lunar Laser Ranging experiments

Image:ALSEP AS15-85-11468.jpg

Image:Laser Ranging Retroreflector Apollo 15.svg

The first successful lunar ranging tests were carried out in 1962 when Louis Smullin and Giorgio Fiocco from the Massachusetts Institute of Technology succeeded in observing laser pulses reflected from the Moon's surface using a laser with a 50J 0.5 millisecond pulse length.{{cite journal |last1=Smullin |first1=Louis D. |last2=Fiocco |first2=Giorgio |year=1962 |title=Optical Echoes from the Moon |journal=Nature |volume=194 |issue=4835 |pages=1267 |bibcode=1962Natur.194.1267S |doi=10.1038/1941267a0|s2cid=4145783 |doi-access=free }} Similar measurements were obtained later the same year by a Soviet team at the Crimean Astrophysical Observatory using a Q-switched ruby laser.{{cite journal |last1=Bender |first1=P. L. |display-authors=etal |year=1973 |title=The Lunar Laser Ranging Experiment: Accurate ranges have given a large improvement in the lunar orbit and new selenophysical information |url=http://www.physics.ucsd.edu/~tmurphy/apollo/doc/Bender.pdf |journal=Science |volume=182 |issue=4109 |pages=229–238 |bibcode=1973Sci...182..229B |doi=10.1126/science.182.4109.229 |pmid=17749298|s2cid=32027563 }}

Shortly thereafter, Princeton University graduate student James Faller proposed placing optical reflectors on the Moon to improve the accuracy of the measurements.{{Cite journal|last=Newman|first=Michael E.|date=2017-09-26|title=To the Moon and Back … in 2.5 Seconds|url=https://www.nist.gov/nist-time-capsule/any-object-any-need-call-nist/moon-and-back-25-seconds|access-date=2021-01-27|journal=NIST|language=en}} This was achieved following the installation of a retroreflector array on July 21, 1969 by the crew of Apollo 11. Two more retroreflector arrays were left by the Apollo 14 and Apollo 15 missions. Successful lunar laser range measurements to the retroreflectors were first reported on Aug. 1, 1969 by the 3.1 m telescope at Lick Observatory. Observations from 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 soon followed.

The uncrewed Soviet Lunokhod 1 and Lunokhod 2 rovers carried smaller arrays. Reflected signals were initially received from Lunokhod 1 by the Soviet Union up to 1974, but not by western observatories that did not have precise information about location. In 2010 NASA's Lunar Reconnaissance Orbiter located the Lunokhod 1 rover on images and in April 2010 a team from University of California ranged the array.{{cite news |last=McDonald |first=K. |date=26 April 2010 |title=UC San Diego Physicists Locate Long Lost Soviet Reflector on Moon |url=http://ucsdnews.ucsd.edu/newsrel/science/04-26SovietReflector.asp |publisher=University of California, San Diego |access-date=27 April 2010|archive-date=30 April 2010|archive-url=https://web.archive.org/web/20100430164946/http://ucsdnews.ucsd.edu/newsrel/science/04-26SovietReflector.asp|url-status=dead}} Lunokhod 2{{'s}} array continues to return signals to Earth.{{cite conference |url=https://ilrs.cddis.eosdis.nasa.gov/docs/williams_lw13.pdf |title=Lunar Geophysics, Geodesy, and Dynamics |conference=13th International Workshop on Laser Ranging. 7–11 October 2002. Washington, D. C. |first1=James G. |last1=Williams |first2=Jean O. |last2=Dickey |date=2002}} The Lunokhod arrays suffer from decreased performance in direct sunlight—a factor considered in reflector placement during the Apollo missions.{{cite news |title=It's Not Just The Astronauts That Are Getting Older |date=10 March 2010 |work=Universe Today |url=http://www.universetoday.com/59310/its-not-just-the-astronauts-that-are-getting-older/ |access-date=24 August 2012}}

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 Nice, France; and the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) at the Apache Point Observatory in New Mexico.

In the 2010s several new retroreflectors were planned. The MoonLIGHT reflector, which was to be placed by the private MX-1E lander, was designed to increase measurement accuracy up to 100 times over existing systems.{{cite journal|last1=Currie|first1=Douglas|last2=Dell'Agnello|first2=Simone|last3=Delle Monache|first3=Giovanni|date=April–May 2011|title=A Lunar Laser Ranging Retroreflector Array for the 21st Century|journal=Acta Astronautica|volume=68|issue=7–8|pages=667–680|bibcode=2011AcAau..68..667C|doi=10.1016/j.actaastro.2010.09.001|url=https://www.openaccessrepository.it/record/137265 }}{{cite news|last=Tune|first=Lee|date=10 June 2015|title=UMD, Italy & MoonEx Join to Put New Laser-Reflecting Arrays on Moon|work=UMD Right Now|publisher=University of Maryland|url=https://umdrightnow.umd.edu/news/umd-italy-moonex-join-put-new-laser-reflecting-arrays-moon|access-date=21 March 2018|archive-date=22 March 2018|archive-url=https://web.archive.org/web/20180322081731/https://umdrightnow.umd.edu/news/umd-italy-moonex-join-put-new-laser-reflecting-arrays-moon|url-status=dead}}{{Cite news|last=Boyle|first=Alan|date=12 July 2017|title=Moon Express unveils its roadmap for giant leaps to the lunar surface ... and back again|work=GeekWire|url=https://www.geekwire.com/2017/moon-express-unveils-roadmap-giant-leaps-lunar-surface-back/|access-date=15 March 2018}} MX-1E was set to launch in July 2020,{{Citation|title=Moon Express Lunar Scout (MX-1E)|url=https://www.rocketlaunch.live/launch/lunar-scout|publisher=RocketLaunch.Live|access-date=27 July 2019|archive-date=27 July 2019|archive-url=https://web.archive.org/web/20190727095927/https://www.rocketlaunch.live/launch/lunar-scout|url-status=dead}} however, as of February 2020, the launch of the MX-1E has been canceled.{{cite web|title=MX-1E 1, 2, 3|url=https://space.skyrocket.de/doc_sdat/mx-1e.htm|access-date=24 May 2020}} India's Chandrayaan-3 lunar lander successfully placed a sixth reflector on the Moon in August 2023. MoonLIGHT will be launched in early 2024 with a Commercial Lunar Payload Services (CLPS) mission.{{Cite web |title=NASA Payloads for (CLPS PRISM) CP-11 |url=https://science.nasa.gov/lunar-discovery/deliveries/cp-11}}

{{see also|Apache Point Observatory Lunar Laser-ranging Operation#Principles of operation}}

File:Lunar_retroreflector_locations.jpg |date=6 May 2004 |url-status=live |archive-url=https://web.archive.org/web/20220430111349/https://science.nasa.gov/science-news/science-at-nasa/2004/06may_lunarranging/ |archive-date=30 April 2022 }}]]

The distance to the Moon is calculated {{em|approximately}} using the equation:

{{serif|1=distance {{=}} (speed of light × duration of delay due to reflection) / 2}}. Since the speed of light is a defined constant, conversion between distance and time of flight can be made without ambiguity.

To compute the lunar distance precisely, many factors must be considered in addition to the round-trip time of about 2.5 seconds. These factors include the location of the Moon in the sky, the relative motion of Earth and the Moon, Earth's rotation, lunar libration, polar motion, weather, speed of light in various parts of air, propagation delay through Earth's atmosphere, the location of the observing station and its motion due to crustal motion and tides, and relativistic effects.{{cite book |title=Satellite Geodesy |url=https://archive.org/details/satellitegeodesy00seeb |url-access=limited |publisher=de Gruyter |first=Günter |last=Seeber |edition=2nd |page=[https://archive.org/details/satellitegeodesy00seeb/page/n458 439] |date=2003 |isbn=978-3-11-017549-3 |oclc=52258226}}{{Cite web|last1=Williams|first1=James G.|last2=Boggs|first2=Dale H.|date=2020|title=The JPL Lunar Laser range model 2020|url=https://ssd.jpl.nasa.gov/ftp/eph/planets/ioms/|access-date=2021-05-24|website=ssd.jpl.nasa.gov}} The distance continually changes for a number of reasons, but averages {{convert|385000.6|km|mi|abbr=on}} between the center of the Earth and the center of the Moon.{{cite journal |last1=Murphy |first1=T. W. |date=2013 |title=Lunar laser ranging: the millimeter challenge |url=http://physics.ucsd.edu/~tmurphy/papers/rop-llr.pdf |journal=Reports on Progress in Physics |volume=76 |issue=7 |page=2 |arxiv=1309.6294 |bibcode=2013RPPh...76g6901M |doi=10.1088/0034-4885/76/7/076901|pmid=23764926 |s2cid=15744316 }} The orbits of the Moon and planets are integrated numerically along with the orientation of the Moon called physical libration.{{Cite journal|last1=Park|first1=Ryan S.|last2=Folkner|first2=William M.|last3=Williams|first3=James G.|last4=Boggs|first4=Dale H.|date=2021|title=The JPL Planetary and Lunar Ephemerides DE440 and DE441|journal=The Astronomical Journal|language=en|volume=161|issue=3|pages=105|doi=10.3847/1538-3881/abd414|bibcode=2021AJ....161..105P|s2cid=233943954|issn=1538-3881|doi-access=free}}

