Lunar Laser Ranging experiments

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Lunar Laser Ranging Experiment from the Apollo 11 mission

Lunar Laser Ranging (LLR) is the practice of measuring

the distance between the surfaces of the Earth and the Moon using laser ranging. The distance can be calculated from the round-trip time of laser light pulses travelling at the speed of light, which are reflected back to Earth by the Moon's surface or by one of several retroreflectors installed on the Moon. Three were placed by the United States' Apollo program (11, 14, and 15), two by the Soviet Lunokhod 1 and 2 missions,[1] and one by India's Chandrayaan-3 mission.[2][3]

Although it is possible to reflect light or radio waves directly from the Moon's surface (a process known as EME), a much more precise range measurement can be made using retroreflectors, since because of their small size, the temporal spread in the reflected signal is much smaller.[4]

Laser ranging measurements can also be made with retroreflectors installed on Moon-orbiting satellites such as the LRO.[5][6]

History

Apollo 15 LRRR
Apollo 15 LRRR schematic

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.[7] Similar measurements were obtained later the same year by a Soviet team at the Crimean Astrophysical Observatory using a Q-switched ruby laser.[8]

Shortly thereafter,

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.[10] Lunokhod 2's array continues to return signals to Earth.[11] The Lunokhod arrays suffer from decreased performance in direct sunlight—a factor considered in reflector placement during the Apollo missions.[12]

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

MX-1E lander, was designed to increase measurement accuracy up to 100 times over existing systems.[13][14][15] MX-1E was set to launch in July 2020,[16] however, as of February 2020, the launch of the MX-1E has been canceled.[17] India's Chandrayaan-3 lunar lander successfully placed a sixth reflector on the Moon in August 2023.[3] MoonLIGHT will be launched in early 2024 with a Commercial Lunar Payload Services (CLPS) mission.[18]

Principle

Annotated image of the near side of the Moon showing the location of retroreflectors left on the surface by Apollo and Lunokhod missions[19]

The distance to the Moon is calculated approximately using the equation: 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.[20][21] The distance continually changes for a number of reasons, but averages 385,000.6 km (239,228.3 mi) between the center of the Earth and the center of the Moon.[22] The orbits of the Moon and planets are integrated numerically along with the orientation of the Moon called physical libration.[23]

At the Moon's surface, the beam is about 6.5 kilometers (4.0 mi) wide

monochromatic
.

As of 2009, the distance to the Moon can be measured with millimeter precision.[27] 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.

List of retroreflectors

List of observatories

The table below presents a list of active and inactive Lunar Laser Ranging stations on Earth.[22][28]

Lunar Laser Ranging stations
Observatory Project Operating timespan Telescope Laser Range accuracy Ref.
McDonald Observatory, Texas, US MLRS 1969–1985

1985–2013

2.7 m 694 nm, 7 J

532 nm, 200 ps, 150 mJ

[29]

[22]

Crimean Astrophysical Observatory (CrAO), USSR 1974, 1982–1984 694 nm 3.0–0.6 m [30]
Côte d'Azur Observatory (OCA), Grasse, France MeO 1984–1986

1986–2010

2010–present (2021)

694 nm

532 nm, 70 ps, 75 mJ

532/1064 nm

[22][31]
Haleakala Observatory, Hawaii, US LURE 1984–1990 532 nm, 200 ps, 140 mJ 2.0 cm [22][32]
Matera Laser Ranging Observatory (MLRO), Italy 2003–present (2021) 532 nm
Apache Point Observatory, New Mexico, US APOLLO 2006–2021

2021–present (2023)

532 nm, 100 ps, 115 mJ 1.1 mm [22]

[33]

Geodetic Observatory Wettzell, Germany WLRS 2018–present (2021) 1064 nm, 10 ps, 75 mJ [34]
Yunnan Astronomical Observatory, Kunming, China 2018 1.2 m 532 nm, 10 ns, 3 J meter level [35]

Data analysis

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.[36][23]
  • The same program integrates the 3-axis orientation of the Moon called physical Libration.

