D'oh, I buggered up my sums there didn't I? 🙂
How many square KM could you make a 2D grid at, say, 10,000m (Edited originally put 10,000km so buggered that up again) between each mine? you'd never be less than 5,000m from a mine in that case and if they move at 1000m/s that's 5 seconds to impact.