Decades after a silent Moon rover, a laser reflector finally answers Earth

A Soviet Lunokhod 1 rover carried a passive laser retroreflector that went silent in 1971; nearly forty years later NASA’s Lunar Reconnaissance Orbiter pinpointed Lunokhod 1’s location, enabling the APOLLO laser-ranging program to re-measure the Earth–Moon distance. The 2010 return was far stronger than typical retroreflector signals (about 2,000 photons, four to five times Lunokhod 2), due to favorable orientation and less degradation. Since then Lunokhod 1 has become a valuable, ongoing part of lunar ranging, supporting tests of general relativity and precise Earth–Moon distance measurements.