Gravity Probe B was a satellite launched by NASA in 2004 to test two predictions of Albert Einstein's general theory of relativity. It aimed to measure the effects of frame-dragging and geodetic precession caused by the Earth's gravity on the spin of gyroscopes onboard the spacecraft. The mission involved four ultra-precise gyroscopes that were monitored for changes in their orientation as they orbited the Earth. The results confirmed Einstein's predictions, providing strong evidence for the effects of gravity on the fabric of space and time, and enhancing our understanding of the universe.