Prudent small celestial body landing strategy with risk precautions

In small body landing missions, the nominal landing scheme may turn out to be inapplicable in reality due to the limited prior information of the target and possible system errors after long-term operation. In this paper, a partial breakdown of the main thruster in the presence of large state deviations and complex terrain conditions at the landing site is considered. In order to ensure lander safety, a risk-precautionary landing strategy that identifies landing anomalies, generates onboard warnings, and plans an emergency transfer is proposed. First, lander safety is repetitively evaluated during descent through a computation of onboard deceleration capability and landing site safety. Upon a predicted landing risk, an emergency mode is activated. Three emergency modes are developed for the strategy, including an orbiting mode, a diverting mode, and a hovering mode. For each mode, a customized goal state is designated and a corresponding emergency transfer trajectory is generated in real-time. At last, an asteroid 433 Eros-based landing scenario is designed, and the effectiveness of the strategy is verified under three different thruster and terrain conditions.