The moon drifts away. Every year it gets about an inch and a half away from us. Hundreds of millions of years from now, our companion in the sky will be so far away that there will be no more total solar eclipses.
For decades, scientists measured the retreat of the moon by firing a laser at light-reflecting panels, known as retroreflectors, that were left on the lunar surface, and then timing the orbit of the light. But the moon’s five retroreflectors are old, and they are now much less efficient at reflecting light. To determine if a layer of lunar dust is the culprit, researchers devised a playful plan: They bounced off laser light from a much smaller but newer retroreflector aboard a NASA spacecraft that flew thousands of miles per hour across the surface of the moon shone. And it worked.
These results were published this month in the journal Earth, Planets and Space.
Of all the goods that humans have left on the moon, the five retroreflectors, supplied by Apollo astronauts and two Soviet robot revolvers, are one of the most scientifically important. They are equal to really long measurements: By precisely timing how long the laser light takes to travel to the moon, turn off a retroreflector and return to Earth (roughly 2.5 seconds, give or take), scientists can measure the distance calculate between the moon and earth.
Arrays of glass corner-cube prisms make this cosmic ricochet possible. These optical devices reflect incoming light back to exactly where it came from, and ensure that retroreflectors send photons on a tight, neat flip turn.
By taking repeated measurements over time, researchers can bring together a better picture of the moon’s orbit, its exact orientation in space and even its interior structure.
But the suitcase-sized retroreflectors of the month, supplied from 1969 to 1973, now show their age. In some cases, they are only about a tenth as efficient as expected, said Tom Murphy, a physicist at the University of California, San Diego, who was not involved in the study. “The returns are severely depressing.”
One obvious culprit is lunar dust built up on the retroreflectors. Dust can build up by meteorites hitting the moon’s surface. It covers the moon packs of astronauts during their visits, and it is expected that it will be a significant problem if humans ever colonize the moon.
Although it has been almost 50 years since a retroreflector was placed on the surface of the moon, a NASA spacecraft launched in 2009 carries a retroreflector about the size of a paperback book. That spacecraft, the Lunar Reconnaissance Orbiter, orbits the moon once every two hours, and it has flown millions of high-resolution images of the lunar surface.
The Lunar Reconnaissance Orbiter “provides an unmistakable objective,” said Erwan Mazarico, a planetary scientist at NASA’s Goddard Space Flight Center, who, along with his colleagues, hypothesized that lunar matter could affect the moon’s retroreflectors.
But it is also a moving target. The orbiter shimmers across the moon’s surface at 3600 mph “It’s hard enough to reach a stationary target,” said Dr. Murphy, who leads the Apache Point Observatory Lunar Laser Ranging Operation, or APOLLO, a project that the retroreflectors used on the lunar surface. “We’ll give you a smaller array and let it move on you.”
In 2017, Dr. Mazarico and his collaborators shoot an infrared laser from a station near Grasse, France – about a half-hour drive from Cannes – towards the orbiter’s retroreflector. At about 3 a.m. on September 4, 2018, they recorded their first success: a detection of 25 photons making the round trip.
The researchers noted three more successes through the fall of 2019. After accounting for the smaller size of the orbiter’s retroreflector, Drs. Mazarico and his colleagues that it often returned photons more efficiently than the Apollo retroreflectors.
There is still not enough evidence to categorically blame the substance for the lower performance of the moon’s retroreflectors, said Drs. Mazarico, and more observations are collected. Mar Dr. Murphy and other scientists said the new findings helped build the case.
“For me, the material reflection idea is more supported than refuted by these results,” he said.
