Mr Red: near earth misses are not a pressing problem. Would would be a problem would be earth hits. And to work out whether they would be a problem you need to know where the asteroids are. Beardedbruce seems to think nobody is doing anything about this, but this is wrong. Take for example the largest asteroid threat, from the list given by Iains. This is called Toutatis. We know where this is. We know where it is going to be. And for the next 600 years, the probability that its orbit will intersect earth is zero.

Chelyabinsk, we might forget that but it will happen again. Big flash in the sky, some windows get broken.

Tunguska, well if something like that happened over a city, big problem for that city. But most of the world isn't cities.

The only problem that we cannot predict or prepare for is long period comets. One thing about these is that their orbits tend not to lie in the ecliptic, as opposed to the orbits of asteroids and periodic comets, which do). Which vastly reduces the probability that these orbits will intersect earth.