A lunar space elevator is a proposed cable running from the surface of the Moon into space.
It is similar in concept to the better known Earth space elevator idea (a cable suspended above Earth, with its center of gravity slightly above geostationary orbit). It would instead be constructed with its center of gravity in a stationary position above the surface of the Moon, providing a controlled means to transport people and/or materials between the surface and lunar orbit.
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 cable would possess a microgravity point, these and other drives can reach the cable from low Earth orbit (LEO) with very 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.
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There are two lunar-synchronous points where an elevator could be placed that would be stable: the libration points L1 and L2. L1 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 up. In these positions, the forces of gravity and centrifugal force are equal, and as long as the system remained balanced (L1 and L2 are in unstable equilibrium along the line between Earth and Moon), it would 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. 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 L1. (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 L2 cable's lowest kilogram would require 1,000 kg of counterweight at a distance of 120,000 km from the Moon.
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.
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). Whereas, a lunar elevator could be constructed using high-strength commercially available materials such as Kevlar or Spectra.
Compared to Earth, 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.
Jerome Pearson has proposed a cable design using M5 fiber (See Materials, below) that would have a mass of 6,100 tonnes including a massive counterweight, that would be capable of lifting or depositing loads of 2,000 newtons (450 lbf, or at lunar surface gravity, masses of 1233 kg / 2700 lbm) at the base. The counterweight could potentially be lifted from the lunar surface.
The idea of space elevators has been around since 1960 when Yuri Artsutanov wrote a Sunday supplement to Pravda on how to build such a structure and the utility of geosynchronous orbit. His article however, was not known in the West. Then in 1966, John Isaacs, a leader of a group of American Oceanographers at Scripps Institute, published an article in Science about the concept of using thin wires hanging from a geostationary satellite. In that concept, the wires were to be thin (thin wires/tethers are now understood to be more susceptible to micrometeoroid damage). Like Artsutanov, Isaacs’ article also wasn’t well known to the aerospace community.
In 1972, James Cline submitted a paper to NASA describing a lunar elevator concept [1]. NASA responded negatively to the idea citing technical risk and lack of funds.[2]
In 1975, Jerome Pearson independently came up with the Space elevator concept and published it in Acta Astronautica. That made the aerospace community at large aware of the space elevator for the first time. His article inspired Sir Arthur Clarke to write the novel The Fountains of Paradise. Later, Pearson extended his theory to the moon and changed to using the Lagrangian points instead of having it in geostationary orbit.
In October 2011 on the the LiftPort website Laine announced that LiftPort is pursuing a Lunar space elevator as an interim goal before attempting a terrestrial elevator. At the 2011 Annual Meeting of the Lunar Exploration Analysis Group (LEAG), LiftPort CTO Marshall Eubanks presented a paper on the prototype Lunar Elevator co-authored by Michael Laine.[3]
Unlike earth-anchored space elevators, the materials for lunar space elevators won’t require a lot of strength. Lunar elevators can be made with materials available today. Carbon nanotubes aren’t required to build the structure.[4] This would make it possible to build the elevator much sooner, since available carbon nanotube materials in sufficient quantities are still years away.[5]
One material that has great potential is M5 fiber. This is a synthetic fiber that is lighter than Kevlar or Spectra.[6] According to Pearson, Levin, Oldson, and Wykes in their article The Lunar Space Elevator, an M5 ribbon 30 mm wide and 0.023 mm thick, would be able to support 2000 kg on the lunar surface (2005). It would also be able to hold 100 cargo vehicles, each with a mass of 580 kg, evenly spaced along the length of the elevator.[4] Other materials that could be used are T1000G carbon fiber, Spectra 200, or Zylon. All of these materials have breaking lengths of several hundred kilometers under 1g.[4]
| Material | Density ρ kg/m3 |
Stress Limit σ GPa |
Breaking height (h = σ/ρg, km) |
|---|---|---|---|
| Single-wall carbon nanotubes (laboratory measurements) | 2266 | 50 | 2200 |
| Toray Carbon fiber (T1000G) | 1810 | 6.4 | 361 |
