Since comets behave predictably(they'll always stay on course, unless someone bumps into them), their location can be determined at any time, if the velocity, mass and mass of the star are known at one time, and the current time is also known. All of this can be achieved easily. If they clip through a station or planet while the sector is not loaded, I doubt many would notice or complain :p
Comets do not stay on course (if you mean a straight line). The course bends depending on the distance from heavy bodies, like stars. So calculating the new location when needed will be a complex set of interactions between stars, planets, and possible collisions in between due to stations.
I never understood what actually rotates when dealing with planets, but depending on time, a planet has a different location and thus interacts differently to the comet's trajectory.
Predictability? Out the window.
Another thing, where do these comets start?
When should the server despawn a comet?