#25c.     The Space Tether Experiment

    The space tether experiment, a joint venture of the US and Italy, called for a scientific payload--a large, spherical satellite--to be deployed from the US space shuttle at the end of a conducting cable (tether) 20 km (12.5 miles) long. The idea was to let the shuttle drag the tether across the Earth's magnetic field, producing one part of a dynamo circuit. The return current, from the shuttle to the payload, would flow in the Earth's ionosphere, which also conducted electricity, even though not as well as the wire.



    One purpose of such a set-up might be to produce electric power, generating current to run equipment aboard the space shuttle. That electric comes at a price: it is taken away from the motion energy ("kinetic energy") of the shuttle, since the magnetic force on the tether opposes the motion and slows it down. In principle, it should also be possible to reverse this process: a future space station could use solar cells to produce an electric current, which would be pumped into the tether in the opposite direction, so that the magnetic force would boost the orbital motion and would raise the orbit to a higher altitude.

    It took a considerable amount of detective work to figure out what had happened. Back on Earth the frayed end of the tether aboard the space shuttle was examined, and pieces of the cable were tested in a vacuum chamber. The nature of the break suggested it was not caused by excessive tension, but rather that an electric current had melted the tether.

    The electric conductor of the tether was a copper braid wound around a nylon string. It was encased in teflon-like insulation, with an outer cover of kevlar, a tough plastic also used in bullet-proof vests, all this inside a nylon sheath. The culprit turned out to be the innermost core, made of a porous material which, during its manufacture, trapped many bubbles of air, at atmospheric pressure.

    Later vacuum-chamber experiments suggested that the unwinding of the reel uncovered pinholes in the insulation. That in itself would not have caused a major problem, because the ionosphere around the tether, under normal circumstance, was too rarefied to divert much of the current. However, the air trapped in the insulation changed that. As it bubbled out of the pinholes, the high voltage ("electric pressure") of the nearby tether, about 3500 volts, converted it into a plasma (in a way similar to the ignition of a fluorescent tube), a relatively dense one and therefore a much better conductor of electricity.

    The instruments aboard the tether satelite showed that this plasma diverted through the pinhole about 1 ampere, a current comparable to that of a 100-watt bulb (but at 3500 volts!), to the metal of the shuttle and from there to the ionospheric return circuit. That current was enough to melt the cable.

    As the broken end whipped away from the shuttle, the plasma established electric contact with the ionosphere directly. The satellite on the distant end monitored the current: after about half a minute it stopped, then it reignited and flowed again for about another half minute, stopping for good when (presumably) all the trapped air was gone.

    Because of the unexpected break, the tether experiment at the time was widely viewed by the press as an expensive failure. True, the planned operation at full deployment, for several hours, could not take place, nor could the tether and its satellite be retrieved, which was to have demonstrated the feasibility of deployable tethers.

    However, many of the scientific experiments had already begun during deployment and yielded good data. And the break itself, though unfortunate, added an unscheduled experiment to the mission, one which highlighted the risks and complexities of operating scientific equipment in space.