[myrmidon]

> And remember, you got time to build up momentum, a ion drive accelerating in a spiral with swingbys, with one final push, can get you pretty high on the C if you dont want it right now.

No. You're not even getting close to your ion drives exhaust velocity, which is <100km/s.

So you are not accelerating ANY kind of asteroid for more than ~10km/s, because the closer you want to get to your exhaust velocity, the harder your scheme gets slapped down by the rocket equation-- "unlimited energy over time" does not even enter the equation.

So what you are left with is the option of finding an asteroid that you can get into a collision orbit with earth as cheaply as possible.

Even to just break even with the weakest nuclear weapons, you need at least 500 metric tons at 20km/s. You'll probably need something around 300m/s of delta-v (that would be a mars/earth transfer).

And this needs to happen quickly, because there is no stealth in space and everything you do can be observed and reverted (leaving aside the fact that any weapon system with a deployment delay measured in years is absurdly impractical).