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1.INTRODUCTION

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 be placed by a rocket into 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 lead 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.

Electrodynamic tether provides a simple and reliable alternative to the conventional

rocket thrusters. Functionally, electrons flow from the space plasma into the conductive tether,

are passed through a resistive load in a control unit and are emitted into the space plasma by an

electron emitter as free electrons. In principle, compact high-current tether power generators are

possible and, with basic hardware, tens, hundreds, and thousands of kilowatts appears to be

attainable.

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 de-orbit old satellites.

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Electrodynamic tethers have strong potential for providing propellantless propulsion to

spacecraft in low-Earth orbit for applications such as satellite de-orbit orbit boosting, and station

keeping. The tether, however, is not a rigid rod held above or below the spacecraft. It is a very

long, thin cable, and has little or no flexural rigidity. The transverse electrodynamic forces

therefore cause the tether to bow and to swing away from the local vertical. Gravity gradient

forces produce a restoring force that pulls the tether back towards the local vertical, but this

results in a pendulum-like motion. Because the direction of the geomagnetic field varies as the

tether orbits the Earth, the direction and magnitude of the electrodynamic forces also varies, and

so this pendulum motion develops into complex librations in both the in-plane and out-of-plane

directions. Due to coupling between the in plane motion and longitudinal elastic oscillations, as

well as coupling between in-plane and out-of-plane motions, an electrodynamic tether operated

at a constant current will continually add energy to the libration motions, causing the libration

amplitudes to build until the tether begins rotating or oscillating wildly. In addition, orbital

variations in the strength and magnitude of the electrodynamic force will drive transverse,

higher-order oscillations in the tether which can lead to the unstable growth of "skip-rope"

modes.

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2. REVIEW OF EXISTING ROCKET PROPULSION MECHANISM

The existing rocket propulsion mechanism derives energy from rocket fuels. Hydrogen

peroxide is one of the commonly used rocket fuels. The disadvantage of these rocket fuels is that

it produces low thrust. Kerosene is a liquid propellant, requires cryogenic systems for their

implementation. Nuclear energy can be used as a propellant. But it produces radiations, which

are very harmful.

There are several disadvantages in dealing with liquid propulsion systems. Spills or leaks

or several hazardous problems occur in it. Usually requires more volume due to lower average

propellant density and the relatively inefficient. A few propellants give toxic vapors or fumes.

Even in solid propellant system there are many backlogs such as these propellants are

largely prone to explosion and exhaust gases are usually toxic. Some propellants or propellant

ingredients can deteriorate.

An electrodynamic tether overcoming all such difficulties and risks with its unique

features put forward is a better option.

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3. HISTORY

Tsiolkovsky once proposed a tower so tall that it reached into space, so that it would be

held there by the rotation of the Earth. However, there was no realistic way to build it. To try to

solve the problems in Komsomolskaya Pravda, another Russian, Yuri Artsutanov, wrote in

greater detail about the idea of a tensile cable to be deployed from a geosynchronous sate llite

downwards towards the ground, and upwards away keeping the cable balanced. This is the space

elevatoridea, a type of synchronous tether that would rotate with the Earth. However, given the

materials, this too was impractical on Earth.

In the 1970s Jerome Pearson explored synchronous tethers further, and in particular

analysed the lunar elevator, and this was found to be possible with materials then existing.

In 1977 Hans Moravec and later Robert L. Forward investigated the physics of

synchronous and non synchronous skyhook tethers, and performed detailed simulations of

tapered tethers that could pick objects off and place objects onto the Moon, Mars and other

planets, with little, or even a net gain of energy.

In 1979 the USA NASA examined the feasibility of the idea and gave direction to the

study of tethered systems, especially tethered satellites. In 2000, the NASA and Boeing

considered a HASTOL concept where a tether would take payloads from a hypersonic aircraft (at

half of orbital velocity) to orbit.

Since then, a series of interesting space tether applications have been proposed and

analysed. Particularly in the last decade, the study of space tether has received significant

attention from researchers covering a broad range of applications. Some examples of

applications have considerable promise including the deployment and retrieval of subsatellites,

aerobraking, electrodynamic boost, deorbit of satellites, and momentum-transfer with libration

and rotation analysis.

