Space tether

Space tethers are long cables that can be used in space for many important jobs. They help give a spacecraft a push, change its direction, keep it steady, or even hold parts of a big satellite system in the right place. These cables can do many things that usually need special engines on a spacecraft.

Because of this, some people think that flying in space using tethers could cost much less than using regular rocket engines. This could make future space trips more affordable and help us explore space in new ways. Scientists and engineers are very interested in learning more about how tethers can be used to make space travel better and cheaper.

Main techniques

Tether satellites can be used for many purposes, such as learning about space movement and how space works. There are five main ways scientists are working on using space tethers:

Electrodynamic tethers
Main article: Electrodynamic tether

These tethers help move spacecraft by using electric currents and magnetic fields from planets.

Momentum exchange tethers
Main article: Momentum exchange tether

These tethers can catch a spacecraft and then let it go again, helping it change its path or speed.

Tethered formation flying
Main article: Tethered formation flying

This uses a special tether to keep several spacecraft at just the right distance from each other as they fly together.

Electric sail
Main article: Electric sail

This is like a sail for spacecraft, using electric tethers pushed by particles from the sun.

Universal Orbital Support System
Main article: Universal Orbital Support System

This idea uses a tether to hold an object steady while it orbits in space.

Scientists have thought of many uses for space tethers, like building tall towers in space or helping spacecraft move without using fuel.

History

Konstantin Tsiolkovsky (1857–1935) once imagined a very tall tower that reached into space, held up by Earth rotating. But at that time, there was no way to build such a tower.

In 1960, Yuri Artsutanov described an idea for a strong cable stretching from a geosynchronous satellite down to Earth and up beyond, balancing itself. This was called the space elevator idea, but it was not practical with the materials available then.

Later, in the 1970s, Jerome Pearson also thought about space elevators, especially for the Moon, and found it possible with materials at the time. Hans Moravec and Robert L. Forward studied rotating cables, called skyhooks, that could move objects to and from the Moon, Mars, and other planets efficiently.

In 1979, NASA started looking into these ideas more seriously. In 1990, Eagle Sarmont suggested a system where a cable could help move spacecraft between Earth and higher orbits. By 2000, NASA and Boeing explored a plan using a rotating cable to send payloads into orbit from high-speed aircraft.

Missions

A tether satellite is a satellite linked to another by a special cable in space. Several satellites have been launched to test these cable ideas, with different results.

Types

Main article: Momentum exchange tether

There are many different types of space tethers, and some of them overlap.

Main article: Skyhook (structure)

A skyhook is a special idea for using tethers in space. It could help carry things up to very high places and speeds. Some ideas for skyhooks include spinning tethers really fast to catch fast-moving things and put them into space.

Main article: Electrodynamic tether

Electrodynamic tethers are long wires that can be used in space. They can change movement into electricity or use electricity to move. These tethers create electricity as they move through Earth's magnetic field. The metal used for these tethers needs to be good at conducting electricity and not be too heavy.

Main article: Tethered formation flying

Formation flying uses a tether to connect several spacecraft together. One idea from 2011 was to test this with a special experiment for studying space travel.

Main article: Universal Orbital Support System

The Universal Orbital Support System is a new idea for a type of tether. It would help support things high above a planet or moon, but not as high as the main spacecraft orbiting above.

Technical difficulties

Gravitational gradient stabilization

Main article: Gravity-gradient stabilization

Space tethers can stay straight because of small changes in how strong gravity is along their length. When two spacecraft at different heights are linked by a tether, they must move at the same speed. This means the lower spacecraft slows down a bit, and the higher one speeds up. The forces change, causing the tether system to line up with the direction of gravity.

Atomic oxygen

Further information: Atomic oxygen

In low Earth orbit, tiny oxygen particles can wear away at objects because they move very fast. This can hurt space tethers over time.

Micrometeorites and space junk

Simple tethers can be damaged by tiny space rocks and bits of old satellites. Some ideas to make tethers stronger include special nets that spread the force if a piece breaks. Big pieces of space junk can still cut tethers, but we can track these with radar and move the tether out of the way when needed.

Radiation

Radiation, like ultraviolet light, can wear down tether materials. Tethers that pass through certain areas around Earth may not last as long because of this.

Construction

Space tethers are long cables that can be used for moving spacecraft, keeping things steady, or holding parts of a big satellite in place. To work well and cost less than rockets, these tethers need to be made from materials that are both strong and light.

Some materials that work well for tethers include special plastics like ultra-high-molecular-weight polyethylene, aramid, carbon fiber, and maybe even carbon nanotubes in the future. These materials need to be strong enough to handle pulling forces and also light so they don’t weigh too much. Designers also have to think about protecting the tethers from space debris and tiny space particles.

For some uses, the tether doesn’t need to be as strong, so different materials might be chosen based on what the mission needs. There are also equations that help designers figure out the best materials and sizes for different kinds of tethers.

Control and modelling

Electrodynamic tethers that hang straight up might shake and get wobbly because of how they interact with magnetic and gravity fields. To stop this, scientists think we could change the electric current in the tether to balance out the shaking. This needs special tools to measure the shaking, like tiny GPS on the tether.

Another idea is to spin the tether instead of letting it hang, which can help keep it steady without extra control.

Sometimes, tethers can get damaged by sudden electric jumps, which can break them or hurt the machines that handle them. Computer models also show that tethers can break from shaking too much. Special machines can help control this shaking by changing how they move along the tether.

Tethers are long and not like a small ball, so their balance points aren’t in the same spot. This makes their paths around Earth a bit tricky to predict, especially if they spin in certain ways.