I don't understand the comment about comets/asteroids. Why would they be a viable way for interstellar travel? In order to catch one, you need to match speed with it, and then you are already on your way. You can't somehow hitch a ride with interstellar comets/asteroids and avoid using energy.
You can use them for gravity assist, and save on propellant. You can't literally "hitch a ride", but the next best is to steal some of their momentum, which is precisely what a gravity assist does.
