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The story of Space Tethers is relatively new in the illustrious field of space technology. This technology was thought about when the cost of manOeuvreing spacecrafts and other objects in space was proving to be very costly and the removal of space debris was a necessity.
A space tether is a long cable used to couple space craft to each other or to other masses, such as a spent booster rocket, space station, or an asteroid. Space tethers are usually made of thin strands of high-strength fibers or conducting wires. The tethers can provide a mechanical connection between two space objects that enables the transfer of energy and momentum from one object to the other, and as a result they can be used to provide space propulsion without consuming propellant. Additionally, conductive space tethers can interact with the Earth's magnetic field and ionospheric plasma to generate thrust or drag forces without expending propellant.
The effectiveness of this technology is that it does not use any propellants. The problem with propellants is that it is very bulky, dangerous and exhaustable. The use of space tethers is the answer to all these problems.
TYPES OF TETHERS
There are mainly two types of Space Tethers. They are: Momentum-Exchange Tethers
These allow momentum and energy to be transferred between objects in space, enabling a tether system to toss spacecraft from one orbit to another.
Electro Dynamic Tethers
These interact with the Earth's magnetosphere to generate power or pollution without consuming propellant.
PRINCIPLES OF OPERATION
The Lorentz Force
When some charge 'q" moves in a magnetic field of intensity B, with a velocity V at an angle '0' with the direction of field, a force 'Bqv Sin 0' acts on it in a direction perpendicular to the plane of 'v' and 'B'.
F = Bqv
when 9 ~ 90Ã‚Â°. i.e, when v and B are perpendicular. Here F is the force acting on charge called Lorentz Force. The Right Hand Rule can be used to figure out the direction in which the force is acting.
Right Hand Rule
For a positively charged particle moving ( velocity v) in a magnetic field ( field B) the direction of the resultant force ( force F) can be found by:
1)THUMB of right hand indicating the direction of the velocity, direction of current 2)INDEX FINGER indicating the direction of the field.B 3)MIDDLE FINGER indicating the direction of the force, F
This is the Right Hand Rule used to find the direction of the force.
The force will AL WA YS be perpendicular to the PLANE of the vectors v and B no matter what the angle between v and B is. Just pretend the following picture is of your right hand:
Right Hand Rule
Electro Dynamic Tether
An electro dynamic tether is essentially a long conducting wire extended from a spacecraft. The gravity gradiet field( also known as the "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 at 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 be up to several hundreds volts per kilometer.
In an "electro dynamic 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 into the plasma at the other end of the tether, the voltage can drive a current along the tether. This current will, in turn, interact with the Earth's magnetic field to cause a Lorentz force which will decrease the orbit of the tether and its host spacecraft. Essentially, the tether converts the orbital energy of the host spacecraft into electrical power, which is dissipated as ohmic heating in the tether.
In an "electro dynamic 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 j.he direction opposite to that which is 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 other space propulsion system is that it doesn't require any propellants. It uses the Earth's magnetic field as its "Reaction Mass". By eliminating the need to launch large amounts of propellants into orbit, electro dynamic tethers can greatly reduce the cost of in-space travel.
Electro dynamic tethers work by virtue of the force a magnetic field exerts on a current-carrying wire. Andre Marie Ampere, a pioneer in the study of electromagnetic phenomenon, first observed the phenomenon in the 19th century. In 1895, Hendrik. Lorentz summarized the phenomenon in the equation that now bears his name. The force acts on any charged particle moving through a magnetic field (including electrons
moving inia wire), in a direction perpendicular to both the direction of current flow and the magnetic field vector.
The basic principle of electro dynamic tether thrusters was first described in 1965 by S.D.Drell, H.M. Foley and M.A.Ruderman. In essence, it is 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 exert a force on and accelerate the wire and hence any payload attached to it. The direction of current flow through the tether, either towards or away from Earth, determines whether the magnetic force will add to or subtract from the tether's orbital energy, and therefore raise or lower its orbit.
