Today's scientists are already hard
at work on new ideas for spaceflight in the future. New technology is being
developed to help propel spacecraft into Earth orbit -- and beyond. NASA researchers
are studying ways to create more powerful and more efficient rockets. Engineers
are considering things such as energy beamed from the ground, ions, and
plasma as possible concepts for next-generation engines. Next to all of
these super-high-tech ideas, one spaceflight possibility sounds remarkably
simple -- string.
|A tether deployed from a rocket stage.
Needless to say, if it's going to
help propel spacecraft and satellites into higher Earth orbits, and beyond,
it has to be some high-tech string. More accurately, the technology is
called space tethers. A tether is essentially just something used to tie
one object to another. On Earth, tethers are generally used to keep something
in place. In space, however, tethers could serve several useful purposes.
One of the uses being examined would be to move satellites into higher orbit,
or even to help send spacecraft out into the solar system. This would
be done using a process known as momentum exchange. The system would
involve placing a satellite in orbit that would be used to boost other
objects further from Earth. This satellite would consist of a tether
with a grappling mechanism on one end that could "grab" the items being
boosted. On the other end of the tether would be a counter mass, which
would serve to keep the tether taut. The tether itself could be tens
to hundreds of kilometers in length. Researchers are still looking into
what materials would make the best space tethers. In fact, one possibility
is based on fishing line! The tether will have to be able to resist the
corrosion caused by atomic oxygen in the upper reaches of Earth's atmosphere,
as well as damage from micrometeoroids.
When the new satellite
or space probe is launched, the spinning tether system captures it and
essentially "flings" it farther from Earth. Using a method such as
this would greatly reduce the altitude to which an object would have to
be launched initially. This, in turn, would cut down on the needed power
of the rocket used to launch it, which, of course, would reduce the cost
of the launch. Researchers believe that satellites could be placed in a
geosynchronous orbit for as little as one-half of the current launch cost.
|A tether hangs from an orbiting satellite.
Of course, nothing
good is ever free, and that includes placing satellites into better orbits.
The system is called momentum exchange because it does just that -- exchanges
momentum from the orbiting tether to the boosted satellite or probe. That
means that the tether loses momentum, and falls closer to the Earth as
the other object moves further from it. Without a way to reboost the tether
satellite, it could only perform the momentum exchange technique a very
limited number of times before dropping into Earth's atmosphere. That's
where another use of space tethers comes in.
Another technology NASA researchers
are developing is the Electrodynamic Tether (EDT). As the long wire of
an EDT passes through Earth's magnetic field, it sets up a voltage along
the tether. This voltage makes electrons flow down the tether, like water
flowing down a pipe. If the tether has the right systems to allow it to
collect and emit electrons, then an electrical current (the flow of electrons)
will move through the tether. Any time an electrical current flows in a
magnetic field, there is a force. This may sound strange, but it is the
same principle that makes every electric motor on Earth work, from the
motor in a blender to the motor that spins your CDs.
Current naturally flowing through
the tether can be used by spacecraft for power. But again, we don't get
something for nothing. The force created by that flowing current slows
the spacecraft down and takes energy out of its orbit. Eventually, the
object would re-enter Earth's atmosphere, where, without heat shielding,
it would burn up. Such a system could be used to prevent space junk from
accumulating in orbit. When a satellite reached the end of its useful life
span, it could deploy a tether that would slow it down until it disintegrated
in the atmosphere, clearing the way for new satellites to be placed in
|Another view of a momentum exchange tether.
If we want to boost
satellites up to higher orbits, we have to force the current to flow through
the tether in the direction opposite the way it wants to go. To
do this, we need energy, which can be collected from the Sun using solar
panels. When a tether forces current in the direction opposite from the
direction it wants to go, it's like pumping water up a pipe. The force
on the tether from "pumping" current adds energy to the orbit of the tether
and boosts it into a higher orbit. In fact, some NASA researchers believe
that by using a tether like this, the orbit of the International Space
Station could be continuously reboosted, countering the effect of atmospheric
drag without requiring any additional propellant.
The same electrodynamic
tether technology would be used to reboost the momentum-exchange tether
after each use. It would use energy collected from solar panels to drive
electrical current through the tether in the right direction to add energy
to its orbit and reboost it for another use. In fact, the type of tether
they are developing is called a Momentum-Exchange/Electrodynamic Reboost
|Artist's conception of a tether used in jovian exploration.
Experiments conducted in space involving
EDT tether concepts have met with limited success when the materials
used proved incapable of handling the strains of the current and space
exposure. However, those experiments did prove that it is possible to
generate a current using a space tether. The next test of the tether
concepts for NASA will be the Propulsive Small Expendable Deployer System
(ProSEDS) experiment, planned for launch in 2004. The ProSEDS experiment
will be launched from an expendable rocket, and will deploy a 15-kilometer
(9.3-mile) tether about the width of dental floss. The experiment is
designed to use EDT technology to drag the Delta rocket stage from which
it deployed back into Earth's atmosphere, where it would burn up.
If successful, ProSEDS
will demonstrate that it is possible to use tether technology to help eliminate
space junk, and will open the door for more advanced space tether applications.
That's not bad for a long piece of high-tech string!
Courtesy of NASA's Aerospace Technology Enterprise/NASAexplores