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Tether is a word, which is not heard often. The word meaning of tether is 'a rope or chain to fasten an animal so that it can graze within a certain limited area'. We can see animals like cows and goats 'tethered' to trees and posts.
In space also tethers have an application similar to their word meaning. But instead of animals, there are spacecrafts and satellites in space. A tether if connected between two spacecrafts (one having smaller orbital altitude and the other at a larger orbital altitude) momentum exchange can take place between them. Then the tether is called momentum exchange space tether. A tether is deployed by pushing one object up or down from the other. The gravitational and centrifugal forces balance each other at the center of mass. Then what happens is that the lower satellite, which orbits faster, tows its companion along like an orbital water skier. The outer satellite thereby gains momentum at the expense of the lower one, causing its orbit to expand and that of the lower to contract. This was the original use of tethers.
But now tethers are being made of electrically conducting materials like aluminium or copper and they provide additional advantages. Electrodynamic tethers, as they are called, can convert orbital energy into electrical energy. It works on the principle of electromagnetic induction. This can be used for power generation. Also when the conductor moves through a magnetic field, charged particles experience an electromagnetic force perpendicular to both the direction of motion and field. This can be used for orbit raising and lowering and debris removal. Another application of tethers discussed here is artificial gravity inside spacecrafts.
NEED AND ORIGIN OF TETHERS
Space tethers have been studied theoretically since early in the 20th century, it wasn't until 1974 that Guiseppe Colombo came up with the idea of using a long tether to support satellite from an orbiting platform. But that was simple momentum exchange space tether. Now lets see what made scientists think of electrodynamic tethers.
Every spacecraft on every mission has to carry all the energy sources required to get its job done, typically in the form of chemical propellants, photovoltaic arrays or nuclear reactors. The sole alternative - delivery service - can be very expensive. For example, a spacecraft orbiting in the International space Station (ISS) will need an estimated 77 metric tons of booster propellant over its anticipated 10 year life span just to keep itself from gradually falling out of orbit. Assuming a minimal price of $7000 a pound (dirt cheap by current standards) to get fuel up to the station's 360 km altitude, i.e. $1.2 billion simply to maintain the orbital status quo.
So scientists have are taking a new look at space tether, making it electrically conductive. In 1996, NASA launched a shuttle to deploy a satellite on a tether to study the electrodynamic effects of a conducting tether as it passes through the earth's magnetic fields. As predicted by the laws of electromagnetism, a current was produced in the tether as it passed through the earth's magnetic field, acting as an electrical generator. This was the origin of electrodynamic tethers