20 year lead time? Huh? Orbital mechanics can't predict where the rocks will be in 20 years with enough accuracy to determine if they need to be moved. That sounds crazy.
No doubt, probability calculations will be part of the threat assessment together with orbital mechanics. Hey, nobody said it would be easy! However, it's worth some effort because an extinction impact event on Earth would be very unpleasant, especially if mankind is snuffed out. :-)