The tether forms a unique type of electric circuits, which NASA demonstrated in space with the Plasma Motor Generator in 1993 and the Tethered Satellite System in 1992 and 1996. All three of these missions deployed long conducting tethers from orbiting spacecraft and generated kilowatts of power. ProSEDS will take the technology one step further, to produce thrust and de-orbit a payload.
The tethers is dragged through the atmosphere's ionospheric plasma, the rarefied medium of electrons through which the whole setup is traveling at a speed of 7-8 km/s. in so doing, the 5 km long aluminium wire extracts electrons from then plasma at the end farthest from the play load and carries them to the near end. There a specially designed device known as a hollow cathode emitter expels the electrons, to ensure their return to space. Currents in the plasma complete the circuit.
Ordinarily, a uniform magnetic field acting on a current-bearing loop of wire yields a net force of zero, since the force on one side of the loop is cancelled by that 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 forces on the plasma currents in space do 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 the Earth and negative at the near end. This polarization is due to the action of Lorentz force on the electron in the tether. Thus, the natural upward current flow is due to the (negatively charged) electrons in the ionosphere being attracted to the tether's far end 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 a small tank, which contains just enough gas for the experiment's two-week run. Electrons from the tether interact with the heated gas to create an ion plasma. At the far end of the tube, a so called keeper electrode, which is positively charged with respect to the tube, draws the electrons and expels them to space. (The xenon ions, meanwhile, are collected by the hollow cathode and used to provide additional heating.)The rapid discharge of electrons invite new electrons to flow from the tether and out through the hollow cathode.
Earth's magnetic field exerts a drag force on the current-carrying tether, decelerating it and the payload and rapidly lowering their orbit. Eventually, they revision-enter Earth's atmosphere.
A key advantage of the electro dynamic tether is that it does not tax a satellite's or spacecraft's on-board power sources. The hollow cathode and other ProSEDS instruments, for example, will run on batteries charged by some of the power generated by the tether. If the tether were being used to raise rather than lower the payload's orbit, the current would have to be forced to flow in the other direction; the addition of a small solar-cell riower supply would do the trick. Electron Capturing
All the same, scooping up electrons from Earth's ionosphere is more easily suggested than done. Previous electro dynamic tether experiments used as the end mass either a large metallic satellite or a second hollow cathode. Unfortunately, a law of diminishing returns soon sets in; the density of electrons in the plasma limits the amount of current that can be collected in the given volume, and that limit is quickly approached as the satellite gets larger. The hollow cathode collector hits the similar limit. To attract
electrons, it emits xenon ions, but as the xenon gas flow is increased to attract more electrons, a collection threshold is soon reached.
PrdSEDS will use a radically different collection scheme, one that promises to be much more efficient and easily scalable to practical applications. Instead of a satellite or hollow cathode, the naked metallic tether itself will collect the electrons. Previous experiments used insulated wire to prevent arcing between it and the space shuttle deploying it. But because the ProSEDS tether will be in contact with the space plasma for most of its length, and because the entire rocket stage will be electrically connected to the tether, arcing should not be a concern.
Plasma chamber tests have verified that thin bare wires can collect current from plasma. Calculations indicate that the ProSEDS tether will be at least five times more efficient in electron collection than were previous space experiments, which used insulated wire and a satellite or hollow cathode as the electron collection device. The 1996 Tethered Satellite System, for example, collected a peak current of 1.1 A. and the 1993 Plasma Motor Generator only 0.3 A, whereas ProSEDS should see an average current of over 1 A, peak currents of 5 A, and average power of 1.46 kW. It is also predicted that ProSEDS will generate an verage thrust of about 1 N, which should dramatically hasten the payload's orbital decay, compared to a tether less reentry.
