Abstract :
The basic
principle of an electrodynamic tether is Lorentz force. It is the force that a
magnetic field exerts on a current carrying wire in a direction perpendicular
to both the direction of current flow and the magnetic field vector.
Electrodynamic
(ED) tether is a long conducting wire extended from spacecraft. It has a strong
potential for providing propellant less propulsion to spacecraft in low earth
orbit. An electrodynamic Tether uses the same principle as electric motor in
toys, appliances and computer disk drives. It works as a thruster, because a
magnetic field exerts a force on a current carrying wire. The magnetic field is
supplied by the earth. By properly controlled the forces generated by this “electrodynamic”
tether can be used to pull or push a spacecraft to act as brake or a booster.
NASA plans to lasso energy from Earth’s atmosphere with a tether act as part of
first demonstration of a propellant-free space propulsion system, potentially
leading to a revolutionary space transportation system. Working with Earth’s
magnetic field would benefit a number of spacecraft including the International
Space Station. Tether propulsion requires no fuel. Is completely reusable and
environmentally clean and provides all these features at low cost.
Satellites
have a major part to play in the present communication system. These satellites
are launched with the help of rockets. Typically a payload will placed by a
rocket in to Low Earth Orbit or LEO (around 400 km) and then boosted higher by
rocket thrusters. But just transporting a satellite from the lower orbit to its
eventual destination can to several thousand dollars per kilogram of payload.
To cut expenses space experts are reconsidering the technology used to place
payload in their final orbits.
There are
over eight thousand satellites and other large objects in orbit around the
Earth, and there are countless smaller pieces of debris generated by spacecraft
explosions between satellites. Until recently it has been standard practices to
put a satellite in to and leave it there. However the number of satellites has
grown quickly, and as a result, the amount of orbital debris is growing
rapidly. Because this debris is traveling at orbital speed (78km/s), it poses a
significant threat to the space shuttle, the International Space Station and the
many satellites in Earth orbit.
One method
of removing a satellite from orbit would be to carry extra propellant so that
the satellite can bring itself down out of orbit. However this method requires
a large mass of propellant and every kilo of propellant that must be carried up
reduces the weight available for revenue-producing transponders. Moreover this
requires that the rocket and satellites guidance systems must be functional
after sitting in orbit for ten years or more.
What can,
without rockets, deploy satellite to Earth-orbit or fling them in to deep
space, can generate electrical power in space, can then catch and eliminate
space junk? String! Sounds impossible, but the development in space-tethers may
be as significant to future space development as rockets were to its
beginnings.
Called an
electrodynamic tether provides a simple and reliable alternative to the
conventional rocket thrusters. Electrodynamic tethers work by virtue of the
force a magnetic field exerts on a current carrying wire. In essence, it is a
clever way of getting an electric current to flow in a long conducting wire
that is orbiting Earth, so that earth’s magnetic field will exerts a force on
and accelerate the wire and hence any payload attached to it. By reversing the
direction of current in it, the same tether can be used to deorbit old
satellites.
HISTORY OF
SPACE TETHERS
While
space-based tethers have been studied theoretically since in the 20th century,
it wasn’t until 1947 that Giuseppe Colombo came up with the idea of using a
long tether to support a satellite System (TSS) to investigate plasma physics
and the generation of electricity in the upper atmosphere.Up until the TSS the
use of tethers in space has been limited. The best-known applications are the
tethers that connect spacewalking astronauts to their spacecraft. Astronauts
can work and fly free of the Space Shuttle using the Manned Maneuvering Unit
(MMU), but for most work activities in the Shuttle payload bay (and during the
assembly of the International Space Station) astronauts still use a safety
tether.
WORKING
An
electrodynamic tether is essentially a long conducting wire extended from a
space craft. The electrodynamic tether is made from aluminium alloy and
typically between 5 and 20 kilometers long. It extends ‘downwards’ from an
orbiting platform. Aluminium alloy is used since it is strong, lightweight,
inexpensive and easily machined.
