The long-awaited launch of Gravity Probe B, a satellite designed to provide two new tests of general relativity, took place on 20 April. Carrying four ultra-precise gyroscopes, the satellite should provide the first direct measurements of the geodetic effect and the frame-dragging effect. The geodetic effect is a change in the direction of the spin axis that occurs as the gyroscope follows the curvature of space-time near any massive object. The frame-dragging, or Lense-Thirring effect, first predicted in 1918, is caused by the Earth's rotation effectively pulling space-time along with it as it turns.
During its mission, Gravity Probe B will monitor any drift in the gyroscopes' spin axes alignment in relation to a guide star. The size of the frame-dragging effect is miniscule. In a polar orbit of 640 km, it should make the spin axis change by just over 40 milliarcseconds in one year; in the same time period, the geodetic effect will produce a relatively huge change of more than 6600 milliarcseconds. Gravity Probe B will measure the minute frame-dragging effect to a precision of 1% or better and the geodetic effect to one part in 10,000.