Satellite On A String

On 31 July 1992, Space Shuttle Atlantis headed into orbit with one of the most unusual experimental payloads on record - which STS-46 Mission Specialist Marsha Ivins described as "weird science'' - known as the Tethered Satellite System (TSS). Originally conceived by the late Professor Guiseppe Colombo of Padua University, it was intended to demonstrate the 'electrodynamics' of a conducting tether in an electrically charged portion of Earth's atmosphere called the ionosphere. It was envisioned that Colombo's idea might ultimately lead to systems that would use tethers to generate electricity for spacecraft, using Earth's magnetic field as a power source. Furthermore, by reversing the direction of the current in the tether, the force

_ created by its interaction with Earth's magnetic field could potentially put objects in motion, thus boosting a spacecraft's velocity without the need for precious fuel, thereby counteracting air drag.

In the late 1980s and into the 1990s, this was particularly appealing to the designers of the International Space Station, as a means of compensating for the effects of atmospheric drag on the colossal outpost. Additionally, it was optimistically hoped that the concept could lead to the development of devices to trail scien-- tifie platforms far below orbital altitudes in difficult-to-study zones, such as the fragile ozone region over the South Pole. Other applications for such tethers included serving as extremely low-frequency antennas, capable of penetrating land and seawater, and perhaps generating artificial gravity or boosting payloads into higher orbits.

During the eight-day STS-46 mission in the summer of 1992 - five years later than

Artist's concept of TSS leaving the Shuttle's payload bay.

planned, due to the Challenger disaster - the satellite underwent its first demonstration in Earth orbit. Unfortunately, it achieved only partial success, when its 20.5-km tether snagged on a bolt in the deployment reel mechanism and refused to unroll more than about 258 m.

Nevertheless, the mission proved the concept sufficiently satisfactorily for a reflight to be proposed in the spring of 1996. A two-week-long flight was required to allow additional time for full deployment and a few days of ionospheric research. Most of the original STS-46 crew were also kept together for the reflight: Andy Allen, who had flown as Pilot of the first mission, was promoted to Commander of its second trip into orbit. He was joined by Payload Commander Franklin Chang-Diaz and Mission Specialists Jeff Hoffman and Claude Nicollier, all of whom had been on STS-46.

Rounding out Allen's crew on STS-75 were a rookie Pilot named Scott 'Doc' Horowitz - the first Shuttle pilot to hold a PhD - and two Italian astronauts: Mission Specialist Maurizio Cheli and Payload Specialist Umberto Guidoni, the latter of whom had also served as an STS-46 backup crew member. Together with Swiss-born Nicollier, this became the first Shuttle mission with three ESA astronauts on board. Unusually, the seven men were split into three separate shifts for the TSS deployment operation, reverting to a Spacelab-style dual-shift system after the completion of operations with the tethered satellite.

The Red Team consisted of Horowitz, Cheli and Guidoni, the Blue Team of Chang-Diaz and Nicollier and a unique 'White' Team of Allen and Hoffman; this unusual, staggered shift was added to enable the crew to operate a suite of instruments on the TSS to gather real-world data about how conducting tethers might some day be used to generate electrical power in space. After completion of deployment operations, it was planned for Allen to rejoin the Blue Team and Hoffman the Reds.

The satellite that was the primary focus of these three shifts was a 1.6-m-diameter, Italian-built sphere, weighing 517 kg, with an outer skin of aluminium alloy and coated with an electrically conducting layer of white paint. It was, however, far more than just an oversized metallic football. Piercing its shell were windows for Sun, Earth and charged-particle sensors, a connector for the umbilical tether and doors that provided access to its onboard batteries. Extending from one side of the TSS was a long, fixed instrument boom, while a shorter antenna sprouted from its other side.

To assist with the thermal control of the satellite, the interior of the spherical shell was painted black. If one were to break open the satellite, like an egg, one would see two separate compartments: a Payload Module, housing its scientific instruments, and a Service Module for its subsystems. Additionally, in the centre of the spherical shell was a pressurised nitrogen tank, which provided propellant for the satellite's 12 cold-gas manoeuvring thrusters. If the satellite could be termed an engineering marvel, the 2-mm-thick conducting tether that connected it to a support mast in Columbia's payload bay was no less.

Surrounding its Nomex core was electrically conducting copper wire, insulated with Teflon and coated with ultra-strong braided Kevlar 29 - a similar kind of

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