Introduction
The term orbital tether refers to an extremely long length of cable linking a fixed point on a planet’s surface with a satellite in orbit around the planet. Orbital tethers are a form of space tether, which is a length of cable linking spacecraft, space stations, asteroids and other objects together.
Space tethers have a number of applications. The most obvious is the orbital tether (also known as a space elevator or beanstalk), which uses the cable in the most straightforward way: to transfer payloads to and from orbit. Other possibilities include space bolas, which transfer momentum like a gigantic slingshot, propelling spacecraft between orbits. Finally, an orbital tether that is also a conductor could be used to generate electrical power via its movement through the earth’s magnetic field (electromagnetic induction). This article is concerned with orbital tethers.
Orbital tethers have great practical benefits for surface-orbit transfer, as well as significant economic benefits: using a space elevator to launch satellites or space station parts and supplies would be considerably cheaper than using existing rocket technology (on the order of a few hundred dollars per kg compared to thousands for conventional rocket transfer to LEO, or $20,000/kg for along a cable”>gravity-gradient.
For a total payload of 100kg (including the tether), the tether tension would be of the order 100 N at a minimal LEO of 350km. The tensile force on the tether due to gravity-gradient increases with the density of the planet and the proximity to its center of mass. Ideally, tethers should be tapered exponentially, with a cross-sectional area greatest at the center of gravity and smallest at the terminal end.
Other potential problems with an orbital tether would be the risk of impact from meteorites or space debris (the US Military maintains a database of over 10,000 debris objects of significant size in LEO). Orbital tethers could be hazardous to conventional air and space flight, especially in less favourable visibility conditions, though the possibility of collision could be reduced by deploying warning beacons anchored to the tether at regular intervals. Bad weather, especially thunderstorms, could be dangerous, and vibrational harmonics (feedback loops that cause the vibrations to increase), which could be compensated for with intelligent damping systems.
Material science and nanotechnology are highly relevant fields. The cable used in an orbital tether must have an extremely high tensile strength to density ratio. A comparison of materials with their tensile strength/density ratio follows:
| Type of material | Strength/density ratio, in Required for space elevator | 65-120 |
| Steel | under 1 | |
| Kevlar | 2.5 - 4.1 | |
| quartz fiber | > 20 | |
| carbon nanotubes | 30 - 50; theoretically > 120 |
From this table it is evident that carbon nanotubes are an excellent candidate for orbital tether construction. There remain significant problems with fabrication, however: technology to manufacture nanotubes in bulk has not yet been developed, and the cost of carbon nanotubes is around $100/gram (greater than gold). However, this is mainly an issue of scale and straightforward R&D, which is certainly being done. The applications of high tensile strength cable is by no means limited to space elevators, and has many practical uses in earthbound engineering and construction.
Because the ideal tether cross-section area varies with distance from the center of gravity, the actual elevator "cabin" needs a considered approach to navigating the length of the cable. Such pods, known as climbers, have a number of proposed designs:
- Moving arms with pads of hooks, similar to insect legs
- Rollers, with or without retracting hooks
- Maglev, difficult because of the bulk of track required along the cable
Future use and development of orbital tethers
The future looks rosy for space tethers and elevators. There is a highly active, competitive market for producing the most important component: high tensile strength cable. NASA and other respected institutions have conducted experimental research into space tether feasibility, and commercial interests are beginning to appear (such as Tethers.com). What could the long-term hold for orbital tethers?
The reduction in cost by a factor of ten or greater for LEO and geosynchronous orbit transfer will result in a beneficial knock-on effect in satellite market sectors such as television, communications, meteorology, astronomy and more. Microgravity experiments will be much cheaper to carry out by educational or R&D institutions. Space elevators will provide efficient, cost-effective access to space, enabling space tourism (orbital hotels or space cruises), and bringing down the cost barriers to orbital factories and larger R&D stations such as the ISS. The necessity for co-operation between nations to foot the cost of initial prototype orbital tethers (estimated to be in the billions) will hopefully bring governments together in the manner of other global projects like the ISS or ITER, which despite their difficulties have shown co-operation on such a scale is possible, and indeed desirable.