Gravity Probe B satellite (GP-B). Composite image of the GP-B satellite in Earth orbit over Florida, USA. This NASA probe, built to test Einstein's General Theory of Relativity, was launched on 20 April 2004. The main part of the probe is a 2.7- metre-tall central vacuum flask that keeps four gyroscopes at very low temperatures. Just under 4 centimetres wide, each gyroscope houses a spinning metal sphere. A telescope (shield at upper centre) will position the probe using a guide star. 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. Power comes from 4 solar panels (blue).