3 Gravity Probe B
The Gravity Probe B (GP-B) satellite (left) is currently measuring two predictions of general relativity: frame-dragging and geodetic precession. The experiment involves detecting tiny changes in the motion of four gyroscopes that contain extremely smooth spheres made of fused quartz (right). The satellite, which is currently in a polar orbit at an altitude of 640 km, is a collaboration between Stanford University, Lockheed-Martin and NASA. The first results from GP-B are expected next year.