Gravity Probe B

Gravity Probe B was a space experiment that measured the slow drift of orbiting gyroscope axes relative to a distant guide star to test general relativity. It targeted geodetic precession from motion through curved spacetime and frame dragging associated with Earth's rotation.

The apparatus used four nearly spherical gyroscopes operating at cryogenic temperature, a telescope tracking the guide star and superconducting magnetic readout of spin orientation. Drag-free spacecraft control reduced unwanted forces. A crucial challenge was separating the tiny relativistic drift from classical torques, including electrostatic surface-patch effects and changing gyroscope orientation.^[ref]

The 2011 final analysis reported drift components $-6601.8\pm 18.3$ and $-37.2\pm 7.2$ milliarcseconds per year, compared with predicted geodetic and frame-dragging values $-6606.1$ and $-39.2$. The signs depend on the stated sky-coordinate convention. Dividing uncertainty by the measured magnitude shows why the experiment tested the much larger geodetic effect more precisely than the smaller frame-dragging effect.^[ref]