For Release: May 23, 1996

Douglas Isbell
Headquarters, Washington, DC
(Phone: 202/358-1547)

Jerry Berg
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 205/544-0034)

RELEASE: 96-43


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 in Huntsville, Ala.

"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 backup 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.