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Lunar/Planetary Space Elevator Systems
With the advent of terrestrial Space Elevator technology will come the prospect of similar systems deployed for the Moon or Mars and likewise offering the benefits of potentially continuous electric powered transportation between surface and orbit. The lighter gravity, particularly for the Moon, offers potentially easier deployment, a more benign environment for the system to deal with, and development using currently common materials such as Kevlar rather than the more advanced nanofiber. However, the prospect of establishing settlement of sufficient scale to justify Space Elevator development in these places before the realization of bulk long/continuous-strand nanofiber production seems relatively remote and it is more likely that these too will employ the more advanced material and their development preceded by their terrestrial counterpart.
Though largely the same as a terrestrial Space Elevator, the Lunar and Mars elevators would have some important differences. A lunar Space Elevator would need a ribbon/tether structure much longer in length owing to a center of gravity location at the Lagrange points L1 and L2 -which, of course, are logical locations for waystations to support asynchronous transorbital traffic. And since the Moon is believed short on carbon, materials for this are likely to be imported from elsewhere in space with tether/ribbons fabricated on-orbit. Also, polar locations are expected to be very desirable for lunar settlement while the elevator system would require equatorial location. But, again thanks to the reduced gravity and smaller radius of the Moon, it is possible to deploy the elevator on an arcing rather than straight path so its downstation terminus would approach either of the poles, the remaining distance spanned by tracked transport integrating the elevator ribbon/tether vehicles. It remains an open question, however, whether this is actually preferable to simply running a surface transport system to the equator -something for future research to explore.
A Mars Space Elevator would have the interesting problem of an equatorial orbit that parallels that of the planet’s two small moons; Phobos and Deimos. This would lead to potential intersection of the elevator and the moons on a regular interval. This issue was noted in Arthur C. Clarke’s novel The Fountains of Paradise which popularized the Space Elevator concept. As suggested in that novel, one solution to this problem may be to deliberately introduce and maintain a wobble in the elevator structure that causes it to continuously dodge the small moons as they pass, rather like a jump rope. This wobble might be introduced by active propulsion on the tether/ribbon structure or by making the downstation terminus mobile on a circular track of some tens of kilometers in circumference. Obviously, either option would be complex to implement. Some have suggested that, given the small size of these asteroid-like moons, they could me moved whole in orbit to keep them out of the way or one might even reposition them for use as counterweight structures for space elevators themselves. An attractive concept as it would coincide with the development of orbit stabilization or repositioning for purposes of mining purposes, but would still be a tremendously great project with high stakes given the potential hazards for early Mars settlements from failure.
The Moon and Mars also offer prospects for another variant of the Space Elevator concept that would be uniquely suited to their low gravity environments; the Rotovator or momentum exchange tether. A Rotovator is a rotating orbital tether structure that has its retrograde rotation matched to the orbital velocity such that, at a series of points on or near the ground, it is momentarily stationary and can thus pick up and carry away payloads into space, imparting momentum on them on release. Conversely, the system can rendezvous with payloads on higher orbit and transfer them to the surface. A series of fixed-position station points can then be used on the surface. Earth’s atmosphere and higher complicates the use of this approach, limiting such systems to high altitude rendezvous at hypersonic speeds. But on the Moon and Mars this might prove a more practical alternative to the stationary Space Elevator as the transfers would be much faster than with a tether/ribbon climber and long chains of Rotovators may be established to provide transorbital propulsion. But perpetual maintenance of such moving systems may be complicated and, unlike the Space Elevator, they would not be able to incrementally grow in transit capacity.
Certainly, the scale of Space Elevator or Rotovator development as industrial projects may preclude their pursuit on the Moon and Mars until quite extensive settlement is established to justify it. Thus early settlements will likely need to function within the logistics of rocket propelled or magnetic mass driver systems for some time. But once such technology has been deployed somewhere, it may create impetus for its use in many variations about the solar system. Time will tell as to what transit technologies our future inhabitants in space will ultimately realize.