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 effective. This is especially true if one considers the unknown fight environment when the spacecraft is first encountering the body. Most asteroids and comets have a very irregular shape and unknown mass distribution. Therefore, knowledge about its irregular gravity field will be directly beneficial as input to orbital corrections and manoeuvre planning. This paper addresses the estimation process of gravity-field 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 filtering it is shown that all degree and order coefficients up to degree 8 can be estimated with an error below 10%. This is the first step towards an autonomous navigation system that can operate in a highly-perturbed environment close to the asteroid.