Lunar laser ranging Experiment - wikipedia Lunar laser ranging Experiment The Lunar Laser Ranging Experiment from the Apollo 11 mission. The ongoing Lunar Laser Ranging Experiment measures the distance between Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the Moon during the Apollo program ( 11, 14, and 15 ) and the two Lunokhod missions. The time for the reflected light to return is measured. Apollo 15 LRRR Apollo 15 LRRR schematic The first successful tests were carried out in 1962 when a team from the Massachusetts Institute of Technology succeeded in observing laser pulses reflected from the Moon 's surface 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, and two more retroreflector arrays left by the Apollo 14 and Apollo 15 missions have also contributed to the experiment. Successful lunar laser range measurements to the retroreflectors were first reported by the 3.1 m telescope at Lick Observatory, Air Force Cambridge Research Laboratories Lunar Ranging Observatory in Arizona, the Pic du Midi Observatory in France, the Tokyo Astronomical Observatory, and McDonald Observatory in Texas. 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 considered in reflector placement 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. Contents 1 Details 2 Results 3 Photo gallery 4 See also 5 References 6 External links Details The distance to the Moon is calculated approximately using this equation : distance = ( speed of light × time taken for light to reflect ) / 2 In actuality, the round - trip time of about 2.5 seconds is affected by the location of the Moon in the sky, the relative motion of Earth and the Moon, Earth 's rotation, lunar libration, weather, polar motion, propagation delay through Earth 's atmosphere, the motion of the observing station due to crustal motion and tides, velocity of light in various parts of air and relativistic effects. Nonetheless, the Earth -- Moon distance has been measured with increasing accuracy for more than 35 years. The distance continually changes for a number of reasons, but averages 385,000.6 km ( 239,228.3 mi ). At the Moon 's surface, the beam is about 6.5 kilometers ( 4.0 mi ) wide and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 3 kilometers ( 1.9 mi ) away. The reflected light is too weak to see with the human eye. Out of 10 photons aimed at the reflector, only one is 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 0.25 mm ( 0.0098 in ). 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. Results Lunar laser ranging measurement data is available from the Paris Observatory Lunar Analysis Center, and the active stations. Some of the findings of this long - term experiment are : The Moon is spiraling away from Earth at a rate of 3.8 cm per year. This rate has been described as anomalously high. The Moon probably has a liquid core of about 20 % of the Moon 's radius. The universal force of gravity is very stable. The experiments have constrained the change in Newton 's gravitational constant G to a factor of ( 2 ± 7 ) × 10 per year. The likelihood of any `` Nordtvedt effect '' ( a differential acceleration of the Moon and Earth towards the Sun caused by their different degrees of compactness ) has been ruled out to high precision, strongly supporting the validity of the Strong Equivalence Principle. Einstein 's theory of gravity ( the general theory of relativity ) predicts the Moon 's orbit to within the accuracy of the laser ranging measurements. Gauge freedom plays a major role in a correct physical interpretation of the relativistic effects in the Earth - Moon system observed with LLR technique Photo gallery Apollo 14 Lunar Ranging Retro Reflector ( LRRR ). APOLLO Collaboration photon pulse return times Laser ranging facility at Wettzell fundamental station, Bavaria, Germany. Laser