Guidance and Control design for CubeSat small body landing using discrete firings of sliding mode control

Small body exploration has seen a renewed interest in recent years. Landing a spacecraft on the body's surface enhances the mission's scientific return, yet it is a high-risk operation. CubeSats offer a cheaper alternative to bigger spacecraft, and when used in tandem with a mothercraft, they may perform risky operations without exposing the mission to a complete loss. Nonetheless, their small size as well as their limited operational and computational capabilities pose hard limitations to their efficient exploitation. Historically, powered and controlled landing has often been key to soft landing success. In an asteroid complex dynamical environment, manoeuvre execution benefits from a dedicated Guidance and Control (G&C) scheme that is robust to dynamical uncertainty, autonomous and reliable. To robustly and reliably soft-land a CubeSat on a small body, we propose a novel Guidance & Control strategy. It alternates closed-loop control segments using the existing Multiple Surface Sliding Guidance algorithm with intervals of ballistic trajectory. We show that very good landing performance in terms of landing ellipse, collision velocity and fuel consumption can be achieved. The method has been tested for Hera's Milani CubeSat landing attempt on Dimorphos, the moon of (65803) Didymos binary asteroid, using Monte Carlo analysis. The Guidance & Control strategy is shown to comply with stringent requirements for targeted soft landing.