Far side of the Moon

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Far side of the Moon, photographed by Apollo 16 Back side of the Moon AS16-3021.jpg
Far side of the Moon, photographed by Apollo 16

The far side of the Moon is the hemisphere of the Moon that always faces away from Earth. The far side's terrain is rugged with a multitude of impact craters and relatively few flat lunar maria. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. Both sides of the Moon experience two weeks of sunlight followed by two weeks of night; the far side is sometimes called the "dark side of the Moon", meaning unseen rather than lacking light. [1] [2] [3] [4]

Moon Earths natural satellite

The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits. The Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known.

Lunar mare large, dark, basaltic plains on Earths Moon

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh), and sinus (bay). The last three are smaller than maria, but have the same nature and characteristics.

Solar System planetary system of the Sun

The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.


About 18 percent of the far side is occasionally visible from Earth due to libration. The remaining 82 percent remained unobserved until 1959, when it was photographed by the Soviet Luna 3 space probe. The Soviet Academy of Sciences published the first atlas of the far side in 1960. The Apollo 8 astronauts were the first humans to see the far side with the naked eye when they orbited the Moon in 1968. All manned and unmanned soft landings had taken place on the near side of the Moon, until 3 January 2019 when the Chang'e 4 spacecraft made the first landing on the far side. [5]

Libration perceived oscillating motion of orbiting bodies relative to each other

In astronomy, libration is the wagging of the Moon perceived by Earth-bound observers caused by changes in their perspective. It permits an observer to see slightly different halves of the surface at different times. It is similar in both cause and effect to the changes in the Moon's apparent size due to changes in distance. It is caused by three mechanisms detailed below, two of which are causing a relatively tiny physical libration via tidal forces exerted by the Earth. Such true librations are known as well for other moons with locked rotation.

Luna 3 lunar probe

Luna 3, or E-2A No.1 was a Soviet spacecraft launched in 1959 as part of the Luna programme. It was the first-ever mission to photograph the far side of the Moon and the third Soviet space probe to be sent to the neighborhood of the Moon. Though it returned rather poor pictures by later standards, the historic, never-before-seen views of the far side of the Moon caused excitement and interest when they were published around the world, and a tentative Atlas of the Far Side of the Moon was created after image processing improved the pictures.

Apollo 8 manned spaceflight mission in the Apollo program and first manned mission to orbit the Moon

Apollo 8, the second manned spaceflight mission flown in the United States Apollo space program, was launched on December 21, 1968, and became the first manned spacecraft to leave low Earth orbit, reach the Moon, orbit it, and return. The three-astronaut crew—Frank Borman, James Lovell, and William Anders—were the first humans to witness and photograph an Earthrise and to escape the gravity of a celestial body. Apollo 8 was the third flight and the first crewed launch of the Saturn V rocket and was the first human spaceflight from the Kennedy Space Center, located adjacent to Cape Canaveral Air Force Station in Florida.

Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth. [6]

Radio telescope form of directional radio antenna used in radio astronomy

A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky in radio astronomy. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects, just as optical telescopes are the main observing instrument used in traditional optical astronomy which studies the light wave portion of the spectrum coming from astronomical objects. Radio telescopes are typically large parabolic ("dish") antennas similar to those employed in tracking and communicating with satellites and space probes. They may be used singly or linked together electronically in an array. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night. Since astronomical radio sources such as planets, stars, nebulas and galaxies are very far away, the radio waves coming from them are extremely weak, so radio telescopes require very large antennas to collect enough radio energy to study them, and extremely sensitive receiving equipment. Radio observatories are preferentially located far from major centers of population to avoid electromagnetic interference (EMI) from radio, television, radar, motor vehicles, and other manmade electronic devices.


Due to tidal locking, the inhabitants of the central body (Earth) will never be able to see the satellite's (Moon) green area Synchronous rotation.svg
Due to tidal locking, the inhabitants of the central body (Earth) will never be able to see the satellite's (Moon) green area

Tidal forces from Earth have slowed down the Moon's rotation to the point where the same side is always facing the Earth—a phenomenon called tidal locking. The other face, most of which is never visible from the Earth, is therefore called the "far side of the Moon". Over time, some parts of the far side can be seen due to libration. [7] In total, 59 percent of the Moon's surface is visible from Earth at one time or another. Useful observation of the parts of the far side of the Moon occasionally visible from Earth is difficult because of the low viewing angle from Earth (they cannot be observed "full on").

