How do you solve a moon mystery? Fire a laser at it
By Katherine Kornei
The moon is drifting away. Every year, it gets about an inch and a half farther from us.
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.
These results were published 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.
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.
The Lunar Reconnaissance Orbiter “provides a pristine target,” said Erwan Mazarico, a planetary scientist at NASA Goddard Space Flight Center who, along with his colleagues, tested the hypothesis that lunar dust might be affecting the moon’s retroreflectors.
But it’s also a moving target. The orbiter skims over the moon’s surface at 3,600 mph.
In 2017, Mazarico 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. Sept. 4, 2018, they recorded their first success: a detection of 25 photons that made the round trip.
The researchers notched three more successes by the fall of 2019. After accounting for the smaller size of the orbiter’s retroreflector, Mazarico and his colleagues found it often returned photons more efficiently than the Apollo retroreflectors.