anyone that knows anything about orbital mechanics would know that you would have to drive the mass retrograde in order for it to actually hit the plannet, unless of course you put your ship into a freefall first. if youve ever played orbiter, take the delta glider up to a stable orbit, point the nose to the dirt (90 degrees) and throttle up to maximum. you will never hit the plannet! to actually make planetfall you need to point your nose oppisite of your momentum vector and burn. so essentially every planetary bombardment scene in every tv show is completely wrong.
now i did say you can put the ship into a freefall, for this you would need to put your ship into an eccentric orbit, at the peak of which you would burn retrograde to stop your forward velocity. at this point you wouldnt even need to fire your weapons, simply release them, earths gravity will accelerate them just fine. and of course you as well. fortunately sence your orbital velosity is slowest at apogee, it wouldnt take much thrust to return you to your previous orbit, simply burn in the opposite direction that you burned to enter the freefall (and hope you have time to finish it). this of course is a total waste of fuel when all you need to do is fire your ordinance retrograde. it woulda been easyer and more effective just to put engines on some rocks from the asteroid belt and put em on a collision course.
