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SPACE TETHERS


Electrodynamic Tethers
Tech Level: 12
Simple Momentum-Exchange Orbital Tether
Tech Level: 12
Spinning Momentum-Exchange Orbital Tether
Tech Level: 13
Multiple-Tether "Stepladder" Space Launch System
Tech Level: 13
Rotovator
Tech Level: 14

Tethers are one of the least romantic ideas ever about space travel: basically they’re long, orbiting pieces of wire. Yet, one cannot deny how imminently practical the whole concept is. To my knowledge, tethers in one form or another have been used on at least 19 space missions, proving their practical place in the future of space exploration. Tethers as long as 20 kilometers have already been used in various space missions.



TETHER BASICS

Besides the obvious uses for towing, tethers in space can also be used for two fundamental purposes: momentum transfer and power generation.

Simple momentum transfers have already been demonstrated on various missions, most dramatically in 1996 when an experimental satellite was extended from the Space Shuttle on a 20 kilometer wire. A mishap caused the tether to snap, causing the satellite to zoom out into an orbit 140 kilometers higher while the Shuttle lost a few hundred meters from its own orbit.

The reason for this can be envisioned by understanding how orbital mechanics work. We can consider orbits to be gigantic circles (in reality, almost all are ellipses, but the same principle applies). The closer in, or lower, the orbit is, the faster the object must travel in order to maintain orbit. The farther out, the slower the orbital speed needs to be.

When two objects are connected by a tether, one in a lower orbit and the other farther out, the lower one "drags" the higher one around at its own required orbital speed, meaning the higher object is travelling faster than it needs to in order to maintain orbit at that altitude. In other words, it "picks up" momentum from its lower, tethered companion. When the connection is severed, conservation of momentum is observed; the higher object zooms higher while the lower object, having given up some of its speed to its companion, slows down and slips into a lower orbit.

A good analogy for this process would be an Olympic hammer thrower. The "hammer" used in the sport is a crossbar connected to a heavy metal ball by a length of chain. The hammer thrower spins around, holding onto the crossbar, imparting momentum to the ball. When he releases the hammer, the hammer sails down field, while the thrower is forced back a step or two by the momentum transfer of the throw. This is basically what happens to two tethered satellites in orbit, but of course it is carried out on a much vaster scale.

Also, while in orbit the tether is passing through Earth’s magnetic field. If the tether contains or is made up of conductive material, this motion generates electric current along the tether. Thus, tethers have ready-made power sources to help them maintain their systems and to power attached spacecraft.

Space tethers aren’t made of just any kind of wire, of course. They need to be made of very strong yet flexible material. Kevlar, Spectra (used in fishing lines), and metal alloy fiber wires have all been used. In the near future, tethers may be made of materials like spider silk or carbon nanotube composite fibers.

Just having one strand of wire in a tether is impractical. In 1994, a payload was left hanging on the end of a 20-kilometer, single-strand tether to see how long it would stand up to collisions with micrometeoroids and space debris. At the orbital speeds involved, the strand could be cut by a particle as small as a grain of sand. It was expected to last at least 12 days. It didn’t even last four.

In order to prevent debris and meteoroids from endangering future tether-based missions, tethers with multiple strands are being designed. On scheme involves a "tape"-like configurations, of interwoven fibers connected side by side. Another option is the so-called Hoytether(TM), developed by Robert P. Hoyt of Tethers Unlimited, which uses a tubular interwoven lattice much like a fishing net to minimize localized damage to any one strand.

The Hoytether(TM) concept

ELECTRODYNAMIC TETHERS
Tech Level: 12
Tethers Unlimited's Terminator Tether(TM) Concept

As stated above, tethers that contain conductive materials can be used to generate electricity as they pass through Earth’s magnetic field. This was dramatically demonstrated in the 1996 space shuttle/tether mishap described above, as the tether was severed by an unforeseen 3000-volt electrical surge caused by the tether’s motion through the magnetic field. Longer tethers, with much better power regulation equipment, should be easily capable of generating kilowatts worth of power, allowing them to supplement or even supplant solar cells on certain space missions.

According to the Tethers Unlimited website, electrodynamic tethers can also provide modest "propellantless" propulsion for micro satellites (a micro satellite weighs 100 kg or less.) The exact details of this have proven sketchy to find, but it appears to use the electric current generated by the tether to trap electrons from Earth’s magnetic field and then propel them out of the satellite proper (perhaps using a souped-up electron gun like on a TV?), providing a small amount of thrust that can be used to (very) slowly alter the satellite’s orbit.

An interesting application being developed using this principle is Tethers Unlimited’s "Terminator Tether" Satellite Deorbiter(TM). The Terminator Tether is actually a small device attached to a satellite prior to launch. After the satellite reaches the end of its operational lifetime, the device is activated, unspooling a 5-km long electrodynamic tether. The tether produces current by interacting with earth's electromagnetic field, which in turn creates an electromagnetic field radiating out from the tether. This magnetic field interacts with ionospheric plasma (charged particles on the extreme outer layer of the atmosphere, reaching far out into space,) inducing drag forces that slows the satellite down. The satellite gradually loses altitude until it burns up in the atmosphere after a few weeks or months.


