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Far side of the Moon is turned away from the Earth.

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Far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The opposite to the far side of the Moon is known as the near side of the Moon. The far side of the moon hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon in 1968. The rugged terrain on the far side of the moon is distinguished by a multitude of crater impacts, as well as a relative paucity of lunar maria. It includes the largest impact feature in the Solar System: the South Pole-Aitken basin. The far side of the moon has been suggested as a potential location for a large Radio telescope, as it would be shielded from possible radio interference from Earth.

History of the far side of the moon.

Far side of the Moon.
Far side of the Moon.

Tidal forces between Earth and the Moon have slowed the moon's rotation so that the same side is always facing the Earth. The other face, which is never visible from the Earth in its entirety, is therefore called the "far side of the Moon". The far side should not be confused with the "dark side" (the hemisphere that is not illuminated by the Sun), as the two are the same only during a full moon. Both the near and far sides receive (on average) almost equal amounts of light from the Sun.

The two hemispheres have distinctly different appearances, with the near side covered in multiple, large maria (Latin for 'seas,' since the earliest astronomers thought, wrongly, that these plains were seas of lunar water). The far side has a battered, densely cratered appearance with few maria. Only 2.5% of the surface of the far side is covered by maria, compared to 31.2% on the near side. The most likely 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 farside 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 (for a more detailed discussion, see Lunar mare).

Exploration of the far side of the moon.

The far side of the moon.
One of the first images of the far side of the moon taken from Luna 3. The dark patch at the upper right is Mare Moscoviense and the dark areas below and left of center are Mare Marginus and Mare Smythii.

Until the late 1950s little was known about properties of the far side of the Moon. Librations of the Moon periodically allowed limited glimpses of features that are located near the lunar limb on the far side. 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 viewed through libration is the Mare Orientale, which is a prominent impact basin spanning almost 1,000 kilometers. 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. It was photographed in fine detail by Lunar Orbiter 4 in 1967.

On October 7, 1959 the Soviet probe Luna 3 took the first photographs of the lunar far side, seventeen of them being resolvable ones covering one-third of the surface invisible from the Earth. 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. It included a catalog of 500 distinguished features of the landscape. A year later the first globe (1:13 600 000 scale) containing lunar features invisible from the Earth was released in the USSR, based on images from Luna 3. On July 20, 1965 another Soviet probe Zond 3 transmitted 25 pictures of very good quality of the lunar far side, with much better resolution than those from Luna 3. In particular, they revealed chains of craters, hundreds of kilometers in length. In 1967 the second part of the "Atlas of the Far Side of the Moon" was published in Moscow, based on data from Zond 3, with the catalog now including 4,000 newly discovered features of lunar far side landscape. In the same year the first "Complete Map of the Moon" (1:5 000 000 scale) and updated complete globe (1:10 000 000 scale), featuring 95 percent of the lunar surface were released in the Soviet Union.

As a lot of 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.

The far side was first observed 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.

Potential of the far side of the moon.

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 crater Daedalus is situated near the center of the far side, and the 3-km-high rim would help to block stray communications from orbiting satellites. Another potential candidate for a radio telescope is the Saha crater.

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 about 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 above the far side. This has also been proposed as the location of a future radio telescope, performing 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 kilometers across. The size 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.

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. 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 presence of this material as a reason for development of a Moon base.

Fictional references to the far side of the moon.

  • The novel Space by James Michener tells the fictional story of an Apollo 18 mission to the far side of the Moon. The novel was the source for a 1985 TV mini-series of the same name.
  • The scientifically impossible premise for the TV program Space: 1999 was the explosion of a nuclear waste dump on the far side of the Moon. This propelled the Moon out of Earth's orbit and deep into space.
  • "Ideas Die Hard" (1957), a short story by Isaac Asimov, described an ill-fated trip to the dark side of the Moon. First appearing in Galaxy magazine, it was reprinted in The Winds of Change and Other Stories, ISBN 0-586-05743-9.
  • In the anime show Grendizer, the Vega galactic empire has set up a base on the far side of the Moon from which they launch attacks on Earth.
  • Pink Floyd had a seminal album titled The Dark Side of the Moon, that, apart from the title and some lines in the songs "Brain Damage" and "Eclipse", had nothing to do with the Earth's physical moon. The use of the word lunatic in the album's lyrics, gains its stem from "luna" for moon, which denotes the traditional link made in folklore between madness and the phases of the moon.
  • "The Far Side of the Moon" is the english title of a french-canadian movie "La Face cachée de la lune".

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