Would that not blacken the skies for centuries?

Personally I think it is arrogant to think that we should alter another planet in our solar system, we are already doing a good enough job destroying our own.
Although I am sure that eventually we will move out into space I think that a better way to do it would be to find another planet that can support life and go there before we start destroying other planets in our own solar system.

You should really work on a theory for travel near the speed of light or for creating worm holes instead of theorizing about destroying other planets.

Also Haleys comet is big and travelling very fast, wouldnt running it into a planet possibly alter that planets rotation around the sun which in turn may alter the rotation of other planets ie (Earth).