Lunar Laser Ranging

Lunar Laser Ranging
Name	Lunar Laser Ranging
Caption	The Apollo 11 retroreflector array, the first placed on the Moon.
Location	Observatories on Earth targeting the Moon
Date	Operational since 1969
Participants	NASA, Academy of Sciences of the Soviet Union, Observatoire de la Côte d'Azur, Apache Point Observatory

Lunar Laser Ranging. It is a long-term experiment that measures the distance between the Earth and the Moon using laser pulses. The technique relies on retroreflector arrays placed on the lunar surface by Apollo program astronauts and Soviet space program robotic missions. These precise measurements have provided critical tests of gravitational physics and have refined our understanding of the Moon's internal structure and Earth's rotation.

Overview

This experiment constitutes one of the most precise tests of gravitational theory, primarily Albert Einstein's general relativity. By timing the round-trip of light between terrestrial observatories and lunar retroreflectors, scientists can detect minute changes in the Earth-Moon separation. The data has been instrumental in studying celestial mechanics, geophysics, and fundamental physics, offering constraints that are unattainable by any other method. Key participating institutions include the University of California, San Diego, the Observatoire de la Côte d'Azur, and the Apache Point Observatory.

History and Development

The concept was proposed shortly after the invention of the laser, with early attempts made by the Massachusetts Institute of Technology and the Crimean Astrophysical Observatory. The successful deployment of a retroreflector array by the crew of Apollo 11 in 1969, designed by a team led by James Faller, provided the first permanent target. Subsequent arrays were deployed by Apollo 14 and Apollo 15, with the latter being the largest. The Soviet Union also contributed two French-built arrays via the Lunokhod 1 and Lunokhod 2 rovers as part of the Luna program.

Technical Principles

A high-powered pulsed laser, such as those at the McDonald Observatory or the Grasse station, is aimed at a specific retroreflector array on the Moon. The laser pulse, lasting nanoseconds, travels through the Earth's atmosphere, reflects off the corner-cube retroreflectors, and returns to the originating observatory. The elapsed time, measured by precise atomic clocks, is used to calculate the distance, accounting for effects like atmospheric refraction and relativistic time delay. This process requires extremely accurate pointing and timing systems to detect the few returning photons.

Retroreflector Arrays

Five functional retroreflector arrays remain on the lunar surface. The three Apollo program arrays were deployed by the crews of Apollo 11, Apollo 14, and Apollo 15, with the latter provided by a team from the University of California, Berkeley. The two Lunokhod rovers, part of the Soviet space program, carried smaller arrays built by the Institut d'Optique in France. These arrays consist of hundreds of corner cubes made from fused silica or quartz, designed to reflect light directly back to its source regardless of the angle of incidence.

Scientific Results

The data have provided stringent tests of general relativity, including the strong equivalence principle and the time-rate change of the gravitational constant. It has precisely measured the Moon's recession from Earth at about 3.8 cm per year due to tidal acceleration. The experiments have revealed details of the Moon's liquid core and physical libration. Furthermore, they have contributed to the study of Earth's rotation, including polar motion and the length of day, tying into systems like the International Terrestrial Reference Frame.

Ongoing and Future Missions

Ongoing measurements are conducted by stations like the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) in New Mexico and the Lunar Laser Ranging station at the Matera Laser Ranging Observatory in Italy. Future missions aim to deploy next-generation retroreflectors, such as those proposed for NASA's Commercial Lunar Payload Services flights. These new arrays, potentially using advanced materials like borosilicate glass, are designed to improve the signal return by orders of magnitude, enabling even more precise tests of fundamental physics.

Category:Astronomical imaging Category:Apollo program Category:Experimental physics Category:Moon