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TSS-1: the science mission that got away.

Any fisherman knows that a line on a reel can snag. Stretch the average fishing line over 20 kilometers, take away the familiarity of gravity, add a 518-kilogram satellite: Then things begin to get really difficult.

Shuttle astronauts hit mechanical snags from the start of their attempts to deploy the first tethered satellite last week. As originally designed, the Tethered Satellite System (TSS-1) resembles a giant fishing kit, with the 5-foot-diameter satellite attached to the shuttle by a shoelace-thin, 20-kilometer-long cord. The TSS-1 team managed to unreel only 256 meters of this tether before several knotty problems halted all progress, forcing them to retrieve the satellite with its primary scientific goals unaccomplished.

"The science team was very disappointed in the results of the mission," said TSS-1 mission scientist Nobie Stone of Marshall Space Flight Center in Huntsville, Ala., speaking at a NASA briefing last week. "We did not achieve our primary objectives. ... We didn't approach those."

A joint venture between NASA and the Italian Space Agency, the mission set out to test the physical stability of long-tethered systems and to conduct electro-dynamic experiments in the space environment.

Although the unreeled tether stretched less than 2 percent of its planned distance, the mission still achieved two major engineering objectives: the deployment and docking of the satellite, noted lead flight director Chuck Shaw of Johnson Space Center in Houston at the briefing. The system was easier to control in space and more stable than predicted, he added.

The problems encountered involved the satellite-deployment mechanism, built by Martin Marietta Astronautics Group of Denver. After several delays due to technical glitches, deployment stopped entirely when a snag seemed to develop at the end of the deployment boom. By the time the TSS-1 team managed to free the tether, the satellite had used up most of its thruster fuel. The team elected to retrieve the satellite rather than risk its loss.

NASA has formed an official board of investigation to assess the problems and recommend corrective measures.

Ironically, all science instruments functioned well. Researchers measured the electrical environment around the shuttle, but the satellite got nowhere near the minimum 10-kilometer distance needed to conduct the primary scientific experiments, says Rick Howard, TSS-1 mission scientist at NASA headquarters in Washington, D.C.

Scientists had hoped to investigate the power-generating capabilities of the tethered system. They had predicted a buildup of 5,000 volts across the electrically conducting tether as it moved through Earth's magnetic field. While unable to observe the phenomena associated with these high voltages, they did demonstrate the system's power-generating properties with a measured electrical potential of 40 volts and a current of 0.015 amp across the tether.

Such results only whetted the scientific appetite, however. "We just got a taste of what we were really out there to do," says Howard. He adds that TSS-1 scientists strongly support another tethered satellite mission. Stone concurs, stating that "the science team is more convinced than ever that the tether system offers us a unique tool for research."

While the Italian satellite was designed to fly three times, NASA has yet to commit to another attempt. The mission will have to compete with other science experiments for funds and space on the shuttle, which is currently booked for the next several years, says George Withbroe, director of NASA's Space Physics Division in Washington, D.C.
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Title Annotation:Tethered Satellite System's deployment mechanism fails
Author:Hoppe, Kathryn
Publication:Science News
Date:Aug 15, 1992
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