| Aramid, Ltd. polybenzoxazole fiber (Zylon PBO) | 1560 | 5.8 | 379 |
| Honeywell extended chain polyethylene fiber (Spectra 2000) | 970 | 3.0 | 316 |
| Magellan honeycomb polymer M5 (with planned values) | 1700 | 5.7(9.5) | 342(570) |
| DuPont Aramid fiber (Kevlar 49) | 1440 | 3.6 | 255 |
| Glass fibre (Ref Specific strength) | 2600 | 3.4 | 133 |
The materials will be used to build the ribbons which will connect from the L1 and L2 balance points to the surface of the moon.[4] The ribbons would be used by the robotic climbing vehicle to get from the surface into orbit.[4] The vehicles would be slow, compared to chemical rockets, but it is a good speed for transferring cargo.[4]
The ribbons are going to be prone to damage by micrometeoroids from space so one way to improve their survivability is to make a multi-ribbon system instead of one.[4] They will have interconnections at regular intervals, so that if one section is damaged, the parallel sections would carry the load until robotic vehicles can come and replace the missing ribbon.[4] The interconnections would be spaced about 100 km apart, which is small enough to allow a robotic climber to carry the mass of the replacement 100 km of ribbon.[4]
One method of getting materials needed from the moon into orbit would be the use of robotic climbing vehicles.[4] These vehicles would consist of two large wheels pressing against the ribbons of the elevator to provide enough friction for lift.[4] The climbers could be set for horizontal or vertical ribbons.
The wheels would be driven by electric motors, which would obtain their power from solar energy or beamed energy. The power required to climb the ribbon would depend upon the lunar gravity field, which drops off the first few percent of the distance to L1.[4] The power that a climber would require to traverse the ribbon drops in proportion to proximity to the L1 point. If a 540 kg climber traveled at a velocity of fifteen meters per second, by the time it was seven percent of the way to the L1 point, the required power would drop to less than a hundred watts, versus 10 kilowatts at the surface.
One problem with using a solar powered vehicle is the lack of sunlight during some parts of the trip. For half of every month, the solar arrays on the lower part of the ribbon would be in the shade.[4] One way to fix this problem would be to launch the vehicle at the base with a certain velocity then at the peak of the trajectory, attach it to the ribbon.[4]
Materials from Earth may be sent into orbit and then down to the Moon to be used by lunar bases and installations.[4]
Former U.S. President George W. Bush, addressing his Vision for Space Exploration, noted[7] that the Moon may serve as a cost-effective construction, launching and fueling site for future space exploration missions.[8] As President Bush announced,[7] "Its soil contains raw materials that might be harvested and processed into rocket fuel or breathable air." For example, future Ares V missions could cost-effectively[9] deliver raw materials from Earth for future spacecraft and missions to a Moon-based[9] space dock positioned as a counterweight[10] to a lunar space elevator,[11] while fuel and breathable air could be shipped up from the Moon's surface to the same Moon-based dock along the same lunar space elevator. As well, the total energy needed for transit between the Moon and Mars is actually much less than between the Moon and Earth, so lunar base activity could make a large impact on building a Mars base. Since millions of tonnes of water ice have been found on the moon's poles, there is a much more accessible form of water than the regolith.[12] The proximity of the polar base on the lunar space elevator to the water ice could make mining the ice far more efficient.
The lunar elevator could also be used to transport supplies and materials from the surface of the moon into the Earth’s orbit and vice versa.[4] According to Jerome Pearson, there are plenty of resources on the moon that would be easier to gather and send into Earth orbit rather than launch from Earth.[4] He claims that one such material which would be very valuable is lunar regolith, also known as moon dirt.[4] One particular use for the regolith would be for massive material to shield space stations or crewed deep space missions against solar flares, Van Allen trapped or cosmic radiation. Other materials such as metals and minerals could be mined and sent up for construction. Silicon for solar cells, as would be needed for the construction of massive satellite solar power stations, seems particularly promising.
One disadvantage of the lunar elevator is that it may not be able to carry human passengers. The rate at which cargo is transferred would be too slow, normally taking weeks to reach its destination.[5] Humans would be able to get there faster by using rockets to and from the moon.
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