Lunar space elevator
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The lunar space elevator (LSE) is also called a moonstalk is an analogy to the better known space elevator idea – a cable suspended above the Earth, with its center of gravity at GEO.
The Moon version, because of its lower gravity, has less stringent requirements on the material then the Earth-tethered cable, however there are only two lunar synchronous points where the elevator could be placed that would be stable; the Lagrange points L1 and L2. L1, the center of gravity for a cable system on the Earth side of the Moon, is 56,000 km up from the surface, and L2, on the far side, is 67,000 km – substantially farther than the 36,000 km from Earth to GEO.
Also, the limb of the cable system extending down to the Moon has to be held up by the cable extending further up, and the Moon's slow rotation means the upper limb has to be much longer than the Earth-based system. To suspend a kilogram of cable or payload just above the surface of the Moon requires 1000 kg of counterweight, 26,000 km beyond L1 (Or an equivalent force from a smaller counterweight on a longer cable. e.g. 100 kg at a distance of 230,000 km – more than halfway to Earth). Without the Earth's gravity to attract it, the L2 cable's lowest kilogram requires 1000 kg of counterweight 120,000 km out from the Moon.
A major advantage a Lunar space elevator has over an Earth-based one is a much less stringent requirement for the tensile strength of the material making up its cable. An Earth-based elevator requires materials which are right at the edge of what is theoretically possible (eg, carbon nanotubes), whereas a Lunar elevator could be constructed using high-strength commercially available materials such as Kevlar or Spectra.
In addition, due to the low gravity of the moon; the connection point of a lunar elevator can be over a wide area of the lunar surface- even quite near the poles. There is some controversial evidence that the poles of the moon has ice in the craters that never see sunlight; if so, this might be collected and employed as rocket fuel.
Jerome Pearson has proposed a cable design using M5 fiber that would weigh only 6,800 kilograms (not including climbers and other equipment) that would be capable of lifting or depositing 200 kilograms at the Lunar surface; such an elevator could be sent to the Moon with just one launch of the largest existing launchers. This could massively reduce the rocketry costs for more reliably and cheaply soft-landing equipment on the lunar surface, using only high impulse/low thrust drives such as Ion drives to reach L1 from LEO. It would be an extremely minimalist cable, the design is intended to be strengthened and expanded over time using the Lunar materials it lifts off the surface.
External links
- An introductory review (http://www.spaceelevator.com/docs/acclarke.092079.se.3.html) of the whole gamut of skyhook contraptions by Arthur C. Clarke
- Universe Today article on a Lunar space elevator (http://www.universetoday.com/am/publish/lunar_space_elevator.html)