What can we do about it

The requirements for intercept have been defined. Most can be met with existing technology. Them are significant uncertainties in coupling efficiency and fragmentation limits. The best approach depends on warning, NEO size and composition, and cost. Optimal defenses generally involve both detection and defense. They are effective to large diameters and justify expenditures on the order of $50-100M/yr. Flyby and landing precursor experiments are scientifically justified. Coupling and deflection experiments are also needed and feasible.