A Little
Physics and A Lot of String
One day space tethers may be used
for boosting orbits, powering satellites, and even
sending payloads to the Moon or Mars -- all without
the expense of propellants.
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.
|
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.
Related 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
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3-day workshop.
May 29, 2000: What's
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April 11, 2000: Where's
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Stories from the 1999 Space
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April 6, 1999: Far
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April 8, 1999: Setting
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April 12, 1999: Reaching
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April 16, 1999: Riding
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