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Control research on space tether applications was one of the most important aspects of

space tether study, and each control method suited each application or mission requirement, such

as liberation, oscillation, attitude, motion, and deployment.

Fig. 3.1

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4. 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.

4.1 Lorentz Force Law:

The Lorentz Force Law can be used to describe the effect of a charged particle moving in

a constant magnetic field.

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,

Thus in above equation represents q is 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. The simplest form of this law given by the scalar equation,

F = QvB

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‘F’ is the force acting on the particle (vector)

‘V’ is the velocity of the particle (vector)

‘Q’ is charge of particle (scalar)

‘B’ is magnetic field (vector).

NOTE: In this case is for v and B perpendicular to each other otherwise use F = QvB (sin (X) )

where X is the angle between v and B, when v and B are perpendicular X =90 deg. So sin (x) =1.

Fleming’s left hand rule comes in to play here to figure out which way the force is acting.

4.2 Fleming’s Left Hand Rule:

For a charged particle moving (velocity v) in a magnetic field (field B) the direction of

the resultant force (force F) can be found by: middle finger of left hand in direction of current

index finger of left hand in direction of field B, thumb now points in direction of the force or

motion F. The force will always be perpendicular to the plane of vector v and B no matter what

the angle between v and B is.

Fig.4.1 Electrodynamic tether propulsion

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5. WORKING

Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a

tether satellite, which can operate on electromagnetic principles, by converting their kinetic

energy to electrical energy(as generators), and vice versa (as motors). Electric potential is

generated across a conductive tether by its motion through the Earth's magnetic field.

As part of a tether propulsion system, crafts can use long, strong conductors (though not

all tethers are conductive) to change the orbits of spacecraft. 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. It can be used either to accelerate or brake an orbiting spacecraft. When

direct current is pumped through the tether, it exerts a force against the magnetic field, and the

tether accelerates the spacecraft.

Fig. 5.1: Generation of electric potential across conductive tether

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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. The voltage thus created along its length can be

up to several hundred volts per kilometer.

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, interacts 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 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

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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 flow. 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 ‘keeper electrode’ is present, 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.

Fig. 5.2

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5.1 Mathematics Of EDT

A motional electromotive force (EMF) is generated across a tether element as it moves

relative to a magnetic field. The force is given by Faraday's Law of Induction,

V(𝒆𝒎𝒇) = ∫ (�⃗⃗� × �⃗⃗� )𝒅�⃗⃗� 𝑳

𝟎

Without loss of generality, it is assumed the tether system is in Earth orbit and it moves

relative to Earth’s magnetic field. Similarly, if current flows in the tether element, a force can be

generated in accordance with the Lorentz Force Equation,

�⃗⃗� = ∫ 𝑰(𝑳)m d�⃗⃗� 𝑳

𝟎

× �⃗⃗�

In self-powered mode (de-orbit mode), this EMF can be used by the tether system to drive

the current through the tether and other electrical loads (e.g. resistors, batteries), emit electrons at the

emitting end, or collect electrons at the opposite. In boost mode, on-board power supplies must

overcome this motional EMF to drive current in the opposite direction, thus creating a force in the

opposite direction, as seen in below figure, and boosting the system.

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Fig. 5.3 Illustratin of EDT concept

At 300-km altitude, the Earth’s magnetic field, in the north-south direction, is

approximately 0.18 – 0.32 Gauss up to ~40º inclination, and the orbital velocity with respect to

the local plasma is about 7500 m/s. This results in a Vemf range of 35 – 250 V/km along the 5-

km length of tether. This EMF dictates the potential difference across the bare tether which

controls where electrons are collected and / or repelled. Here, the ProSEDS de-boost tether

system is configured to enable electron collection to the positively biased higher altitude section

of the bare tether, and returned to the ionosphere at the lower altitude end. This flow of electrons

through the length of the tether in the presence of the Earth’s magnetic field creates a force that

produces a drag thrust that helps de-orbit the system, as given by the above equation. The boost

mode is similar to the de-orbit mode, except for the fact that a High Voltage Power Supply

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(HVPS) is also inserted in series with the tether system between the tether and the higher positive

potential end. The power supply voltage must be greater than the EMF and the polar opposite.

