material to that used in bullet-proof vests - which made it much hardier than it looked. Over the top of the Kevlar was a final, outermost 'jacket' of braided Nomex, which protected it from abrasion and the corrosive effects of atomic oxygen in Earth's rarefied upper atmosphere. During deployment operations, the tether was unreeled from a 2,027-kg mechanism affixed to a Spacelab pallet and the Mission-Peculiar Equipment Support Structure (MPESS) in the payload bay.
Essentially, the mechanism took the form of a four-sided erectable tower, looking like a small broadcasting pylon, which unfolded slowly out of its storage canister using a series of rollers. As the canister rotated, fibreglass batons popped out of their stowed, bent-in-half positions to form cross-members - 'longerons' - that supported the tower's vertical pieces. The tower deployed to a height of 11.8 m above Columbia's payload bay, so that when the satellite was released, there was no risk of it hitting any part of the Shuttle's structure. ''The complexity of the experiment is extreme,'' Andy Allen had said before the flight.
That, however, was the easy part. Deploying the tower and even releasing the TSS itself had already been done by the STS-46 crew nearly four years earlier; what Allen's team planned to do on their 14-day mission in February 1996 was finish the job by getting the satellite and tether to their full 20.5-km length and demonstrating Giuseppe Colombo's concept. In fact, no fewer than 12 separate experiments were planned during the reflight - known as TSS-1R - of which six had been provided by NASA, five by the Italian Space Agency (ASI) and one by the US Air Force.
Several of these experiments were mounted on the MPESS: two were designed to investigate the dynamics of the tether during its deployment phase, another provided theoretical support in the area of electrodynamics, a couple more employed ground-based equipment to measure electromagnetic emissions from the satellite and seven others stimulated or monitored the entire assembly as it reeled its way out of the payload bay. Nearly 22 km of cable was in the deployment mechanism for STS-75, although 'only' 20.5 km of that would actually be unravelled.
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