If an asteroid is going to hit Earth I doubt there is much they will be able to do to stop it despite what Hollywood says. It may be the only time the whole planet finds religion at the same time...: )
See my post #4. IF we (or at present, the Russians) had a rocket ready to go, then I’d say we have to have the offending(!) asteroid accurately tracked at least a month B4 it impacts Earth. If it’s a “rogue” (not previously discovered, or its orbit has changed since the last time tracked), then, as this one last night demonstrated, finding and tracking ‘em is tough. It probably requires a space or moon-based observatory to properly do the job. Best in the long run might be an observatory on the Moon and another in (roughly) Venus’ orbit.