On this day, May 9, in 1962, a pulse light laser beam sent by a team of scientists from MIT successfully bounced off the moon, the first lunar laser ranging experiment. Later similar experiments used a reflector left on the moon’s surface by the Apollo 11 mission to increase accuracy.
The Laser Ranging Retroreflector mirrors were also deployed on Apollo, 14, and 15 missions and a similar device was also included on the Soviet Union’s Lunakhod 2 spacecraft. But the original Apollo reflector is the only one still operational. Readings collected over the years have shown the moon is slowly spiraling away from the earth, at the rate of around 38 millimeters per year, and that is contains a liquid center, which makes up about 20% of its radius.