The ongoing
Lunar Laser Ranging Experiment measures the
distanceIn astronomy, a lunar distance is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,403 kilometers...
between the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
using
laser rangingLIDAR is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses...
.
LaserA laser is a device that emits light through a process called stimulated emission. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses...
s on Earth are aimed at
retroreflectorA retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
s previously planted on the Moon and the time delay for the reflected light to return is determined. Since the
speed of lightIn physics, the speed of light is a physical constant, the speed at which electromagnetic radiation, such as light, travels in free space . Its value is 299,792,458 metres per second...
is known with very high accuracy, the distance to the moon can be calculated. This distance has been measured with increasing accuracy for more than 35 years.
The distance continually changes for a number of reasons, but averages about 384,467 kilometers (238,897 miles).
The experiment was first made possible by a retroreflector array installed on July 21, 1969, by the crew of
Apollo 11The Apollo 11 mission was the first human spaceflight to land on the Moon. Launched on July 16, 1969, it carried Mission Commander Neil Alden Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin Eugene 'Buzz' Aldrin, Jr...
. Two more retroreflector arrays left by the
Apollo 14Apollo 14 was the eighth manned mission in the Apollo program and the third mission to land on the Moon. The nine-day mission was launched on January 31, 1971, with lunar touch down on February 5. The Lunar Module landed in the Fra Mauro formation; this had originally been the target of the...
and
Apollo 15Apollo 15 was the ninth manned mission in the Apollo program and the fourth mission to land on the Moon. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous missions. The mission began on July 26,...
missions have contributed to the experiment.
The unmanned Soviet
Lunokhod 1Lunokhod 1 was the first of two unmanned lunar rovers landed on the Moon by the Soviet Union as part of its Lunokhod program. The spacecraft which carried Lunokhod 1 was named Luna 17...
and
Lunokhod 2Lunokhod 2 was the second of two unmanned lunar rovers landed on the Moon by the Soviet Union as part of the Lunokhod program....
rovers carried smaller arrays. Reflected signals were initially received from
Lunokhod 1, but no return signals have been detected since 1971, at least in part due to some uncertainty in its location on the Moon.
Lunokhod 2's array continues to return signals to Earth.
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
McDonald ObservatoryThe McDonald Observatory is an astronomical observatory located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Fowlkes and Mount Locke in the Davis Mountains of West Texas....
and the OCA Laser-Lune
telemetryTelemetry is a technology that allows remote measurement and reporting of information. The word is derived from Greek roots tele = remote, and metron = measure. Systems that need external instructions and data to operate require the counterpart of telemetry, telecommand.Although the term commonly...
station affiliated with the
Côte d'Azur ObservatoryThe Observatoire de la Côte d'Azur originated in 1988 with the merger of two observatories:# Observatoire de Nice# The CERGA - External links :*...
.
At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide and scientists liken the task of aiming the beam to using a rifle to hit a moving
dimeDime or DIME may refer to:* Dime , ten cents* Dime , ten cents* Dime Bag , slang for ten dollars worth of Marijuana* Dime , an assist in basketball...
3 kilometers (two miles) away. The reflected light is too weak to be seen with the human eye, but under good conditions, one photon will be received every few seconds (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 one hundredth of an inch. As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy.
Some of the findings of this long-term experiment are:
- The moon is spiralling away from Earth at a rate of 38 mm per year.
- The moon probably has a liquid core of about 20% of the Moon's radius.
- The universal force of gravity
Gravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
is very stable. The experiments have put an upper limit on the change in Newton'sSir Isaac Newton FRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian who is perceived and considered by a substantial number of scholars and the general public as one of the most influential men in history...
gravitational constantThe gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
G of less than 1 part in 1011 since 1969.
- The likelihood of any "Nordtvedt effect"
In theoretical astrophysics, the Nordtvedt effect refers to the relative motion between the Earth and the Moon which would be observed if the gravitational self-energy of a body contributed to its gravitational mass but not its inertial mass...
(a composition-dependent differential acceleration of the Moon and Earth towards the Sun) has been ruled out to high precision, strongly supporting the validity of the Strong Equivalence Principle.
- Einstein's
Albert Einstein was a theoretical physicist. His many contributions to physics include the special and general theories of relativity, the founding of relativistic cosmology, the first post-Newtonian expansion, explaining the perihelion advance of Mercury, prediction of the deflection of...
theory of gravity (the general theory of relativityGeneral relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics. It unifies special relativity and Newton's law of universal gravitation, and describes gravity as a...
) predicts the moon's orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
to within the accuracy of the laser ranging measurements.
Additionally, the accuracy of these experiments has improved historic knowledge of the Moon's orbit enough to permit timing of solar eclipses up to 3,400 years ago.
The presence of reflectors on the Moon has been used to rebut claims that the Apollo landings were faked.
For example, the
APOLLO CollaborationThe Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO, is a project at the Apache Point Observatory in New Mexico. It is an extension and advancement of previous Lunar Laser Ranging Experiment, which uses retroreflectors on the Moon to track changes in lunar orbital distance and...
photon pulse return graph, shown here, has a pattern consistent with a retroreflector array near a known landing site.
See also
- Lunar distance (astronomy)
In astronomy, a lunar distance is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,403 kilometers...
- LIDAR
LIDAR is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses...
- Carroll Alley
Carroll Alley is an American physicist. He was the principal investigator on the Apollo Program's Lunar Laser Ranging Experiment, which significantly restricted the possible range of spatial variation of the strength of the gravitational interaction...
, (principal investigator of Apollo's reflector experiment)
- Lunokhod programme
Lunokhod 1 and 2 were a pair of Soviet robotic lunar rovers landed on the Moon in 1970 and 1973, respectively. They were in operation concurrently with the Zond series of flyby missions. The Lunokhod missions were primarily designed to explore the surface and return pictures...
- Apache Point Observatory Lunar Laser-ranging Operation
The Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO, is a project at the Apache Point Observatory in New Mexico. It is an extension and advancement of previous Lunar Laser Ranging Experiment, which uses retroreflectors on the Moon to track changes in lunar orbital distance and...
- Apollo Lunar Surface Experiments Package
- Independent evidence for Apollo Moon landings
Third-party evidence for Apollo Moon landings is evidence or analysis of evidence from groups external to NASA or the U.S. government, that demonstrates that NASA conducted manned Moon landings. One of the reasons for such an endeavor is to counter the Apollo Moon Landing hoax theories, by showing...
External links