At the Moon's surface, the beam is about {{convert|6.5|km|mi|sp=us}} wide{{cite web

|last=Espenek |first=F. |date=August 1994 |title=NASA – Accuracy of Eclipse Predictions

|url=http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html

|publisher=NASA/GSFC

|access-date=4 May 2008

}}{{efn-lr|During the round-trip time, an Earth observer will have moved by around {{val|1|u=km}} (depending on their latitude). This has been presented, incorrectly, as a 'disproof' of the ranging experiment, the claim being that the beam to such a small reflector cannot hit such a moving target. However the size of the beam is far larger than any movement, especially for the returned beam.}} and scientists liken the task of aiming the beam to using a rifle to hit a moving dime {{convert|3|km|mi|sp=us}} away. The reflected light is too weak to see with the human eye. Out of a pulse of 3×10¹⁷ photons{{cite web |title=The Basics of Lunar Ranging |url=https://tmurphy.physics.ucsd.edu/apollo/basics.html |access-date=21 July 2023}} aimed at the reflector, only about 1{{ndash}}5 are received back on Earth, even under good conditions.{{Cite journal|last=Merkowitz|first=Stephen M.|date=2010-11-02|title=Tests of Gravity Using Lunar Laser Ranging|url= |journal=Living Reviews in Relativity|language=en|volume=13|issue=1|pages=7|doi=10.12942/lrr-2010-7|doi-access=free |issn=1433-8351|pmc=5253913|pmid=28163616|bibcode=2010LRR....13....7M}} They can be identified as originating from the laser because the laser is highly monochromatic.

As of 2009, the distance to the Moon can be measured with millimeter precision. In a relative sense, 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 within the width of a human hair.

{{main article|List of retroreflectors on the Moon}}

The table below presents a list of active and inactive Lunar Laser Ranging stations on Earth.{{Cite journal|last1=Biskupek|first1=Liliane|last2=Müller|first2=Jürgen|last3=Torre|first3=Jean-Marie|date=2021-02-03|title=Benefit of New High-Precision LLR Data for the Determination of Relativistic Parameters|journal=Universe|language=en|volume=7|issue=2|pages=34|doi=10.3390/universe7020034|arxiv=2012.12032|bibcode=2021Univ....7...34B|doi-access=free}}

The Lunar Laser Ranging data is collected in order to extract numerical values for a number of parameters. Analyzing the range data involves dynamics, terrestrial geophysics, and lunar geophysics. The modeling problem involves two aspects: an accurate computation of the lunar orbit and lunar orientation, and an accurate model for the time of flight from an observing station to a retroreflector and back to the station. Modern Lunar Laser Ranging data can be fit with a 1 cm weighted rms residual.

