Short circuit severed satellite tether.

When an electrified tether connecting a small satellite and the space shuttle Columbia broke in February, it left behind a mystery. Now, officials say they have a good idea what happened. A jagged flaw in the tether's copper strands or an errant metal flake pierced its insulating layer, a NASA investigating board reported in Washington, D.C., last week. When a spark leaped from the unprotected cable to the shuttle's hull, the surge in electricity burned through the tether's layers of fiber, copper, and insulating plastic. In 9 seconds, investigators said, the tether broke.

Tests done later, on Earth, showed that any gap in the insulation leads to the same result. The report notes that sections of the tether's copper conductor "were nicked up to one-third of their diameter" during manufacturing. Also, sharp bits of aluminum, copper, iron, nickel, silver, and titanium were found embedded in the tether's jacket or loose in its reel mechanism. A nick in the copper or a sharp metal flake probably sliced through the insulation as the tether was wound onto its spool, the report concludes. Exactly what happened will never be known because the evidence melted away. "The threat to the insulation was just not appreciated prior to flight," said Kenneth J. Szalai, director of NASA's Dryden Flight Research Center in Edwards, Calif., and head of the investigating board.

The same Tethered Satellite System flew aboard a shuttle in 1992, but the tether snagged like a tangled fishing line after unreeling less than 300 meters. This year, it snapped while passing through a series of pulleys in Columbia's cargo bay-just 1 kilometer short of the planned 20.7 km. Nobie H. Stone, NASA's mission scientist for the tether test, noted that the tether was almost 10 years old.

While the report declines to place blame for the failure, tether specialist Brian E. Gilchrist of the University of Michigan at Ann Arbor says more rigorous preflight testing might have prevented it. A "spark gap test" that detects holes in the insulation after manufacture could have been repeated before the flight, he noted.

Data collected before the tether snapped nevertheless proved that an orbiting satellite system can reap electric energy from Earth's magnetic field, says project scientist Carlo Bonifazi of the Italian Space Agency in Rome. The system generated electric current as it swept through the field at 8 km per second. The same principle governs a spinning dynamo in an Earth-bound power plant.

The electricity comes at a cost, however. As the satellite system gathers electrons, it loses a small amount of momentum and its orbit decays slightly. If scientists reversed the current, the system would become an "electrodynamic motor" that could push a spacecraft to a higher orbit, says Adam T. Drobot of Science Applications International Corp. in McLean, Va.

A pleasantly surprised Bonifazi said the data show that the electrical conversion process "is very efficient." The experiment recorded a peak current of 1 amp, he added, far surpassing preflight estimates.