:''This article concerns the '''far side''' of the Moon. For other uses see Far_side_(disambiguation).''
Image:Moon_PIA00304.jpg
The '''far side of the Moon''' is the lunar hemisphere
that is permanently turned away from the Earth. This face is
not visible because the rotation of the moon about its axis is
synchronized with its orbital period. This lock-step
synchronization was achieved by the tidal forces
between the Earth and the Moon.
The two hemispheres have a distinctly different appearance, with
the near side covered in multiple, large ''maria''.
The far side has a battered, densely-cratered appearance with
few mares. 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 that the crust of the Moon is
40 km thicker on the far side. Thus it was more difficult for
molten materials to penetrate to the surface.
The far side was sometimes referred to as the "Dark Side", due to
the lack of human knowledge concerning that hemisphere. The word
"Dark" in a cultural context was meant to express a lack of information, rather than the actual lighting conditions.
==Exploration==
Until the far side of the Moon was Photographed by the Soviet
probe Luna_3 in 1959, little was known about its properties.
Librations of the Moon periodically allowed limited glimpses of
features that are located near the lunar limb on the far side.
But these features were seen from a low angle, hindering useful
observation. (It proved difficult to distinguish a crater from a
mountain range.) The remaining 41% 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 wasn't 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.
As the far side was first viewed by Soviet space probes, the Russians selected many of the names for the prominent features. This action provoked some controversy, and so the International_Astronomical_Union later assumed the role of naming lunar features on this hemisphere. However many of the names selected by the Soviets are still recognized.
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|"''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.''"
—Astronaut William_Anders, describing the view during the Apollo_8 mission. |} The far side was first observed directly by Human eyes during the Apollo_8 mission in 1968. It has been seen by all crew members of the 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== 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 sitated near the center of the far side, and the 3-km-high rim would help to block stray communications from orbiting satellites. Other potential candidates for a radio telescope include the Saha crater and the South_Pole-Aitken_Basin. Before deploying radio telescopes to the far side, several problems must be overcome. The fine lunar dust can serve to 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 ("Halo") 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 Lunar_mares 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== *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-questionable 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 ''Dark_Side_Of_The_Moon'', that, apart from the title and a line in the songs ''Brain Damage'' and ''Eclipse'' , actually had nothing to do with the Earth's moon. ==External links== *Prospecting for Helium-3 on the Moon Category:Moon
—Astronaut William_Anders, describing the view during the Apollo_8 mission. |} The far side was first observed directly by Human eyes during the Apollo_8 mission in 1968. It has been seen by all crew members of the 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== 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 sitated near the center of the far side, and the 3-km-high rim would help to block stray communications from orbiting satellites. Other potential candidates for a radio telescope include the Saha crater and the South_Pole-Aitken_Basin. Before deploying radio telescopes to the far side, several problems must be overcome. The fine lunar dust can serve to 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 ("Halo") 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 Lunar_mares 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== *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-questionable 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 ''Dark_Side_Of_The_Moon'', that, apart from the title and a line in the songs ''Brain Damage'' and ''Eclipse'' , actually had nothing to do with the Earth's moon. ==External links== *Prospecting for Helium-3 on the Moon Category:Moon