Forty years ago, we sent a map to Earth sailing deep into the cosmos.

Copies of this map are etched into each of the twin Voyager spacecraft, which launched in the late 1970s and are now the farthest spacecraft from home. One of the probes has already slipped into interstellar space, and the other is skirting the fringes of our sun’s immediate neighborhood.

If it’s ever intercepted and decoded by extraterrestrials, the map will not only reveal where to find our watery little world, but also when the space probe that delivered it to alien hands left home.

“We needed to put something on the Voyager that said where it came from, and how long it was traveling,” says my dad, Frank