As every UFO fanatic knows, the Earth received a short message from outer space back in 1977 - and now you've got a chance to reply.
The so-called 'Wow!' signal was recorded by the Big Ear Observatory at Ohio State University and lasted for 72 seconds. The narrowband signal came from the direction of the Sagittarius constellation, and has never been satisfactorily explained.
It got its name from the note jotted in the margin of the data printout by astronomer Jerry Ehman, who first spotted it.
And now, as a tie-in with its new series 'Chasing UFOs', National Geographic has decided that it's about time the Sagittarians got a reply. It's inviting people to help crowd-source a reply, and tweet it back 'using the biggest radio telescope we can get our hands on'.
"Tweet us a message with hashtag #chasingufos. We’ll collect all tweets starting at 8P ET to Midnight on June 29th," says National Geographic.
"Then, on the 35th anniversary of the Wow! Signal, we’ll collect a digital packet of all those tweets and beam them into outer space."
Anybody that already knows what they want to say - in 140 characters or less - can use this tool.
The Wow! signal may have been short, but it's the best candidate so far for an extra-terrestrial message, despite 40 years of looking. It was was 30 times more powerful than ambient radiation from deep space, but has never been repeated.
Its frequency was around 1420 MHz, leading some people to suggest that it refers to hydrogen, the most common element in the universe, which resonates at about that frequency.