a 2016 paper in the Journal of the Washington Academy of Sciences.
Paris proposed that the signal was produced by hydrogen emission from comets 266P/Christensen and P/2008 Y2, which he calculated were in the vicinity of the Wow! signal’s sky position in August 1977. Comets emit hydrogen from the photodissociation of water in their comas, and the hydrogen emission could in principle produce a radio signal near the 1420 MHz hydrogen line frequency.
The Paris hypothesis attracted significant critical attention from SETI researchers who found it inadequate to explain the signal’s specific characteristics. The primary objections are documented in published responses:
The signal’s intensity. The Wow! signal’s intensity was approximately 30 times above background noise, which is unusually strong for a cometary hydrogen emission at any documented comet’s distance from Earth. No known comet has been documented to produce a radio emission of the Wow! signal’s observed intensity.
The narrowband character. Cometary hydrogen emission produces broad spectral emission, not the narrowband signal concentrated in a single frequency channel that the Wow! data shows. The spectral characteristics of the Wow! signal are inconsistent with the expected spectral profile of cometary hydrogen emission.
The Big Ear team’s contemporaneous comet checking. Ehman and his colleagues documented that they checked for known solar system objects in the signal’s direction as part of their immediate follow-up investigation. No known comets were documented in the relevant sky position.
Paris attempted to test his hypothesis by observing comet 266P/Christensen when it returned to the Wow! signal region in 2017. The results he published claimed to show hydrogen emission from the comet at the relevant frequency. Independent replication of his 2017 observations has not been published in a form that confirms his specific findings at the sensitivity level required to make cometary emission a plausible explanation for the original Wow! signal’s intensity.
Whether Paris’s comet hypothesis adequately explains the Wow! signal is contested in the published literature. Seth Shostak of the SETI Institute expressed skepticism. Multiple independent SETI researchers have published critiques of the specific characteristics that the comet hypothesis cannot account for.
The comet hypothesis is documented. Its adequacy is contested. The signal remains unexplained.
The Arecibo Context
The Wow! signal arrived in August 1977, three years after the Arecibo Message was transmitted on November 16, 1974.
The Arecibo Message was a deliberate interstellar radio transmission designed by Frank Drake and Carl Sagan and broadcast at 2380 MHz toward the globular cluster M13 in the Hercules constellation using the Arecibo Observatory in Puerto Rico. The message encoded information about human biology, the solar system, the Arecibo telescope itself, and the structure of DNA in a 1,679-bit binary string whose specific length, the product of two prime numbers 23 and 73, was chosen to make its intended 23-by-73 bitmap format recognizable to any recipient capable of factoring the transmission length.
Whether the Wow! signal three years later represents a response to the Arecibo Message is physically impossible as a direct connection: M13 is approximately 25,000 light years from Earth, and a response traveling at the speed of light from M13 would not arrive for 25,000 years. The Sagittarius constellation direction of the Wow! signal does not correspond to M13’s position. The timing and direction eliminate any direct causal connection between the Arecibo Message and the Wow! signal.
The coincidence of timing, three years between humanity’s first deliberate interstellar transmission and the most anomalous SETI detection in the program’s history, is noted in the SETI literature as coincidental rather than causal.
Whether a civilization monitoring Earth’s radio emissions would have detected the Arecibo Message, identified it as a deliberate transmission, located its source, and transmitted a response in the direction of Earth within three years is a different question from the direct connection hypothesis. Any civilization monitoring Earth’s radio environment would have been aware of terrestrial radio transmissions since the early twentieth century, and the Arecibo Message was not the first deliberate transmission from Earth into space. Whether the Wow! signal’s specific timing is meaningful in this broader context is a question the available evidence cannot answer.
The Big Ear and What Was Lost
The Big Ear Radio Observatory’s history is worth developing because the specific circumstances of its funding and demolition provide the institutional context for understanding why the Wow! signal remains unresolved.
The telescope was designed by John Kraus, a pioneering radio astronomer at Ohio State University, and was built primarily by volunteers and university staff in the 1960s with National Science Foundation funding. Its specific design, a large flat reflector with two fixed feed horns rather than the steerable dish of most radio telescopes, made it economical to build and operate but limited its ability to follow up on individual detections in real time.
The SETI program at Big Ear ran from 1973 to 1995 on a budget of approximately $100,000 per year from NASA, supplemented by volunteer labor whose total contribution to the program is documented as thousands of person-hours over more than two decades. It was, as Ehman described it, a shoestring operation whose specific constraints, no real-time processing, no rapid follow-up capability, a limited frequency range, directly shaped the way the Wow! signal was detected and the limitations of the subsequent investigation.
Congress eliminated NASA’s SETI funding in 1993, citing the program’s cost and the speculative nature of its objective. The specific congressional vote that ended Big Ear’s funding has been documented in the congressional record and has been cited repeatedly in subsequent discussions about the institutional barriers to SETI research. Big Ear lost its $100,000 annual NASA grant and a separate $50,000 earmarked for an upgrade that would have given it real-time processing capability.
Without funding, Big Ear continued operating on a reduced basis until 1997, when Ohio State University sold the land on which it stood to a real estate developer for a golf course. The telescope was demolished in 1998.
The specific instrument that detected the Wow! signal no longer exists. The sky region where the signal originated has been monitored by subsequent instruments, none of which have detected a repetition. Whether a real-time capable instrument at Big Ear’s sensitivity would have been able to characterize the Wow! signal’s properties more precisely if the 1993 funding had continued is a counterfactual that Ehman himself has addressed: he has stated that the specific upgrade that was planned before the funding was cut would have given the telescope the capability to immediately re-examine any anomalous detection in real time rather than waiting days for the printout review.
The telescope that could have answered the question was demolished. The printout that preserves the question is in an archive.
What 47 Years of Investigation Has Established
The Wow! signal investigation’s documented record across 47 years establishes a specific set of conclusions and a specific residue of unresolved questions.
What is established: the signal was detected on August 15, 1977 by the Big Ear telescope at 1420 MHz. Its intensity, narrowband character, frequency stability, and single-horn detection are all documented. The signal matches the theoretical profile expected for an extraterrestrial transmission. It has not been detected again in 47 years of subsequent monitoring. The comet hypothesis proposed by Paris in 2016 is contested by published critiques that identify specific characteristics the hypothesis cannot account for. No other natural explanation has been proposed that satisfactorily addresses all the signal’s documented characteristics.
What is not established: the signal’s origin, whether it was intentionally transmitted, whether it originated from outside the solar system, and whether its non-repetition reflects the absence of a repeating source or the specific observational constraints of the instruments that have searched for it.
Seth Shostak’s analogy is the most honest available summary: it is like hearing chains rattling in an attic and never hearing them again. The absence of repetition prevents confirmation. The absence of an alternative explanation prevents dismissal.
Jerry Ehman, who first saw the data and wrote Wow! in the margin of a printout on August 17, 1977, has maintained throughout the subsequent decades that no conclusion was possible except that the signal had the potential to be a sign of extraterrestrial intelligence.
He wrote that word in red pen 47 years ago. He has never retracted it.
The printout is still in the archive. The signal is still unexplained. Whatever produced 6EQUJ5 in the direction of Sagittarius on August 15, 1977 has not repeated itself in the 47 years since.
Whether it was waiting for a response, whether it was a one-time transmission whose source has moved on, whether it was a natural phenomenon whose specific character has simply not been reproduced in subsequent observations, or whether it was the most significant 72 seconds in the history of human observation, is the question that the documented evidence leaves permanently open.
The answer, if there is one, is in the direction of Sagittarius.
