|
 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."
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.
|
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. |
