The shape of asteroids can at best be described as irregular. However, for certain target opportunities, it is often that a complete characterization the shape may not be necessary for the purpose of mosaicking. In case of slow spinning objects, a simple rectangular bounding is sufficient. Eigenvectors of the scatter matrix from the boundary points of an object can be used to determine the orientation of the bounding rectangle. These eigenvectors correspond physically to the directions about which the 2D projection of the object has maximum and minimum moments of inertia. An optimal mosaic size can then be determined from the aspect ratio of the bounding rectangle, and the size of the rectangle can be used to assist us in determining the starting mosaicking time. In a simple asteroid flyby scenario which the spacecraft travels in a linear trajectory with constant speed, the apparent size of the asteroid can be parameterized in a closed form. The parameter estimation can be solved by a least- squares fit using the size information derived from images taken when the angular diameter of the asteroid is less than the camera's field of view.
David Q. Zhu,
"Characterization of irregularly shaped bodies", Proc. SPIE 2466, Space Guidance, Control, and Tracking II, (12 June 1995); doi: 10.1117/12.211501; https://doi.org/10.1117/12.211501