June 9, 2000 -- It's amazing what you can do with a little physics and a lot of string.

You could generate electrical power for orbiting satellites. Or you could prevent the International Space Station's orbit from deteriorating. You could also force an object in orbit around the Earth to fall into the atmosphere and burn up.

These are just some of the applications being explored by NASA scientists and private companies for a remarkably elegant technology called space tethers.

Right: This artist's concept shows a satellite attached to the space shuttle by means of a conducting tether. Learn more about tethers from NASA Liftoff.

"A space tether is a long string or a wire that connects two objects that are in orbit together," said Dr. Dennis Gallagher, a research scientist at NASA's Marshall Space Flight Center. "An orbiting tether tends to straighten out along a radial line because the force of gravity varies slightly along its length. The pull of gravity is stronger nearest the Earth and weakest furthest away. That means there is a net force on a tether which stretches it and keeps the line taut. This isn't just an exercise in physics, though, these tethers have lots of useful applications."

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Space tethers were an important topic of discussion last week at the 11th Advanced Space Propulsion Research Workshop in Pasadena, CA, which was sponsored by the Jet Propulsion Laboratory and the Marshall Space Flight Center.

"There are two types of tethers: electromagnetic tethers and momentum-exchange tethers," says Dr. Robert Hoyt, president of Tethers Unlimited, Inc., who presented a paper at the Propulsion Workshop entitled Design and Simulation of a Tether Boost Facility for GEO, Lunar, and Mars Transport. "Momentum exchange tethers allow momentum and energy to be transferred between objects in space. Electrodynamic tethers interact with the Earth's magnetosphere to generate power or propulsion."

Electrodynamic tethers have already been flight tested, and the concept has proven to be a viable technology. In fact, it is being considered as a means to counteract the slight aerodynamic drag on the International Space Station, reducing the need for reboosts that rely on conventional chemical propellants. Such a system could possibly save the program about a billion dollars in operating costs over the life of the station.

These papers about space tethers were presented at the 11th Advanced Space Propulsion Workshop May 31 - June 2, in Pasadena, CA: Overview of Advanced Space Propulsion Activities in the Space Environmental Effects Team at MSFC, David L. Edwards, et al. (NASA MSFC) Design and Simulation of a Tether Boost Facility for GEO, Lunar, and Mars Transport, Robert P. Hoyt and Robert L. Forward (Tethers Unlimited) Simulated Bare Electrodynamic Tethers in a Dense, Flowing, High- Speed Plasma, B. E. Gilchrist (University of Michigan) and S. G. Bilén (Pennsylvania State University) Conductive Tether Coating for Electrodynamic Tethers, Jason A. Vaughn (NASA MSFC)

Momentum-exchange tethers are still in the conceptual phase and probably won't be ready for in-orbit experiments until at least five years from now, according to scientists working on the technology.

Both types of tethers promise to reduce the cost of getting satellites into orbit and keeping them there or removing them.

"Right now if you needed to get a big payload out to geosynchronous orbit, you might need a $200 million rocket," said Hoyt, "but using a [momentum-exchange] tether system you could maybe do it with a $20 million rocket."

In one variant of a momentum-exchange tether, the faster-moving tether system grabs a slower-moving satellite in a lower orbit using a grapple at the end of a tether line between 20 and 200 kilometers long.

After orbiting around the Earth once together, the rotating tether system tosses the satellite forward into a higher orbit, somewhat like a roller derby skater grabbing a teammate and slinging them forward. The first skater transfers some of their momentum to the second skater, leaving the first skater going slower afterward. Similarly, the tether system gives some of its momentum to the satellite, ending up in a lower orbit.

Above: One illustration of a possible "tether transport node facility" that could add or subtract velocity from space payloads. [more information from tethers.com]

The momentum-exchange tether then needs a way to return to its original orbit so that it can grab the next satellite.

In current designs, the momentum-exchange tether system will get the push it needs by acting as the other kind of tether -- a conducting electrodynamic tether.

Electrodynamic tethers are typically between five and 20 kilometers long. As the long wire moves through Earth's magnetic field, the changing magnetic field in the vicinity of the wire induces a current that flows up the tether. If a power supply is added to the tether system and used to drive current in the other direction, an electrodynamic 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 systems is that it doesn't require any propellant.

Above: The blue orb is the Earth and the red curves denote planetary magnetic field lines. Currents are induced in conducting wires as they orbit through the magnetic field. This connection between electric currents and magnetic fields has many applications in everyday life. Speakers, microphones, ceiling fans, electric motors and most power plants rely on the same principle.

The momentum gained by these tether systems is ultimately taken from the rotational momentum of the Earth.

