Tethers are one of the least romantic ideas ever about space travel: basically they're long, orbiting pieces of wire. Tethers in one form or another have been used on at least 17 space missions, proving their practical place in the future of space exploration. Tethers as long as 20 kilometers have already been used in orbit for various purposes. But advocates suggest that they could be used for much more, even whisking payloads from the Earth to the moon using a minimal amount of fuel. After reading all that they're capable of, it's hard to deny how eminently practical the idea seems.

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 one-ton 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.

This can be explained by thinking about how orbital mechanics work. We can consider orbits to be gigantic circles. (In reality, they're ellipses, but the same principle applies.) The deeper into a gravitational field, 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, they can be considered to be a single object whose center of mass exists roughly halfway between them. When the connection is severed, conservation of momentum is observed and the momentum shifts to the two masses at the ends of the connection. The higher object zooms up, as its former center of mass was moving faster than its required orbital velocity. It is given extra oomph from this momentum shift and heads to a higher orbit. Meanwhile the lower object experiences the reverse, as its former center of mass was moving slower than its required orbital velocity. So as it slows down, gravitational attraction is able to dig its hooks in deeper and the object falls 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 or she 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.

Space tethers can't be made of just any kind of wire, of course. They need to be constructed of very strong yet flexible material. Kevlar, Nomex, 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.

Tethers made of a single strand of wire have proven impractical. In 1994, a payload in orbit 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 make it to four.

In order to prevent debris and meteoroids from endangering future tether-based missions, tethers with multiple strands are being designed. One scheme involves a tape-like configuration of interwoven fibers connected side by side. Another uses a tubular interwoven lattice much like a fishing net to minimize localized damage to any one strand.

Electrodynamic Tethers

Image: NASA

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.

This was dramatically demonstrated in the 1996 space shuttle/tether mission described above, one of whose main purposes was to test the power generation capabilities of the concept. Using only insulated copper wire around a Nomex core, the tether generated an electrical potential of 3500 volts across its length at a current strength of about 480 milliamps before it was severed. Longer tethers, with much better conductive materials and design, 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 orbital objects massing 100 kg or less. The details are unclear, but the plan 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, 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. 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 Tethers

These are orbiting tethers that impart added momentum to a satellite as described in the Basics section 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 where it detaches, 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 Tethers

A spinning orbital tether is also sometimes called a bolo.

Spinning tethers can act as orbiting momentum-energy banks. Like a Simple Orbital Tether, bolos 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 than by static momentum transfer alone.

The primary scheme for this is to have a long, vertically spinning (i.e., always perpendicular to Earth's surface) tether already in Low Earth Orbit. A satellite or space ship launched in a conventional way can then rendezvous with the end of the cable at the low point of its spin, where electromagnetic "grapples" 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, perhaps powered by the tether's electrodynamic properties, would then have to correct these losses before it is 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 might also be possible.

"Stepladder" Space 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 traveled highway to and from the Moon.

This type of multiple-tether "stepladder" would 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 could be provided a steady stream of needed supplies in a relatively cheap and reliable way.

Rotovators

Image: islandone.org

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, to 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.

Rotovators by necessity would have to be built of extremely strong but flexible materials, as the stresses they would have to endure both from their spin and picking up significant masses are enormous. In fact, the calculated stresses a rotovator would have to endure exceed the performance characteristics of any material now in widespread use. One way to deal with this is by tapering the cable, i.e., making it thicker in the middle than at its ends. For the hundreds of kilometers of tether material needed for a rotovator, the taper ratio can be as much as 10 or 12 to 1 (in other words, it would be about 10 or 12 times as thick in the middle than at its ends.) But even this technique only mitigates a small portion of the structural stress. What's needed is better building material.

A gigapascal (GPa) is a unit that measures a material's tensile strength. Steel cable has a tensile strength of about 2 GPa. Diamond filaments (materials made out of hair-thin strands of diamond) have a tensile strength of around 20 GPa. The tensile strength required for a Rotovator system is thought to be between 50 and 60 GPa.

Fortunately, one material meeting this requirement has recently been synthesized, albeit only in microscopic quantities: carbon nanotubes. These are composites which have a theoretical upper tensile strength of 200 GPas, over 100 times that of steel cable at only a small fraction of the weight. In the future, small fibers of this material could be set down side by side, then interconnected to form a growing ribbon.

There are still a number of hurdles to overcome before we see carbon nanotube tethers, however, not the least of which is economic. The current cost of carbon nanotubes is about $500 per gram, or $500 million per ton. And a rotovator is bound to weigh many, many tons.

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 the minimum.

Lunavators

Rotovators need not be built solely in Earth Orbit. In fact, because of the lesser gravity of smaller bodies such as the moon, rotovators there would have to endure far less stress, so substances of much more modest and inexpensive means than carbon nanotubes can be used for construction. Kevlar or Spectra 200, with tensile strengths of about 3.25 GPa, would be sufficient to build a moon-based rotovator, or "lunavator" as some have dubbed it. A lunavator could greatly facilitate the settling and exploitation of the moon.

Conclusion

In the coming decades, tethers could become as invaluable to space travel as chemical rockets are today. Even further into the future, a vast array of spinning tethers and rotovators could eventually be strategically placed around every significant body in the solar system, creating well-traveled and inexpensive highways for cargo and passengers between worlds that would require very little expenditure of fuel.

An impressive rope trick by anyone's standards.

References:

In Print:

Robert L. Forward, Indistinguishable from Magic and Timemaster

On the Web:

NASA page on orbital tethers

Tethers Unlimited, Inc. Homepage

An excellent article on Space Tethers in general

Links to many Space Tether articles and sites:

Information on the Lunavator

Paul Lucas is a freelance writer and artist hailing from the wilds of Erie, Pennsylvania.