In 1977, the Big Ear observatory was listening to the stars when it recorded a signal that would later be known as the WOW Signal. The Big Ear Observatory is run and controlled by the Ohio State University in the United States. Its site is a bit basic to look at and hasn't been updated since 2008 according to a note on the page. The actual telescope was demolished in 1998 by land developers.
The signal was noticed by Jerry Ehman on August 15th, 1977 when he was pouring over the information that had been received. Jerry noticed successive values of intensity and circled them and wrote WOW on the print out hence how it got its name.
Signals were encoded in numbers and letters so that they could be deciphered easily enough. A 1 is any value of intensity between 1.0 to 1.9999 and then following that 2 meaning 2.0 etc.... etc... When the numbers were exhausted, they turned to letters so A meant any intensity between 10.0 and 10.9999 and so on up the alphabet. The "U" was the highest intensity ever recorded. There was nothing for over 35 which would have been for Z.
The signal was identified as coming from a three star star cluster that included Chi Sagittarii in the constellation of the archer, Sagittarius. This is the same constellation that contains the Galactic Centre, the supermassive black hole at the centre of the milky way.
The signal was only heard once, it was not heard of again. Scientists and astronomers all listened in on that area of space but nothing was heard again. Some joked that it might have been aliens telling us that they aren't interested and they don't want to hear from us again which would go some way to explaining why we only heard from them the once.
The record set off a chain of events that went round the world. Some people saw it as being a sign that aliens were out there and they were communicating back. Our radio signals have been travelling through space since roughly the beginning of the last century and maybe an alien race had picked them up and responded. People were hopeful that it was a sign that there was intelligent life out there who had responded back to us.
In 2016, a new explanation was put forward, one that ruled out aliens. the explanation was that the signals came from hydrogen gas encompassing two Comets that were found to be in the vicinity of the star at the time. The comets were 266P/Christiansen and P/2008 Y2 (Gibbs). Both of these comets were discovered in 2006 and 2008 which why they were identified as a possible source back when the news first broke.
Christiansen will next come close (Perihelion) in April 2020 when Professor Antonio Paris is hoping to have a radio telescope trained on the object to see if it creates the same sort of noise as it did in 1977. He started a crowd funded venture but when it was checked, it no longer existed. Christiansen has been and gone (Jan 2017) but Gibbs is expected on or about the 7th January 2018 but probably won't be visible to the naked eye sadly. If anyone knows of any results or anything, do share by putting comments it the box below.