Autonomous spacecraft navigation and control for comet landing

The autonomous guidance, navigation, and control operations during the landing phase of the Comet Nucleus Sample Return mission are described and the associated performance of the onboard system is analyzed in terms of landing accuracy and relative velocity at touchdown. Two descent scenarios are developed and investigated: a slow descent strategy that optimizes the operation of an onboard laser mapper and a fast descent strategy that relies only on microwave measurements for navigation. It is demonstrated by means of Monte Carlo simulations that the first scenario provides a more accurate landing, whereas the second one is a suitable alternative to the case where the complex laser system has failed. Equatorial and polar descent orbits to a landing site on the comet equator are considered. Models of the comet, spacecraft dynamics, and instrument measurements are described, and the main features of the onboard algorithms are outlined. These models and algorithms are incorporated into a simulation software that is used to generate typical time histories of the spacecraft dynamical state during the landing phase.