Why Are We Firing Lasers At The Moon?

Scientists are measuring how far the moon has gotten away from us. Yes, the moon is drifting away from us every year. For decades, scientists have been measuring the moon’s retreat by firing a laser at the moon’s surface. Don’t worry, they’re not aiming to destroy the lunar surface. The laser hits the light-reflecting panels left on the surface, as The New York Times detail:

But the moon’s five retroreflectors are old, and they’re now much less efficient at flinging back light. To determine whether a layer of moon dust might be the culprit, researchers devised an audacious plan: They bounced laser light off a much smaller but newer retroreflector mounted aboard a NASA spacecraft that was skimming over the moon’s surface at thousands of miles per hour. And it worked.

These results were published this month in the journal Earth, Planets and Space.

Of all the stuff humans have left on the moon, the five retroreflectors, which were delivered by Apollo astronauts and two Soviet robotic rovers, are among the most scientifically important. They’re akin to really long yardsticks: By precisely timing how long it takes laser light to travel to the moon, bounce off a retroreflector and return to Earth (roughly 2.5 seconds, give or take), scientists can calculate the distance between the moon and Earth.