It's all a question of how well you've measured the trajectory and how much computing power you can throw at the problem. If you've tracked it long enough, you can predict where it's going to go to within a gnat's eyebrow.
. . . and if you've tracked it long enough, it has already either hit or, more likely by far, missed by hundreds of miles. The bigger object you find, the less effect you can have on its trajectory with a given bopper--and the sooner you'd better bop it, so the effect accumulates over time.The conclusion is that you do well to detect early, and to refine your estimate of trajectory rapidly when you do detect. Does Hubble's different perspective help, and would a deep space probe be enough better to justify its cost?