The NASA/ESA MiniSTEP project Original Research Article

The Equivalence Principle states that gravitational mass and inertial mass are identical quantities. However, modern theory cannot reconcile gravity with the other three fundamental forces found in nature. One possibility is that the present theory of gravity, rooted in general relativity, is incomplete or incorrect. As a consequence of this, the Equivalence Principle may be violated at some level beyond the one part in ∼1012 that has been experimentally verified. The objective of the Satellite Test of the Equivalence Principle (STEP) is to extend the range of experiment to one part in 1018. This is accomplished by allowing two concentric, cylindrical test masses to “fall” around the Earth in a drag-free satellite. The test masses are cooled to < 2K and are supported by frictionless, superconducting, linear bearings. Relative motion between the test masses is measured by ultra-sensitive SQUID position detectors. The consequences of a confirmed violation will be to provide key information toward the possible unification of the four fundamental forces. A null result may rule out several competing theories and set better limits for new theories. Of the several STEP missions studied, MiniSTEP is the least expensive mission to date that still achieves the basic scientific goals. It uses four pairs of test masses (four differential accelerometers) to compare at least four different materials. An existing superfluid helium dewar, containing the four differential accelerometers, will be used along with a small, semi-production spacecraft to form the drag free satellite. The satellite will be placed in a 400 km, sun-synchronous, Earth orbit by a small commercial launch vehicle early in 2002. The nominal mission lifetime is four months.