1 NEIGHBORING OPTIMAL GUIDANCE FOR SOFT LUNAR LANDING

This paper presents a general-purpose neighboring o ptimal guidance algorithm that is capable of driving a space vehicle along a specified nominal, optimal path. This goal is achieved by minimizing the secon d differential of the objective function along the perturbed trajectory. This minimization principle leads to deriving all the corrective maneuvers, in the conte xt of a closed-loop guidance scheme. Original analytical developments, based on optimal control theory, constitute the theoretical foundation for two relevant features: (i) a new, efficient law for the real-time update of the time of flight (the so called time-to-go), and (ii) a new formulation of the sweep method. Some ch allenging but nevertheless promising projects have the purpose of building a s table lunar base for future interplanetary missions. For soft lunar landing, the nominal trajectory is represented by the minimum-time path departing from the periselenium of a given elliptic orbit and arriving at the Moon surface. Pert urbations arising from the imperfect knowledge of the propulsive parameters and from errors in the initial conditions are considered. Extensive Monte Carlo te sts are performed and definitely prove the effectiveness, robustness, and acc ur y of the neighboring optimal guidance, also in comparison with the well-es tablished linear tangent steering law.