Wow! Signal

In 1977, a radio telescope in Ohio picked up a strong signal, that appeared to come from the constellation of Sagittarius. The astronomer who discovered it was so impressed that he circled the printout of the data, and added "Wow!". It caused a lot of excitement among folks who were searching for signs of extraterrestrial intelligence.

Even though astronomers searched for another signal like that, it was never detected again. The source of the signal is unknown. Theories include a military transmission that was reflected on a piece of space debris, but the best technical explanation appears to be a transmission actually originating from deep space, whatever the origin.

In 2012, on the 35th anniversary of the signal, astronomers beamed a transmission into the direction of Sagittarius that consisted of 10,000 Twitter messages written for this purpose.