"You're actually transferring the rotational momentum of the Earth to the satellite," said Kirk Sorensen, an aerospace engineer involved with momentum-exchange tether research at NASA's Marshall Space Flight Center in Huntsville, Alabama. "You're spinning down the Earth."

However, since the mass of the Earth is so many times greater than the satellite, the impact on the Earth's rotation is infinitesimal, Sorensen noted.

Right: The cornerstone of the International Space Station - the combined Zarya (bottom) and Unity (top) modules - sails around the world after the crew of STS-88 completed assembly operations in December 1998. A propulsive tether system could replace most or all of the propellant refills that ISS will need for regular orbital reboosts. (NASA)

Electrodynamic tethers can be used as a brake as well as an accelerator.

If a current is not forced down the tether, the motion of the tether through the Earth's magnetic field will create a current traveling upwards. This produces a force that slows the system down rather than speeding it up.

Slowing a satellite down renders it unable to circle the Earth fast enough to "beat" gravity and so it falls back into the atmosphere. Without heat shielding, it will burn up.

Installing such a "suicide" device on satellites is actually more useful than it may sound.

"Commercial satellite companies have already recognized that if they leave the satellites up there, pretty soon they're going to get in the way of the other satellites that they want to put up," Hoyt said. Without one of these electrodynamic "brakes," an expired satellite can take months or years to fall out of orbit.

Left: The "Terminator Tether," designed to remove satellites from Earth orbit. Click for a 1.6 MB Quicktime animation. Credit: Tethers Unlimited

As elegant and useful as space tethers might someday be, however, the technology isn't ready for the big time yet.

"There are a few open issues preventing this from becoming a routinely usable technology," said Dr. Nobie H. Stone, a senior scientist at MSFC.

One question is the long-term survival of the tethers. While the atmosphere at Low Earth Orbit (LEO) altitudes 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 micro-meteorites pose an even more recalcitrant problem. Exactly how to protect the thin tether material of electrodynamic tethers from small grains traveling at tens to hundreds of thousands of miles per hour is not clear.

"I don't think we have a good handle on that problem," Stone said.

Dr. Robert Forward, vice-president of Tethers Unlimited, noted that his company is working on a momentum-exchange tether design that uses redundant, interconnected lines to give the tether a high tolerance for micro-meteorite impacts.

The next flight test for tether technologies will be the Propulsive Small Expendable Deployer System mission (ProSEDS), now scheduled to launch in December 2000.

Above: One day lunar payloads could be delivered with a system of three tethers. This artist's concept shows a package first launched from Earth and then picked up by a tether in low orbit. This cartwheeling tether hands off the payload to another cartwheeling tether that is in a higher orbit (1). Like a hunter hurling a rock with a sling, the second tether catapults the payload (2) toward the moon (3), where it is picked up by another tether in orbit there (4). This third cartwheeling tether then deposits the package onto the moon's surface (5). Credit: Tethers Unlimited and Scientific American.Web Links

Tethers in Space -- an excellent overview of space tethers for non-experts

Highway2Space.com -- news and information about space transportation research from the Marshall Space Flight Center

Recent Science@NASA Stories about Space Transportation:

May 31, 2000: Advanced Space Propulsion Workshop- an overview of the 3-day workshop.

May 29, 2000: What's the Matter with Antimatter?- It may be the ultimate fuel for space travel, but right now antimatter is fleeting, difficult to work with, and measured in atoms not pounds!

April 11, 2000: Where's the Edge?- NASA's Advanced Space Transportation Program looks at ways to turn science fiction into reality.

Stories from the 1999 Space Propulsion Workshop:

April 6, 1999: Ion Propulsion -- 50 Years in the Making- The concept of ion propulsion, currently being demonstrated on the Deep Space 1 mission, goes back to the very beginning of NASA and beyond.

April 6, 1999: Far Out Space Propulsion Conference Blasts Off - Atoms locked in snow, a teaspoon from the heart of the sun, and the stuff that drives a starship will be on the agenda of an advanced space propulsion conference that opens today in Huntsville.

April 7, 1999: Darwinian Design - Survival of the Fittest Spacecraft

April 7, 1999: Coach-class tickets for space? - Scientists discuss new ideas for high-performance, low-cost space transportation

April 8, 1999: Setting Sail for the Stars - Cracking the whip and unfurling gray sails are among new techniques under discussion at the 1999 Advanced Propulsion Research Workshop

April 12, 1999: Reaching for the stars - Scientists examine using antimatter and fusion to propel future spacecraft.

April 16, 1999: Riding the Highways of Light - Science mimics science fiction as a Rensselaer Professor builds and tests a working model flying disc. The disc, or "Lightcraft," is an early prototype for Earth-friendly spacecraft of the future.