STEP (satellite)
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In physics, the Satellite Test of the Equivalence Principle is a planned (as of 2005) space science experiment to test the equivalence principle of general relativity. The experiment should be sensitive enough to distinguish Einstein's theory of gravity from many other competing theories.
The basic configuration is that of a drag-free satellite: 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 satellite.