Initial Pole Axis and Spin Direction Estimation of Asteroids Using Infrared Imagery

Knowing the pole axis of an asteroid is vital to autonomous asteroid exploration efforts. Ground-based initial pole estimation methods are time and data intensive and produce estimates with large uncertainties. These errors have a significant impact on proximity navigation, shape modeling, and scientific data for small body missions. In this paper, a new method of obtaining this information from onboard spacecraft imagery is presented. The proposed method estimates the pole from onboard infrared imagery using the camera-asteroid geometry. This method does not require a prior and is designed to work in a vast majority approach trajectories due to the use of infrared images. The method is applied to simulated infrared images of asteroids 101955 Bennu and 25143 Itokawa as well as real infrared images of asteroid 162173 Ryugu from the Hayabusa2 mission. The average pole errors using this method on Bennu and Itokawa images are approximately 2 and 6 deg, respectively. The pole estimate error on the Ryugu images is approximately 8 deg. The algorithm is shown to be sensitive to the percentage of spin period imaged and the spacing between the images.