Evolution of the rotational state of irregular cometary nuclei

A simplified thermal model has been used to calculate the non-gravitational forces acting on small irregular nuclei in the orbit of Comet 46P/Wirtanen. The torque of the non-gravitational force has been calculated and the Euler equations have been solved in order to investigate the rotational evolution of several irregular nuclei during a single orbital step. Several initial spin axis orientations and activity patterns on their surfaces have been considered. The nuclei considered have a mean radius of 1 km and their inertia moments have been calculated assuming a homogeneous bulk density of 500 kg/m3. In all the simulations, the initial spin period is 6h and the nuclei initially rotate around their shortest axis. Under these assumptions, significant changes in the angular momentum and in spin period have been obtained in all the simulations, but the nucleus is found to practically remain in its spin state of lowest energy during the entire orbital period.