LISA Pathfinder test mass injection in geodesic motion: status of the on-ground testing

The LISA Technology Package (LTP) onboard the LISA Pathfinder mission aims to demonstrate, in orbit, several critical technology milestones for LISA, including the purest geodesic motion ever achieved for a macroscopic body. The gravitational reference sensor in the LTP hosts a heavy test mass (TM) surrounded by electrodes, at a relatively large ‘gap’ distance of several mm, which are used to measure and control the TM position and attitude. The large gaps-–necessary to minimize the force noise acting on the TM-–limit the available level of electrostatic actuation force that can be applied to the TM and thus the authority to control its position and velocity. Due to the large mass and gaps, a caging mechanism is required to securely hold the TM during the launch phase, when the whole payload endures large accelerations. Later in orbit, the TM must be injected into its geodesic trajectory, through the release from the caging mechanism and subsequent capture by the electrostatic actuation. During the release phase, the constraining device must limit adhesion forces that exert a net impulse upon rupture, such that the required forces needed to control the TM do not exceed the actuation authority. The TM injection into geodesic motion, and most critically the release phase, constitutes a potential point of failure for the mission. The on-ground verification of this phase is performed by measuring the momentum transferred between TM-representative surfaces and the release device, reproducing the dynamics that will take place in flight. This paper reports on the testing activities performed at the Department of Mechanical and Structural Engineering of the University of Trento.