A Newly Identified Disturbing Effect in the LISA Pathfinder Test Mass Release into Geodesic

In LISA (Laser Interferometer Space Antenna), the first observatory of gravitational waves from space, a Grabbing, Positioning, and Release Mechanism (GPRM) is designed to inject a 1.96kg test mass (TM) into a geodesic trajectory, with residual velocities (with respect to the hosting spacecraft) below 15μm s−1(linear) and 500μrad s−1(angular). During LISA Pathfinder (LPF), a technology demonstrator mission flown by ESA in 2015, several injections exceeded these requirements due to unintended recontacts between the mechanism main end-effector (plunger) and the released TM. This was caused by the erosion of the micrometric gap between the TM and the plungers in the preliminary maneuvers, preparatory to the nominal TM injection to be performed by dedicated end-effectors (release tips). We identify premature TM releases, evidenced by TM displacements and rotations non compatible with the mechanism holding action, taking place before the nominal injection instant, i.e. the separation of the release tips from the TM. At the same time, an interfering fictitious force signal, generated during the release tip extension, can mimic the presence of contact between the tips and the TM, producing conflicting diagnostic signals (i.e. false confirmation of held TM). Ground tests and FEM analyses show that this interference originates from the plunger deformation near the force sensing element. Using the LPF telemetry and a multibody model including the origin of the interference, we reproduce the in-flight LPF release tests signals and trace the root cause of the premature releases. Our results highlight the opportunity to integrate force sensing into the control loop during the TM handover from plungers to release tips, and identify the mechanism stiffness required to suppress the fictitious forces for the LISA GPRM. We also outline improvements to the release procedure to enhance its reliability in the upcoming LISA mission.