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 planted on the moon during the Apollo program and the time delay for the reflected light to return is determined. Because the speed of light is known with a high degree of precision, the distance to the Moon can be calculated using this simple equation:
The 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 were detected after 1971 until a team from University of California rediscovered the array in April 2010 using images from NASA’s Lunar Reconnaissance Orbiter. Lunokhod 2's array continues to return signals to Earth. The Lunokhod arrays suffer from decreased performance in direct sunlight, a factor which was considered in the reflectors placed during the Apollo missions.
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 Lunar Laser-ranging Operation (APOLLO) at 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: out of 1017 photons aimed at the reflector, only one will be received back on Earth every few seconds, even under good conditions (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.
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