Release: June 4, 1996
TETHERED SATELLITE INVESTIGATION REPORT IS RELEASED
NASA and the Italian Space Agency today released the report of the investigative board appointed to determine factors which resulted in the Feb. 25 tether break and loss of the Tethered Satellite during the STS-75 Space Shuttle mission.
Findings of the board, included in a 358-page document, identified primary causes which accounted for the tether break during deployment of the Tethered Satellite.
"The tether failed as a result of arcing and burning of the tether, leading to a tensile failure after a significant portion of the tether had burned away," the report concludes. The arcing occurred because either external foreign object penetration (but not orbital debris or micrometeoroids) or a defect in the tether caused a breach in the layer of insulation surrounding the tether conductor. The insulation breach provided a path for the current to jump, or arc, from the copper wire in the tether to a nearby electrical ground.
The board found that the arcing burned away most of the tether material at that location, leading to separation of the tether from tensile or pulling force. The break occurred when approximately 12.2 miles (19.7 km) of tether was unreeled, in a period when the tether was experiencing normal stresses of\ approximately 15 pounds (65 newtons).
In addition to the two primary causes for the tether break, the board cited, as one contributing factor, that "the degree of vulnerability of the tether insulation to damage was not fully appreciated." The board noted that the actual environment that the tether was exposed to in flight made it more vulnerable to damage than was expected. And, it noted that the high voltages under which the system was operating could, over a period of time, have reduced the ability of the tether insulation to withstand electrical breakdown due to contamination found in the tether.
"The tether itself was a remarkable engineering achievement," said Ken Szalai, who chaired the investigative board, "and produced some startling scientific discoveries." Scientific papers recently presented at an American Geophysical Union conference reported that currents generated by the tether were three times higher than theoretical models had predicted prior to the flight.
"Constructing a tether that was strong, lightweight and electrically conducting took the project into technical and engineering areas where they had never been before," said Szalai. "Now, with 20/20 hindsight, they know where the system is vulnerable and can improve the design."
The Tethered Satellite System is a joint NASA-Italian Space Agency (ASI) system that was flown aboard Space Shuttle Columbia in an experiment to better understand the electrically charged environment of Earth's ionosphere, and how tether systems behave in it. ASI had the responsibility of providing the satellite, while NASA had the responsibility of the Deployer, which includes the tether, and the overall responsibility for payload integration and operations. The provision of science investigations was shared by ASI and NASA.
The system was generating 3,500 volts DC and up to 0.5 amps of current during satellite deployment. That high level of electrical energy resulted from the length of conducting tether extending from the Shuttle, coupled with the 17,500-mile-per-hour speed at which the Shuttle and tether were cutting through Earth's magnetic field lines.
The board found sufficient evidence to identify two possible causes of the breach in the insulation -- foreign object damage, or a defect in the tether itself. Debris and contamination found in the deployer mechanisms and in the tether itself could have been pushed into the insulation layer while the tether was still wound on its reel. The investigation found evidence of damage to copper wire in the tether, and also established that normal forces on the tether while on the reel could push a single copper strand or foreign debris through the insulation.
The arcing, which began in an intricate part of the Tethered Satellite System known as the lower tether control mechanism, sputtered intermittently for nine seconds as the moving tether passed through deployer mechanisms and then into the boom area of the tether system. At the time, tether was continuing to play out at one meter per second, or slightly more than three feet per second.
"This arcing produced significant burning of most of the tether material in the area of the arc," the board found. The tether was designed to carry up to 15,000 volts DC and handle tensile forces of up to 400 pounds (1780 newtons). It used super-strong strands of Kevlar as a strength-providing member, wound around the copper and insulation. However, postflight inspection of the tether end which remained aboard Columbia showed it to be charred. The board concluded that after arcing had burned through most of the Kevlar, the few remaining strands were not enough to withstand forces being exerted by satellite deployment.
Extensive, rigorous tests performed in support of the investigation established that undamaged tether would not arc, even when subjected to electrical potentials much higher than the 3500 volts experienced during the mission.
The board was able to exonerate a number of factors which clearly did not cause the break. These factors include the satellite, the science equipment hardware and operations, which were being conducted prior to the break, in addition to micrometeoroids or orbital debris impact, and electrical storm activity.
The investigation panel made several detailed recommendations which it said should be followed for any future space missions involving electrodynamic tether systems such as that flown aboard Columbia. These include more precautions to ensure any such tether systems in the future do not suffer from possible debris or contamination damage and specific attention during design to minimize the possibility of high-voltage arcing.
The board offered, in the form of observations, its assessment that the STS-75 tether problem "is not indicative of any fundamental problem in using electrodynamic tethers." It also noted that in spite of the break, a "significant amount" of scientific data was obtained from the Tethered Satellite operations during STS-75.
The nine-member independent review panel was formed in consultation with ASI and appointed by NASA's Associate Administrator for the Office of Space Flight, Wilbur Trafton, shortly after the tether break. The board was chaired by Ken Szalai, director of the Dryden Flight Research Center, Edwards, CA, and included representation from NASA and the ASI.