Launch Date: 1969
Mission Duration: Continuous

Apollo 11 astronauts placed a reflector array in the Sea of Tranquillity, on the Moon, to begin lunar laser ranging. Additional retroreflectors were placed on the moon by the Apollo 14 and 15 missions. Also, a French-built retroreflector was carried aboard the Soviet Lunakhod 2. These four are used regularly.

Lunar ranging involves sending a laser beam through an optical telescope. The beam enters the telescope where the eyepiece would be, and the transmitted beam is expanded to become the diameter of the main mirror, then bounced off the surface toward the reflector on the Moon.

Once the laser beam hits a reflector, scientists at the ranging observatories use extremely sensitive filtering and amplification equipment to detect the return signal, which is far too weak to be seen with the human eye. Even under good atmospheric viewing conditions, only one photon is received every few seconds.

Ranging has provided the most accurate technique available for measuring the Moon's rotation and orbit. It has also determined that the length of an Earth day has distinct small-scale variations. Also observed are crustal plate drifts on Earth. Measurements show that the Moon is moving away from the Earth at a rate of about 3.8 centimeters per year. Laser ranging has also made possible a wealth of new information about the dynamics and structure of the Moon.

One objective is to see if the Earth - with its iron core - and the Moon - which is mostly silicate material - fall around the Sun at the same rate. There is strong evidence that the Moon has a liquid core, and laser ranging has allowed us to determine with great accuracy the rate at which the Moon is gradually receding from the Earth.

Analysis of the laser ranges has allowed us to verify the Equivalence Principle - an assumption of Einstein's Theory - with an accuracy of 2x10^-13. Several tests of Einstein's theory of General Relativity have also been made. The measurements so far are in agreement, within their uncertainties, with Einstein's theory. Also, no change in the gravitational constant - a measurement of the force of gravity - has been detected with the lunar laser data.

By beaming laser pulses at the reflector from Earth, scientists have been able to determine the round-trip travel time that gives the distance between the observatory and retroreflector.

For additional information:

• http://cddis.gsfc.nasa.gov/slr_summary.html
• http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html
• http://www.ifag.de/Geodaesie/earth_rotation/three.htm
• http://ilrs.gsfc.nasa.gov

LLR Observatories:

• http://almagest.as.utexas.edu/~rlr/mlrs.html MacDonald Laser Ranging - first site.
• http://www.lpi.usra.edu/pub/expmoon/Apollo11/laserFS.gif

LLR site at Observatoire de la Côte d'Azur, near Grasse France

• http://www.obs-nice.fr/laserlune.jpg picture
• http://wwwrc.obs-azur.fr/cerga/default.html CERGA

LLR as an Apollo Experiment

• http://www.lpi.usra.edu/pub/expmoon/Apollo11 /A11_Experiments_LRRR.html
• http://www.lpi.usra.edu/pub/expmoon/Apollo14 /A14_Experiments_LRRR.html
• http://www.lpi.usra.edu/pub/expmoon/Apollo15 /A15_Experiments_LRRR.html

Apollo

Apollo 11 30th anniversary

• http://www.hq.nasa.gov/office/pao/History/ap11ann/ introduction.htm

Apollo missions

• http://www.lpi.usra.edu/expmoon/apollo_landings.html

Apollo pictures

• http://www.ksc.nasa.gov/mirrors/images/html/apollo.htm
• http://images.jsc.nasa.gov/iams/html/pao/as11.htm
• http://images.jsc.nasa.gov/iams/html/pao/as14.htm
• http://images.jsc.nasa.gov/iams/html/pao/as15.htm
• http://www.retroweb.com/apollo_gallery.html