Deep Impact/Fridge


 * Fridge Logic: The Messiah has an Orion Propulsion System and was assembled in space. It's way too small to be one. (Orion was a military project to build a deep space craft in the late 50's, it works by exploding nuclear bombs behind a very thick metal plate and getting hit by the explosion. Most designs are assembled on the Earth and are about 1 mile in Radius).
 * Further, if it was an Orion Craft, which could achieve the course required by the ship to not only reach the comet, but match course with it, there would be a heck of a lot more nuclear weapons on the ship. No reason to not simply try again at altering the course of the comet fragments.
 * Other than running out of personnel, you mean?