Space tether

Space tethers are long cables which can be used for propulsion, momentum exchange, stabilization and attitude control, or maintaining the relative positions of the components of a large dispersed satellite/spacecraft sensor system.

These are conducting tethers that carry a current that can generate either thrust or drag from a planetary magnetic field, in much the same way as an electric motor does.

Momentum exchange tethers can be used for orbital maneuvering, or as part of a planetary-surface-to-orbit / orbit-to-escape-velocity space transportation system.

This is typically a non-conductive tether that accurately maintains a set distance between multiple space vehicles flying in formation.

Konstantin Tsiolkovsky (1857–1935) once proposed a tower so tall that it reached into space, so that it would be held there by the rotation of Earth.

In 1977, Hans Moravec[6] and later Robert L. Forward investigated the physics of non-synchronous skyhooks, also known as rotating skyhooks, and performed detailed simulations of tapered rotating tethers that could pick objects off, and place objects onto, the Moon, Mars and other planets, with little loss, or even a net gain of energy.

A number of satellites have been launched to test tether technologies, with varying degrees of success.

A rotating tether will create a controlled force on the end-masses of the system due to centrifugal acceleration.

However, using electrodynamic tether thrusting, or ion propulsion the system can then re-boost itself with little or no expenditure of consumable reaction mass.

A skyhook is a theoretical class of orbiting tether propulsion intended to lift payloads to high altitudes and speeds.

[1] Electric potential is generated across a conductive tether by its motion through the Earth's magnetic field.

An electrodynamic tether was profiled in the documentary film Orphans of Apollo as technology that was to be used to keep the Russian space station Mir in orbit.

A theoretical type of non-rotating tethered satellite system, it is a concept for providing space-based support to things suspended above an astronomical object.

A non-rotating tether system has a stable orientation that is aligned along the local vertical (of the earth or other body).

Normally, each spacecraft would have a balance of gravitational (e.g. Fg1) and centrifugal (e.g. Fc1) forces, but when tied together by a tether, these values begin to change with respect to one another.

Although thrusters could be used to change the orbit of the system, a tether could also be temporally wiggled in the right place, using less energy, to dodge known pieces of junk.

For applications that exert high tensile forces on the tether, the materials need to be strong and light.

Some current tether designs use crystalline plastics such as ultra-high-molecular-weight polyethylene, aramid or carbon fiber.

Electrodynamic tethers, such as the one used on TSS-1R,[clarification needed] may use thin copper wires for high conductivity (see EDT).

Eventually however, the mass of the tether propulsion system will be limited at the low end by other factors such as momentum storage.

Proposed materials include Kevlar, ultra-high-molecular-weight polyethylene,[citation needed] carbon nanotubes and M5 fiber.

It would also be able to hold 100 cargo vehicles, each with a mass of 580 kg (1,280 lb), evenly spaced along the length of the elevator.

Several systems have been proposed, such as shooting nets at the cargo, but all add weight, complexity, and another failure mode.

[36] Currently, the strongest materials in tension are plastics that require a coating for protection from UV radiation and (depending on the orbit) erosion by atomic oxygen.

Disposal of waste heat is difficult in a vacuum, so overheating may cause tether failures or damage.

Pendular motion causes the tether vibration amplitude to build up under the action of electromagnetic interaction.

The one ton climber proposed by Brad Edwards for his Space Elevator may detect and suppress most vibrations by changing speed and direction.

[37] Tethers are nearly always tapered, and this can greatly amplify the movement at the thinnest tip in whip-like ways.