A 30 year project to boldly go where no gravity probe has... well, anyway, you get the point.

Gravity Probe B is a satellite that should be launched near the end of 2000. It's a collaborative project between Stanford University and Lockheed-Martin, and it has been 30 years in production.

It plans to test one of the remaining untested predictions of Einstein's theory of General Relativity. In layman's terms (which is how it was explained to me), a heavy spinning object (like the Earth) will twist spacetime around it, affecting spinning objects near it. The effect is very small, even for things the size of Earth, but might be significant in some galaxy-scale events like quasars.

To measure this frame-dragging effect, the satellite will contain 4 gyroscopes. These gyroscopes, spheres of fused quartz, are the smoothest objects ever made by man. If one was magnified to the size of the Earth, the height difference between the deepest valley and the highest mountain would only be about 10 feet. The gyroscopes are isolated from all external forces, cooled down by the surrounding liquid helium, and spun to 10,000 rpm. Superconducting shields eliminate all magnetic and electric fields, and complex stationkeeping systems eliminate effects of solar wind and micrometeorites. The gyroscopes are free-floating, and kept centered in their housing by electrodes. Their orientation is read with SQUIDS (superconducting quantum interference devices), which are extremely sensitive, and no, I don't know how they work either. A very sensitive telescope, also made of quartz, keeps the satellite oriented in the right direction.

In the end, the satellite needs to be able to detect a change of rotation axis in the gyroscopes of about the width of a human hair seen from a mile away over the period of a year.

In short, it was a very cool place to work for a summer job.
2003 Update: No, it hasn't launched yet. But it's pretty likely that the current Fall 2003 launch plan will actually stick. Update 2: Make that December 6, 2003.