[quote=Asuras]
An homage to life before civilization. How ironic. xDIf you're attaching rockets to an asteroid and then sending it off to the enemy, I feel as though they could easily and well in advance blow the asteroid up before it became a problem. It seems much more feasible to project a much smaller projectile at much faster speeds (aka space guns) than to fly big rocks at the enemy.Not to mention the fact that using the asteroids as such seems like a big waste of good minerals.
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I'd be putting rockets on the sides for lateral maneuvering of the asteroid, but yeah, you could still shoot it down. The question becomes could you shoot down a hundred, or a thousand, or a million? Still yes, but it would require a lot of energy -- more than it would take to launch the rocks in the first place. Intercepting is harder than throwing -- that's why batters get three strikes. Energy-wise, you can build up a lot of power in deep space that defenders can't really match. 

For instance, let's say I have a fleet and you have a fleet. I'm deciding to attack earth, and you move your fleet around Sol to stop me.

First thing I do is launch a bunch of rocks at earth. You can shoot them down from Earth -- but it takes tremendous (man-on-the-moon-ish) power to do so.

The thing to do would be to intercept my attack with your fleet -- lasers and defense missiles and railguns and whatever else you wanna use, human shields, whatever. So you do that, and maybe you stop *all* my rocks. Good job!

Now I bring in my fleet; you've already burned all that energy stopping my rocks, and I've got Deep Space kinetics at my back, plus totally un-spent weaponry and power and whatever. I mop you up in a heartbeat, and then move into bombardment mode and glass Planet Earth.

The attacker (almost) always wins, which then begs the question: if I can destroy an equal force at will, why would I ever let someone build up enough of a standing fleet to present a challenge? It's the same result that happened with the F-15 -- its combat record is a whopping 102 kills and [i]zero[/i] losses. Fifty years later (an impossibly long time, for a war machine), nobody can fight it, and the politics follow the power. Space empire seems like the only logical conclusion to me.