A Feature Tracking and Trajectory Selection Based Rotation Axis Estimation Method for Small Bodies Using Optical Remote Sensing Images From the Approach Phase

Determining the rotation axis of small bodies during the approach phase is essential for both mission operations and scientific investigations. Estimating the axis from the motion trajectories of image features has proven effective, but challenges remain due to limited image availability, weak surface textures, and uncertain observation geometries. In particular, tracking errors, unreliable trajectories, and dependence on accurately known rotation periods reduce the robustness and efficiency of existing methods. To address these challenges, this study proposes a rotation-axis estimation method for small bodies during the approach phase, based on image feature tracking and trajectory selection. The method employs sparse optical flow to extract feature trajectories and removes unstable tracks using image masks and bidirectional flow. An adaptive trajectory selection and shape classification are then performed based on the statistical distribution of fitted parameters using the histogram. Finally, a geometry-based optimization model identifies the correct rotation axis solution via a genetic algorithm, without requiring prior knowledge of the rotation period. The proposed algorithm was tested on over 400 simulated cases considering varying sun phase angles, approach angles, image numbers per rotation period, and small body shapes. The results demonstrate that the proposed method significantly outperforms the existing algorithms. The proposed algorithm achieved estimation errors below 3° in 89% of the cases and below 5° in 92% of the cases, and the running time of all the cases was less than 3 min. Validation using in-orbit data from the OSIRIS-REx mission confirmed that the proposed algorithm can estimate the rotation axis of asteroid Bennu with an error of only 2.69°. The results validate the proposed algorithm's effectiveness and efficiency, proving its potential for small body exploration missions.