The moon is drifting away. 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.
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.
Arrays of glass corner-cube prisms make this cosmic ricochet possible. These optical devices reflect incoming light back to exactly where it came from, ensuring that retroreflectors send photons on a tight, neat flip turn.
Making repeated measurements over time allows researchers to piece together a better picture of the moon’s orbit, its precise orientation in space and even its interior structure.
But the moon’s suitcase-size retroreflectors, delivered from 1969 through 1973, are now showing their age. In some instances, they’re only about one-tenth as efficient as expected, said Tom Murphy, a physicist at the University of California, San Diego, who was not involved in the research. “The returns are severely depressed.”