Development status of the differential accelerometer for the MICROSCOPE mission Original Research Article

Tests of the Equivalence Principle are essential to fundamental physics theories, but as performed previously with torsion balances and laser ranging they have been limited by the vibrations inherent to any Earth-based environment. The MICROSCOPE mission will take advantage of the space environment to extend the EP test accuracy to 10−15, by placing two masses of different materials in a drag compensated orbit. A violation of the Equivalence Principle will appear as a difference in the electrostatic forces necessary to maintain both masses on the same orbit. The satellite will be launched in 2008 and carry as the primary science instrument a differential electrostatic accelerometer. The accelerometer is composed of two coaxial cylindrical proof masses surrounded by silica cages, in a vacuum housing. The two proof masses, one in platinum–rhodium within one in titanium, are maintained at the centre of their cages using electrodes engraved in the silica cage. These electrodes are used to capacitively sense the proof-mass position and to apply electrostatic forces to control the position. The accuracy and stability of the silica cage is therefore essential to the quality of the EP test. This paper presents the current design of the accelerometer, specifically the critical areas for the instrument design, integration, and final performance requirements. Also discussed is the status of the analytical and theoretical models, as well as the experimental investigations, which are developed to overcome these critical areas.