A special NASA spacecraft designed to test two aspects of Einstein's General Theory of Relativity, Gravity Probe B, lifted off Tuesday from Vandenberg Air Force Base on board a Boeing Delta II rocket. The spacecraft was inserted into a perfectly circular polar orbit, and operators will begin calibrating its instruments over the next 60-days. If everything checks out, the spacecraft will begin making precise measurements about the effect of the Earth's gravity for 12 months - analysis of the data will take a further year.