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June
11, 2007: NASA is joining a Japanese team in a space
experiment that uses reverse origami to show the way to help
keep satellites in their proper orbits, or to return spent
rocket stages quickly to Earth.
Les
Johnson of NASA's Marshall Space Flight Center is working
with Prof. Hironori A. Fujii of the Tokyo Metropolitan University
on the Foldaway Flat Tether Deployment System--or Fortissimo,
as it is sometimes called. "It's a new method for rolling
out tethers in space," says Johnson. The mission is sponsored
by the Japanese Aerospace Exploration Agency (ISAS/JAXA).
Right:
An artist's concept of a satellite tethered to the space shuttle.
Fortissimo
-- ff in music -- means strong in Italian. This tether will
have to be strong considering its gossamer construction. While
the tether is 1 km (3280 feet) long, it's only 0.05 mm thick
and 50 mm (almost 2 inches) wide. "It looks like a strip
of aluminum foil, almost like a tape measure," says Johnson.
This is a departure from most previous tethers, "which
have been braided wires of some sort."
Johnson
says that many details have to be worked out before the planned
2009 launch atop an S-520 sounding rocket.
"The
Japanese team has designed a deployer that looks like the
way firemen store their hoses. It's very different from anything
we've used before," Johnson continues. This is the reverse
origami part: The product starts folded, and is then pulled
up from the top to produce a nearly straight line. In this
fashion, researchers believe, the tether can be deployed 1
km in only a few minutes.
The
first space tethers were webbed lines that connected the Gemini
11 and 12 spacecraft to their Agena docking targets on separate
missions in 1966. These demonstrated that tethers could be
used to connect spacecraft for artificial gravity or to stabilize
a spacecraft.
![see caption](images/ff/ff_strip.jpg)
Above:
(L) A schematic diagram of the Fortissimo concept. (R) A ground
test of the system.
The
next major effort was the 20km NASA-Italy Tethered Satellite
System flown in 1992 and again in 1996. This used a complex
reel system that resembled a deck winch. (TSS-1 experienced
a mechanical jam early in its deployment. TSS-1R collected
a large volume of data before a different mishap cut the tether.)
NASA
moved on to ProSEDS, the Propulsive Small Expendable Deployer
System, which built on the simpler and highly successful SEDS-1
and -2 tests. ProSEDS involved a 20 km (12 mi) tether deployed
on a bobbin from a Delta II second stage. It was to fly in
2003, but after the Columbia tragedy NASA re-evaluated several
missions and decided that ProSEDS posed too great a risk to
the International Space Station.
Still,
as Robert Goddard wrote when his space travel concepts were
ridiculed, "The dream would not [go] down." Other
nations have experimented with tethers in more than 20 missions
overall, including Japanese participation in the Charge 1 and
2 rocket experiments in 1983 and 1984. Recently,
Fujii asked Johnson and fellow NASA researcher George Khazanov
to join the project as co-investigators. They will help the
Japanese team figure out how to use the tether as a means
of propulsion and, in particular, assist in the modeling of
the tether's reaction to Earth's magnetic field.
The
principle is similar to what happens in an electrical dynamo
on Earth: A wire moving through a magnetic field will produce
an electrical current in the wire. In low Earth orbit, the
Fortissimo tether will move through Earth's magnetic field
as well as Earth's ionosphere, a conducting layer of ionized
gas in the upper atmosphere. The resulting current will decelerate
the spacecraft. (Deceleration by tethers is a trick that would
come in handy for de-orbiting space junk. Acceleration is
also possible by pumping current in the opposite direction
through the tether, but testing tether acceleration is not
a goal of this particular mission, notes Johnson.)
"We
expect a low average current--about 1 to 3 amps," he
continues. That will vary, depending on the time of day the
mission launches. The ionosphere contracts at night and the
tether would pass through 10 times more electrons during the
day than it would in a night launch.
Right:
The blue orb is Earth; red lines denote Earth's magnetic field.
A tether orbiting through this field is subject to electrodynamic
currents.
There
also is uncertainty about friction, electrostatic charging,
and other forces involved in deploying the tether. A model
has been tested on the ground, but an experiment in the vacuum
and free-fall of space is needed to verify predictions before
possible orbital experiments.
Also
different from past tethers will be the manner of the experiment:
It will last just five minutes, from about 100 km (62 mi)
altitude, just above most of Earth's atmosphere, to an apex
of 300 km (186 mi), and until it is destroyed during re-entry.
Once
in space, the probe will deploy a satellite that rises then
falls at the same velocity as the probe, but moves outward,
gently pulling the aluminum tape with it, and hopefully unfolding
new opportunities in space.
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Author: Dave Dooling | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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