AbstractThe Equivalence Principle, which states that gravitational mass and inertial mass are different ways of measuring the same property, is the experimental foundation for modern gravitational theory. Modern theory cannot reconcile gravity with the other fundamental forces. One possibility is that the present theory of gravity, general relativity, is incomplete or incorrect. If this is the case, the Equivalence Principle may be violated at some level beyond the one part in 10¹² that has been experimentally verified. The goal of the Satellite Test of the Equivalence Principle (STEP) is to improve this result to better than one part in 10¹⁸. In STEP two or more concentric, cylindrical test masses “fall” around the Earth in a drag-free satellite. The masses are cooled to < 2K and supported by frictionless, superconducting, linear bearings. Ultra-sensitive SQUID position detectors measure their relative motion. A confirmed violation could provide key information toward the possible unification of the four fundamental forces; conversely, a null result can rule out competing theories or constrain new theories. MiniSTEP is the least expensive mission proposed to date that still achieves STEP's basic scientific goal. It compares four different materials in four differential accelerometers. Cost saving is achieved by operating the accelerometers sequentially, thereby avoiding duplicated equipment. The drag-free satellite will be built around an existing superfluid helium dewar and a small, semi-production spacecraft. A small commercial launch vehicle will place the satellite in a 400 km, sun-synchronous, polar, Earth orbit. Nominal mission lifetime is four months - but a six to eight month life is expected.