Forty years ago, on Nov. 16, 1974, Earth beamed its first postcard to the stars.

The message left our home planet on a warm and sticky day in Puerto Rico and has been flying through the galaxy at the speed of light ever since. In about 25,000 years, it will collide with a cluster of 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. 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