The tidal force is an apparent force that stretches a body towards and away from the center of mass of another body due to a gradient in gravitational field from the other body; it is responsible for diverse phenomena, including tides, tidal locking, breaking apart of celestial bodies and formation of ring systems within Roche limit, and in extreme cases, spaghettification of objects. It arises because the gravitational field exerted on one body by another is not constant across its parts: the nearest side is attracted more strongly than the farthest side. It is this difference that causes a body to get stretched. Thus, the tidal force is also known as the differential force, as well as a secondary effect of the gravitational field.

Earth Third planet from the Sun in the Solar System

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.

Tidal locking situation in which an astronomical objects orbital period matches its rotational period

Tidal locking occurs when the long-term interaction between a pair of co-orbiting astronomical bodies drives the rotation rate of at least one of them into the state where there is no more net transfer of angular momentum between this body and its orbit around the second body ; this condition of "no net transfer" must be satisfied over the course of one orbit around the second body. This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit, as there can be some back and forth transfer over the course of an orbit. This effect arises from the gravitational gradient between the co-orbiting bodies, acting over a sufficiently long period of time.

The phrase "dark side of the Moon" does not refer to "dark" as in the absence of light, but rather "dark" as in unknown: until humans were able to send spacecraft around the Moon, this area had never been seen. [1] [2] [3] [4] While many misconstrue this to think that the "dark side" receives little to no sunlight, in reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. However, the near side also receives sunlight reflected from the Earth, known as earthshine. Earthshine does not reach the area of the far side that cannot be seen from Earth. Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark. The word "dark" has expanded to refer also to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon, during Apollo space missions for example. [8]

Full moon lunar phase: completely illuminated disc

The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon. This means that the lunar hemisphere facing Earth – the near side – is completely sunlit and appears as a circular disk, while the far side is dark. The full moon occurs once roughly every month.


Detailed view by the Lunar Reconnaissance Orbiter (LRO) Moon Farside LRO.jpg
Detailed view by the Lunar Reconnaissance Orbiter (LRO)
The Moon transits across the Earth as seen by the DSCOVR satellite, with its far side entirely visible Dscovrepicmoontransitfull.gif
The Moon transits across the Earth as seen by the DSCOVR satellite, with its far side entirely visible

The two hemispheres of the Moon have distinctly different appearances, with the near side covered in multiple, large maria (Latin for 'seas,' since the earliest astronomers incorrectly thought that these plains were seas of lunar water). The far side has a battered, densely cratered appearance with few maria. Only 1% of the surface of the far side is covered by maria, [9] [10] compared to 31.2% on the near side. One commonly accepted explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer. While other factors, such as surface elevation and crustal thickness, could also affect where basalts erupt, these do not explain why the far side South Pole–Aitken basin (which contains the lowest elevations of the Moon and possesses a thin crust) was not as volcanically active as Oceanus Procellarum on the near side.

Lunar water water molecules present on the Moon, either as ice in permanently shadowed craters at the lunar poles, or as vapor in the thin lunar atmosphere

Lunar water is water that is present on the Moon. Liquid water cannot persist at the Moon's surface, and water vapor is decomposed by sunlight, with hydrogen quickly lost to outer space. However, scientists have conjectured since the 1960s that water ice could survive in cold, permanently shadowed craters at the Moon's poles. Water molecules are also detected in the thin layer of gases above the lunar surface.

Geochemical modeling is the practice of using chemical thermodynamics, chemical kinetics, or both, to analyze the chemical reactions that affect geologic systems, commonly with the aid of a computer. It is used in high-temperature geochemistry to simulate reactions occurring deep in the Earth's interior, in magma, for instance, or to model low-temperature reactions in aqueous solutions near the Earth's surface, the subject of this article.

<i>Lunar Prospector</i> Third mission of the Discovery program; polar orbital reconnaissance of the Moon

Lunar Prospector was the third mission selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition including polar ice deposits, measurements of magnetic and gravity fields, and study of lunar outgassing events. The mission ended July 31, 1999, when the orbiter was deliberately crashed into a crater near the lunar south pole after the presence of water ice was successfully detected.

It has also been proposed that the differences between the two hemispheres may have been caused by a collision with a smaller companion moon that also originated from the Theia collision. [11] In this model, the impact led to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness that may be consistent with the dimensions of the far side highlands.