PRINCIPLE OF ORBIT SHIFTING
Using Electro Dynamic Tether
Consider a satellite with mass Msat orbiting a central body with mass of mass Mcentrai. The central body could be planet, the sun or some other large mass capable of causing sufficient acceleration on a less massive nearby object. If the satellite moves in circular motion, then the net centripetal force acting upon this orbiting satellite is given by the relationship
Fnet = (Msat X Vsat2)/R
This net centripetal force is the result of the gravitational force which attracts the satellite towards the central body, and can be represented as
, Fgrav = (GX MsatX Mcentrai)/R2
Since Fgrav = Fnet, the above expressions for centripetal force and gravitational force are equal. Thus,
(Msat X v2) / R = (G X Msat X MCentral) / R2
Observe that the mass of the satellite is present on both sides of the equation; thus it can be cancelled by dividing through by Msat. Then both sides of the equation can be multiplied by R, leaving the following equation. Velocity of a satellite moving around a central body is
I V2 = (GX Mcentrai)/R
Where G = 6.67 X 10-11 Nm2/Kg2, Mcentrai = the mass of the central body about which the satellite orbits, and R = the radius of the orbit for the satellite. In our case MCentrai is the mass of the Earth.
The direction of the velocity vector at every instant is in the direction tangent to the circle.
Therefore V2a 1/R
When v increases R decreases, i.e, the radius of the orbit decreases, i
A conductive tether material acts as a long wire moving through a magnetic field. This induces an electromotive force and corresponding current to move through the wire, with the surrounding plasma completing the circuit. The electromotive force or voltage potential depends directly on the field strength, the orbital velocity, and the tether length.
EMF= V = $v~ XBttl Where,
V is induced e.m.f across the tether length v is the velocity of the tether B is the magnetic field induced dl is the differential length
Now the direction of current flow due to the induced emf is given by Fleming's Right hand rule which states that when the forefinger, middle finger and thumb are mutually perpendicular to each other then the forefinger represents the direction of the field, the
thumb represents the direction of motion; the middle finger represents the direction of induced current.
In our case as the satellite is moving the current will be flowing g outwards, i.e., away from the Earth.
Now taking Lorentz right hand rule stated earlier F = Bqv
When G 90Ã‚Â°., i.e., when v and B are perpendicular.
Here F is the force acting on the charge called Lorentz Force. The Right Hand Rule can be used to figure out the direction in which the force is acting.
Clearly, forces are required to overcome the Earth's gravitational attraction and to propel a satellite into orbit, with higher orbits requiring greater forces. Consider a satellite moving in a circular orbit. If the satellite is subjected to some force in the same direction as its motion, it will be propelled into a higher orbit and will travel at a slower speed according to the equation derived for orbital velocity. Conversely, if the satellite is subjected to some force in a direction opposite to its motion, it will be driven in to a lower orbit and will move at a greater speed.
Now in our case as the current is flowing outwards, the force will be opposite in direction to motion. So the satellites descend in orbit.
Now for propelling the satellite, the direction of current will have to be reversed so that the force is in the same direction as the motion of the satellite. This can be achieved by adding a battery to the tether which will work on solar energy and drive the current in the opposite direction.
The motion of the satellite can be controlled by the hollow cathode which completes the circuit.
Using Momentum Exchange Tethers
The Tether Launch Assist concept combines the techniques Tarzen used to swing through the jungle with the principles of a simple electric motor to create a system capable of repeatedly picking payloads up from a sub orbital trajectory and boosting them to higher orbits. In this concept, illustrated in the figure to the right, a long high strength is deployed from an orbiting facility, and the tethered system is set into rotation so that, at the bottom of its swing, the tip of the tether is moving much slower than the center of mass of the system. A grapple system attached to the tip of the tether can thus reach down below the facility and rendezvous with a payload moving in a slower, sub orbital trajectory. The grapple would then capture the payload and pull it into orbit along with
the tether system. Later, it could release the payload at the top of the swing, tossing it into a higher orbit. When the tether system captures and releases the payload. it transfers some of its momentum and energy to the payload.