• The center of Earth to center of Moon distance is computed by a program that numerically integrates the lunar and planetary orbits accounting for the gravitational attraction of the Sun, planets, and a selection of asteroids.{{Cite journal|last1=Pavlov|first1=Dmitry A.|last2=Williams|first2=James G.|last3=Suvorkin|first3=Vladimir V.|date=2016|title=Determining parameters of Moon's orbital and rotational motion from LLR observations using GRAIL and IERS-recommended models|url=http://link.springer.com/10.1007/s10569-016-9712-1|journal=Celestial Mechanics and Dynamical Astronomy|language=en|volume=126|issue=1|pages=61–88|doi=10.1007/s10569-016-9712-1|issn=0923-2958|arxiv=1606.08376|bibcode=2016CeMDA.126...61P|s2cid=119116627}}
• The same program integrates the 3-axis orientation of the Moon called physical Libration.

The range model includes{{Cite web|last1=Williams|first1=James G.|last2=Boggs|first2=Dale H.|date=2020|title=The JPL Lunar Laser range model 2020|url=https://ssd.jpl.nasa.gov/ftp/eph/planets/ioms/|access-date=2021-06-01|website=ssd.jpl.nasa.gov}}

• The position of the ranging station accounting for motion due to plate tectonics, Earth rotation, precession, nutation, and polar motion.
• Tides in the solid Earth and seasonal motion of the solid Earth with respect to its center of mass.
• Relativistic transformation of time and space coordinates from a frame moving with the station to a frame fixed with respect to the solar system center of mass. Lorentz contraction of the Earth is part of this transformation.
• Delay in the Earth's atmosphere.
• Relativistic delay due to the gravity fields of the Sun, Earth, and Moon.
• The position of the retroreflector accounting for orientation of the Moon and solid-body tides.
• Lorentz contraction of the Moon.
• Thermal expansion and contraction of the retroreflector mounts.

For the terrestrial model, the IERS Conventions (2010) is a source of detailed information.{{Cite web|title=IERS - IERS Technical Notes - IERS Conventions (2010)|url=https://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36.html|access-date=2021-06-01|website=www.iers.org}}

Lunar laser ranging measurement data is available from the Paris Observatory Lunar Analysis Center,{{cite web |url=http://polac.obspm.fr/llrdatae.html |title=Lunar Laser Ranging Observations from 1969 to May 2013 |publisher=SYRTE Paris Observatory |access-date=3 June 2014}} the International Laser Ranging Service archives,{{Cite web|title=International Laser Ranging Service|url=ftp://cddis.gsfc.nasa.gov/pub/slr/data/npt_crd/}}{{Cite web|title=International Laser Ranging Service|url=ftp://edc.dgfi.tum.de/pub/slr/data/npt_crd/}} and the active stations. Some of the findings of this long-term experiment are:

