Has there been a well-documented encounter with extraterrestrials?

The map that can show aliens the way to Earth

It was then that my father and Carl Sagan drafted a message for the Pioneer 10 and 11 space probes that would be catapulted out of the solar system after an encounter with Jupiter. One of the things he and Sagan wanted to include was a map that showed the way to Earth both in time and space.

The question was how to write such a map in units that aliens could understand.

Earth years would be absolutely meaningless as they result from the path of our planet around the sun. The question of coordinates also had to be clarified - after all, nobody in space can find you using terms like above, below, west or east.

Even the stars do not stand still in space or time. “The second star from the right and then straight on until dawn” doesn't really work if the map is found a billion years from now and the star in question - Betelgeuse, for example - has long since exploded and died.

THE MAGIC OF THE PULSARS

The answer to that was obvious to my father: pulsars. These condensed shells of burned out stars were discovered by Joycelyn Bell Burnell in 1967 and served as perfect beacons in space and time.

Pulsars are incredibly long-lived and remain active for many millions of years to several billion years.

In addition, each pulsar is unique. Many rotate faster than anything on earth, sometimes several thousand times per second, and emit pulses of electromagnetic rays like lighthouses. By measuring these impulses, astronomers can calculate the rate of rotation of a pulsar extremely accurately - and no two are alike.

But pulsars slow down over time, sometimes by an imperceptible but measurable billionth of a second per year. Should an intelligent being find the map at some point, it could calculate the difference between the current rate of rotation of a pulsar and its rate of rotation on the map and thus find out how much time has passed since the map was created.

"There was something magical about the pulsars ... Nothing else in the sky had such markers," says Drake. "Everyone has their own characteristic pulse frequency, so that they can be identified by everyone, even after a long time from other, distant living beings."