The far side has more visible craters. This was thought to be a result of the effects of lunar lava flows, which cover and obscure craters, rather than a shielding effect from the Earth. NASA calculates that the Earth obscures only about 4 square degrees out of 41,000 square degrees of the sky as seen from the Moon. "This makes the Earth negligible as a shield for the Moon [and] it is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters visible on the near side than the far side, even though both sides have received the same number of impacts." [12]

Newer research suggests that heat from Earth at the time when the Moon was formed is the reason the near side has fewer impact craters. The lunar crust consists primarily of plagioclases formed when aluminium and calcium condensed and combined with silicates in the mantle. The cooler, far side experienced condensation of these elements sooner and so formed a thicker crust; meteoroid impacts on the near side would sometimes penetrate the thinner crust here and release basaltic lava that created the maria, but would rarely do so on the far side. [13]


The October 7, 1959, image by Luna 3 which revealed, for the first time, the far side of the Moon Luna 3 moon.jpg
The October 7, 1959, image by Luna 3 which revealed, for the first time, the far side of the Moon

Until the late 1950s, little was known about the far side of the Moon. Librations of the Moon periodically allowed limited glimpses of features near the lunar limb on the far side, but only up to 59% of the total surface of the moon. [14] These features, however, were seen from a low angle, hindering useful observation. (It proved difficult to distinguish a crater from a mountain range.) The remaining 82% of the surface on the far side remained unknown, and its properties were subject to much speculation.

An example of a far side feature that can be seen through libration is the Mare Orientale, which is a prominent impact basin spanning almost 1,000 km (600 miles), yet this was not even named as a feature until 1906, by Julius Franz in Der Mond. The true nature of the basin was discovered in the 1960s when rectified images were projected onto a globe. The basin was photographed in fine detail by Lunar Orbiter 4 in 1967.

Before space exploration began, astronomers did not expect that the far side would be different from the side visible to Earth. [10] On October 7, 1959, the Soviet probe Luna 3 took the first photographs of the lunar far side, eighteen of them resolvable, [15] [10] covering one-third of the surface invisible from the Earth. [16] The images were analysed, and the first atlas of the far side of the Moon was published by the USSR Academy of Sciences on November 6, 1960. [17] [18] It included a catalog of 500 distinguished features of the landscape. [19] A year later, the first globe (1:13600000 scale) [20] containing lunar features invisible from the Earth was released in the USSR, based on images from Luna 3. [21] On July 20, 1965, another Soviet probe, Zond 3, transmitted 25 pictures of very good quality of the lunar far side, [22] with much better resolution than those from Luna 3. In particular, they revealed chains of craters, hundreds of kilometers in length, [16] but, unexpectedly, no mare plains like those visible from Earth with the naked eye. [10] In 1967, the second part of the Atlas of the Far Side of the Moon was published in Moscow, [23] [24] based on data from Zond 3, with the catalog now including 4,000 newly discovered features of the lunar far side landscape. [16] In the same year, the first Complete Map of the Moon (1:5000000 scale [20] ) and updated complete globe (1:10000000 scale), featuring 95 percent of the lunar surface, [20] were released in the Soviet Union. [25] [26]

As many prominent landscape features of the far side were discovered by Soviet space probes, Soviet scientists selected names for them. This caused some controversy, and the International Astronomical Union, leaving many of those names intact, later assumed the role of naming lunar features on this hemisphere.

On April 26, 1962, NASA's Ranger 4 space probe became the first spacecraft to impact the far side of the Moon, although it failed to return any scientific data before impact. [27]

The first truly comprehensive and detailed mapping survey of the far side was undertaken by the American unmanned Lunar Orbiter program launched by NASA from 1966 to 1967. Most of the coverage of the far side was provided by the final probe in the series, Lunar Orbiter 5.

The far side was first seen directly by human eyes during the Apollo 8 mission in 1968. Astronaut William Anders described the view:

The backside looks like a sand pile my kids have played in for some time. It's all beat up, no definition, just a lot of bumps and holes.

It has been seen by all crew members of the Apollo 8 and Apollo 10 through Apollo 17 missions since that time, and photographed by multiple lunar probes. Spacecraft passing behind the Moon were out of direct radio communication with the Earth, and had to wait until the orbit allowed transmission. During the Apollo missions, the main engine of the Service Module was fired when the vessel was behind the Moon, producing some tense moments in Mission Control before the craft reappeared.

Geologist-astronaut Harrison Schmitt, who became the last to step onto the Moon, had aggressively lobbied for his landing site to be on the far side of the Moon, targeting the lava-filled crater Tsiolkovskiy. Schmitt's ambitious proposal included a special communications satellite based on the existing TIROS satellites to be launched into a Farquhar–Lissajous halo orbit around the L2 point so as to maintain line-of-sight contact with the astronauts during their powered descent and lunar surface operations. NASA administrators rejected these plans on the grounds of added risk and lack of funding.