Tether Transport Facility
Â¢ 420 km altitude
Â¢ 5 tons for deployer and winch
Â¢ +145 tons ballast (ballast mass can be
supplied by shuttle external tanks) i
Â¢ 5 tons
Â¢ Captured from a suborbital tra jectory
Tether Payload Released
Â¢ 290 km long one-half revolution later
Â¢ 5 tons with+1.2 km/sec
Propellant less Propulsion for LEO Spacecraft
ED tether system can provide propellant less propulsion for spacecraft operating in low Earth orbit. Because the tether system does not consume propellant, it can provide very large delta-V's with a very small total marks, dramatically reducing costs for missions that involve delta-V hungry maneuvers such as formation flying, low-altitude station keeping , orbit raising, and end-of-mission de-orbit. TUI is developing several ED tether products including the uPET propulsion system and the Terminator Tether Satellite De-orbit Device.
Power Generation in Low Earth Orbit
Electro dynamic tethers may also provide an economical means of electrical power in orbit. Essentially, the tether can be used to convert some of the spacecraft's orbital energy into electrical power. However, since converting the orbital energy into electrical power will lower the orbit of the spacecraft (there is no such thing as a free lunch), this technique is probably only useful for providing high power energy bursts to short duration experiments.
Electro Dynamic Revision-Boost of the International Space Station
The International Space Station (ISS) will experience a small but constant aerodynamic drag force as it moves through the thin upper reaches of the Earth's atmosphere. This drag force will cause the station's orbit to decay. If nothing were done to counter act this, the station would fall out of orbit within several months. 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. These launches, however, will be very costly.
Tethers Unlimited Inc. has helped NASA to explore the potential for using electro dynamic tether propulsion to maintain the orbit of the ISS. By using excess power generated by the ISS's solar panels to drive current through the conducting tether, a tether re-boost system could counter act the drag forces or even raise the station's orbit. NASA and TUI's studies revealed that such a tether re-boost system could reduce or eliminate the need for dedicated launches for re-boost propellant. potentially saving up to $2 billion over the first ten years of the station's operation.
Space Junk Cleanup
The most direct application of ProSEDS would be to get rid of space junk. Over the past half century of space exploration, the region around Earth has become cluttered with debris, which could take years, and in some cases centuries, to fall from orbit. The
danger is that all satellites and rocket stages and trash thrown over board by early space travelers could collide with working satellites, space shuttle, and orbiting space station. The International Space Station, for example, maneuvers several times a year to avoid hitting debris, burning up precious rocket fuel each time. NASA and the European Space Agency have recommended that governments require future spacecraft to be able to take themselves otit of orbit at the end of their life spans.
To do that, a satellite could be loaded up with extra propellant, to thrust to revision-entry. But that would add as much as 25% to the satellite's weight. The propulsion system, too, would need to remain functional after sitting in orbit for ten years or more.
Using a tether to de-orbit would be inherently more reliable. For one thing, it is an electromechanical system, with no complex valves, plumbing, or circuitry that must stay operational and leak free for years. Also, ED tethers are much lighter and more compact than conventional thrusters: a tether system would account for as little as 2 % of the satellite total weight and could be easily bolted to the satellite. Once the end of the satellite's useful life is reached, the tether would unreel, and the tether driven orbital decay would begin. The satellite would then burn up during revision-entry. One of the commercial partners on the ProSEDS effort, Tethers Unlimited Inc., based in Clinton, Wash., is developing a commercial version of the de-orbit system, known as the Terminator Tether.
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. Conceivably, several such orbital revision-assignments could be performed without the need for rocket propellant, making the tug relatively inexpensive to operate. But because plasma density diminishes rapidly with distance from Earth, the tug could only operate below about 2,300km. That would still make it practical
for about half the payload now scheduled for launch, which have destined for orbits of around 2200km. The lowest usable altitude would be about 250km, below which the atmosphere would begin to exert too much aerodynamic drag.
Exploring the Outer Planet
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 velocities. With the magnetic field moving much faster than the spacecraft, the tether
would essentially be stealing energy from the planet's magnetic field.
In theory the tether could power the craft's instruments and generate thrust at one and the same time for a circular orbit close to the planet, tether propulsive forces have been calculated to be as high as 50 N and power levels as high as 1 MW. This level of power would sustain a whole new suit of science instruments such as high power radars, but it also means having to deal with power conversion, energy dissipation and tether, overheating.