This drives the current in the opposite direction, which in turn causes the higher altitude

end to be negatively charged, while the lower altitude end is positively charged(Assuming a

standard east to west orbit around Earth).

5.2 Voltage across conductor

With a long conducting wire of length L, an electric field E is generated in the wire. It

produces a voltage V between the opposite ends of the wire. This can be expressed as:

V=E . L=EL 𝐜𝐨𝐬 𝝉 = 𝝑𝑩𝑳𝐜𝐨𝐬𝝉

where the angle τ is between the length vector (L) of the tether and the electric field vector (E),

assumed to be in the vertical direction at right angles to the velocity vector (v) in plane and the

magnetic field vector (B) is out of the plane.

5.3 Current in conductor

An electrodynamic tether can be described as a type of thermodynamically "open

system". Electrodynamic tether circuits cannot be completed by simply using another wire, since

another tether will develop a similar voltage. Fortunately, the Earth's magnetosphere is not

"empty", and, in near-Earth regions (especially near the Earth's atmosphere) there exist highly

electrically conductive plasmas which are kept partially ionized by solar radiation or other

radiant energy. The electron and ion density varies according to various factors, such as the

location, altitude, season, sunspot cycle, and contamination levels. It is known that a positively

charged bare conductorcan readily remove free electrons out of the plasma. Thus, to complete

the electrical circuit, a sufficiently large area of uninsulated conductor is needed at the upper,

positively charged end of the tether, thereby permitting current to flow through the tether.

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However, it is more difficult for the opposite (negative) end of the tether to eject free

electrons or to collect positive ions from the plasma. It is plausible that, by using a very large

collection area at one end of the tether, enough ions can be collected to permit significant current

through the plasma. This was demonstrated during the Shuttle orbiter's TSS-1R mission, when

the shuttle itself was used as a large plasma contactor to provide over an ampere of current.

Improved methods include creating an electron emitter, such as a thermionic cathode, plasma

cathode, plasma contactor, or field electron emission device. Since both ends of the tether are

"open" to the surrounding plasma, electrons can flow out of one end of the tether while a

corresponding flow of electrons enters the other end. In this fashion, the voltage that is

electromagnetically induced within the tether can cause current to flow through the surrounding

space environment, completing an electrical circuit through what appears to be, at first glance, an

open circuit.

5.4 Tether current

The amount of current (I) flowing through a tether depends on various factors. One of these

is the circuit's total resistance (R). The circuit's resistance consist of three components:

1. The effective resistance of the plasma.

2. The resistance of the tether.

3. A control variable resistor.

In addition, a parasitic load is needed. The load on the current may take the form of a charging

device which, in turn, charges reserve power sources such as batter ies. The batteries in return

will be used to control power and communication circuits, as well as drive the electron emitting

devices at the negative end of the tether. As such the tether can be completely self-powered,

besides the initial charge in the batteries to provide electrical power for the deployment and

startup procedure. The charging battery load can be viewed as a resistor which absorbs power,

but stores this for later use (instead of immediately dissipating heat). It is included as part of the

"control resistor". The charging battery load is not treated as a "base resistance" though, as the

charging circuit can be turned off at anytime. When off, the operations can be continued without

interruption using the power stored in the batteries.

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5.5 Circuit Diagram

The below figure describes a typical EDT system in a series bias grounded gate

configuration (further description of the various types of configurations analyzed have been

presented) with a blow-up of an infinitesimal section of bare tether. This figure is symmetrically

set up so either end can be used as the anode. This tether system is symmetrical because rotating

tether systems will need to use both ends as anodes and cathodes at so me point in its rotation.

The V_hvps will only be used in the cathode end of the EDT system, and is turned off otherwise.

