Scientists Solve a Mystery By Firing a Laser at the Moon

"The moon is drifting away," reports the New York Times.

Every year, it gets about an inch and a half farther from us. Hundreds of millions of years from now, our companion in the sky will be distant enough that there will be no more total solar eclipses.

For decades, scientists have measured the moon's retreat by firing a laser at light-reflecting panels, known as retroreflectors, that were left on the lunar surface, and then timing the light's round trip. 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...


Dust can be kicked up by meteorites striking the moon's surface. It coated the astronauts' moon suits during their visits, and it is expected to be a significant problem if humans ever colonize the moon. While it has been nearly 50 years since a retroreflector was placed on the moon's surface, a NASA spacecraft launched in 2009 carries a retroreflector roughly the size of a paperback book. That spacecraft, the Lunar Reconnaissance Orbiter, circles the moon once every two hours, and it has beamed home millions of high-resolution images of the lunar surface...
In 2017, Dr. Erwan Mazarico, a planetary scientist at NASA Goddard Space Flight Center and his collaborators began firing an infrared laser from a station near Grasse, France — about a half-hour drive from Cannes — toward the orbiter's retroreflector. At roughly 3 a.m. on Sept. 4, 2018, they recorded their first success: a detection of 25 photons that made the round trip... After accounting for the smaller size of the orbiterâ(TM)s retroreflector, Dr. Mazarico and his colleagues found that it often returned photons more efficiently than the Apollo retroreflectors...
"For me, the dusty reflector idea is more supported than refuted by these results" he said.
Laser-reflection measurements over long periods of time and across several reflectors "have revealed that the Moon has a fluid core," NASA notes. "Scientists can tell by monitoring the slightest wobbles as the Moon rotates."

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