To address the many remaining performances and operational issues, a follow-on experiment to ProSEDS will need to be flown. Such an experiment would demonstrate the tether's use in raising altitudes and changing orbital inclinations in a series of predictable, repeatable flight profiles. NASA researchers have proposed such an experiment, but it has yet to receive funding.
Momentum Exchange Space Tethers
Net Ten si en
stronger oentrifu gal force
A momentum exchange tether is a long thin cable used to couple two objects in space together so that one transfers momentum and energy to the other. A tether is deployed by pushing one object up or down from the other. Once the two objects are separated by enough distance, the difference in the gravitational force at the two locations will cause the objects to be "pulled" apart. This is called the "gravity gradient force". The tether can then be let out at a controlled rate, pulled by the tension caused by the gravity gradient force. Once the tether is deployed, if there are no other forces on the tether it will have an equilibrium orientation that is aligned vertically. There are a number of different concepts for momentum exchange using tethers. Some general categories are:
A stationary tether is one that connects two masses together and remains at constant length, except, of course, for deployment and retrieval. A stationary tether could drag a payload through the upper atmosphere of a planet and lower payloads to the surface of an asteroid. If the tether is conducting and is moving through electric or magnetic fields, then it can be used as a generator to provide electrical power, or as a motor to provide propulsion. If the tether and its masses are orbiting a massive body, then
typically the system will be gravity gradient stabilized, with the tether pointed along the radius vector to the massive body. Thus, although the tether is stationary in the orbital reference frame, it is really rotating once per orbit in inertial space, and so is a slowly rotating bolo.
A bolo is a long rotating cable anywhere in space that is used as a "momentum-energy bank". It could be used to "catch" a payload coming from any given direction (in its plane of rotation) at any given speed (less than its maximum tip speed), and then some time later, "launch" the payload off in some other direction at some other speed. A gravity gradient stabilized bolo orbiting some planet has the property that if the tether is cut, then one-half an orbit later, the separation distance between the two masses is seven times larger than the initial separation. This can be used to deorbit the lower mass, or throw the upper mass to a rendezvous or to escape.
A "rotovator" is a long bolo in low orbit around a planet (or moon) that is used as
a giant elevator to reach down from space to lift payloads from a planet or to deposit i
payloads onto a planet. To reach the surface of the planet, the orbital altitude should be equal to half the length of the rotating cable. By proper adjustment of the cable rotation period to the orbital period of the center of mass of the cable (plus or minus the planetary rotation period), the relative velocity of the planetary surface and the tip of the cable can be made zero at the time of touchdown, allowing for easy payload transfer. A half-rotation later, the payload is at the top of the trajectory with a cable tip velocity that is twice the orbital velocity. Although present day material strengths do not allow the construction of rotovators around Earth or the major planets, they can be built for Mars, Mercury, and most moons, especially including Earth's Moon.
Tip Velocity and Material Strength
The maximum tip speed of all these systems is a function of the "launcher to payload mass ratio" of the tether system and the "characteristic velocity" of the material used. The characteristic velocity of the material in a tether is given by the square root of the ratio of the design tensile strength T of the tether to the density D of the tether material, u = (T_d/D) Al/2. In practice, the design tensile strength is usually chosen to be 50% of the measured strength for metals and 25% of the measured short-term individual fiber strength for other materials. Thus, using imperfect materials with reasonable safety margins, the characteristic velocity of most metals and fibers is around 1 km/s, with" optimistic predictions for graphite and improved polymers reaching 3 km/s. With the development of a design for a high strength-to-weight tapered Hoy tether, the design tensile strength can be safely chosen to be 60% of the measured strength of the individual fibers, allowing commercially available fibers to have characteristic velocities up to 4 km/s.