The China National Space Administration's Chang'e 4 made the first soft landing on the lunar far side on 3 January 2019. [28] The craft included a lander equipped with a low-frequency radio spectrograph and geological research tools. [29]


Some of the features of the geography of the far side of the Moon are labeled in this image Craters of the Far Side of the Moon.jpg
Some of the features of the geography of the far side of the Moon are labeled in this image

Because the far side of the Moon is shielded from radio transmissions from the Earth, it is considered a good location for placing radio telescopes for use by astronomers. Small, bowl-shaped craters provide a natural formation for a stationary telescope similar to Arecibo in Puerto Rico. For much larger-scale telescopes, the 100-kilometer-diameter (60 mi) crater Daedalus is situated near the center of the far side, and the 3-kilometer-high (2 mi) rim would help to block stray communications from orbiting satellites. Another potential candidate for a radio telescope is the Saha crater. [30]

Before deploying radio telescopes to the far side, several problems must be overcome. The fine lunar dust can contaminate equipment, vehicles, and space suits. The conducting materials used for the radio dishes must also be carefully shielded against the effects of solar flares. Finally, the area around the telescopes must be protected against contamination by other radio sources.

The L2 Lagrange point of the Earth–Moon system is located about 62,800 km (39,000 mi) above the far side, which has also been proposed as a location for a future radio telescope which would perform a Lissajous orbit about the Lagrangian point.

One of the NASA missions to the Moon under study would send a sample-return lander to the South Pole–Aitken basin, the location of a major impact event that created a formation nearly 2,400 km (1,500 mi) across. The force of this impact has created a deep penetration into the lunar surface, and a sample returned from this site could be analyzed for information concerning the interior of the Moon. [31]

Because the near side is partly shielded from the solar wind by the Earth, the far side maria are expected to have the highest concentration of helium-3 on the surface of the Moon. [32] This isotope is relatively rare on the Earth, but has good potential for use as a fuel in fusion reactors. Proponents of lunar settlement have cited the presence of this material as a reason for developing a Moon base. [33]

Conspiracy theories

Some conspiracy theorists, notably Milton William Cooper, have alleged that some Apollo astronauts had seen UFOs on the far side of the Moon but were told to keep quiet about them. [34] [35] Some have allegedly reported seeing an alien base (code named "Luna") and even encountered aliens who told them to stay off the Moon. Some photographs circulated on the Internet purport to show a large "castle" on the Moon. The Apollo 20 hoax claims that a manned mission landed on the far side. NASA states that these claims are hoaxes. [36]

Former United States Secretary of Defense Robert McNamara stated that several unspecified officials ("Chiefs") within The Pentagon were opposed to a Nuclear Test Ban Treaty between the United States and the Soviet Union, on the premise that the Soviets would continue nuclear weapons testing on the far side of the Moon, far from the observations of American observers. McNamara considered this premise "absurd" and that "[they were] out of [their] minds," but he believed that it was an example of the state of mind of some Pentagon officials during the Cold War. [37] Ironically, it was later revealed that the Pentagon had their own plan to detonate a nuclear weapon on the Moon as part of the experiment Project A119. The project was created not only to help in answering some of the mysteries in planetary astronomy and astrogeology, but also as a show of force intended to boost domestic confidence in the astro-capabilities of the United States, a boost that was thought to be needed after the Soviet Union took an early lead in the Space Race and who were thought by some to be working on a similar project. [38]

Named features

See also

Related Research Articles

Apollo 17 Final mission of the Apollo program

Apollo 17 was the final mission of NASA's Apollo program and the last mission as of 2019 in which humans have travelled to and walked on the Moon. Launched at 12:33 a.m. Eastern Standard Time (EST) on December 7, 1972, with a crew made up of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, it was the last use of Apollo hardware for its original purpose; after Apollo 17, extra Apollo spacecraft were used in the Skylab and Apollo–Soyuz programs.

Mare Imbrium vast lunar mare filling a basin on Earths Moon

Mare Imbrium is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to 3938 ± 4 million years ago, the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it was originally because later events have altered its surface.

Ranger program series of unmanned space missions by the United States in the 1960s

The Ranger program was a series of unmanned space missions by the United States in the 1960s whose objective was to obtain the first close-up images of the surface of the Moon. The Ranger spacecraft were designed to take images of the lunar surface, transmitting those images to Earth until the spacecraft were destroyed upon impact. A series of mishaps, however, led to the failure of the first six flights. At one point, the program was called "shoot and hope". Congress launched an investigation into "problems of management" at NASA Headquarters and Jet Propulsion Laboratory. After two reorganizations of the agencies, Ranger 7 successfully returned images in July 1964, followed by two more successful missions.

<i>Luna 16</i> space probe

Luna 16, also known as Lunnik 16, was an unmanned space mission, part of the Soviet Luna program.

