Lunar Laser Ranging experiments

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The Lunar Laser Ranging Experiment from the Apollo 11 mission

The ongoing Lunar Laser Ranging Experiment was first made possible by a lunar laser ranging retroreflector array planted on the Moon on July 21, 1969, by the crew of the Apollo 11. Since then, the distance between the Earth and the Moon has been measured repeatedly over a period of more than 35 years.

Three more retroreflector arrays, left by the Apollo 14 and Apollo 15 missions and by the unmanned Soviet Lunokhod 2 rover, have contributed to the experiment.

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.

Lunokhod 2 also left an array, but it was poorly positioned and has not been of use to the experiment[1].

Lunar ranging also provides data useful for other experiments, including tests of general relativity designed by physicists such as Carrol Alley.

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. Estimates of Newton's gravitational constant G have changed less than 1 part in 100-billion since the laser experiments began.
• Einstein's theory of gravity and the general theory of relativity match the moon's orbit as well as laser ranging can measure it.

See also

• Lunar distance

External links and sources

• 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