EARLY FINDINGS FROM TETHERED SATELLITE MISSION
POINT TO REVAMPING OF SPACE PHYSICS THEORIES
May 23, 1996 / posted May 28, 1996
Source: NASA HQ Public Affairs Office
Numerous space physics and plasma theories are being
revised or overturned by data gathered during the Tethered
Satellite System Reflight (TSS-1R) experiments on Space
Shuttle Columbia's STS-75 mission last March.
Models, accepted by scientists for more than 30 years,
are incorrect and must be rewritten. This assessment follows
analysis by a joint U.S.-Italian Tethered Satellite
investigating team of the information gathered during the mission.
During STS-75, a tether system was being unreeled to
nearly 13 miles above Columbia's payload bay. Just short of
the full distance, its tether broke. Nevertheless, the
science instruments on the satellite and Shuttle, which had
been operating during the five hours of deployment
operations, sent a flood of readings that were received and
recorded by scientists on the ground. "Even the quick-look
made to date reveals that this data harvest is rich in
content," said Dr. Nobie Stone, NASA TSS-1R mission scientist
at the Marshall Space Flight Center, Huntsville, AL.
"Perhaps the most significant finding," Stone said, "is
that tether currents proved to be up to three times greater
than existing theoretical models predicted prior to the
mission. With the amount of power generated being directly
proportional to the current, this bodes well for
technological applications."
"Reversing the direction of current flow puts the system
into an electric-motor mode," Stone explained. This harnessed
energy could furnish thrust for reboosting a space station,
satellite or Shuttle in a decaying orbit.
"Traditionally, the primary source of power for long-
term space platforms has been solar arrays," Stone said.
"Those cells can only produce power when exposed to sunlight
during the two-thirds of each 90-minute orbit when a space
station, for instance, is not on Earth's dark side. However,
a tether system might provide a constant source of energy,"
he noted. "It is very efficient and might serve as an
effective back up power system."
Other important revelations from the STS-75 mission
include observations of the satellite's thrusters interacting
with the ionosphere while moving rapidly in Earth orbit.
Stone said that, when the thrusters were fired to adjust the
satellite's spin rate, the neutral gas emitted became ionized.
The tethered satellite researchers noted that, at that
point, "a sudden jump" took place in the level of current
flow, while the satellite's potential (voltage) dropped
several hundred volts. They traced this effect to the small
amount of gas, released from the thrusters, becoming ionized
in the vicinity of the satellite. A greater, more efficient
current flow was observed. "The effect of neutral-gas ionization
is not taken into consideration by existing theoretical models
of current collection in the ionosphere," Stone said.
Also, for the first time ever, the high voltage plasma
sheath and wake of a high-voltage satellite moving rapidly in
the ionosphere was measured. "This is virtually impossible to
study in a laboratory and is difficult to model
mathematically," Stone said.
Tethered Satellite System investigators have just begun
to scrutinize the data from STS-75. They expect that it will
reveal more answers to questions about the workings of the
Earth's upper atmosphere, its physics and the electrodynamic
applications of tethered systems in space.
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