Fig. 5.4(a) A circuit diagram of a bare tether segment (b) An equivalent EDT system

circuit model showing the series bias grounded gate configuration.

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6. TETHER STABILIZATION

Electrodynmic tethers are inherently unstable. The electrodynamic forces also varies and

so this pendulum motion develops in to complex librations in both the in-plane and out-of-plane

direction. The Tether configuration feedback algorithm calculates a gain factor based upon the

network that the electrodynamic forces will perform on the tether dynamics. The second

algorithm requires only periodic measurements of the acceleration of the tether end mass called

End mass acceleration feedback method. These form heart of EDTS provide thermal stability and

efficiency.

A tether system deployed in orbit around the Earth will be pulled by gravity gradient

forces towards an equilibrium configuration oriented along the local vertical. In an

electrodynamic tether system, currents in the tether flowing across the planetary magnetic field

will generate JxB forces acting in a direction perpendicular to both the magnetic field and the

tether. These forces will push the tether away from the local vertical orientation, and the tether

will have a new equilibrium configuration roughly like that shown in the figure. Because the

strength and relative direction of the geomagnetic field will vary as the tether orbits the Earth,

however, this "equilibrium" configuration is really only an instantaneous equilibrium

configuration, and it changes constantly during the orbit. The tether will thus naturally develop

oscillations as it seeks to follow its changing "equilibrium" configuration.

The first criteria for electrodynamic tether stability is that the tether libration angles

cannot exceed 45°. If the tether librations exceed this level, the tether will begin to go slack at the

peak of its librations, much like the cables on a child's swing will go slack as the angle

approaches 90°. If the tether goes slack, it will generate complex, difficult-to-predict higher-

order oscillations and large tension excursions when the tether rebounds.

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Fig. 6.1 Drag effects on an Electrodynamic Tether system

6.1 Physics in EDTS :

To calculate the in-plane and out-of-plane directions, the components of the velocity and

magnetic field vectors must be obtained and the force values calculated. The component of the

force in the direction of travel will serve to enhance the orbit raising capabilities, while the out-

of-plane component of thrust will alter the inclination. In the below figure, the magnetic field

vector is solely in the north (or y-axis) direction, and the resulting forces on an orbit, with some

inclination, can be seen. An orbit with no inclination would have all the thrust in the in-plane

direction.

There has been work conducted to stabilize the librations of the tether system to prevent

misalignment of the tether with the gravity gradient. The below figure above displays the drag

effects an EDT system will encounter for a typical orbit. The in-plane angle, α_ip, and out-of-

plane angle, α_op, can be reduced by increasing the endmass of the system, or by employing

feedback technology. Any deviations in the gravity alignment must be understood, and

accounted for in the system design.

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Fig. 6.2 Description of an in-plane and out-plane force. .

NASA has developed two new feedback algorithms that evaluate the work being

performed on the tether oscillations and calculate a feedback modulation that is applied to the

tether current to achieve net damping of the unstable tether modes. The first algorithm requires

periodic measurements of the location of several points along the tether; this algorithm is

referred to as the “Tether Configuration” feedback method. The second algorithm requires only

periodic measurements of the acceleration of the tether endmass; this algorithm is referred to as

the “Endmass Acceleration” feedback method.

These stabilization algorithms form the heart of the Electrodynamic Tether Stabilization

(EDTS) System, illustrated in Figure, which will enable electrodynamic tethers to provide long-

term propellantless propulsion while maintaining tether stability and efficiency.

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Fig. 6.3 Electrodynamic Tether Stabilization (EDTS)

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7. APPLICATIONS

7.1 Propellant less Propulsion for LEO Spacecraft : ED tether system can provide propellant

less propulsion for spacecraft operating in low Earth orbit as shown in fig.8.1

Fig. 8.1

7.2 Propellant less propulsion system for microsatellites: The µPET (fig.8.2) Propulsion

System is a small, low-power electrodynamic tether system designed to provide long-

duration boost, deboost, inclination change, and stationkeeping propulsion for small

satellites. Because the system uses electrodynamic interactions with the Earth's magnetic

field to propel the spacecraft, it does not require consumption of propellant, and thus can

provide long-duration operation and large capability with low mass requirements.

