Thanks to NASA's Kepler mission, astronomers have identified more than 1,000 Earth-size planets in our tiny corner of the Milky Way galaxy alone [source: NASA]. Multiply those 1,000 potentially inhabitable planets by the vastness of space and time and it seems inconceivable that we are the only intelligent life forms in creation.
Are we alone in the universe? This tantalizing question is what launched the first serious scientific search for extraterrestrial intelligence (SETI) in the 1970s. In 1973, the Ohio State University Radio Observatory — known as "Big Ear" — began scanning the skies for the slightest traces of extraterrestrial transmissions, blips in the deafening silence of space [source: Kawa]. And on an August night in 1977, the first astonishing signal they received wasn't a whimper, but a roar.
Jerry Ehman was an Ohio State professor volunteering with the Big Ear SETI experiment that summer in 1977 [source: Krulwich]. Every couple of days, a bike messenger would arrive at Ehman's office with a pile of printouts generated by the telescope's mainframe computer. Ehman's thankless job was to scan the mind-numbing numbers for anomalies, anything that stood out from the constant low hum of background radiation.
On Aug. 18, 1977, Ehman was scanning readouts from three days earlier when he came across something radically different. Instead of the usual 1s and 2s and occasional 4s, there was a stream of both letters and numbers signaling a radio transmission 30 times louder than the background buzz of deep space [source: Krulwich]. Grabbing a red pen — he's a teacher, after all — Ehman circled the mysterious sequence "6EQUJ5" and excitedly scribbled next to it the single word "Wow!"
More than 35 years later, the so-called "Wow!" signal remains the "closest encounter" mankind has ever had with what may or may not be an alien species. The powerful blast of radio waves only lasted 72 seconds, but many astronomers and amateur UFOlogists believe the unique characteristics of the signal point to a celestial origin [source: Kiger]. In the three decades since that original wow moment, no one has been able to replicate the signal or identify its definitive source, cosmic or earthly.
The "Wow!" signal is one of the greatest unsolved mysteries of modern astronomy. Keep reading for more tantalizing clues to its true origin and to learn why this type of transmission is exactly what an alien life-form would use to communicate across the chasm of space.