On Aug. 15, 1977, a telescope at Ohio State University detected a strong narrowband radio signal in the constellation Sagittarius — one so unusual that astronomer Jerry Ehman marked the printout with an exclamation.

A source at this distance using a transmitting dish as big as the Arecibo radio telescope (the largest in the world) would require a 2.2 gigawatt transmitter-extraordinarily powerful but not out of the question. Another possibility is that a much weaker signal from a more remote source may have experienced gravitational microlensing by an intervening star. More than 100 subsequent searches of the same region of sky have failed to recover the signal. However, this is not surprising. A telescope such as the Big Ear listens to only about one millionth of the sky at a time and a similar dish on another planet would broadcast to only about one millionth of the sky. The chance of an alignment between receiver and transmitter, therefore, would only be about one in a trillion. (Encyclopedia of Science)

“Either the Wow! signal was the intercepted radiation from another civilization, or it’s a previously undiscovered astrophysical phenomenon. Either possibility is mind-boggling.” – Paul Shuch of the SETI League

Further Reading:
The Big Ear Wow! Signal What We Know and Don’t Know About It After 20 Years
Written by Dr. Jerry R. Ehman