Robust Interplanetary Trajectory Design for Missed Thrust Events using Reachable Sets with Solar Electric Propulsion and Solar Sail

Low-thrust propulsion systems are widely used in deep-space missions but are vulnerable to missed thrust events, which can cause significant trajectory deviations and mission failure. Existing robust trajectory design methods often incur prohibitive computational costs or rely on conservative heuristic margins, limiting their applicability to complex missions. This study proposes a computationally efficient robust trajectory design framework for spacecraft equipped with hybrid solar electric propulsion and solar sails by explicitly incorporating reachable set analysis into low-thrust trajectory optimization. A quasi-analytical reachable set is computed by backward propagation from the destination, and post-MTE recoverability is enforced by evaluating whether the deviated state lies within this set. The reachable set boundary is approximated using a convex hull and formulated as a differentiable signed distance function, enabling direct integration into the optimization problem while substantially reducing the number of optimization variables. Conditions under which the convex hull approximation remains valid are also identified. Numerical simulations for an interplanetary mission employing a one-year synchronous Earth-return orbit demonstrate that the proposed method significantly enhances robustness against MTEs while maintaining computational tractability.