Ranging at Goddard Spaceflight Center. See also Moon portal Apache Point Observatory Lunar Laser - ranging Operation Apollo Lunar Surface Experiments Package Tom Murphy ( Physicist ) ( principal investigator of Apollo 's reflector experiment ) Carroll Alley ( previous principal investigator of Apollo 's reflector experiment ) EME ( communications ) Lidar Lunar distance ( astronomy ) Lunokhod programme Satellite laser ranging Space geodesy Third - party evidence for Apollo Moon landings List of retroreflectors on the Moon References Jump up ^ Chapront, J. ; Chapront - Touzé, M. ; Francou, G. ( 1999 ). `` Determination of the lunar orbital and rotational parameters and of the ecliptic reference system orientation from LLR measurements and IERS data ''. Astronomy and Astrophysics. 343 : 624 -- 633. Bibcode : 1999A&A... 343... 624C. Jump up ^ Smullin, Louis D. ; Fiocco, Giorgio ( 1962 ). `` Optical Echoes from the Moon ''. Nature. 194 ( 4835 ) : 1267. Bibcode : 1962Natur. 194.1267 S. doi : 10.1038 / 1941267a0. Jump up ^ Bender, P.L. ; et al. ( 1973 ). `` The Lunar Laser Ranging Experiment : Accurate ranges have given a large improvement in the lunar orbit and new selenophysical information '' ( PDF ). Science. 182 ( 4109 ) : 229 -- 238. Bibcode : 1973Sci... 182... 229B. doi : 10.1126 / science. 182.4109. 229. PMID 17749298. Jump up ^ McDonald, K. ( April 26, 2010 ). `` UC San Diego Physicists Locate Long Lost Soviet Reflector on Moon ''. UCSD. Retrieved 27 April 2010. ^ Jump up to : Williams, J.G. ; Dickey, J.O. `` Lunar Geophysics, Geodesy, and Dynamics '' ( PDF ). ilrs.gsfc.nasa.gov. Retrieved 2008 - 05 - 04. 13th International Workshop on Laser Ranging, October 7 -- 11, 2002, Washington, D.C. ^ Jump up to : `` It 's Not Just The Astronauts That Are Getting Older ''. Universe Today. March 10, 2010. Retrieved 24 August 2012. Jump up ^ Seeber, Gunter. Satellite Geodesy 2nd Edition. de Gruyter, 2003, p. 439 Jump up ^ Murphy, T.W. ( 2013 ). `` Lunar laser ranging : the millimeter challenge '' ( PDF ). Reports on Progress in Physics. 76 ( 7 ) : 2. arXiv : 1309.6294. Bibcode : 2013RPPh... 76g6901M. doi : 10.1088 / 0034 - 4885 / 76 / 7 / 076901. ^ Jump up to : Espenek, F. ( August 1994 ). `` NASA - Accuracy of Eclipse Predictions ''. eclipse.gsfc.nasa.gov. Retrieved 2008 - 05 - 04. Jump up ^ `` Apollo 11 Experiment Still Going Strong after 35 Years ''. www.jpl.nasa.gov. July 20, 2004. Retrieved 2008 - 05 - 04. Jump up ^ `` LUNAR LASER RANGING OBSERVATIONS FROM 1969 TO MAY 2013 '' SYRTE Paris Observatory, retrieved 3 June 2014 Jump up ^ Bills, B.G. ; Ray, R.D. ( 1999 ). `` Lunar Orbital Evolution : A Synthesis of Recent Results ''. Geophysical Research Letters. 26 ( 19 ) : 3045 -- 3048. Bibcode : 1999GeoRL... 26.3045 B. doi : 10.1029 / 1999GL008348. Jump up ^ Müller, J. ; Biskupek, L. ( 2007 ). `` Variations of the gravitational constant from lunar laser ranging data ''. Classical and Quantum Gravity. 24 ( 17 ) : 4533. doi : 10.1088 / 0264 - 9381 / 24 / 17 / 017. Jump up ^ Adelberger, E.G. ; Heckel, B.R. ; Smith, G. ; Su, Y. ; Swanson, H.E. ( 1990 ). `` Eötvös experiments, lunar ranging and the strong equivalence principle ''. Nature. 347 ( 6290 ) : 261 -- 263. Bibcode : 1990Natur. 347... 261A. doi : 10.1038 / 347261a0. Jump up ^ Williams, J.G. ; Newhall, X.X. ; Dickey, J.O. ( 1996 ). `` Relativity parameters determined from lunar laser ranging ''. Physical Review D. 53 ( 12 ) : 6730 -- 6739. Bibcode : 1996PhRvD... 53.6730 W. doi : 10.1103 / PhysRevD. 53.6730. Jump up ^ Kopeikin, S. ; Xie, Y. ( 2010 ). `` Celestial reference frames and the gauge freedom in the post-Newtonian mechanics of the Earth -- Moon system ''. Celestial Mechanics and Dynamical Astronomy. 108 ( 3 ) : 245 -- 263. Bibcode : 2010CeMDA. 108... 