Check out http://www.spaceguarduk.com/ for info on the UK's efforts in tracking objects.
Also quite a few of these are being initially found by amateurs, the smaller end of the range being harder though as you need a bigger 'scope == more money.
The money is needed to do the accurate tracking required to accurately predict if the thing is going to hit or not.
If an object that's going to hit the earth is identified early enough then there are things that can be done to shift it to an orbit that misses. A gentle push over a long time, using for example "Gravity Tractors", is much better than trying to nuke it (you just get hit by lots of smaller objects, same total energy though).
Aliens cos they'll help us out (or are they the ones pushing them our direction?).