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6/12/96

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Space tether redux

STANFORD -- A small programming change could have prevented the loss of the space tether that broke last February while being tested by the space shuttle Columbia, prematurely ending the $443-million experiment. That's the opinion of Roger Williamson, a senior research scientist at the Hansen Experimental Physics Laboratory, who helped lay the scientific and technical foundation for the experiment.

On June 4, NASA television carried a news conference announcing the results of the official investigation into the failure. Williamson and Scott Williams, a research and development engineer who served as project engineer for the first tether experiment in 1994 and as a scientific co-investigator on the most recent mission, critiqued the findings.

According to the investigation board, the break was caused when an electrical arc ­ a stream of sparks formed when a strong electrical current jumps from one electrical conductor to another ­ started at a spot on the tether where the electrical insulation was weak or punctured. The arcing began in a pulley mechanism located in the shuttle bay that was used to deploy the 13-mile-long tether. The arcing continued for nine seconds as the tether continued to pay out, and the line was weakened to the point that it broke.

Because the tether had a very strong electrical charge of 3,500 volts at the time, it only took a pinhole-sized puncture in the cable's insulation to cause the damaging electrical discharge. The puncture, in turn, appears to have been made by a small piece of contaminated material embedded in the outer layer of the cable. This could have occurred during manufacture, or the material might have been picked up from the unsealed deployment mechanism in the shuttle bay.

Williamson and Williams reported that several times in the course of the experiment's development, the scientific team had fought to have computer instructions included that would detect such arcing and automatically ground the tether to the orbiter.

"All that was needed is a small change in the computer program. But we couldn't get the change made," said Williamson.

The shuttle test did lay to rest a number of concerns about the practicality of space tethers, the researchers said. NASA engineers had been concerned about the dynamics of such a large tether, worrying that an accumulation of small forces might cause it to begin rotating like a giant jump rope. If such rotations became large enough, the tether might wrap itself around the shuttle with unforeseen consequences. During the mission, however, the methods used to control the tether appeared to work well, Williamson said.

Although the board found that the tether's insulation was more vulnerable to damage than the experiment's designers had believed, they also found that the problem "is not indicative of any fundamental problem in using electrodynamic tethers." In fact, while the tether was operating it produced currents three times higher than theoretical models had predicted prior to the flight, the board reported.

The tether's stability, combined with the generous amounts of electrical current that it produced before it broke, means that the tether system has a promising future as an alternative source of electricity for orbiting spacecraft, Williamson and Williams agreed.

"The flight demonstrated that tethered systems will work, both as an electricity source for scientific experiments and for more practical applications as well," Williamson said.

Overall, the two Stanford researchers said the agree with the board's basic recommendations, particularly the suggestion that science teams be given increased oversight of engineering plans in return for giving engineers more say in the design of science payloads.

-dfs-

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