THE ALENIA-BUILT Italian Tethered Satellite was lost in space during the STS75/Columbia Space Shuttle mission on 25 February, when its tether broke after suffering "arcing and burning", says an investigation team appointed by NASA (Flight International, 6-12 March).
The arcing occurred because the tether was penetrated by either a "foreign object" - not a piece of orbital debris or a micrometeoroid - or there was a defect in the tether itself, which caused a breach in its insulation and Kevlar outer layer as the satellite was being deployed.
Higher-than-planned voltages experienced by the system may have reduced the ability of the tether insulation to withstand electrical breakdown caused by contamination (discovered after the Columbia's return to Earth) in the remaining part of the tether and the deployment mechanism. The contaminated part could have been pushed into the insulation layer while the tether was still on the reel.
The investigation found evidence of damage to copper wire in the remaining part of the unreeled tether and also established that normal forces on the tether while it was on the reel could push a single copper strand or foreign debris through the insulation from the inside or outside. The breach in the insulation provided a path for the planned electrical current generated by the tether, to jump, or arc, from the copper wire inside to "ground" in the deployment mechanism.
The arcing spluttered intermittently for 9s as the moving tether passed through deployer mechanisms and then into the boom area. The tether and deployment mechanism were built by Lockheed Martin. Most of the material was burned away at the location of the arcing and it broke as a result of "tensile or pulling force" when the satellite had been unreeled to a distance of 19.7km - almost the full length - and the tether was experiencing "normal stresses of approximately 65 Newtons [13lb]".
The investigation team adds that, as a contributing factor to the loss, "the degree of vulnerability of the tether insulation to damage was not appreciated fully". The tether was also exposed to an environment which made it more vulnerable to damage than expected.
Currents generated by the tether were three times higher than models had predicted. The system generated 3,500V DC and up to 0.5A of current during deployment. This resulted from the 28,000km/h speed at which the Shuttle and tether were cutting through the Earth's magnetic-field lines.
Source: Flight International
