A SUPERCRITICAL SUPERFLUID-HELIUM CRYOSTAT FOR GEOSTEP - PRELIMINARY-STUDY

The test of the equivalence principle to one part in 10(17) requires t he use of a drag-free spacecraft to shield free-falling test masses fr om any environmental disturbances. Potential violations of the equival ence principle are detected by very small relative displacements of th ese test masses measured by high-sensitivity differential acceleromete rs. The accelerometer assembly is contained in a superfluid helium cry ostat. The cryogenic temperature (about 2 K) is required to provide a high degree of mechanical (i.e. thermal) stability, to provide superco nducting shielding against electromagnetic radiation and to ensure an ultra-high vacuum environment reducing gas-molecule impacts. For GEOST EP, electrostatic suspension and capacitive sensing of the test masses are baselined. Helium cooling may also allow for the use of supercond ucting magnetic suspension and sensing. The cryogen boil-off also prov ides a source of propellant for the proportional gas thrusters used fo r the drag-free and attitude control.