Lunar space elevator

Diagram showing equatorial and polar Lunar space elevators running to L₁. An L₂ elevator would mirror this arrangement on the Lunar farside, and cargo dropped from its end would be flung outward into the solar system.

A lunar space elevator (also called a moonstalk) 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 massively reduce the costs for reliably and cheaply soft-landing equipment on the lunar surface. For example, it permits the use of mass-efficient high 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 slash launch costs for payloads bound for the lunar surface by a similar factor.

Location

There are two lunar-synchronous points where the 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 1000 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 weight would require 1000 kg of counterweight at a distance of 120,000 km from the Moon.

The anchor point of a space elevator is normally 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.

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 materials at the edge of what is even theoretically possible (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 few geographic and no political restrictions on the location of the surface connection. Due to the lower gravity, 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 that would weigh only 6100 tonnes including a massive counterweight, that would be capable of lifting or depositing loads of 2000 N (about 1200 kg mass) at the base. The counterweight could potentially be lifted from the lunar surface.

History

The idea of building a space elevator has been around for about 50 years. It was Yuri Artsutanov who in 1960 wrote a Sunday supplement to Pravda on how to build a structure for the space elevator 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. As the wires were to be thin, micrometeoroids would have had no trouble cutting them, which makes it impractical. Like Artsutanov, Isaacs’ article also wasn’t known to the aerospace community. It wasn’t until 1975, when Jerome Pearson independently came up with the concept and published it in Acta Astronautica, which finally made the aerospace community at large aware of the space elevator for the first time. His article inspired Sir Arthur Clarke to write his famous novel The Fountains of Paradise. It wasn’t until later that Pearson extended his theory to the moon and changed to using the Lagrangian points instead of having it in geostationary orbit.

Materials

Unlike the earth space elevator, the materials for the lunar space elevators won’t require a lot of strength. The elevator can be made with materials available today. Carbon nanotubes aren’t required to build the structure. This would make it possible to build the elevator much sooner, since available carbon nanotube materials in sufficient quantities are still years away.

One material that has great potential is M5 fiber. This is a synthetic fiber that is lighter than Kevlar or Spectra. According to Pearson, Levin, Oldson, and Wykes in their article The Lunar Space Elevator, an M5 ribbon 30mm wide and .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. 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.

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. The ribbons would be used by the robotic climbing vehicle to get from the surface into orbit. The vehicles would be slow, compared to chemical rockets, but it is a good speed for transferring cargo.

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. 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. 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.

Climbing vehicles

One method of getting materials needed from the moon into orbit would be the use of robotic climbing vehicles. These vehicles would consist of two large wheels pressing against the ribbons of the elevator to provide enough friction for lift. 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. 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. 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.

Possible uses

The Lunar elevator would be used to transport supplies and materials from the surface of the moon into the Earth’s orbit and vice versa. 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 (2005). He claims that one such material which would be very valuable is lunar regolith, also known as moon dirt (2005). One particular use for the regolith would be to shield space stations. Other materials such as metals and minerals would be mined and sent up for construction. Then there are materials from earth that can be sent into orbit and then down to the moon to be used by lunar bases and installations. 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.

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. Humans would be able to get there faster by using rockets to and from the moon.