EXPERIMENTS AND SUCCESSES
The first space tether flew with the Gemini II astronauts in 1996. that was a 30 meter long non conducting tether made of parachute webbing, and it linked the piloted spacecraft with a rockets upper stage. In doing so, the tether stabilized the spacecraft as it orbited the earth. Since then there have been at least 17 space missions with tethers. The Small Expendable Deployer System (SEDS), on which the upcoming Propulsive SEDS experiment is based, has flown successfully four times. The first two missions, in 1993 and 1994, were mainly intended to validate tether development. They used a 20 Km long non-conducting tether.
The deployer was most recently used in 1996, for the U.S.Naval Research Laboratory's Tether Physics and Survivability (TiPS) experiment. Two small end masses, nicknamed Ralph and Norton, were connected by a 4.2Km long non-conducting tether. TiPS was designed to demonstrate a tether's longevity. It remains in orbit today.
The idea of Ed tether has been around for 35 years, but it was the 1990s before one flew in space. The 1993 Plasma Motor Generator mission used an insulated ED tether equipped with a hollow cathode end mass to collect electrons.
In 1992 and 1996, NASA flew the Tethered Satellite System (TSS) on the space shuttle orbiter.
The p PET Propulsion System
Propellant less Electro Dynamic Tether Propulsion for Micro satellites
TUI is currently developing a propulsion system called the '"Micro satellite Propellant less Electro dynamic Tether (pPETâ€žÂ¢) Propulsion System" that will provide
propulsive capabilities to micro satellites and other small spacecraft without consuming
How it works
ED tethers can provide long-term propellant less propulsion capability for orbital maneuvering and station keeping of small satellites in low-Earth-orbit. The uPETâ€žÂ¢ Propulsion System is a small, low-power ED tether system designed to provide long-duration boost, deboost, inclination change, and station keeping 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 total delta-V capability with low mass requirements. Furthermore, because the uPETâ€žÂ¢ system does not require propellant, it can easily meet stringent safety requirements such as are imposed upon Shuttle payloads. In addition, the tether system can also serve as a gravity-gradient attitude control element, reducing the ACS requirements of the spacecraft.
The mass, size, and power requirements of the uPETIM Propulsion System depends upon the size of the satellite and the propulsive mission. TUI has developed a prototype of a uPETâ€žÂ¢ sized for a 125 kg micro satellite, which could raise the orbit of this satellite from a 350 km drop-off orbit to a 700 km operational orbit within 50 days.
Electro Dynamic Tether
BARRIERS TO OVERCOME
One question is the long-term survival of the tethers. While the atmosphere at Low Earth Orbit (LEO) altitude is extremely thin -millions of times thinner than the air at sea level -it is largely composed of atomic oxygen, which is very corrosive.
High velocity micrometeorites pose an even more prominent problem. High velocity meteorites can easily rupture the tether and can tear the tether apart materials with greater strength are being developed to overcome this barrier.
To Explore Jovian System
Researchers at the Marshall Center also 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 i
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 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.
Tethers for Rapid Autonomous Deorbit of Leo Satellites
Tethers Unlimited Inc. is currently developing a system called "Terminator Tether" that will provide a low cost, light weight and reliable method of removing objects from Low Earth Orbit (LEO).
Electrodynamic tether now becoming the most popular fuel carrier for space crafts. ED tethers can provide long-term propellant less propulsion capability for orbital maneuvering and station keeping of small satellites in low-Earth-orbit. Electro dynamic tethers may also provide an economical means of electrical power in orbit. TUI is currently developing a propulsion system called the "Micro satellite Propellant less Electro dynamic Tether (uPETâ€žÂ¢) Propulsion System". The universe is eagerly waiting for the application of these tethers in future.
Books an,d Magazines:
Â¢ THE TERMINATOR TETHERâ€žÂ¢ - HOYT, R.P., FORWARD, R.L
Â¢ DYNAMICS OF SPACE TETHER SYSTEMS - BELETSKII, V.V.. LEVIN,
Â¢ STABILIZATION OF ELECTRODYNAMIC - HOYT. R.P & HEINEN
Â¢ ELECTRON ICS FOR YOU - JANUARY 2005
Â¢ ELECTRONICS TODAY - JUNE 2004