Microthruster-Based Control for Precision Pointing of Next-Generation Space Telescopes

The next generation of space telescopes, such as the Habitable Exoplanet Observatory mission concept (HabEx), are expected to have milli-arcsecond-level pointing requirements at the spacecraft level. The pointing performance of such telescopes tends to be driven by internal rather than external disturbances, in particular reaction wheel jitter. This paper considers an architecture that relies on microthrusters instead of reaction wheels for spacecraft fine pointing, focusing on the HabEx mission concept as a case study. The proposed implementation uses micro-Newton-class colloidal thrusters that were tested in orbit as part of the ST7 payload on the LISA Pathfinder mission to reach Technical Readiness Level (TRL) 7. Key characteristics of these thrusters and the associated system trade-offs are first discussed. The resulting pointing control loop design is then described. Its ability to meet applicable pointing requirements is shown using three-degree-of-freedom simulations, where relevant disturbances are modelled. The control system is shown to reduce the pointing error by several orders of magnitude in about two hours, starting from coarse reaction control system (RCS) handover conditions. Once settled, the system meets the steady-state milli-arcsecond-level performance requirements with significant margin. Overall, the results show that microthruster-based pointing control architectures offer a promising alternative to traditional reaction-wheel-based designs for space telescope applications.