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The far side of the Moon, sometimes called the dark side of the Moon, is the lunar hemisphere that is permanently turned away, and not visible from the surface of the Earth. The far 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 is distinguished by a multitude of crater impacts, as well as relatively few lunar maria. It includes the second largest known impact feature in the Solar System, the South Pole-Aitken basin. The far side has been suggested as a potential location for a large radio telescope, as it would be shielded from possible radio interference from Earth. To date, there has been no ground exploration of the far side of the Moon.
Tidal forces from Earth have slowed down the Moon's rotation so that 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". It should be noted that over time some parts of the far side can be seen due to libration, in total 59% of the Moon's surface. However, useful observation of those parts is difficult due to the low angle from Earth.
The commonly used term "dark side of the Moon" does not refer to "dark" as in the absence of light, but rather as the unknown, as until man was able to send spacecraft around the Moon, this area had never been seen. 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 from the Sun. Only during a full moon (as viewed from Earth) is the far side of the Moon physically dark. The word "dark" may also refer to the fact that communication with spacecraft can be blocked while on the far side of the Moon, during Apollo space missions for example.
The two hemispheres 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, 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 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.
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. 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 farside highlands.
Though the far side has more visible craters, this is due to 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. . . . 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 the both sides have received the same number of impacts."
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. 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 kilometres (600 mi), 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, eighteen of them resolvable, 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 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.
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 second to 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 based on added risk and lack of funding.
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 (62 mi) diameter crater Daedalus is situated near the center of the far side, and the 3 km (2 mi)-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 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 kilometers (1,491 mi) 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.
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