While the spheres are shielded from the outside world, the spacecraft will be subject to drag from the upper reaches of the Earth's atmosphere. At an altitude of 600 kilometres this is a small effect but one that would eventually cause the spacecraft to collide with the spheres within it. To prevent this, the spacecraft's position must be continuously measured relative to its cargo, and corrected.

This will be done by monitoring the position of a test mass a quartz sphere identical to the ones used as gyroscopes that will sit at the spacecraft's centre of mass. This is the point at which the centrifugal force resulting from the probe's circular motion precisely balances the Earth's gravitational field and so the test mass should follow a perfect orbit. By centring itself on the test mass the spacecraft will follow the same orbit. "In essence we're getting the spacecraft to chase the test mass," says Mac Keiser, a physicist at Stanford.

Then there is the measurement itself. During the course of the experiment, the changes in the London moment will be tiny but well within the measuring capability of superconducting measuring devices, known as SQUIDs, which will sit within the quartz housing. SQUIDs (superconducting quantum interference devices) are ideal for measuring small signals because they do not create the "noise" that is usually associated with electronic components and which could swamp the signal. "A SQUID has the best signal-to-noise ratio of any detector," says Jim Lockhart, co-director of the team at Stanford that designed the measuring equipment. The SQUIDs are capable of measuring a change in the magnetic field corresponding to a movement of only 0.1 milliarcsecond.

But because SQUIDs are so sensitive and the effect they are measuring so small, the Earth's magnetic field must not be allowed to swamp the results. Excluding this field was one of the major challenges that the team had to tackle and, again, they turned to superconductors for help.

One of the properties of superconductors is that they are impervious to magnetic fields and so can be used as magnetic shields. But creating a superconducting box and cooling it simply traps whatever ambient fields there are inside. The trick is to trap the field inside a superconducting "balloon" and then dilute the field by inflating it. Inserting another balloon inside the first, cooling and then inflating it, reduces the field even further. The Gravity Probe B experiment will sit inside four lead balloons inflated in this way that will help to reduce the ambient magnetic field by some 13 orders of magnitude. Of course, to make all this work the entire experiment must be cooled to superconducting temperatures. The main bulk of the 3-tonne satellite is like a huge Thermos flask filled with 230 litres of superfluid helium at 2.3 kelvin. This surrounds and cools the lead balloons, inside which the experiment sits in a vacuum. Maintaining the flask and its contents at this temperature would be a simple matter if it could be perfectly sealed. But the equipment must be monitored and this information and the results of the experiment passed through the walls of the flask so that they can be broadcast to Earth. The only way this can be done is to use wires carrying a current. Inevitably this will create heat, and even a small amount of heat energy could increase the temperature significantly, says Richard Parmley, a physicist at Stanford. This would boil away the helium dramatically reducing the lifetime of the experiment.

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