The range model includes[36][37]

  • 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.[38]

Results

Lunar laser ranging measurement data is available from the Paris Observatory Lunar Analysis Center,[39] the International Laser Ranging Service archives,[40][41] and the active stations. Some of the findings of this long-term experiment are:[22]

Properties of the Moon

  • The distance to the Moon can be measured with millimeter precision.[27]
  • The Moon is spiraling away from Earth at a rate of 3.8 cm/year.[24][42] This rate has been described as anomalously high.[43]
  • The fluid core of the Moon was detected from the effects of core/mantle boundary dissipation.[44]
  • The Moon has free physical
    librations that require one or more stimulating mechanisms.[45]
  • Tidal dissipation in the Moon depends on tidal frequency.[42]
  • The Moon probably has a liquid core of about 20% of the Moon's radius.[11] The radius of the lunar core-mantle boundary is determined as 381±12 km.[46]
  • The polar flattening of the lunar core-mantle boundary is determined as (2.2±0.6)×10−4.[46]
  • The free core nutation of the Moon is determined as 367±100 yr.[46]
  • Accurate locations for retroreflectors serve as reference points visible to orbiting spacecraft.[47]

Gravitational physics

Gallery

See also

References

  1. ^ During the round-trip time, an Earth observer will have moved by around 1 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.
  1. .
  2. ^ "Chandrayaan-3". ISRO. Retrieved 15 August 2023.
  3. ^ a b Dhillon, Amrit (23 August 2023). "India lands spacecraft near south pole of moon in historic first". The Guardian. Retrieved 23 August 2023.
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  9. ^ a b Newman, Michael E. (26 September 2017). "To the Moon and Back … in 2.5 Seconds". NIST. Retrieved 27 January 2021.
  10. ^ McDonald, K. (26 April 2010). "UC San Diego Physicists Locate Long Lost Soviet Reflector on Moon". University of California, San Diego. Archived from the original on 30 April 2010. Retrieved 27 April 2010.
  11. ^ a b c Williams, James G.; Dickey, Jean O. (2002). Lunar Geophysics, Geodesy, and Dynamics (PDF). 13th International Workshop on Laser Ranging. 7–11 October 2002. Washington, D. C.
  12. ^ "It's Not Just The Astronauts That Are Getting Older". Universe Today. 10 March 2010. Retrieved 24 August 2012.
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  14. ^ Tune, Lee (10 June 2015). "UMD, Italy & MoonEx Join to Put New Laser-Reflecting Arrays on Moon". UMD Right Now. University of Maryland. Archived from the original on 22 March 2018. Retrieved 21 March 2018.
  15. ^ Boyle, Alan (12 July 2017). "Moon Express unveils its roadmap for giant leaps to the lunar surface ... and back again". GeekWire. Retrieved 15 March 2018.
  16. ^ Moon Express Lunar Scout (MX-1E), RocketLaunch.Live, archived from the original on 27 July 2019, retrieved 27 July 2019
  17. ^ "MX-1E 1, 2, 3". Retrieved 24 May 2020.
  18. ^ "NASA Payloads for (CLPS PRISM) CP-11".
  19. ^ "Was Galileo Wrong?". NASA. 6 May 2004. Archived from the original on 30 April 2022.
  20. OCLC 52258226
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  21. ^ Williams, James G.; Boggs, Dale H. (2020). "The JPL Lunar Laser range model 2020". ssd.jpl.nasa.gov. Retrieved 24 May 2021.
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  24. ^ a b Espenek, F. (August 1994). "NASA – Accuracy of Eclipse Predictions". NASA/GSFC. Retrieved 4 May 2008.
  25. ^ "The Basics of Lunar Ranging". Retrieved 21 July 2023.
  26. PMID 28163616
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  30. ^ Yagudina (2018). "Processing and analysis of lunar laser ranging observations in Crimea in 1974-1984". Institute of Applied Astronomy of the Russian Academy of Sciences. Retrieved 1 June 2021.
  31. S2CID 212785296
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  32. ^ "Lure Observatory". Institute for Astronomy, University of Hawaii. 29 January 2002. Retrieved 3 June 2021.
  33. ^ "APOL - Apache Point Observatory".
  34. S2CID 155720383
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  37. ^ Williams, James G.; Boggs, Dale H. (2020). "The JPL Lunar Laser range model 2020". ssd.jpl.nasa.gov. Retrieved 1 June 2021.
  38. ^ "IERS - IERS Technical Notes - IERS Conventions (2010)". www.iers.org. Retrieved 1 June 2021.
  39. ^ "Lunar Laser Ranging Observations from 1969 to May 2013". SYRTE Paris Observatory. Retrieved 3 June 2014.
  40. ^ "International Laser Ranging Service".
  41. ^ "International Laser Ranging Service".
  42. ^
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External links