Moon (Earth's natural satellite)

Moon (Earth's natural satellite)
Name	Moon
Other names	Luna, Selene
Type	Natural satellite
Radius km	1737.4
Mass kg	7.34767309e22
Gravity m s2	1.62
Orbital period days	27.321661
Discovered	Prehistory

Moon (Earth's natural satellite) The Moon is Earth's only permanent natural satellite, visible from Earth as a dominant celestial object and central to historical navigation, calendar systems, and cultural mythology. It influences Earth's tides, stabilizes axial tilt, and serves as a primary target for space exploration programs led by organizations and nations including NASA, Roscosmos, European Space Agency, China National Space Administration, and private companies such as SpaceX.

Overview

The Moon occupies a unique place in comparative planetology and in studies by institutions like the Smithsonian Institution and observatories such as Mount Wilson Observatory, Palomar Observatory, and Mauna Kea Observatories. Its synchronous rotation produces a near side and far side that were mapped by missions such as Luna 3, Apollo 8, Lunar Reconnaissance Orbiter, and Chang'e 4. Scientific inquiries by researchers affiliated with universities including Massachusetts Institute of Technology, Caltech, University of Oxford, and University of Cambridge explore lunar geology, regolith, and impact history. The Moon also features in treaties and policies shaped by bodies like the United Nations Office for Outer Space Affairs and agreements such as the Outer Space Treaty.

Formation and Origin

Leading hypotheses from planetary scientists at institutions like California Institute of Technology and University of Arizona support the Giant Impact hypothesis, proposing a collision between proto-Earth and a Mars-sized body often called Theia. Computer simulations run on supercomputers at labs such as Los Alamos National Laboratory and published by teams linked to Harvard University and Princeton University model debris disk evolution and angular momentum transfer that produced the current Earth–Moon system. Alternative models invoking capture or co-formation were advanced historically in works at Royal Society meetings and journals like Nature and Science, but isotopic similarities in oxygen, titanium, and tungsten between lunar samples returned by Apollo program missions and terrestrial specimens bolster the giant impact scenario.

Physical Characteristics

The Moon's mean radius (about 1,737 km) and mass define a surface gravity of ~1.62 m/s², measured and refined by instruments on Apollo 11, Apollo 12, and Lunar Prospector. Its bulk composition—dominated by silicate minerals similar to Earth's mantle—was characterized by analyses at labs including Jet Propulsion Laboratory and the Johnson Space Center. The lunar crust, mantle, and possible small core were constrained by seismic data from Apollo seismometers and modern gravity mapping by GRAIL. Surface temperature extremes were recorded by missions such as Lunar Reconnaissance Orbiter and instruments developed at Jet Propulsion Laboratory and European Space Agency labs.

Orbit and Rotation

The Moon orbits Earth with a sidereal period of ~27.3 days and a synodic period of ~29.5 days; its orbital elements, including eccentricity and inclination, are tracked by observatories like Greenwich Observatory and radar facilities such as Arecibo Observatory (prior to collapse). Tidal interactions between Earth and the Moon cause the lunar orbit to recede at ~3.8 cm/yr, a rate measured through lunar laser ranging experiments conducted by teams at MIT, University of Maryland, and University of Hawaii. The Moon's synchronous rotation results from tidal locking, a phenomenon discussed in literature from researchers affiliated with Royal Astronomical Society and modeled in papers in The Astrophysical Journal.

Geology and Surface Features

Lunar geology includes maria, highlands, rilles, and cratered terrains named and cataloged in atlases produced by US Geological Survey and imagery from missions like Clementine and Lunar Reconnaissance Orbiter. Major maria such as Mare Imbrium, Mare Tranquillitatis, and Oceanus Procellarum were sites of sample return by Apollo 11, Apollo 12, and Chang'e 5, enabling geochronology by teams at Smithsonian Institution and university isotope labs. Impact basins including the South Pole–Aitken Basin were studied by researchers at Brown University and University of Colorado Boulder for insights into late heavy bombardment hypotheses debated in conferences of the International Astronomical Union. Volcanic features and basaltic plains were examined in analyses published by groups at University of California, Los Angeles and Stanford University.

Atmosphere and Exosphere

The Moon lacks a substantial atmosphere; instead it possesses a tenuous exosphere composed of trace gases such as helium, argon, sodium, and potassium detected by instruments from missions including Apollo, LADEE, and Chandrayaan-1. Studies by scientists at Arizona State University and University of Bern measured transient phenomena like lunar horizon glow and surface-bound exospheric processes. Solar wind interactions with the regolith, investigated by teams at Max Planck Institute for Solar System Research and Instituto de Astrofísica de Canarias, contribute to sputtering and implantation effects that alter surface chemistry.

Exploration and Human Interaction

Human interaction with the Moon spans observation by ancient cultures in regions now part of China, Mesopotamia, and Greece to modern exploration programs conducted by NASA, Roscosmos, CNSA, and private firms. The Apollo program achieved crewed landings at sites including Tranquility Base and returned samples curated at the Smithsonian Institution and Lunar Sample Laboratory Facility. Recent missions—Lunar Reconnaissance Orbiter, Chandrayaan-2, Chang'e 4, and the Artemis program—represent international and commercial collaboration, with scientific payloads developed by teams at European Space Agency, Canadian Space Agency, and research universities worldwide. Future plans for lunar bases, resource prospecting, and in-situ science involve partnerships among entities such as NASA, ESA, JAXA, and commercial contractors including SpaceX and Blue Origin.

Category:Natural satellites of Earth