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Moon
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The Moon was -- and remains -- the most popular target for any spacefaring nation's first adventurous steps into planetary exploration. It was, of course, the target of the space race between the United States and Soviet Union in the 1960s and 1970s. It is the only place in the solar system other than Earth walked upon by humans. Lately, the Moon has become a prime destination for efforts by many nations seeking to expand into space: Japan (Kaguya), China (Chang'E), India (Chandrayaan-1), and the United States (Lunar Reconnaissance Orbiter).
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The Moon is 384,403 kilometers (238,857 miles) distant from the Earth. Its diameter is 3,476 kilometers (2,160 miles). Both the rotation of the Moon and its revolution around Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned toward Earth. Optical librations have been observed telescopically since the mid-17th century. Very small but real librations (maximum about 0°.04) are caused by the effect of the Sun's gravity and the eccentricity of Earth's orbit, perturbing the Moon's orbit and allowing cyclical preponderances of torque in both east-west and north-south directions.
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The Moon has an external magnetic field of the order of one to a hundred nanotesla—more than 100 times smaller than the Earth's, which is 30-60 microtesla. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin.[31] One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core... is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.[32]
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The Moon has a small iron-rich core, but is composed mostly of rock. Its heavily cratered surface was caused by the bombardment of asteroids when the solar system was young, about 500-700 million years after its formation. Volcanic activity that continued until approximately 2 billion years ago is responsible for the basalt lava floes that flooded the surface, cooled, and solidified into level plains. These plains are known as "seas" though they contain no water. Not only does the Moon lack water, it ... has no permanent atmosphere. The pull of gravity at the surface of the Moon is only 1/6 as strong as gravity's pull at the Earth's surface.
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A large portion of the Moon's post–magma-ocean geologic evolution was dominated by impact cratering. The lunar geologic timescale is largely divided in time on the basis of prominent basin-forming impact events, such as Nectaris, Imbrium, and Orientale. These impact structures are characterised by multiple rings of uplifted material, and are typically hundreds to thousands of kilometres in diameter. Each multi-ring basin is associated with a broad apron of ejecta deposits that forms a regional stratigraphic horizon. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages on the basis of stratigraphic grounds. The continuous effects of impact cratering are responsible for forming the regolith.
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The gravitational force of the Moon causes the ocean tides; these tidal forces have other effects as well. These tidal forces have the effect of creating resistance which slows down the rotation of both objects. In the case of the Earth, the result is that the Earth's rotation slows on the order of one millisecond per century. The Moon's rotation is locked in to its period of revolution, and the same side of the Moon always faces the Earth.
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