Fig. 8.2

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7.3 Power Generation in Low Earth Orbit: Electro dynamic tethers may also provide an

economical means of electrical power in orbit.

7.4 Electro Dynamic Revision-Boost of the International Space Station: NASA currently

plans to launch several large rockets every year to carry fuel up to the station so that it can re-

boost its orbit.

7.5 Space Junk Cleanup: The most direct application of ProSEDS(propulsive small expendable

deployer system) would be to get rid of space junk.

7.6 Satellite Tugboat: Another idea is for the ED tether to be attached to an unmanned space

tugboat that would ferry satellites to higher orbits. After being launched in to low Earth orbit,

the so called Orbital Transfer Vehicle would grapple the satellite and maneuver it to a new

altitude or inclination. The tug could then lower its own orbit to rendezvous with another

payload and repeat the process.

Fig. 8.3

7.7 Exploring the Outer Planets: Perhaps the most exotic use of ED tether technology would

be to propel and power spacecraft exploring the outer planets. Existing vessels have relied on

solar cells, but at distances far from the Sun, the power available is typically less than 100 W.

Jupiter and its moons have an environment particularly favorable to ED tethers; the planet

has a strong magnetic field and a rapid rotation rate, and its mass dictates high orbital

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velocities. With the magnetic field moving much faster than the spacecraft, the tether would

essentially be stealing energy from the planet's magnetic field.

7.8 Terminator tether: The Terminator Tether is a small, lightweight system that will use

passive electrodynamic tether drag to rapidly de-orbit spacecraft from low Earth orbit.

More applications of ED tethers are as shown in the below table,

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8. BENEFITS OF USING EDTS

1. Saving the Mass: The major advantage of the tether technology compared to other

propulsion systems is that it does not require any propellant. While conventional

chemical thrusters need a mass allocation. A typical electrodynamic tether system,

weighting about 30-50 kg, can achieve de-orbit of spacecraft requiring only a few percent

(1-5%) of the carrier vehicle mass at launch. A tethered system, would be inactive during

the mission, while waiting for a command to de-orbit the spacecraft at the end-of-life.

Therefore, by eliminating the need to launch and store in orbit for many years a large

amount of propellant, electrodynamic tethers can greatly reduce the cost and improve the

reliability of in-space propulsion and operations.

2. Reducing the De-orbit times: Another benefit of using electrodynamic tethers is that the

time to de-orbit spacecraft from LEO can be more. For a typical satellite above 500 km,

affected by the sole natural orbit perturbations, the orbital lifetime can be tens to

thousands of years. On the other hand, the Terminator Tether might de-orbit a satellite

from various LEO orbits within a few weeks to a few months.

3. Increasing the Effectiveness in Terms of the Area-Time-Product: The main objective of a

de-orbit technology is to remove dead and unwanted spacecraft from orbit so that they

cannot pose a collision threat to other operational spacecraft. Of course, the use of a long

tether will greatly increase the cross-sectional area of the spacecraft system, raising, in

turn, the probability that the system will suffer an accidental collision during the mission.

However, this probability depends not only on the cross-sectional area, but also upon the

amount of time the satellite spends in orbit. For a satellite left to de-orbit by aerodynamic

drag only, the cross-sectional area is relatively small, but the amount of time needed to

re-enter the Earth’s atmosphere can be many hundreds or thousands of years.

Nonetheless, even if an electrodynamic tether increases the satellite system cross-

sectional area, the orbital decay rate is large enough to compensate the first effect and to

greatly reduce the risk for the tethered system to collide with other spacecraft.

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Therefore, the criterion to evaluate the effectiveness of a de-orbit technique is not just

whether it reduces the orbital lifetime compared to the atmospheric drag decay, but whether it

reduces the product of the orbital lifetime and collision cross-sectional area of the spacecraft,

namely the Area-Time-Product: ATP. As a matter of fact, they also demonstra ted that the

Terminator Tether can significantly reduce the ATP value for most LEO orbits.