South Pole–Aitken basin impact crater

The South Pole–Aitken basin is an impact crater on the far side of the Moon. At roughly 2,500 km (1,600 mi) in diameter and 13 km (8.1 mi) deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and deepest basin recognized on the Moon. It was named for two features on opposing sides: the crater Aitken on the northern end and the lunar south pole at the other end. The outer rim of this basin can be seen from Earth as a huge chain of mountains located on the Moon's southern limb, sometimes informally called "Leibnitz mountains".

Mare Orientale Lunar mare on the western border of the near side and far side of the Moon

Mare Orientale is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have revealed it to be one of the most striking large scale lunar features, resembling a target ring bullseye.

Mare Crisium lunar mare

Mare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. The basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago.

Aitken (crater) impact crater

Aitken is a large lunar impact crater that lies on the far side of the Moon, named for Robert Grant Aitken, an American astronomer specializing in binary stellar systems. It is located to the southeast of the crater Heaviside, and north of the unusual formation Van de Graaff. Attached to the southwest rim is Vertregt. To the southeast is the smaller Bergstrand.

Shackleton (crater) impact crater

Shackleton is an impact crater that lies at the south pole of the Moon. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow. The low-temperature interior of this crater functions as a cold trap that may capture and freeze volatiles shed during comet impacts on the Moon. Measurements by the Lunar Prospector spacecraft showed higher than normal amounts of hydrogen within the crater, which may indicate the presence of water ice. The crater is named after Antarctic explorer Ernest Shackleton.

Bowditch (crater) lunar crater

Bowditch is a lunar impact crater that lies on the far side of the Moon, just beyond the eastern limb. It is located on a region of the lunar surface that is brought into view due to libration, but at such times the area is viewed from the edge and so not much detail can be observed. It lies just to the north of the small Lacus Solitudinis lunar mare, between the craters Titius to the southwest and Perel'man to the east-northeast.

Moon landing arrival of a spacecraft on the surface of the Moon

A Moon landing is the arrival of a spacecraft on the surface of the Moon. This includes both manned and unmanned (robotic) missions. The first human-made object to reach the surface of the Moon was the Soviet Union's Luna 2 mission, on 13 September 1959.

Lunar craters craters on Earths moon

Lunar craters are impact craters on Earth's Moon. The Moon's surface has many craters, almost all of which were formed by impacts.

Geology of the Moon Structure and composition of the Moon

The geology of the Moon is quite different from that of Earth. The Moon lacks a significant atmosphere, which eliminates erosion due to weather; it does not have any form of plate tectonics, it has a lower gravity, and because of its small size, it cooled more rapidly. The complex geomorphology of the lunar surface has been formed by a combination of processes, especially impact cratering and volcanism. The Moon is a differentiated body, with a crust, mantle, and core.

Sundman (crater) lunar crater

Sundman is a lunar impact crater that lies just past the western limb of the Moon. Although it lies on the far side from the Earth, this part of the surface is brought into view during periods of favorable libration and illumination. Sundman lies to the southwest of the walled plain Einstein, and to the west of the Vallis Bohr cleft. It was named after Finnish mathematician and astronomer Karl F. Sundman.

Exploration of the Moon various missions to the Moon

The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation from Earth. The invention of the optical telescope brought about the first leap in the quality of lunar observations. Galileo Galilei is generally credited as the first person to use a telescope for astronomical purposes; having made his own telescope in 1609, the mountains and craters on the lunar surface were among his first observations using it.

Near side of the Moon

The near side of the Moon is the lunar hemisphere that is permanently turned towards Earth, whereas the opposite side is the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth – a situation known as synchronous rotation, or tidal locking.

Halo orbit periodic, three-dimensional orbit near the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics

A halo orbit is a periodic, three-dimensional orbit near the L1, L2 or L3 Lagrange point in the three-body problem of orbital mechanics. Although the Lagrange point is just a point in empty space, its peculiar characteristic is that it can be orbited. Halo orbits can be thought of as resulting from an interaction between the gravitational pull of the two planetary bodies and the Coriolis and centrifugal accelerations on a spacecraft. Halo orbits exist in any three-body system, e.g. the Sun–Earth–Orbiting Satellite system and the Earth–Moon–Orbiting Satellite system. Continuous "families" of both Northern and Southern halo orbits exist at each Lagrange point. Because halo orbits tend to be unstable, stationkeeping is required to keep a satellite on the orbit.

Lunar Reconnaissance Orbiter NASA robotic spacecraft orbiting the Moon

The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO has been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

The following outline is provided as an overview of and topical guide to the Moon:


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