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A lunar space elevator is a proposed transportation system for moving a mechanical climbing vehicle up and down a ribbon-shaped, tethered cable, in between the surface of the Moon "at the bottom," and a docking port, suspended tens of thousands of kilometers above, in space, at the top. It is similar in concept to the better known Earth-based space elevator idea, but since the Moon's surface gravity is much lower than the Earth's, the engineering requirements for constructing a lunar elevator system can be met using currently available materials and technology. For a lunar elevator, the cable or tether extends considerably farther out from the lunar surface into space than one that would be used in an Earth-based system. However, the main function of a space elevator system is the same in either case; both allow for a reusable, controlled means of transporting payloads of cargo, or possibly people, between a base station at the bottom of a gravity well and a docking port in outer space. A lunar elevator could significantly reduce the costs and improve reliability of soft-landing equipment on the lunar surface. For example, it would permit the use of mass-efficient (high specific impulse), low thrust drives such as ion drives which otherwise cannot land on the Moon. Since the docking port would be connected to the cable in a microgravity environment, these and other drives can reach the cable from low Earth orbit (LEO) with minimal launched fuel from Earth. With conventional rockets, the fuel needed to reach the lunar surface from LEO is many times the landed mass, thus the elevator can reduce launch costs for payloads bound for the lunar surface by a similar factor. ==Location== There are two points in space where an elevator's docking port could maintain a stable, lunar-synchronous position: the Earth-Moon Lagrange points and . is 56,000 km away from the Earth-facing side of the Moon, (at the lunar equator) and is 67,000 km from the center of the Moon's far side, in the exact opposite direction. At these points, the effect of the Moon's gravity and the effect of the centrifugal force resulting from the elevator system's synchronous, rigid body rotation cancel each other out. The gravitational stability of these Lagrange points are not permanent, ( and are in unstable equilibrium along a straight line between Earth and Moon,) but so long as small inertial adjustments are made to account for minor gravitational perturbations, any object positioned there can remain stationary. Both of these positions are substantially farther up than the 36,000 km from Earth to geostationary orbit. Furthermore, the weight of the limb of the cable system extending down to the Moon would have to be balanced by the cable extending further up, and the Moon's slow rotation means the upper limb would have to be much longer than for an Earth-based system, or be topped by a much more massive counterweight. To suspend a kilogram of cable or payload just above the surface of the Moon would require 1,000 kg of counterweight, 26,000 km beyond . (A smaller counterweight on a longer cable, e.g., 100 kg at a distance of 230,000 km — more than halfway to Earth — would have the same balancing effect.) Without the Earth's gravity to attract it, an cable's lowest kilogram would require 1,000 kg of counterweight at a distance of 120,000 km from the Moon. The average Earth-Moon distance is 384,400 km. The anchor point of a space elevator is normally considered to be at the equator. However, there are several possible cases to be made for locating a lunar base at one of the Moon's poles; a base on a peak of eternal light could take advantage of continuous solar power, for example, or small quantities of water and other volatiles may be trapped in permanently shaded crater bottoms. A space elevator could be anchored near a lunar pole, though not directly at it. A tramway could be used to bring the cable the rest of the way to the pole, with the Moon's low gravity allowing much taller support towers and wider spans between them than would be possible on Earth. 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 space elevator」の詳細全文を読む スポンサード リンク
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