• The distance to the Moon can be measured with millimeter precision.{{cite journal|last1=Battat|first1=J. B. R.|last2=Murphy|first2=T. W.|last3=Adelberger|first3=E. G.|last4=Gillespie|first4=B.|last5=Hoyle|first5=C. D.|last6=McMillan|first6=R. J.|last7=Michelsen|first7=E. L.|last8=Nordtvedt|first8=K.|last9=Orin|first9=A. E.|last10=Stubbs|first10=C. W.|last11=Swanson|first11=H. E.|display-authors=3|date=January 2009|title=The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range1|journal=Publications of the Astronomical Society of the Pacific|volume=121|issue=875|pages=29–40|bibcode=2009PASP..121...29B|doi=10.1086/596748|jstor=10.1086/596748|doi-access=free}}
• The Moon is spiraling away from Earth at a rate of {{val|3.8|u=cm|up=year}}.{{r|ApolloLaser}}{{Cite journal|last1=Williams|first1=James G.|last2=Boggs|first2=Dale H.|date=2016|title=Secular tidal changes in lunar orbit and Earth rotation|url=http://link.springer.com/10.1007/s10569-016-9702-3|journal=Celestial Mechanics and Dynamical Astronomy|language=en|volume=126|issue=1|pages=89–129|doi=10.1007/s10569-016-9702-3|bibcode=2016CeMDA.126...89W|s2cid=124256137|issn=0923-2958}} This rate has been described as anomalously high.{{Cite journal |last1=Bills |first1=B. G. |last2=Ray |first2=R. D. |year=1999 |title=Lunar Orbital Evolution: A Synthesis of Recent Results |journal=Geophysical Research Letters |volume=26 |issue=19 |pages=3045–3048 |bibcode=1999GeoRL..26.3045B |doi=10.1029/1999GL008348|doi-access=free }}
• The fluid core of the Moon was detected from the effects of core/mantle boundary dissipation.{{Cite journal|last1=Williams|first1=James G.|last2=Boggs|first2=Dale H.|last3=Yoder|first3=Charles F.|last4=Ratcliff|first4=J. Todd|last5=Dickey|first5=Jean O.|date=2001|title=Lunar rotational dissipation in solid body and molten core|journal=Journal of Geophysical Research: Planets|language=en|volume=106|issue=E11|pages=27933–27968|doi=10.1029/2000JE001396|bibcode=2001JGR...10627933W|doi-access=free}}
• The Moon has free physical librations that require one or more stimulating mechanisms.{{Cite journal|last1=Rambaux|first1=N.|last2=Williams|first2=J. G.|date=2011|title=The Moon's physical librations and determination of their free modes|journal=Celestial Mechanics and Dynamical Astronomy|volume=109|issue=1 |pages=85–100|doi=10.1007/s10569-010-9314-2|bibcode=2011CeMDA.109...85R |s2cid=45209988|url=https://hal.archives-ouvertes.fr/hal-00588671/file/PEER_stage2_10.1007%252Fs10569-010-9314-2.pdf}}
• Tidal dissipation in the Moon depends on tidal frequency.
• The Moon probably has a liquid core of about 20% of the Moon's radius.{{r|jwjd1}} The radius of the lunar core-mantle boundary is determined as {{val|381|12|u=km}}.{{r|viswanathan2019grl}}
• The polar flattening of the lunar core-mantle boundary is determined as {{val|2.2|0.6|e=-4}}.{{r|viswanathan2019grl}}
• The free core nutation of the Moon is determined as {{val|367|100|u=yr}}.{{cite journal|last1=Viswanathan|first1=V.|last2=Rambaux|first2=N.|last3=Fienga|first3=A.|last4=Laskar|first4=J.|last5=Gastineau|first5=M.|date=9 July 2019|title=Observational Constraint on the Radius and Oblateness of the Lunar Core-Mantle Boundary|journal=Geophysical Research Letters|volume=46|issue=13|pages=7295–7303|arxiv=1903.07205|doi=10.1029/2019GL082677|bibcode=2019GeoRL..46.7295V|s2cid=119508748}}
• Accurate locations for retroreflectors serve as reference points visible to orbiting spacecraft.{{Cite journal|last1=Wagner|first1=R. V.|last2=Nelson|first2=D. M.|last3=Plescia|first3=J. B.|last4=Robinson|first4=M. S.|last5=Speyerer|first5=E. J.|last6=Mazarico|first6=E.|date=2017|title=Coordinates of anthropogenic features on the Moon|journal=Icarus|language=en|volume=283|pages=92–103|doi=10.1016/j.icarus.2016.05.011|bibcode=2017Icar..283...92W|issn=0019-1035|doi-access=free}}

