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In astronomy, lunar orbit (also known as a Selenocentric orbit) refers to the orbit of an object around the Moon. As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon. The altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune, apocynthion or aposelene, while the periapsis (point closest to the surface) is known as perilune, pericynthion or periselene. Low Lunar orbit (LLO)—orbits below altitude—are of particular interest in exploration of the moon, but suffer from gravitational perturbation effects that make most unstable, and leave only a few orbital inclinations possible for indefinite ''frozen orbits'', useful for long-term stays in LLO.〔 ==Robotic spacecraft== The Soviet Union sent the first spacecraft to the vicinity of the Moon, the robotic vehicle Luna 1, on January 4, 1959. It passed within of the Moon's surface, but did not achieve lunar orbit.〔 Luna 3, launched on October 4, 1959, was the first robotic spacecraft to complete a circumlunar free return trajectory, still not a lunar orbit, but a figure-8 trajectory which swung around the far side of the Moon and returned to the Earth. This craft provided the first pictures of the far side of the Lunar surface.〔 The Soviet Luna 10 became the first spacecraft to actually orbit the Moon in April 1966. It studied micrometeoroid flux, and lunar environment until May 30, 1966.〔 The first United States spacecraft to orbit the Moon was Lunar Orbiter 1 on August 14, 1966. The first orbit was an elliptical orbit, with an apolune of and a perilune of . Then the orbit was circularized at around to obtain suitable imagery. Five such spacecraft were launched over a period of thirteen months, all of which successfully mapped the Moon, primarily for the purpose of finding suitable Apollo program landing sites.〔 The most recent was the Lunar Atmosphere and Dust Environment Explorer (LADEE), which became a ballistic impact experiment in 2014. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lunar orbit」の詳細全文を読む スポンサード リンク
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