Nadia Drake in No Place Like Home:
The message left our home planet on a warm and sticky mid-November day in Puerto Rico. It has been flying through the galaxy at the speed of light ever since, and in about 25,000 years will arrive at a cluster filled with more than 300,000 stars.
Unlike the radio signals that had been leaking from Earth since the late 1930s, this postcard was the first deliberate transmission to an alien civilization. Written in a way meant to be decipherable by extraterrestrial beings, the message contained some key information about the species that had sent it.
“It was a message that would actually inform anyone who did receive it that we existed, and tell them a little bit about what we were like,” says my dad Frank, who had the responsibility of constructing and sending what’s now known as the Arecibo Message. “And it was also a message to ourselves in that it showed what an intelligent civilization can do to contact other civilizations.”
He had just one month to write Earth’s first radio greeting to the stars.
It was 1974, and the Arecibo Observatory’s giant radio telescope had just gotten a major upgrade. Beamed into space by the Observatory’s powerful, one million-watt transmitter, the message would cap a ceremony marking the completion of the improvements (you can listen to it being sent, below). But it was a secret – only the ceremony’s organizers knew ahead of time what would happen, and they envisioned a transmission lasting about 3 minutes.
More here.