NASA scientists have been firing a laser beam at a reflector on the Lunar Reconnaissance Orbiter (LRO) for the last decade, and recently they received a signal back for the first time. This is the latest step forward in the surprising science of lasers and reflectors on the moon.
Over the years, many craft have carried reflectors to the moon. These panels or cubes are generally small and are covered in mirrors, which is how they can reflect light including lasers. During the Apollo era, reflectors were delivered by the Apollo 11 and Apollo 14 crews, and during this time Soviet landers Lunokhod 1 and 2 delivered reflectors as well.
The tradition of taking reflectors to the moon continues to this day, with the Israeli lunar craft Beresheet carrying one last year. The craft crashed into the moon, but scientists believe that the reflector device may have survived the impact.
These reflectors can be used to take extremely accurate measurements of the distance to the moon, and show that the moon does not stay in the same orbit relative to Earth. In fact, the moon is slowly drifting away from us at a rate of 1.5 inches per year — or, as NASA puts it, the rate at which fingernails grow.
“Now that we’ve been collecting data for 50 years, we can see trends that we wouldn’t have been able to see otherwise,” Erwan Mazarico, a planetary scientist from NASA’s Goddard Space Flight Center who worked on the LRO experiment, said in a statement. “Laser-ranging science is a long game.”
To measure the distance between the Earth and the moon, the scientists time how long it takes for the laser signal they send to bounce off the reflector and be received on Earth again. This takes a few seconds and allows highly accurate measurements of long distances. The problem is that the laser beams spread out over the distance and only a few photons, if any, make it back to Earth. This is why it took so long for the LRO experiment to get results.
The wait is worth it, however, given how laser measurements could reveal information about the moon such as finding out about its internal structure and evolution. Getting an even more accurate measurement is key, according to Xiaoli Sun, a Goddard planetary scientist who helped design LRO’s reflector: “The precision of this one measurement has the potential to refine our understanding of gravity and the evolution of the solar system,” Sun said.
