Launch of Gravity Probe B satellite (GP-B). This NASA probe was built to test Einstein's General Theory of Relativity. It was launched on a Boeing Delta 2 rocket at Vandenberg Air Base, California, USA, at 1657 GMT on 20 April 2004. The ascending rocket is leaving a trail of exhaust gases. The probe contains four gyroscopes, each just under 4 centimetres wide, and each containing a spinning metal sphere. The probe will orient itself with a guide star using a telescope. During the year the probe spends in its polar orbit, it is predicted that the warped space-time around the rotating Earth will drag the gyroscopes out of position. The probe was built by Stanford University, USA.