Retroreflectors are devices which reflect light back to its source. Seven retroreflectors were left at seven sites on the Moon by three crews of the Apollo program, two by remote landers of the Lunokhod program, one by the Commercial Lunar Payload Services program and one by the Chandrayaan program. Lunar reflectors have enabled precise measurement of the Earth–Moon distance since 1969 using lunar laser ranging.
There have been several additional attempts to land retroreflectors on the lunar surface which were unsuccessful, and several future attempts are planned.