The Role of Friction in the LISA-Pathfinder Release Mechanism Anomaly

Abstract

Release mechanisms are crucial devices for the success of space missions. In LISA, the first gravitational waves observatory in space, the grabbing positioning and release mechanism (GPRM) is responsible for setting all the test masses (TMs) in the free-fall condition necessary for starting science observations. This mechanism is considered a key equipment for the success of the entire space mission. The GPRM was initially tested in LISA Pathfinder (LPF), a space mission flown between 2015 and 2017, and an anomaly was encountered in the TM velocity after the release, which also resulted in some failures. One of the suspected reasons of this behavior is that the trajectory of the release mechanism deviates from the nominally rectilinear one. We find that this anomaly is due to the presence of high and asymmetric friction between moving parts. In this paper, we characterize experimentally the deviation from the linear trajectory, using LPF qualification models of the mechanism available on ground, and we identify the mechanical features that determine the friction-driven spurious lateral motion via a numerical finite element model. The results are one of the basis to improve the design of the GPRM for LISA. Some risk-reduction strategies and design guidelines are also presented and discussed.