The four gyroscopes have become their advertising coup. When you've made probably the most spherical objects in the known universe (outside of astronomical phenomena known as neutron stars and discounting, of course, the possible products of extraterrestrial gyroscope makers), you are going to get attention. After all this time, says Everitt, making the gyroscopes turned out to be straightforward. By that he means "difficult but straightforward."
The rotors--the spinning part of the gyroscope--are a triumph of sphericity. Quartz globes one and a half inches in diameter, coated with a layer of niobium, they look like idealized silver squash balls. They have been lapped and polished to within 50 atomic layers of being perfect spheres. The deviation between their highest and lowest points is less than one- millionth of an inch. And as anyone associated with Gravity Probe B will gladly remind you, if Earth were correspondingly round, the highest mountain would tower all of 20 feet over the depths of the deepest ocean trench.
Perfection, however, comes with inherent complications. For instance, how do you mount and spin a near-perfect ball without ruining the symmetry? And how do you tell in which direction its axis is pointing when you've gone to so much trouble to make it exactly spherical--and therefore unmarked?
Both of these problems are solved by the electrical properties of the niobium coating the rotors. Niobium is a superconductor, which means that if cooled to a few degrees above absolute zero (-459.67 degrees Fahrenheit), it will lose all resistance to electricity, and an electric current, once incited, will run around it endlessly. The researchers will mount the rotors in spherical cavities, with a thousandth of an inch of clearance for the rotor to spin freely. Inside the cavities are three pairs of titanium-coated copper electrodes. An electric charge will be applied to the electrodes, which will then repel the niobium on the rotors and levitate them. Once they're up, a stream of helium gas will be squirted through a channel, setting the rotors spinning at some 10,000 revolutions per minute. As soon as they're going, the remaining gas will be pumped out, and the gyroscopes, now in a near-perfect vacuum, could theoretically go on whirling for a good hundred thousand years, if the coolant supply held out that long, which it won't. The probe is expected to exhaust its coolant in just two years.
The problem of determining the direction of spin turned out to have been solved theoretically back in the 1930s by Fritz London, a German- born physicist working in England. London predicted that a spinning superconductor, unlike any normal metal, would create its own magnetic field and that the north-south axis of this field would be exactly aligned with the axis of the spinning superconductor. In a spinning superconductor, London suggested, the electrons in the metal would rotate slightly more slowly than the positively charged particles, and the rotating electric charge that resulted would create the magnetic field. (Because the electrical resistance in a normal metal causes the electrons to spin along with the rest of the positive charges in the metal, there is no rotating charge and thus no magnetic field.) Fairbank himself discovered in 1963 that this London moment, as it's called, actually exists and suggested using it as a marker. "Without that, we're dealing with an unmarked ball,'' says Brad Parkinson, an engineer and former Air Force colonel who is one of the program's three co-principal investigators. "The bad news, however, is that the London moment is fairly weak."
To detect London moments, the Gravity Probe will have to be outfitted with four superconducting quantum interference devices, or squids, one for each gyroscope. This extraordinarily sensitive tool will measure the magnetic fields and should detect any changes in the direction of the London moment, and therefore in the spin direction of the rotors. The squids are so sensitive that in five hours they can measure the orientation of the London moment to a thousandth of an arc second. (By launch, Everitt hopes to have that read-out time down to even fewer hours. "Straightforward," predicts Everitt. Well, difficult but straightforward.)
The Gravity Probe B gyroscopes will eventually be mounted in a solid quartz structure that includes the telescope designed to keep the probe fixed on a distant star. This structure will then be placed within a nine-foot-long dewar--essentially a big thermos--filled with 2,300 liters of liquid helium, which will cool the device to 2 degrees above absolute zero and last through the mission's 20-month duration. The low temperature will keep the probe's quartz housing stable and keep the superconductors cold enough to superconduct.
Before the probe goes in, however, any magnetic field in the dewar will have to be pumped out by inserting a succession of lead bags into the container and inflating them one after another. This was another of Fairbank's ideas back in 1963. At low enough temperatures, lead can be a superconductor as well, and another remarkable property of superconductors is that magnetic fields cannot pass through them. "As you expand the lead bag so that it has a larger interior volume, any magnetic flux that is trapped in the bag has to expand over a larger volume, so the field strength has to go down,'' explains Stanford physicist John Turneaure. "It's a little bit like if you take a cubic centimeter of water and expand it into a thousand cubic centimeters volume. It's going to be considerably less dense.'' By the time four or five lead bags have been inserted and inflated, and the probe set into the dewar, the magnetic field left around the gyroscopes will have been diluted by a factor of 10 million or so.
