Autonomous nucleus tracking for comet/asteroid encounters: the Stardust example

Stardust is a mission to fly by Comet Wild-2 in early 2004 and return samples of the coma to Earth. During its 120-150 km flyby of the comet nucleus, a secondary science goal is to obtain images of the nucleus using the onboard navigation camera. Due to the 40 minute round-trip light time, ground processing of navigation data to update pointing information to maintain the nucleus in the camera field-of-view is impractical. Thus, a simple, reliable, and fast algorithm was developed to close the navigation loop onboard during encounter. The algorithm uses images of the nucleus during approach to update target relative state information. This involves centroiding on the image to obtain nucleus center-of-figure data and then processing the data through a Kalman filter to update the spacecraft, position and attitude. Monte Carlo simulations were then performed to test the algorithm. These simulations incorporated errors in spacecraft initial position and in attitude knowledge to provide a "truth" model which the filter must recover from. The results of the simulations proved that the algorithm was successful in maintaining the nucleus in the camera field-of-view assuming nominal values for the error sources. Even with worst case errors, the algorithm performed successfully in over 90% of the cases.