1. sketch graphs of the momenta of asteroids a and b before the collision.
2. during the collision, is the magnitude of the force of asteroid a on asteroid b greater than, less than, or equal to the magnitude of the force of asteroid b on asteroid a?
3. sketch a graph of the total momentum in the system of the two asteroids after the collision.

First, there is no external force acting on the asteroids ( I don't think so, since they are travelling at constant velocity). Since there is no external force acting on the individual asteroids, their respective momenta are constant in time as well. So, in your momentum v/s Time graph, the momenta for each asteroid will correspond to a straight line parallel to time axis. This happens until they collide. On collision an impulse acts on each asteroid ( by Newton's third law, the same magnitude of impulse acts on the two bodies). Due to this impulse, their respective momenta will change. You just need a little mathematics to figure out the respective momenta after the collision. Keep in mind there is no external force on the system, so after collision, the asteroids would again move with constant velocities. (Conservation of linear momentum of the system would help! You just need another equation to find out the velocities)