Gravity-Field Estimation of Asteroids

With the increasing interest in the Solar System's smaller bodies, quite a few missions have been sent to comets and asteroids, and more will be send in the near future. Due to the large distances involved, communication to command mission parameters takes a long time, which has a negative impact on operational safety. Autonomous navigation would be one of the key technologies that can make the mission more robust, safe, and cost e ective. This is especially true if one considers the unknown ight environment when the spacecraft is  rst encountering the body. Most asteroids and comets have a very irregular shape and unknown mass distribution. Therefore, knowledge about its irregular gravity  eld will be directly bene cial as input to orbital corrections and manoeuvre planning. This paper addresses the estimation process of gravity- eld parameters that could potentially be implemented in an autonomous navigation system. The focus is on a spherical-harmonic modelling of asteroid Eros-433, most notably outside the Brillouin sphere where the validity of the model is guaranteed. By using Kalman  ltering it is shown that all degree and order coe cients up to degree 8 can be estimated with an error below 10%. This is the  rst step towards an autonomous navigation system that can operate in a highly-perturbed environment close to the asteroid.