Page "Apache Point Observatory Lunar Laser-ranging Operation" Paragraph 5

High precision Lunar Laser Ranging ( LLR ) started soon after the Apollo 11 astronauts left the first retroreflector on the Moon.

Additional reflectors were left by the Apollo 14 and Apollo 15 astronauts, and two French-built reflector arrays were placed on the Moon by the Soviet Luna 17 ( Lunokhod 1 ) and Luna 21 ( Lunokhod 2 ) lunar rover missions.

Over the years since, many groups and experiments have used this technique to study the behavior of the Earth-Moon system, investigating gravitational and other effects.