Micro-thruster ACS Architecture for Precision Pointing of 6-meter Exo-Earth Imaging Space Telescope

The ambitious science goals of the Astro2020 Decadal Survey to conduct direct imagery and spectroscopy of Earth-like exo-planets will dictate a stringent pointing stability requirement for the Habitable Worlds Observatory (HWO). Sufficient starlight suppression is achieved through high-contrast coronagraphy, which drives HWO to reach an order-of-magnitude improvement in pointing stability over state-of-the-art precision pointing architectures based on reaction wheels paired with vibration isolation systems. Since further improvement in observatory stability is fundamentally limited by wheel-induced vibrational disturbances, one proposed solution is to remove the wheels in favor of a “thrusters-only” ACS approach that utilizes conventional hydrazine thrusters for maneuvers and micro-thrusters for precision pointing. This thrusters-only ACS approach was baselined by JPL for the 4-meter Habitable Exoplanet Observatory (HabEx) Astro2020 mission concept study. HabEx examined the feasibility of using the Busek Colloid Micro-Newton Thruster (CMNT), an electrospray micro-thruster successfully flight-demonstrated on LISA Pathfinder in 2016, to enable a precision pointing L2 Great Observatory. Following Astro2020 recommendations for a 6 meter “IR/O/UV telescope optimized for observing habitable exo-planets and general astrophysics,” JPL has developed a scaled-up “6 Meter Space Telescope” (6MST) design concept based on HabEx. Ongoing JPL research is examining the CMNT when paired with the 6MST to design and evaluate a realistic thrusters-only ACS architecture for HWO based on existing technology. As with HabEx, current ACS design work is focused on developing a time and fuel-efficient ACS architecture, given conservative estimates of the environmental torque disturbance from solar radiation pressure and the maximum thrust capability of the CMNT micro-thrusters. Here we summarize the latest CMNT performance capability, the 6MST observatory reference design, and the proposed thrusters-only ACS architecture and evaluation approach.