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Lunar Laser Ranging Experiment

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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. Since the speed of light is known with very high accuracy, the distance to the moon can be calculated. This distance has been measured with increasing accuracy for more than 35 years.

The distance continually changes for a number of reasons, but averages about 384,467 kilometers (238,897 miles). The time delay in the reflected light is about 2½ seconds.

The first successful tests were carried out in 1962 when a team from the Massachusetts Institute of Technology succeeded in observing reflected laser pulses using a laser with a millisecond pulse length. Similar measurements were obtained later the same year by a Soviet team at the Crimean Astrophysical Observatory using a Q-switched ruby laser. Greater accuracy was achieved following the installation of a retroreflector array on July 21, 1969, by the crew of Apollo 11, while two more retroreflector arrays left by the Apollo 14 and Apollo 15 missions have also 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. The Lunokod arrays suffer from declined performance in direct sunlight, a factor which was considered in the reflectors placed during the Apollo missions.

Apollo 15 LRRR

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 Côte d'Azur Observatory in Grasse, France, and the Apache Point Observatory in New Mexico. The first measurements were made by the McDonald Observatory in Texas, although lunar laser ranging at this site stopped in 2009.

At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 3 kilometers (two miles) away. The reflected light is too weak to be seen with the human eye, but under good conditions, one photon of the 10^17 photos aimed at the reflected will be received back on Earth every few seconds (they can be identified as originating from the laser because the laser is highly monochromatic). This is one of the most precise distance measurements ever made, and is equivalent in accuracy to determining the distance between Los Angeles and New York to one hundredth of an inch. As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy, though the performance of the reflectors continues to degrade with age.

Apollo 14 Lunar Ranging Retro Reflector (LRRR)

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

• The moon is spiralling away from Earth at a rate of 38 mm per year.
• The moon probably has a liquid core of about 20% of the Moon's radius.
• The universal force of gravity is very stable. The experiments have put an upper limit on the change in Newton's gravitational constant G of less than 1 part in 10¹¹ since 1969.
• The likelihood of any "Nordtvedt effect" (a composition-dependent differential acceleration of the Moon and Earth towards the Sun) has been ruled out to high precision, strongly supporting the validity of the Strong Equivalence Principle.

• Einstein's theory of gravity (the general theory of relativity) predicts the moon's orbit to within the accuracy of the laser ranging measurements.

APOLLO Collaboration photon pulse return times

Additionally, the accuracy of these experiments has improved historic knowledge of the Moon's orbit enough to permit timing of solar eclipses up to 3,400 years ago.

The presence of reflectors on the Moon has been used to rebut claims that the Apollo landings were faked.

For example, the APOLLO Collaboration photon pulse return graph, shown here, has a pattern consistent with a retroreflector array near a known landing site.

See also

• Lunar distance (astronomy)
• LIDAR
• Carroll Alley (principal investigator of Apollo's reflector experiment)
• Lunokhod programme
• Apache Point Observatory Lunar Laser-ranging Operation
• Apollo Lunar Surface Experiments Package
• Independent evidence for Apollo Moon landings

References

Lunokhod 1 – the small structure on the left is the retroreflector.

External links

• Apollo 14 Laser Ranging Retroreflector Experiment
• History of Laser Ranging
• Lunar Retroreflectors History and Position
• Station de Télémétrie Laser-Lune in Grasse, France
• 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

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