A signal at the hydrogen line
On an otherwise unremarkable summer night in 1977, the Big Ear radio telescope was listening to the cosmos. Located on land belonging to Ohio Wesleyan University's Perkins Observatory, the massive instrument scanned the sky as the Earth turned. A few days later, volunteer astronomer Jerry R. Ehman was reviewing the computer printouts from August 15 when he saw something that made him grab a red pen. He circled a sequence of characters—"6EQUJ5"—and wrote a single word in the margin: "Wow!".
The signal lasted for exactly 72 seconds, the maximum time the Big Ear's design could observe any single point in the sky. Its intensity rose and fell in a perfect curve, consistent with a celestial source moving through the telescope's observation window. The signal was powerful, at one point registering an intensity 30 times higher than the background cosmic noise. Its frequency was 1420.4556 megahertz. This is almost exactly the frequency of neutral hydrogen, the most common element in the universe. Scientists Philip Morrison and Giuseppe Cocconi had theorized in 1959 that any intelligent civilization wanting to broadcast its existence would likely use this "hydrogen line" as a universal marker. The "Wow!" signal was a narrowband emission, just what astronomers expected from an artificial beacon, not the broad-spectrum noise of natural phenomena.
An enduring astronomical mystery
The Big Ear was a unique instrument, known as a Kraus-type telescope, designed by John D. Kraus. Instead of a single steerable dish, it used a massive fixed parabolic reflector measuring 110 meters by 21 meters and a large, tiltable flat reflector. Its total collecting area was equivalent to three football fields. From 1973 to 1995, it conducted the longest-running Search for Extraterrestrial Intelligence (SETI) program in history.
After the initial detection, astronomers repeatedly scanned the same patch of sky in the constellation Sagittarius where the signal originated. They never heard it again. The signal's transient nature has led to numerous alternative explanations. One hypothesis proposed that the signal came from the hydrogen cloud surrounding a passing comet, such as 266P/Christensen. However, this idea has been widely dismissed by other astronomers, including members of the original Big Ear team, who note that the comets in question were not in the correct position at the time and are unlikely to produce such a strong, focused signal.
Other theories include interstellar scintillation—a twinkling effect that could momentarily amplify a much weaker, continuous signal—or even a reflection of a terrestrial signal from space debris. None of these explanations fully account for all the signal's characteristics. The "Wow!" signal is unexplained, a solitary, 72-second whisper from the stars. The Big Ear telescope itself was dismantled in 1998 to make way for the expansion of a golf course.