Lunar Laser Reflectors

Lunar reflectors have enabled precise measurement of the earth moon distance since 1969.

Lunar laser reflectors.

The picture at right of the lunokhod rover shows the reflector jutting out in front left. The purpose was to calculate distance between earth and the moon. Retroreflectors are devices which reflect light back to its source. Science experiments laser ranging retroreflector the laser ranging retroreflector experiment was deployed on apollo 11 14 and 15.

The moonwalking crew of apollo 11 which landed on the moon 50 years ago. Part of the apollo program specifically the apollo 11 14 and 15 missions involved something called the lunar laser ranging experiment where astronauts set up retroreflectors that can reflect a laser beam sent from earth. Lunokhod 1 was successfully ranged during its maneuvering phase but then was not seen for almost 40 years until our project with the help of the lunar reconnaissance. Five were left at five sites on the moon by three crews of the apollo program and two remote landers of the lunokhod program.

Laser light pulses are transmitted and reflected back to earth and the round trip. For the physics community the biggest payoff from the apollo science program has come from the reflector arrays which are part of the lunar laser ranging llr experiment. The ongoing lunar laser ranging experiment or apollo landing mirror measures the distance between surfaces of earth and the moon using laser ranging. An epic lunar laser experiment is still going strong five decades after the apollo astronauts set it up on the surface.

The lunar laser ranging retroreflector array apollo 11 astronauts buzz aldrin and neil armstrong put it there on july 21 1969 about an hour before the end of their final moonwalk. It consists of a series of corner cube reflectors which are a special type of mirror with the property of always reflecting an incoming light beam back in the direction it came from. The close up here shows the apollo 15 landing site. The apollo 15 lunar laser ranging retroreflector lrrr array is one of four such working arrays on the surface of the moon.

For 50 years and counting scientists have been pinging laser pulses off these reflectors to measure the moon earth distance to high precision. Each reflector is 11 cm on a side for a total package 44 cm long and 19 cm across. The reflectors are too small to be seen from earth so even when the beam is correctly aligned in the telescope actually hitting a lunar reflector is quite challenging.