Far side of the Moon
From Wikipedia, the free encyclopedia
- For other uses, see Dark Side of the Moon (disambiguation).
- For the Pink Floyd album, see The Dark Side Of The Moon.
- For Robert Lepage film, see La Face cachée de la lune.
The far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The opposite side is known as the near side of the Moon.
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[edit] History
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 they are relatively flat plains). 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).
[edit] 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. 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 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 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.
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.
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 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.
[edit] 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 situated 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 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 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.
[edit] 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 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.
[edit] External links
- Northwest Africa 482, only meteorite believed to have originated from the far side of the Moon
- Prospecting for Helium-3 on the Moon
| Image:Moon-footer.jpg |
| General: Phases of the Moon · Solar eclipse · Lunar eclipse · The Moon's orbit · Near and Far sides of the Moon · Tides |
| Lunar science: Moon rocks · Geology · Lunar mare · Lunar geologic timescale · Lunar meteorites · KREEP · Giant impact theory |
| Exploration: Exploration of the Moon · Manned exploration (20th Century)(21st Century) · Robotic exploration · Future missions |
| See also solar system, natural satellite, and impact craters. |
