Gravity Probe B A US mission to test two predictions of Einstein's general theory of relativity, launched Apr. 2004. The mission, developed jointly by NASA and Stanford University, aims to verify these predictions: it will seek to assess (1) how the presence of the Earth warps space and time and (2) how the Earth's rotation drags space and time around with it. The effects of the Earth on space and time are minute and have hitherto gone undetected and therefore unmeasured. The Gravity Probe B is designed to measure them for the first time. It will seek to do so using four gyroscopes housed within an artificial satellite orbiting directly over the Earth's poles at an altitude of 640 km. Free from external disturbances and cooled to near absolute zero to eliminate disruptions caused by their own molecular structures, the gyroscopes are near perfect spheres spinning in a vacuum and designed to provide direction measurements that are 30 million times as accurate as any gyroscope ever built before. If Einstein's predictions are correct, the gyroscopes should help to detect that very small amounts of space and time are missing from each orbit. In order to measure each orbit accurately, the gyroscopes are lined up with a guide star by means of a tracking telescope, and a magnetic-field measuring instrument records any changes with respect to the guide star. The mission was scheduled to run for a total of 18 months, including 16 months of data collection. It is set to end in late 2005.