Apollo 15 Mission
Science Experiments - Laser Ranging Retroreflector
The Laser Ranging Retroreflector measured very precisely the distance between the Earth and Moon.
Left: The Apollo 15 Laser Ranging Retroreflector. Right: Laser beam aimed at Moon by a telescope at McDonald Observatory. (Image courtesy of McDonald Observatory.)
The Laser Ranging Retroreflector experiment was deployed on Apollo 11, 14, and 15. 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. A similar device was also included on the Soviet Union's Lunakhod 2 spacecraft. These reflectors can be illuminated by laser beams aimed through large telescopes on Earth. The reflected laser beam is also observed with the telescope, providing a measurement of the round-trip distance between Earth and the Moon. This is the only ALSEP experiment that is still returning data from the Moon. Many of these measurements have been made by McDonald Observatory in Texas. From 1969 to 1985, they were made on a part-time basis using the McDonald Observatory 107-inch telescope. Since 1985, these observations have been made using a dedicated 30-inch telescope. Additional measurements have been made by observatories in Hawai'i, California, France, Australia, and Germany.
Laser beams are used because they remain tightly focused for large distances. Nevertheless, there is enough dispersion of the beam that it is about 7 kilometers in diameter when it reaches the Moon and 20 kilometers in diameter when it returns to Earth. Because of this very weak signal, observations are made for several hours at a time. By averaging the signal for this period, the distance to the Moon can be measured to an accuracy of about 3 centimeters (the average distance from Earth to the Moon is about 385,000 kilometers).
The Laser Ranging Retroreflector experiment has produced many important measurements. These include an improved knowledge of the Moon's orbit and the rate at which the Moon is receding from Earth (currently 3.8 centimeters per year) and of variations in the rotation of the Moon. These variations in rotation are related to the distribution of mass inside the Moon and imply the existence of a small core, with a radius of less than 350 kilometers, somewhat smaller than the limits imposed by the passive seismic and magnetometer experiments. These measurements have also improved our knowledge of changes of the Earth's rotation rate and the precession of its spin axis and have been used to test Einstein's Theory of Relativity.