Development of a differential accelerometer to test the equivalence principle in the microscope mission

Abstract

A violation of the Equivalence Principle (EP), which hypothesizes the equality of inertial mass and gravitational mass, is indicated by current theories in modern physics. The MICROSCOPE mission seeks to extend the accuracy of previous EP tests to 10 - 15 , by avoiding the disturbances inherent to every Earth based test facility. The test will involve the measurement of the electrostatic forces required to maintain two concentric masses on the same orbit. The satellite, to be launched in 2008, will carry two differential accelerometers, one with masses of platinum and titanium, and a second with two platinum masses for baseline measurements. Each accelerometer will contain two coaxial cylindrical proof masses, each encompassed by a silica cage, all in a vacuum housing. The capacitance between electrodes etched into the silica, and the surface of the gold-coated proof masses provides a measurement of the proof mass position, which is then controlled by adjusting the voltages applied to the electrodes. Because an EP violation will appear as a difference between the forces required to keep each mass centred, the quality and stability of the silica cages is essential to achieve the desired test accuracy. This paper presents the overall design of the accelerometer, focusing on areas critical to the instrument core design, integration, and final performance requirements. The models and experimental investigations designed to overcome these issues are also discussed.