245K. doi : 10.1007 / s10569 - 010 - 9303 - 5. External links `` Theory and Model for the New Generation of the Lunar Laser Ranging Data '' by Sergei Kopeikin Apollo 15 Experiments - Laser Ranging Retroreflector by the Lunar and Planetary Institute `` History of Laser Ranging and MLRS '' by the University of Texas at Austin, Center for Space Research `` Lunar Retroreflectors '' by Tom Murphy Station de Télémétrie Laser - Lune in Grasse, France Lunar Laser Ranging from International Laser Ranging Service `` UW researcher plans project to pin down moon 's distance from Earth '' by Vince Stricherz, UW Today, January 14, 2002 `` What Neil & Buzz Left on the Moon '' by Science@NASA, July 20, 2004 `` Apollo 11 Experiment Still Returning Results '' by Robin Lloyd, CNN, July 21, 1999 The Moon Physical features Face on the Moon Internal structure Gravity field Topography Magnetic field Atmosphere Ion tail Moonlight Lunar swirls Orbit Orbit of the Moon Phases Lunation Lunar libration Full Moon Lunar terminator Full moon cycle Earthshine Lunar eclipse Solar eclipse Solar eclipses on the Moon Tide Surface Selenography Near side Far side North pole South pole Lunar mare List Craters List Calderas Mountains Valleys Volcanic features South Pole -- Aitken basin Shackleton crater Water Soil Ray systems Peak of eternal light Crater of eternal darkness Space weathering Transient lunar phenomenon Lunar basalt 70017 Science Geology timescale Giant - impact hypothesis Hill sphere Moon rocks Lunar meteorites KREEP Late Heavy Bombardment Lunar laser ranging ALSEP Observation Origin Lunar sample displays Apollo 11 lunar sample display Apollo 17 lunar sample display Exploration Apollo program Exploration of the Moon Colonization of the Moon List of lunar probes List of missions to the Moon Moon landing conspiracy theories Related topics Lunar calendar Lunisolar calendar Craters named after people Double planet Lunar effect Man in the Moon Memorials on the Moon Month Lunar month Moon illusion Harvest moon Hunter 's moon Moon in art and literature Moon in mythology Hjúki and Bil ( Norse legends ) Moon is made of green cheese Natural satellite Solar System Supermoon Theia Tourism Phases / Occurrence Black moon Blue moon Lunar phase New moon Dark moon Wet moon Lilith ( hypothetical moon ) Moon portal Earth sciences portal Solar System portal Apollo program hardware Vehicles Apollo ( spacecraft ) Apollo Command / Service Module Apollo Lunar Module Lunar Escape Systems Lunar Roving Vehicle Saturn V Vehicle components Apollo Abort Guidance System Apollo Docking Mechanism Apollo Guidance Computer Primary Guidance, Navigation and Control System Apollo Telescope Mount Apollo TV camera Saturn Launch Vehicle Digital Computer Descent Propulsion System Ascent Propulsion System Lunar surface Apollo Lunar Surface Experiments Package Apollo / Skylab A7L ( spacesuit ) Beta cloth Lunar Laser Ranging experiment Modular Equipment Transporter Solar Wind Composition Experiment Earth - based Crawler - transporter Little Joe II Lunar Landing Research Vehicle Mobile Launcher Platform Mobile Quarantine Facility Symbolic Lunar plaque Lunar Flag Assembly Fallen Astronaut Category : Apollo program Retrieved from `` https://en.wikipedia.org/w/index.php?title=Lunar_Laser_Ranging_experiment&oldid=804131779 '' Categories : Lunar science Apollo program Apollo program hardware Tests of general relativity Hidden categories : Articles containing potentially dated statements from 2002 All articles containing potentially dated statements Talk Contents About Wikipedia Català Dansk Deutsch Español فارسی Français 한국어 Bahasa Indonesia Italiano 日本 語 Polski Русский Svenska Українська 中文 Edit links This page was last edited on 6 October 2017, at 23 : 38. 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Question: what three apollo missions left reflectors on the moon
Answer: 11, 14, and 15