Lunar Laser Ranging Experiment

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The Lunar Laser Ranging Experiment from the Apollo 11 mission
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 great 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
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) wide[2] 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 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.

Apollo 14 Lunar Ranging Retro Reflector (LRRR)
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.[2]
  • The moon probably has a liquid core of about 20% of the Moon's radius.[1]
  • 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.[1]
  • Einstein's theory of gravity (the general theory of relativity) predicts the moon's orbit to within the accuracy of the laser ranging measurements.[1]

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 left 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.

Apollo 15 LRRR
Apollo 15 LRRR

[edit] See also

[edit] References

  1. ^ a b c d e James G. Williams and Jean O. Dickey. Lunar Geophysics, Geodesy, and Dynamics. ilrs.gsfc.nasa.gov. Retrieved on 2008-05-04. 13th International Workshop on Laser Ranging, October 7-11, 2002, Washington, D. C.
  2. ^ a b Fred Espenak (August, 1994). NASA - Accuracy of Eclipse Predictions. eclipse.gsfc.nasa.gov. Retrieved on 2008-05-04.
  3. ^ Apollo 11 Experiment Still Going Strong after 35 Years. www.jpl.nasa.gov (July 20, 2004). Retrieved on 2008-05-04.
Lunokhod 1 – the small structure on the left is the retroreflector.
Lunokhod 1 – the small structure on the left is the retroreflector.

[edit] External links

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