Electrostatic accelerometers for the equivalence principle test in space

The concept of the three-axis electrostatic accelerometers based on the full electrostatic suspension of one unique proof mass is very suitable for space applications requiring very high resolution of acceleration measurement or drag-free control of satellite. This concept has been tested in orbit with the accelerometer CACTUS from ONERA in the late seventies and recently with the accelerometer ASTRE on board Columbia shuttle in June 1996. The accelerometer outputs are derived from the measurement of the electrostatic forces, necessary to maintain the mass motionless at the centre of the accelerometer cage. The relative test-mass position and attitude are servo-controlled from measurements of capacitive sensors exhibiting resolutions of better than 10(-10) mHz(-1/2) depending on the geometrical configuration. The test of the weak equivalence principle can be performed in orbit on board a drag-free satellite with two concentric electrostatic accelerometers including two cylindrical test masses made of different materials. The measured common acceleration is controlled to null along the three directions by the drag compensation system of the satellite. The differential acceleration is detected at the orbital frequency (or around the satellite spin frequency) along the common revolution axis with an expected resolution of 10(-14) ms(-2) Hz(-1/2). The differential disturbing acceleration induced by magnetic, electric and thermal disturbances must be limited to this value thanks to the 4 K environment of the sensor-head. The present definition of such an instrument is presented and the expected performances are detailed.