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.

PRINCIPLE

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.

The Dutch physicist Hendrik Androon Lorentz showed that a moving electric charge experiences a force in a magnetic field. (if the charge is at rest, there will not be any force on it due to magnetic field ) Hence it is clear that the force experienced by a current conductor in a magnetic field is due to the drifting of electrons in it. If a current I flows through a conductor of cross-section A then

I = neAv where v is the drift speed of electronics n is number density in the conductor and e the electronic charge.

For an element dI of the conductor

Id = nAdIev

But Adi is the volume of the current element. Therefore, nAdI represents the number (N) of electrons in the element

Hence, nAdIe = Ne = q, the total charge in the element.

Therefore, IdI = qv

But, the force dF on a current carrying element dI in a magnetic field B is given by

dF = IdIB

i.e.,dF = qvB

This fundamental force on a charge q moving with a velocity v in a magnetic field B is called the Magnetic Lorentz Force.

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.