The Satellite Test of the Equivalence Principle (STEP) is a proposed (as of 2014) space science experiment to test the equivalence principle of general relativity. The experiment is thought to be sensitive enough to test Einstein's theory of gravity and other theories. The basic configuration is that of a drag-free satellite where an outer shell around an inner test mass is used to block solar wind, atmospheric drag, the Earth's magnetic field and other effects which might disturb the motion of a freely-falling inner object. It is designed for an expected sensitivity of one part in 1018.