Propulsion systems for a starshade rendezvous mission with the Habitable World Observatory

We assess how propulsion and operational constraints shape the exo-Earth characterization performance of a starshade rendezvous architecture for a Habitable Worlds Observatory (HWO)–like telescope at Sun–Earth L2. Using end-to-end Monte Carlo mission simulations with EXOSIMS, we model two representative starshade point designs: a 60 m occulter optimized for visible-band (VIS) spectroscopy (500 to 1000 nm) and a 35 m occulter optimized for near-ultraviolet (NUV) characterization (250 to 550 nm). For the 60 m case, we compare three electric-propulsion options spanning Hall and gridded-ion technologies (AEPS Hall, NEXT ion, and a Starlink-class Hall system) and quantify yield sensitivity to onboard propellant capacity, retargeting trajectory “burn-portion” (accelerate and decelerate fraction of slew), and the presence or absence of on-orbit refueling. In fuel-limited scenarios (1000 to 3000 kg initial propellant), the higher-Isp NEXT system delivers the largest characterization yield, providing 9% to 13% more characterizations than AEPS and 13% to 16% more than the Starlink-class option by enabling more retargeting slews per kilogram of propellant. When refueling is allowed, cumulative yields converge across thruster choices (about 120 cumulative characterizations over a 10-year campaign), while the propulsion choice primarily trades against the number of refueling events. For the 35 m near-UV starshade (modeled with AEPS), yields are substantially less fuel-sensitive and are instead dominated by mission time and geometric accessibility; tightening the minimum solar keepout (scarf) angle from 40 deg toward ≥50 deg barely impacts yield, from 50 deg to 65 deg modestly impacts yield, and ≥65 deg reduces yield rapidly, with severe degradation near 80 deg. These results identify regimes where starshade performance is fundamentally fuel-limited versus time/keepout-limited and provide quantitative guidance for jointly optimizing propulsion selection, tank size, refueling policy, and keepout constraints in HWO starshade architectures.