- (written as Moon)The Earth’s natural satellite, orbiting the Earth, and rotating on its axis, once per lunar month of 27.3 days. The Moon has a diameter of 3500km and a surface area about equal to that of Africa. Six US space missions have taken people to the Moon, and three unmanned Soviet missions have returned moonrock to the Earth. The Moon has little if any magnetic field and a limited amount of seismic activity, and is geologically quiet compared to the Earth. Its surface consists of two main types of terrain, the Maria, smooth, dark plains of up to 4 billion years of age, and the Highlands, which are older and more heavily cratered. Apart from meteorites, highland rocks are the most available souvenirs we have of the earliest days of the solar system. They are not much younger than the Moon itself, which seems to have formed some 4.54 billion years ago. Most of the Moon’s craters were formed by meteorite impact, but there is ample evidence that the Moon has also had internal forces shaping its own surface, with volcanic craters and other volcanic structures visible in the Highlands and elsewhere. The side of the Moon away from the Earth has fewer large Maria structures and is a few kilometres higher than the near side, so that the Moon is shaped like an egg, pointing away from us. After the great era of lunar exploration, the late 1960s and early 1970s, manned lunar studies went quiet – so much so that no rocket system capable of launching people to the Moon is now in existence. But the subject was revived by the Clementine space probe, which supplied huge amounts of data about the Moon’s surface very cheaply. In addition, the European and US space agencies are collaborating on studies of a return to the Moon, starting with robotic probes and working up to permanent habitation. A range of scientific problems, like the possible existence of a molten lunar core and the existence of water at the Moon’s surface, remain to be solved.
- A natural satellite of a planet.
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