SIMPLE MOMENTUM-EXCHANGE ORBITAL TETHER
Tech Level: 12

These are orbiting tethers that impart added momentum to a satellite as described above. A satellite in orbit deploys a tether attached to a counterweight into a lower orbit. Actually, the satellite and the counterweight will "push" off each other as the tether is deployed, meaning the original satellite gains a bit of altitude from this motion alone. When fully deployed, the satellite will orbit until it hits a desired trajectory window (perhaps at the aphelion of its orbital ellipse) to detach, gaining a substantial momentum "push" from the tether and counterweight. It zooms up to a higher orbit, while the counterweight can rewind the tether and deorbit for pick-up and re-use.

Tethers can also be used in deep space missions, where the upper satellite can drag the lower one through a planet’s atmosphere for samples, or even land the lower satellite directly onto the surface on an airless body such as an asteroid or a moon.


SPINNING MOMENTUM-EXCHANGE
ORBITAL TETHER
Tech Level: 13

A spinning orbital tether is also sometimes called a bolo.

Spinning tethers can act as orbiting momentum-energy "banks." Like a Simple Orbital Tether, they exchange momentum by giving up some of their orbital speed to a satellite at the "high orbit" end of the tether. However, the Spinning Tether also adds the momentum of its rotation to the departing satellite, allowing it to impart much greater speed (perhaps even escape velocity) than by static momentum transfer alone.

The primary scheme for this is to have a long, vertically spinning (ie, always perpendicular to Earth’s surface) tether already in Low Earth Orbit. A satellite or space ship launched in a conventional way rendezvous with the end of the cable at the low point of its spin, where electromagnetic "grapples" (perhaps powered by the tether’s electrodynamic properties) latch onto it as it passes by. The satellite is then swung up by the tether’s centrifugal force and released at the apex of its rotation. It then shoots into a higher orbit, much like a stone released from a sling.

The rotating tether loses altitude and rotational speed both from the pick up and release of the satellite. On-board thrusters, again perhaps powered by the tether’s electrodynamic properties, would then have to correct these losses before its ready for its next pick-up. A scheme where the tether could correct its orbit by modifying the length of the spinning tether at aphelion and perihelion in its orbit was mentioned in one article, but unfortunately it did not go into details.


MULTIPLE TETHER "STEPLADDER" SPACE LAUNCH SYSTEM
Tech Level: 13
A multiple tether Earth-Moon launch system

Very simply, this is a series of spinning tethers that "hand off" payloads from one to the other, providing transport from one point in space to another with very little need for on-board propellant. For example, one tether (perhaps a rotovator; see below) takes a payload from Earth and flings it into Low Earth Orbit; another "catches" the payload and throws it into geosynchronous orbit; still another "catches" it again and this time launches it at escape velocity into deep space. The process can of course be reversed, to deliver an incoming payload to the surface of the Earth with almost no expenditure of fuel.

One such scheme has been proposed by the scientists at Tethers Unlimited to create a steadily--travelled "highway" to and from the Moon.

This type of multiple-tether ‘stepladder’ will most likely be created to steadily exchange payloads between two well-established points, such as Earth and a moonbase, or the Moon and a Lagrange-point station, or Earth and Mars, and so on. In this way, space outposts can be provided a steady stream of needed supplies in a relatively cheap and reliable way.

 


ROTOVATOR
Tech Level: 14

A Rotovator is a spinning orbital tether built on a truly gigantic scale, designed to reach down from space into the lower atmosphere, or perhaps even to the surface of the Earth, pick up and drop off payloads directly. The orbital altitude of the cable’s center of spin is equal to half the length of the cable.

The Rotovator would be orbiting along the equator, perpendicular to Earth’s surface. The rotational velocity of its tips can be matched to the rotational velocity of Earth’s surface spinning under it. Both the forward motion of the tether in its orbit and its carefully timed rotation rate can result in its lower tip "hovering" over a certain fixed point on Earth for a few minutes, allowing smooth transfer of cargo.

It is important to understand that even though the word "hover" is used above, the tether of course never stops spinning, just as the surface of the Earth under it never stops rotating. But the forward orbital motion of the rotovator is synchronized in such a way with its spin that the lower tip "glides" over a fixed spot on the rotating Earth, making it seem stationary for a few moments to observers on the ground. In fact, because of the scale and choreographed motion involved, people on the ground could never tell the rotovator was in fact rotating by eyeball alone; all they’d see is a gigantic column of material reach vertically down from the sky like God’s own arm, pick up its cargo, and retreat back up in exactly the same way.

Robert L. Forward in his novel Timemaster gave extensive details about a rotovator 8000 miles long that "touched down" into the lower atmosphere to pick up cargo and passengers flown up to it on specially-modified jets. This rotovator’s orbit and spin were designed in such a way that it set down three times per 24-hour period. Rotovators need not always be quite on this scale, but a length of several hundred miles is probably minimum.


RELATED INFORMATION

A NASA page on orbital tethers:

http://liftoff.msfc.nasa.gov/academy/tether/tethers.html

Tethers Unlimited, Inc. Homepage:

http://www.tethers.com/

An excellent article on Space Tethers in general:

http://www.vectorsite.net/tarokt4.html

Links to many Space Tether articles and sites:

http://www.spaceref.com/directory/future_technology/tethers/



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