File:Apollo AS11-40-5952HR.jpg

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.[1] 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.[2] Lunokhod 2's array continues to return signals to Earth.[3] The Lunokhod arrays suffer from decreased performance in direct sunlight, a factor which was considered in the reflectors placed during the Apollo missions.[4]

File:ALSEP AS15-85-11468.jpg

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.[5]

At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide[6] 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.[4][7] 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.[4]

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.[6]
  • The moon probably has a liquid core of about 20% of the Moon's radius.[3]
  • 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.[3]
  • 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,[8][9] strongly supporting the validity of the Strong Equivalence Principle.

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 Edit

References Edit

  1. Bender, P. L., The Lunar Laser Ranging Experiment, UCSD [1]
  2. McDonald, Kim (April 26, 2010). "UC San Diego Physicists Locate Long Lost Soviet Reflector on Moon", UCSD. Retrieved on 27 April 2010. 
  3. 3.0 3.1 3.2 3.3 James G. Williams and Jean O. Dickey. "Lunar Geophysics, Geodesy, and Dynamics" (PDF). Retrieved on 2008-05-04. 13th International Workshop on Laser Ranging, October 7-11, 2002, Washington, D. C.
  4. 4.0 4.1 4.2 "It’s Not Just The Astronauts That Are Getting Older". Universe Today (March 10, 2010). Retrieved on 10 March 2010.
  5. McKie, Robin (June 21, 2009), "After 40 years' reflection, laser moon mirror project is axed", The Guardian, .
  6. 6.0 6.1 Fred Espenak (August 1994). "NASA - Accuracy of Eclipse Predictions". Retrieved on 2008-05-04.
  7. "Apollo 11 Experiment Still Going Strong after 35 Years". (July 20, 2004). Retrieved on 2008-05-04.
  8. Adelberger, E.G., Heckel, B.R., Smith, G., Su, Y., and Swanson, H.E. (1990-Sep-20), "Eötvös experiments, lunar ranging and the strong equivalence principle", Nature 347: 261–263, 
  9. Williams, J.G., Newhall, X.X., and Dickey, J.O. (1996), "Relativity parameters determined from lunar laser ranging", Phys. Rev. D 53: 6730–6739, 

External links Edit


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