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9. RISKS OF USING EDTs

Space Debris Related Concerns: Tethers are usually very long and thin, providing increased

opportunities for something to go wrong. The accidental tether severing may be due to a number

of causes including manufacturing defects, system malfunctions, material degradation,

vibrations, and contact with other spacecraft elements. Most of these causes can be prevented

through design, quality check and active control of the tether dynamics and stability during the

mission.

Nonetheless, due to their peculiar characteristics, tethers in space introduce unusual

problems when viewed from the space debris perspective. They present a much greater risk to

operating satellites due to their considerably large collision cross-sectional area. Because of their

small diameter, tethers of normal design may have a high probability of being severed by

impacts with relatively small meteoroids and orbital debris. The resulting tether fragments may

pose additional risks to operating spacecraft.

Therefore, before electrodynamic tethers can be used to mitigate the problem of orbital debris,

various problems have to be investigated:

1. To evaluate the impact of tethers on the space environment, i.e. to determine the tether

collision risk with operating spacecraft.

2. The risk posed by the tether remnants after severing, the chance of collision among the

tethers themselves.

3. To assess the tether survivability, i.e. to evaluate the risk for a tether of being cut during

the mission by orbital debris and meteoroids.

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10. FUTURE SCOPE

Satellite Tugboat: Another idea is for the ED tether to be attached to an unmanned space

tugboat that would ferry satellites to higher orbits

Exploring the outer planets: The most exotic use if ED tether technology would be to

propel and power spacecraft exploring the outer planets.

Researchers are investigating the use of ED tethers to extend and enhance future

scientific missions to Jupiter and its moons. In theory, ED tether propulsion could be used near

any planet with a Previous visits to the largest planet in the solar system - including the "Grand

Tour" flyby missions of Voyager 1 and 2, launched in 1977, and an orbital visit by the Galileo

probe, which left Earth in 1989 and it continues to tour and study the Jovian system today- were

illuminating, but the fuel limitations and minimum maneuverability of those probes hampers

long term, more detailed scientific study.

Development of a propellant free, ED tether propulsion system would make it possible to

put a long term probe in Jupiter's orbit - one that could leverage the planet's powerful magnetic

field and magneto sphere to travel freely among the Jovian moons, providing more information

and new insight about them as well. Tether Transport from Low Earth to the Lunar Surface A

concept developed by Tethers Unlimited wherein several rotating tethers in orbit around the

earth and moon may provide a means of exchanging supplies between low Earth orbit facilities

and Lunar bases without requiring the use of propellants.

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11. CONCLUSION

Electrodynamic tether is now becoming the most popular fuel carrier for spacecrafts. The

use of space tethers is the answer to all the current problems as they don’t require propellents.

ED tethers can provide long-term propellant less propulsion capability for orbital maneuvering

and station keeping of small satellites in low-Earth-orbit. It reduces the de-orbit time unlike other

satellites.

Tether propulsion requires no propellant and is completely reusable. It is also

environmentally clean with low cost. It acts as both booster and braker for a spacecraft. Over the

years, numerous applications for electrodynamic tethers have been identified for potential use in

industry, government, and scientific exploration. Electro dynamic tethers may also provide an

economical means of electrical power in orbit. Beyond these, ETDs are opening new doors in

space explorations and getting close to the universe.

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12. REFERENCES

Websites:

http://www.tethers.com/

http://www.wikipedia.org/

Books and authors :

1. THE TERMINATOR TETHER - HOYT, R.P., FORWARD, R.L

2. DYNAMICS OF SPACE TETHER SYSTEMS - BELETSKII, V.V.. LEVIN,

3. STABILIZATION OF ELECTRODYNAMIC - HOYT. R.P & HEINEN

4. SPACE TETHERS

5. Columbo, G, et al., Investigation of Electrodynamic Stabilization and Control of Long

Orbiting Tethers, Smithsonian Astrophysical Observatory Interim Report on NASA

Contract NAS8-33691, March 1981.