The gravity
gradient field (also known as “tidal force”) will tend to orient the tether in
a vertical position. If the tether is orbiting around the Earth, it will be
crossing the earth’s magnetic field lines orbital velocity (7-8 km/s). The
motion of the conductor across the magnetic field induces a voltage along the
length of the tether. This voltage can up to several hundred volts per
kilometer.
In the
above figure the sphere represents the Earth and the unbroken lines represents
Earth’s magnetic field. The broken line is LEO. As shown in the figure there is
a drag force experienced in the wire in a direction perpendicular to the
current and magnetic field vector.
In an
“electrodynamic tether drag” system such as the terminator Tether, the tether
can be used to reduce the orbit of the spacecraft to which it is attached. If
the system has a means for collecting electrons from the ionospheric plasma at
one end of the tether and expelling them back in to the plasma at the other end
of the tether, the voltage can drive a current along the tether. This current
bill, in turn, interact with the Earth’s magnetic field to cause a Lorentz JXB
force, which will oppose the motion of the tether and whatever it is attached
to. This “electrodynamics drag force” will decrease the orbit of the tether and
its host spacecraft. Essentially, the tether converts the orbital energy of the
host spacecraft in to electrical power, which is dissipated as ohmic heating in
the tether.
In an
“electrodynamic propulsion” system, the tether can be used to boost the orbit
of the spacecraft. If a power supply is added to the tether system and used to
drive current in the direction opposite to that which it normally wants to
flow, the tether can “push” against the Earth’s magnetic field to raise the
spacecraft’s orbit. The major advantage of this technique compared to the other
space propulsion system is that it doesn’t require any propellant. It uses
Earth’s magnetic field as its “reaction mass”. By eliminating the need to
launch large amounts of propellant in to orbit, electrodynamic tethers can
greatly reduce the cost of in-space propulsion
The tether
is dragged through the atmosphere‘s’ ionosheric plasma. The rarefied medium of
electrons through which the whole set up is traveling at a speed of 7-8km/s. In
so doing, the 5-km. long aluminium wire extracts electrons from the plasma at
the end farthest from the payload and carries them to the near end (plasma
chamber tests have verified that thin bare wires can collect current from
plasma). There a specially designed devise known as a hollow cathode emitter
expels the electrons, to ensure their return to space currents in the
circuit.
Ordinarily,
a uniform magnetic field acting on a current-bearing loop of wire yields a net
force of zero, since that cancels the force on one side of the loop on the
other side, in which the current is flowing in the opposite direction However,
since the tethered system is not mechanically attached to the plasma. The
magnetic force on the plasma current in the space does not cancel the forces on
the tether. And so the tether experiences a net force.
As the
tether cuts across the magnetic field, its bias voltage is positive at the end
farthest from Earth and negative at the near end. This polarization is due to
the action of Lorentz force on the electrons in the tether. Thus the “natural”
upward current flow due to the (negatively charged) electrons in the ionosphere
being attracted to the tethers far and then returned to the plasma at the near
end. Aided by the hollow cathode emitter. The hollow cathode is vital: without
it, the wire’s charge distribution would quickly reach equilibrium and no
current would flows.
Switching
on the hollow cathode causes a small tungsten tube to heat up and fill with
xenon gas from small tank. Electrons from the tether interacted with the heated
gas to create ion plasma. At the far end of the tube. a so called keeper
electrode, which is positively charged with respect to the tube. Draw the
electrons and expels them to space. (the xenon ions, mean while are collected
by the hollow cathode and used to provide additional heating). The rapid
discharge of electrons invites new electrons to follow from the tether and out
through the hollow cathode. Earth’s magnetic field exerts a drag force on a
current carrying tether, decelerating it and the payload and rapidly lowering
their orbit Eventually they re-enter Earth’s atmosphere.
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