ZEM/ZEV feedback guidance application to fuel-efficient orbital maneuvers around an irregular-shaped asteroid

The new vision for advanced missions to asteroids, including soft landing, presents many challenges that have essential differences from previous experiences with planetary landing. This paper focuses on two subjects pertaining to asteroid proximity operations: highaccuracy modeling of the gravitational environment and fuel-efficient guidance and control algorithm design. Both a spherical harmonic expansion method and a polyhedron shape model are used for modeling the gravitational environment of an irregular-shaped asteroid. The effects of Coriolis and centripetal accelerations are also examined. The ZEM/ZEV (Zero-Effort-Miss/Zero-Effort-Velocity) feedback guidance algorithm is in general not an optimal control scheme, however it is conceptually simple and easy to implement, and in many cases it approaches optimality. Two mission phases, orbital transfer between observational orbits and soft landing, are numerically simulated using different implementations of the ZEM/ZEV algorithm. These simulations show that the ZEM/ZEV algorithm is suitable for asteroid proximity operations, and important considerations for using the algorithm are discussed.