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.
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
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.
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
PHYSICALLY WHAT IS A
in space is a long, flexible cable connecting two masses. When the cable is electrically
conductive, the ensemble becomes an electrodynamic tether.
can be three main types of electrodynamic tether employed systems providing different
Electrodynamic tether systems - in which two masses are separated by a long flexible
cable electrically conductive cable - can perform many of the same functions as
conventional spacecrafts but without the use of chemical or nuclear fuel sources.
In low earth orbit (LEO) tether systems could provide electrical power and positioning
capability for satellites and manned spacecraft, as well as help get rid the region
of dangerous debris.
3. On long term missions, such as exploration of Jupiter
and its moons, tethers could drastically reduce the amount of fuel needed to maneuver
while also providing a dependable source of electricity.
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