Measuring the distance between the Earth and the Moon with laser light
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] and because the return will be more evenly reflected with less diffusion.
Laser ranging measurements can also be made with retroreflectors installed on Moon-orbiting satellites such as the LRO.[5][6]
^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.
^"Chandrayaan-3". ISRO. Retrieved 15 August 2023.
^Dhillon, Amrit (23 August 2023). "India lands spacecraft near south pole of moon in historic first". The Guardian. Retrieved 23 August 2023.
^Müller, Jürgen; Murphy, Thomas W.; Schreiber, Ulrich; Shelus, Peter J.; Torre, Jean-Marie; Williams, James G.; Boggs, Dale H.; Bouquillon, Sebastien; Bourgoin, Adrien; Hofmann, Franz (2019). "Lunar Laser Ranging: a tool for general relativity, lunar geophysics and Earth science". Journal of Geodesy. 93 (11): 2195–2210. Bibcode:2019JGeod..93.2195M. doi:10.1007/s00190-019-01296-0. ISSN 1432-1394. S2CID 202641440.
^Mazarico, Erwan; Sun, Xiaoli; Torre, Jean-Marie; Courde, Clément; Chabé, Julien; Aimar, Mourad; Mariey, Hervé; Maurice, Nicolas; Barker, Michael K.; Mao, Dandan; Cremons, Daniel R.; Bouquillon, Sébastien; Carlucci, Teddy; Viswanathan, Vishnu; Lemoine, Frank; Bourgoin, Adrien; Exertier, Pierre; Neumann, Gregory; Zuber, Maria; Smith, David (6 August 2020). "First two-way laser ranging to a lunar orbiter: infrared observations from the Grasse station to LRO's retro-reflector array". Earth, Planets and Space. 72 (1): 113. Bibcode:2020EP&S...72..113M. doi:10.1186/s40623-020-01243-w. hdl:11603/19523. ISSN 1880-5981.
^Kornei, Katherine (15 August 2020). "How Do You Solve a Moon Mystery? Fire a Laser at It". The New York Times. ISSN 0362-4331. Retrieved 1 June 2021.
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