Since the equivalence principle is a fundamental postulate of Einstein's general theory of relativity, a precise test is of great scientific interest. An improved test of the equivalence of inertial and gravitational mass by a factor of 10 6 is now possible, using superconducting measurement technology in an earth-orbiting spacecraft. Superfluid liquid helium must be used as a cryogen to maintain the temperature of the four gravimeters at 1.8 K. Because of the extreme sensitivity of the measurement, the gravitational disturbance caused by the motion of the liquid helium in the local gravity-gradient field is of concern. It is proposed to use electrostatic forces to provide the necessary control. We present a description of the design and performance of a proposed flight system.
