Gravity Probe B - A Sphere of Near Perfection

The Gravity Probe B team had to create the roundest gyroscopes ever made, and set them orbiting Earth inside a force-free pocket. No form of atmospheric drag or magnetic forces could be allowed to penetrate the gyro-chambers. That's tricky because Earth's far-flung magnetic field envelops GP-B and, even at an altitude of 400 miles, Earth's outermost atmosphere exerts drag on the spacecraft. Furthermore, it would be necessary to measure the tilt of the gyroscope's spin axis ... without ever touching the gyroscope itself.

One of the spherical gyroscopes used in Gravity Probe B.

The gyroscopes in GP-B are the most perfect spheres ever made by humans. (The experiment actually carries four gyroscopes for redundancy.) These ping pong-sized balls of fused quartz and silicon are 1.5 inches across and never vary from a perfect sphere by more than 40 atomic layers. That means that if these gyroscopes were the size of the Earth, the elevation of the entire surface would vary by no more than 12 feet! If these gyroscopes weren't so spherical, their spin axes would wobble even without the effects of frame-dragging, thus ruining the experiment.

Being in orbit allows the spheres to float within their housings as if weightless, but without other controls, the spinning spheres would still tend to drift and bump into the walls of their containers. The reason is that the spacecraft is being slowed slightly by aerodynamic drag, while the free-floating spheres within the spacecraft's belly are not.

The GP-B team solved this problem by developing a drag-free satellite.

Inside the spacecraft instruments monitor the distance between one of the gyroscopes and its chamber walls with extraordinary precision - to within less than a nanometer (a millionth of a millimetre). The spacecraft's thrusters respond to any changes in that separation. In effect, the spacecraft chases the gyroscope and flies along the same "drag free" orbital path that it does.

The spheres must also be protected from Earth's magnetic field. Why? Because a faint magnetic signal from the gyroscopes themselves will ultimately be used to detect the all-important change in angle of their spin axes. The intrusion of Earth's magnetic field would swamp that signal.

But how do you block a planet's magnetic field?

"We used superconducting bags," says Kolodziejczak. The gyroscope assembly is placed inside lead bags, which in turn are placed inside a large cryogenic container called a "dewar" holding 400 gallons of liquid helium. The helium cools the lead bags to 1.7 degrees above absolute zero (1.7 K, or about -271 °C). At this temperature the lead becomes a superconductor, thus blocking out Earth's magnetic field. The ambient magnetic field within these bags is reduced to less than 3 micro-gauss, which is about the same as in deep interstellar space.