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The Lunar Laser Ranging Experiment from the Apollo 11 mission

The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors previously planted on the Moon and the time delay for the reflected light to return is determined. The distance has been measured repeatedly over a period of more than 35 years.

The experiment was first made possible by a retroreflector array installed on July 21, 1969, by the crew of the Apollo 11. Two more retroreflector arrays left by the Apollo 14 and Apollo 15 missions have contributed to the experiment.

The unmanned Soviet Lunokhod 1 and Lunokhod 2 rovers carried smaller arrays. Reflected signals were initially received from Lunokhod 1, but no return signals have been detected since 1971, at least in part due to some uncertainty in its location on the Moon. Lunokhod 2's array continues to return signals to Earth.[1]

The Apollo 15 array is three times the size of the arrays left by the two earlier Apollo missions. Its size made it the target of three-quarters of the sample measurements taken in the first 25 years of the experiment. Improvements in technology since then have resulted in greater use of the smaller arrays, by sites such as the McDonald Observatory and the OCA Laser-Lune telemetry station affiliated with the Côte d'Azur Observatory.

As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy.

Some of the results of this long-term experiment are:

• The moon is spiraling away from Earth at a rate of 3.8 cm per year, due to the Earth's ocean tides.
• The moon probably has a liquid core.
• The universal force of gravity is very stable. The experiments have put an upper limit of the change in Newton's gravitational constant G of less than 1 part in 100,000,000,000 since 1969.
• Einstein's theory of gravity and the general theory of relativity predict the moon's orbit to within the accuracy of the laser ranging measurements.

• Lunar distance (astronomy)
• LIDAR
• Carroll Alley, principal investigator of Apollo's reflector experiment
• Lunokhod programme
• Apache Point Observatory Lunar Laser-ranging Operation
• ALSEP

• Lunar Geophysics, Geodesy, and Dynamics by James Williams Jean Dickey in 13th International Workshop on Laser Ranging, October 7-11, 2002, Washington, D. C.
• Apollo 15 Laser Ranging Retroreflector Experiment
• History of Laser Ranging
• Lunar Retroreflectors History and Position
• Station de Télémétrie Laser-Lune in Grasse, France
• 1994 article on "Measuring the Moon's Distance"
• 2002 article about "UW researcher plans project to pin down moon's distance from Earth"
• NASA: What Neil & Buzz Left on the Moon
• CNN: Apollo 11 Experiment Still Returning Results after 30 Years
• JPL: Apollo 11 Experiment Still Going Strong after 35 Years

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