Technical challenges of satellite test of the equivalence principle mission

Abstract

The Satellite Test of the Equivalence Principle (STEP) experiment is being proposed to improve our knowledge of the equivalence of inertial and gravitational mass by a factor of 10 6, yielding a crucial test of one of the fundamental postulates of Einstein's theory of general relativity. The experiment will use six differential superconducting accelerometers in a spacecraft in low Earth orbit. The accelerometers will each contain two proof masses of different densities. If the equivalence principle is violated, the masses will oscillate with respect to each other at the orbital frequency. The mission will last six months and is planned for launch in 2000. The entire experiment must be cooled to 1.8 K and must be held to this level within 1 mK per orbit. A 200 dm 3 superfluid helium dewar will maintain this environment. A crucial requirement is that any disturbances in the orbital frequency must generate gravitational variation signals much less than the expected signal. One source of such disturbance is the motion of the liquid helium in the local gravity gradient field, which rotates at orbital frequency with respect to the spacecraft. An electrostatic confinement system has been proposed for this purpose. This system will be described. A back-up system using the superfluid fountain effect will also be discussed.