In 1971, astronomer Frank Drake, the father of the search for extraterrestrial intelligence, drew a map pinpointing Earth in our galaxy. That diagram, a "pulsar map," was etched on a plaque designed by Frank and Carl Sagan and first carried into space in 1972 by the Pioneer 10 and 11 spacecraft. In 1977, the pulsar map would appear again etched on the covers of the golden records affixed to the the Voyager probes. These days, Frank's original pencil drawing of the map is stored in an old tomato box at his house. (In fact, Frank kindly allowed us to scan it for our book included in our new Voyager Golden Record vinyl box set!) Over at National Geographic, Nadia Drake, one of my favorite science journalists who also happens to be Frank's daughter, tells the fascinating story of this iconic piece of cosmic cartography. From National Geographic:
The question was, how do you create such a map in units that an extraterrestrial might understand?
...To my dad, the answer was obvious: pulsars. Discovered in 1967 by Jocelyn Bell Burnell, these dense husks of expired stars were perfect blazes in both space and time.
For starters, pulsars are incredibly long-lived, staying active for tens of millions to multiple billions of years.
Also, each pulsar is unique. They spin almost unbelievably fast, and they emit pulses of electromagnetic radiation like lighthouses. By timing those pulses, astronomers can determine a pulsar’s spin rate to a ridiculous degree of accuracy, and no two are alike.
But pulsars do slow down, sometimes by a mere but measurable billionths of a second a year. By calculating the difference between a pulsar’s spin rate when the map is found versus the spin period inscribed on the map, an intelligent being could figure out how long it had been since the map was made.
“There was a magic about pulsars … no other things in the sky had such labels on them,” Drake says. “Each one had its own distinct pulsing frequency, so it could be identified by anybody, including other creatures after a long period of time and far, far away.”
"How a NASA Spacecraft May Help Aliens Find Earth" (National Geographic)
Below, Frank showing me his original Pulsar Map drawings: