A huge asteroid which is on a course to miss the Earth by a whisker in 2029 could go round its orbit again and score a direct hit a few years later.
Astronomers have calculated that the 1,000ft-wide asteroid called 2004 MN4 will pass by the Earth at a distance of between 15,000 and 25,000 miles — about a tenth of the distance between the Earth and the Moon and close enough to be seen with the naked eye.
Although they are sure that it will miss us, they are worried about the disturbance that such a close pass will give to the asteroid’s orbit. It might put 2004 MN4 on course for a collision in 2034 or a year or two later: the unpredictability of its behaviour means that the danger might not become apparent until it is too late.
This does not look like it would be the "big one" that would wipe out civilization, but 1,000 megatons is twenty times larger than the 50-megaton Tsar Bomba exploded by the Soviets in 1961. Widescale devestation and destruction is likely, either through the asteroid hitting a large metropolitan area, or by the tsunami generated by an oceanic impact.
We need to be ready for such an eventuality if it were to occur. We need to be able to understand how to destroy or deflect such an impact object from hitting earth without inflicting additional damage here on earth, either from the radiation of a nuclear explosion or by a "chain-gun" style reaction from blowing the object into smaller pieces. We should plan now, so we do not have to improvise from training and experience like the Apollo XIII engineers had to do on the fly 35-years ago this month.