Elevators on the Moon and Mars

If space elevators can be deployed around Earth, they could in principle also be used on other planets and moons. Of course it would be more difficult to transport the necessary equipment that far out, but there are also important benefits. The lower a planet's gravity and the faster it rotates, the easier it is to build a space elevator. Theoretically, it will thus be easier to build a space elevator on the Moon than on Earth, as the gravity on its surface is only one sixth of what we feel here on Earth. (On a small asteroid that has barely any gravity, a simple tower built out of conventional materials would suffice, but it would not be a very useful structure.) The forces on the cable are therefore much lower; instead of revolutionary, not-yet-existing carbon nanotubes, a lunar elevator could be made using already-existing tether materials such as Kevlar and Spectra.

Apart from enabling easy access to the Moon's surface, a lunar space elevator could transport into space raw materials mined on the moon. From there it could be transferred to orbiting factories or Earth itself. The lunar regolith contains oxygen that could be used in spacecraft propulsion and life support systems, and lunar helium-3 (extremely rare on Earth) may fuel future nuclear fusion reactors. However, a lunar space elevator would need to be rather different from one based on Earth. While our own planet rotates around its axis every 24 hours, the Moon makes only one turn every 29 days with respect to the Sun. Because this is the same amount of time it takes to complete one orbit around the Earth, we always see the same one side of the Moon. The slow rotational velocity of the Moon compared to Earth means that a longer space elevator is required to generate the same amount of centripetal force in the cable. However, the lower lunar gravity means less centripetal force is necessary to keep the space elevator from falling down, which partly compensates for the slower rotation. If we do the calculations, we find that the Moon's theoretical stationary orbit altitude would be about 88,000 km (55,000 miles) above the surface. However, at such a distance from the Moon, Earth's gravity is certainly apparent and would severely distort the orbit of a satellite or space elevator station put there (the average distance between Earth and the Moon is 384,000 km [239,000 miles]). In other words, there is no equivalent of a geostationary orbit for the Moon. Nevertheless, there are alternative points for positioning the center of mass of a lunar space elevator.

Jerome Pearson, the space elevator pioneer, appears to be the first to have thought up a practical space elevator concept for the Moon. He determined that the center of mass of a lunar space elevator could be placed at either the Moon-Earth L1 or L2 Lagrange point. Lagrange points are special stable points that exist about any two orbiting bodies, such as the Moon and Earth. The gravitational forces of both bodies on a satellite and the centrifugal force caused by its orbit around the Sun are balanced in these points. L1 is a point between Earth and the Moon, 56,000 km (35,000 miles) above the lunar surface. L2 is on the far side of the Moon, 67,000 km (42,000 miles) up (Fig. 6.8). A spacecraft put in a Moon-Earth Lagrange point can thus in principle stay there indefinitely, always keeping the same position with respect to the Moon and Earth. Seen from the lunar surface, the Lagrange points always stays in the same position, making it possible to connect each point with a fixed spot on the lunar surface by a tether—a lunar space elevator.

Pearson's idea for building a lunar space elevator from the L1 or L2 point is similar to what Edwards proposes for a terrestrial version: a spacecraft carrying a huge spool of cable would be launched to L1 or L2, then slowly drift to higher altitude as it deploys a cable down to the lunar surface. Once anchored to the lunar surface, Earth's gravity pulling at the cable and the countermass at the top would ensure it remains taut and aligned toward the L1 point. Solar- or laser-powered climbers could then climb up from the lunar surface to the top of the cable, delivering lunar material there to

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Figure 6.8: Positions of the L1 and L2 Lagrange points of the Earth-Moon system (not drawn to scale).

increase the amount of countermass and thus make it possible for heavier climbers to ascend the cable. Released from the top of the lunar space elevator, payloads would end up in a high Earth orbit and could then be slowed down further (by rocket engines) to enter lower orbits.

According to Pearson, an already-existing fiber material called M5 would be sufficiently strong to build a lunar space elevator. His calculations show that a cable with a lifting capacity of 200 kg (440 pounds) would have a mass of only 6800 kg (15,000 pounds). Once the initial elevator is operational, it could be reinforced with additional materials mined on the Moon, such as glass and boron.

Mars turns once around its axis in 24 hours and 37 minutes, thus spinning only just a bit slower than Earth. At Mars gravity is higher than on the Moon, but still about 62 percent lower than on our planet. The Mars stationary orbit altitude is at about 17,000 km (11,000 miles), thus much lower than for Earth. The net result is that also on Mars a space elevator may be constructed with already-existing materials, and that it can be less than half the height of a space elevator on Earth (at least as far as the distance to its center of mass at stationary orbit altitude is concerned).

Employing an asteroid as a countermass for an Earth space elevator would require somehow capturing such a huge space rock and putting it into Earth orbit above the equator. However, Mars already has two suitable highmass bodies in near-equatorial orbits: its two small moons Phobos and Deimos. These are actually asteroids, captured by Mars gravity in some distant past. It may be possible to link the surface of the red planet with one of its moons by a long tether. Phobos orbits at an altitude of only about 9400 km (5800 miles)—well below the stationary orbit. To act as a space elevator countermass the orbit altitude of Phobos would have to be significantly increased, because only when the moon is placed above the 17,000-km (11,000-mile) stationary orbit can it exert a pulling force on the cable. However, moving Phobos to a dramatically higher orbit would be extremely difficult and expensive. It makes more sense to employ Deimos as a space elevator countermass, because at an altitude of 23,500 km (14,600 miles) it already orbits well (but not too far) beyond Mars's stationary orbit. Moreover, Deimos contains lots of carbon, which could be used to produce the carbon nanotubes needed for the space elevator ribbon.

Because any type of Mars space elevator will need to extend beyond the nominal orbit of Phobos, and this moon orbits the planet more or less in the orbital plane once every 8 hours, there is a real risk of Phobos smashing through the cable. A solution for this may be to induce a constant oscillation in the cable, sort of a skip rope motion, accurately timed so that the space elevator avoids the moon each time it passes. Arthur C. Clarke described this in his novel The Fountains of Paradise in 1979, also suggesting that the highspeed Phobos flybys would offer spectacular views to people on the space elevator.

There seems to be only one tether-technology alternative to the space elevator to launch things from Earth's surface into space. Although it has a lot of technology requirements in common with the space elevator, the way it works is quite different.

Imagine a giant taking one end of an extremely long chain in his hand and whirling it around. Then imagine a large part of the other end is shaped like an airplane wing. Just like on a helicopter, that wing will create lift and go up. Now imagine that the chain is so long that the aerodynamic lift elevates the far end all the way up to the edge of the atmosphere. Not only is that end close to space and virtually in vacuum, it is also rotating at a tremendous speed. If an object would be detached from the tip, it would actually fly away at orbital speed and thus become a satellite. If that object would be able to slide along the chain, it would need only a bit of speed starting from the hand of the giant to send it on its way, with the centrifugal force of the rotation speeding it up and moving it to the end of the chain. Now instead of the giant imagine a rotating hub at Earth's surface, and jet engines to propel the system, and instead of the chain imagine a tether ribbon, and you have an "aerovator'' (Fig. 6.9).

The aerovator concept was invented and developed during an online Yahoo Group Internet discussion on space elevators in May 2006. The design for this megastructure consists of a tether ribbon with a length of about 1000

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