In concept only, the way to do this is to have or create a large mass of individual rocks, each of which can be animated moving in a single direction. The camera must follow the direction of the moving mass. During the sequence of the animation, individual rocks on the outer edges of the mass should have their animation slowed or stopped altogether. This gives the effect of portions of the comet peeling away during the movement. The rest of the work is in applying appropriate materials and glows.