What NASA mission measured the frame-dragging effect of Earth's rotation?

Gravity Probe B was the NASA mission that measured the frame-dragging effect of Earth’s rotation. Frame-dragging, or the Lense–Thirring effect, is the predicted twisting of spacetime caused by a rotating mass such as Earth.

Launched on 20 April 2004, Gravity Probe B carried four ultra-sensitive quartz gyroscopes into a nearly polar orbit. Scientists compared each gyroscope’s spin direction with a guide star, IM Pegasi. General relativity predicted two tiny changes: geodetic precession caused by Earth’s mass and frame-dragging caused by its rotation.

The frame-dragging signal was much smaller than the geodetic effect and was complicated by unexpected electrostatic torques on the gyroscopes. After extensive data analysis, the mission’s final results, announced in 2011, agreed with Einstein’s prediction. LAGEOS satellites also provided important frame-dragging measurements, but Gravity Probe B was the dedicated NASA mission designed to test the effect with orbiting gyroscopes. GRACE and MICROSCOPE studied other aspects of Earth science and equivalence-principle physics.