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The far side of the Moon is the Lunar hemisphere that is permanently turned away from the Earth . 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, is therefore sometimes called the "dark side" of the moon. (''Dark'', here, means ''obscure'' or ''unknown'' rather than lacking light: the far side of the moon receives as much light as the near side). 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. EXPLORATION Until the far side of the Moon was Photograph ed by the Soviet probe Luna 3 in 1959 , little was known about its properties. Libration s 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 Kilometer s. 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 Russia ns 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 . 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 Telescope s for use by Astronomer s. 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 Flare s. 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 Mare s 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
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