Early in August 1992, on the twelfth mission of shuttle orbiter Atlantis (STS-46), a satellite with a difference was deployed from the payload bay. It was a satellite that was kept tethered to the orbiter by a cable. This tethered satellite system (TSS) was developed jointly by NASA and ASI, the Italian Space Agency. The aim of this first tethered mission (TSS-1) was to demonstrate the feasibility of deploying, controlling and retrieving a tethered satellite, which is impossible to simulate on Earth. Potential future uses of a tethered satellite include electrical generation for the space station Freedom, orbital manoeuvring, the creation of artificial gravity, and high- atmosphere aerodynamic testing. The Italian-built satellite is a sphere 1.6 metres across and weighing 500 kg. It has a fixed boom for carrying instruments, and for the TSS-1 mission, two extra instrument booms which can be retracted. The satellite is mounted in the payload bay on a Spacelab-type pallet, which also carries the tether reel assembly. Another carrier provides science support equipment. The cable that forms the tether measures 2.5 mm across. It carries a central conductor made up of 10 strands of copper wire. It is insulated with a layer of Teflon and wrapped in braided Kevlar plastic, which gives it strength. With its copper core, the tether is an electric conductor. In space, it circles the Earth at a speed of some 28,000 km/h and cuts across the lines of force of the Earth's magnetic field. This turns it into an electric generator, setting up an electric current. On release from its carrier in the orbiter's payload bay, the tethered satellite is pushed away from its mounting by an extendible boom, and an on-board thruster is fired to help separation. As the satellite climbs, the tether pays out in its wake. Soon there is no need for thrusters to help pull out the tether. As the satellite orbits higher, it travels slower and thus drags out the tether. The total length of the tether is over 20 km, and on TSS-1 it was intended to extend the tether to this length, an operation estimated to take about five hours. But in the event the release mechanism proved faulty and jammed after the tether had been paid out for only about 260 metres. It then had to be rewound. At full-length deployment it was expected that an electric potential of some 5,000 volts and a current peaking at about 1 amp would have been generated in the tether. But the 260-metre length only managed about 40 volts and 15 milliamps.
