WikiPortallus: Lunar laser ranging experiment.html

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 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

Time when photons arrive after many short laser pulses are directed at the location of a lunar retro-reflector. Credit: Apollo Collaboration

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.

At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide² 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 will be received 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.³ As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy.

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.¹

The presence of reflectors on the Moon has been used to refute claims that the Apollo landings were faked. For example, the figure on the right shows evidence of something very small, located within a few kilometers of where a landing occurred, and which reflects laser light directly back to the source as well as a mirror array.

Lunar laser ranging experiment.html

See also

References

External links