With 20 years warning, a neutron bomb could turn a direct hit into a near miss. Two years warning would make a deflection more difficult, but still possible. If we only have two months, we're in big trouble.
Marsden says that if we didn't specifically search for it, we probably wouldn't detect an incoming asteroid or comet until radar picked it up at the range of the moon.
At that point, we could do nothing more than prepare, in a few hours, for the end of the world.
We worry about earthquakes, hurricanes, tornadoes and other natural disasters because they frequently affect populated areas. But we underestimate the danger of asteroid and comet strikes because they often occur in the ocean.
An example is the alleged Israeli-South African nuclear test in the Indian Ocean in 1978, which now is believed to have been an asteroid strike.
Marsden pessimistically concludes that "our scientific and technological capabilities are infinitely greater than were those of the dinosaurs, [but] I don't know how much more sensible we are."
Perhaps Chicken Little had a point after all.