• Einstein's theory of gravity (the general theory of relativity) predicts the Moon's orbit to within the accuracy of the laser ranging measurements.{{r|jwjd1}}
• Gauge freedom plays a major role in a correct physical interpretation of the relativistic effects in the Earth-Moon system observed with LLR technique.{{cite journal |last1=Kopeikin |first1=S. |last2=Xie |first2=Y. |year=2010 |title=Celestial reference frames and the gauge freedom in the post-Newtonian mechanics of the Earth–Moon system |journal=Celestial Mechanics and Dynamical Astronomy |volume=108 |issue=3 |pages=245–263 |bibcode=2010CeMDA.108..245K |doi=10.1007/s10569-010-9303-5|s2cid=122789819 }}
• The likelihood of any Nordtvedt effect (a hypothetical differential acceleration of the Moon and Earth towards the Sun caused by their different degrees of compactness) has been ruled out to high precision,{{cite journal|last1=Adelberger|first1=E. G.|last2=Heckel|first2=B. R.|last3=Smith|first3=G.|last4=Su|first4=Y.|last5=Swanson|first5=H. E.|year=1990|title=Eötvös experiments, lunar ranging and the strong equivalence principle|journal=Nature|volume=347|issue=6290|pages=261–263|bibcode=1990Natur.347..261A|doi=10.1038/347261a0|s2cid=4286881}}{{cite journal|last1=Williams|first1=J. G.|last2=Newhall|first2=X. X.|last3=Dickey|first3=J. O.|year=1996|title=Relativity parameters determined from lunar laser ranging|journal=Physical Review D|volume=53|issue=12|pages=6730–6739|bibcode=1996PhRvD..53.6730W|doi=10.1103/PhysRevD.53.6730|pmid=10019959}}{{cite journal|last1=Viswanathan|first1=V|last2=Fienga|first2=A|last3=Minazzoli|first3=O|last4=Bernus|first4=L|last5=Laskar|first5=J|last6=Gastineau|first6=M|date=May 2018|title=The new lunar ephemeris INPOP17a and its application to fundamental physics|journal=Monthly Notices of the Royal Astronomical Society|volume=476|issue=2|pages=1877–1888|arxiv=1710.09167|doi=10.1093/mnras/sty096|doi-access=free|bibcode=2018MNRAS.476.1877V}} strongly supporting the strong equivalence principle.
• The universal force of gravity is very stable. The experiments have constrained the change in Newton's gravitational constant G to a factor of {{val|2|7|e=-13}} per year.{{cite journal|last1=Müller|first1=J.|last2=Biskupek|first2=L.|year=2007|title=Variations of the gravitational constant from lunar laser ranging data|journal=Classical and Quantum Gravity|volume=24|issue=17|page=4533|doi=10.1088/0264-9381/24/17/017|s2cid=120195732 }}

File:ALSEP AS14-67-9386.jpg|Apollo 14 Lunar Ranging Retro Reflector (LRRR)

File:LunarPhotons.png|APOLLO collaboration photon pulse return times

File:Wettzell Laser Ranging System.jpg|Laser ranging facility at Wettzell fundamental station, Bavaria, Germany

File:Goddard Spaceflight Center Laser Ranging Facility.jpg|Laser Ranging at Goddard Space Flight Center

{{Portal|Solar System}}

• Carroll Alley (first principal investigator of the Apollo Lunar Laser Ranging team)
• Lidar
• Lunar distance (astronomy)
• Satellite laser ranging
• Space geodesy
• Third-party evidence for Apollo Moon landings
• List of artificial objects on the Moon

{{Notelist-lr}}

{{reflist}}

• [http://www.issibern.ch/teams/lunarlaser/#Team "Theory and Model for the New Generation of the Lunar Laser Ranging Data"] by Sergei Kopeikin
• [http://www.lpi.usra.edu/lunar/missions/apollo/apollo_15/experiments/lrr/ Apollo 15 Experiments - Laser Ranging Retroreflector] by the Lunar and Planetary Institute
• [http://www.csr.utexas.edu/mlrs/history.html "History of Laser Ranging and MLRS"] by the University of Texas at Austin, Center for Space Research
• [http://physics.ucsd.edu/~tmurphy/apollo/lrrr.html "Lunar Retroreflectors"] by Tom Murphy
• [https://web.archive.org/web/20061112025133/http://www.obs-azur.fr/cerga/laser/laslune/llr.htm Station de Télémétrie Laser-Lune] in Grasse, France
• [http://ilrs.gsfc.nasa.gov/science/scienceContributions/lunar.html Lunar Laser Ranging] from International Laser Ranging Service
• [http://www.washington.edu/news/2002/01/14/uw-researcher-plans-project-to-pin-down-moons-distance-from-earth/ "UW researcher plans project to pin down moon's distance from Earth"] by Vince Stricherz, UW Today, 14 January 2002
• [https://science.nasa.gov/science-news/science-at-nasa/2004/21jul_llr/ "What Neil & Buzz Left on the Moon"] by Science@NASA, 20 July 2004
• [http://www.cnn.com/TECH/space/9907/21/apollo.experiment/ "Apollo 11 Experiment Still Returning Results"] by Robin Lloyd, CNN, 21 July 1999
• [https://www.youtube.com/watch?v=WbOBbZ6A51k "Shooting Lasers at the Moon: Hal Walker and the Lunar Retroreflector"] by Smithsonian National Air and Space Museum, YouTube, 20 Aug 2019

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