The Gravity Probe B spacecraft, which is designed to test two predictions of Einstein’s General Theory of Relativity, has been scheduled to launch on April 17. The spacecraft will use four precise gyroscopes to determine how space and time are distorted by the gravity of the Earth and its rotation. The spacecraft will orbit the Earth once every 90 minutes, and gather data for more than a year, comparing any drift in its gyroscopes to the position of a guide star.