Lunar Laser Ranging experiment
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 five retroreflectors installed on the Moon during the Apollo program (11, 14, and 15) and Lunokhod 1 and 2 missions. 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. A review of Lunar Laser Ranging is available. Laser ranging measurements can also be made with retroreflectors installed on Moon-orbiting satellites such as the LRO. History. 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. Similar measurements were obtained later the same year by a Soviet team at the Crimean Astrophysical Observatory using a Q-switched ruby laser. Shortly thereafter, Princeton University graduate student James Faller proposed placing optical reflectors on the Moon to improve the accuracy of the measurements. 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 . "Lunokhod 2" 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 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. MX-1E was set to launch in July 2020, however, as of February 2020, the launch of the MX-1E has been canceled. Principle. The distance to the Moon is calculated using the equation: . 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. The distance continually changes for a number of reasons, but averages between the center of the Earth and the  center of the Moon. The orbits of the Moon and planets are integrated numerically along with the orientation of the Moon called physical Libration. At the Moon's surface, the beam is about wide and scientists liken the task of aiming the beam to using a rifle to hit a moving dime away. The reflected light is too weak to see with the human eye. Out of  photons aimed at the reflector, only one is received back on Earth, even under good conditions. 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. List of observatories. The table below presents a list of active and inactive Lunar Laser Ranging stations on Earth. 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 range model includes For the terrestrial model, the IERS Conventions (2010) is a source of detailed information. Results. Lunar laser ranging measurement data is available from the Paris Observatory Lunar Analysis Center, the International Laser Ranging Service archives, and the active stations. Some of the findings of  this long-term experiment are: