Two cylindrical masses in orbit for the test of the equivalence principle

Two pairs of solid test-masses have been considered to perform in space the test of the universality of free fall with an accuracy of at least 10~15. These cylindrical masses are precisely at the heart of the MICROSCOPE mission instrument comprising two differential electrostatic accelerometers. These masses shall exhibit material quality, shapes, positions and alignments in regard to stringent experimental requirements. Indeed the space experiment is based on the control of the two masses submitted to the same gravity acceleration along the same orbit at 810 km altitude with an accuracy of 10~n m. Thus effects of Earth and satellite gravity gradients shall be contained as well as any other disturbances of the mass motions induced by their magnetic susceptibility or electrical dissymmetries, by outgassing of the materials or radiation emissivity. Furthermore, the electrostatic levitation of the two masses depends dramatically on the mass shapes and electrical properties in particular for the definition of the sensitive axes orientation. All these aspects will be presented from the mass characteristics to the space MICROSCOPE experiment performance.