So reducing the heat transfer along these wires is yet another challenge for the scientists. Ideally, the wires should be made of a material with a low thermal conductivity but with sufficient electrical conductivity. According to Parmley, the best material for this purpose is an alloy of copper which will carry currents of up to 1.4 amps and yet be only half a millimetre thick or even thinner. Each wire will be insulated so that the combined heat from the 700 wires entering the flask will boil off only 7 milligrams of helium per second. This gas will then be used by the craft's thrusters to control its attitude.

An even more serious heating problem could arise during the launch. Parmley says the vibrations during liftoff will send the superfluid helium sloshing around the flask, creating enough energy to significantly raise its temperature. "When you're down near absolute zero it takes almost no energy to increase the temperature dramatically," he explains. If the temperature of the lead balloons rises above 7.2 kelvin, they loose their superconductivity and hence their shielding ability. And if the Earth's magnetic field penetrates the shielding it cannot be removed, even if the temperature is lowered again later.

Baffles inside the flask should minimise sloshing, but whether this will be enough to prevent excessive heating, Parmley cannot say. "It's such a difficult calculation that we don't have any confidence in it. I don't even have a number," he explains. So the flask is designed with an emergency cooling system. Should the temperature of the lead balloons rise dramatically during the launch, they will be flooded with liquid helium so that they are cooled both from the inside and the outside. "If the lead is encased in liquid helium we know it will remain superconducting," says Parmley.

If all goes to plan, Gravity Probe B will be launched in 1999 at a cost of some $500 million. Everitt says it is money well spent. He believes that the results of the mission will have far-reaching implications for scientists studying the nature of matter and the structure of the Universe. In the 80 years since Einstein developed his ideas, several theorists have challenged his ideas with competing theories. So far relativity has survived and few scientists expect it to fall at this hurdle. Nevertheless, only Gravity Probe B will tell us for sure.

Ben Iannotta is a technology journalist based in Florida.August 1996, No.2045 Page 28: Gravity Probe B 31 August 1996, No.2045 Page 28: Expected shifts in rotation Copyright Reed Business Information 1995