A lunar space elevator or lunar spacelift is a proposed transportation system for moving a mechanical climbing vehicle up and down a ribbon-shaped tethered cable that is set 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 materials and technology already available. 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 L1 and L2. The 0.055 eccentricity of the lunar orbit means that these points are not fixed relative to the lunar surface : the L1 is 56,315 km +/- 3,183 km away from the Earth-facing side of the Moon (at the lunar equator) and L2 is 62,851 km +/- 3,539 km from the center of the Moon's far side, in the 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 Lagrangian points L1 and L2 are points of unstable gravitational equilibrium, meaning that small inertial adjustments will be needed to ensure any object positioned there can remain stationary relative to the lunar surface. Both of these positions are substantially farther up (from the Moon) compared to 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. Suspending a kilogram of cable or payload just above the surface of the Moon in the direction of the L1 point would require 1,000 kg of mass as counterweight in an orbit 26,000 km closer to Earth as L1 is. (A smaller mass 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.) Suspending a kilogram of cable or payload just above the surface of the Moon in the direction of the L2 point would require 1,000 kg at a distance of approximately 120,000 km from the Moon as counterbalance (more than 51 thousand kilometer further away from the Moon as the L2 point, almost 59 thousand km when the L2 point is closest to the Moon). These 1,000 kg would be in an orbit around Earth with the same orbital period as the Moon, yet its mean orbital radius would be almost one third larger as that of the Moon and the centripetal force to keep it in such an orbit would equal the weight of 1 kg at Moon's surface. 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 near-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.
Fabrication Because of the Moon's lower gravity and lack of atmosphere, a lunar elevator would have less stringent requirements for the tensile strength of the material making up its cable than an Earth-tethered cable. An Earth-based elevator would require high strength-to-weight materials that are theoretically possible, but not yet fabricated in practice (e.g., carbon nanotubes). A lunar elevator, however, could be constructed using commercially available mass-produced high-strength para-aramid fibres (such as Kevlar and M5) or ultra-high-molecular-weight polyethylene fibre. Compared to an Earth space elevator, there would be fewer geographic and political restrictions on the location of the surface connection. The connection point of a lunar elevator would not necessarily have to be directly under its center of gravity, and could even be near the poles, where evidence suggests there might be frozen water in deep craters that never see sunlight; if so, this might be collected and converted into rocket fuel.
… excerpt ends here. Continue reading the full article.
