The LISA DFACS: effects of micrometeoroid impacts in the drag-free mode

LISA is a space-based gravitational wave observatory under study and prototyping by the European Space Agency and other institutions/companies. It consists in a triangular constellation of three spacecraft traveling in heliocentric orbits, connected through bi-directional laser links in order to detect/measure gravitational waves by means of interferometry. In the science mode (also called drag-free mode), each spacecraft compensates for the disturbances and noises affecting the control loops by performing tiny adjustments around a suitable working point. Micrometeoroid streams may collide with the spacecraft surface and generate impulsive forces and torques on the spacecraft body. Such impulsive disturbances determine attitude perturbations, which in some cases cause the incoming laser beams to move outside the optical sensor ranges. In these cases, the laser links are lost and interferometry cannot be performed anymore. Link recovery may be accomplished by performing the maneuver conceived for the initial constellation acquisition by this maneuver may take a long time (several hours), implying a significant reduction of the science time. In this paper, the effects of micrometeoroid impacts on the LISA spacecraft in the drag-free mode are evaluated and a fast recovery control strategy is proposed to quickly return in science/drag-free mode.