I know most of you have never seen SeaQuest, but this comes to mind:
Politician: How much would it cost to get me to [insert generic place here] 2 hours early?
Captain: About $2 million.
Politician: That's ok, we'll take it out of the space budget
Don't forget the cost of moving the rocks... having a small heat-shielded bomb with a huge payload would be much easier than towing in a multiple-kilometer-wide rock and dropping it on a planet, in terms of time and fuel. Now FS2 ignores fuel, and subspace makes moving big things seem pretty easy, but the point still holds. The Harbringer was designed for planetary bombardment after all, so we have to assume that orbit-to-surface munitions existed up through FS1 time at least.
Sorry Mikhael, I couldn't resist

Oh, and that "render" rocks!