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.
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
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.
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