The Wow! signal was a strong narrowband radio signal detected by Jerry R. Ehman on August 15, 1977, while working on a SETI project at the Big Ear radio telescope of The Ohio State University then located at Ohio Wesleyan University’s Perkins Observatory, Delaware, Ohio.

You can go over the technical details of the signal at the excellent Wikipedia entry at

http://en.wikipedia.org/wiki/Wow!_signal

The Wow! signal is SETI’s only success story. A radio signal of undisputed artificial origin was detected from deep space, at an appropriate frequency. It met all the requirements for a microwave communication from extraterrestrial intelligence–but one: It was heard only once and never again. All SETI protocols include repeatability of the observation as an absolute requirement.

Could we have possibly heard a real radio signal from ETI? The answer is…maybe. For a true SETI discovery to be announced as such, it must be observed independently by different observers, which implies it must transmit for a sufficiently long time that other observatories can be notified and make their own observations. Alternatively, the signal must periodically repeat so someone else has a chance to observe it, and so that scientists can systematically eliminate all possible non-SETI origins of the signal.

Although it appears the Wow! is probably an artificial signal of extraterrestrial origin, we simply can’t say for sure without further study. It might even be a hoax, a diabolical prank devised by radio astronomers or others with the technical expertise and access to equipment to fake the observation. Radio telescopes are so sensitive that any transmitter located nearby would be picked up as if it came from wherever the telescope was pointing at the time. ETI might just be a couple of graduate students with a bootleg UHF microwave oscillator in the trunk of their car, and the astronomical knowledge of precisely when to turn it off and on. It might also be some kind of secret (and illegal, that’s a Federally protected frequency) military transmission from a space probe in the solar system. The location near the ecliptic plane is consistent with the capabilities of our space technology.

The fact that its frequency differs slightly from the 21 cm line of neutral hydrogen (1420 MHz) is also consistent with the real thing. This frequency and its harmonics is precisely where you would expect ETI to be transmitting, and where ETI would expect us to be listening. (Yes, there is solid reasoning to support that last statement.) The deviation could be a Doppler shift due to relative motion between the transmitter and receiver. An alien signaller could (and probably would) modulate his frequency to mask his own world’s motion about the galaxy, but he would have no way of knowing how to correct for an unknown recipient’s velocity.

We cannot rule out the signal as being of extraterrestrial origin simply because it has never been heard before or since. It is conceivable that it was an alien transmission generated for some unknown reason and that it wasn’t aimed at us. We may have just drifted into the beam by accident for a few seconds, and then moved on through.