If astronomers were suddenly to announce that, in 3 weeks a comet or asteroid measuring 300 meters across was due to pass inside the orbit of the Moon, and that they could not predict exactly where on Earth it might impact until about 1 week before impact, what should be our best strategy? I think that world opinion would not say 'Well, lets just wait until we are certain before we mobilize 100 billion dollars worth of resources', after all, those scientists could be wrong!
The dramatic impact of Comet Shoemacher-Levi with Jupiter certainly left us all with a vivid reminder of how devastating such an impact can be, even if we all choose to ignore the Tunguska Impact in 1908. If we are going to take action, what should we do? Many influential people believe that launching nuclear-tipped rockets to deflect, or fragment the intruder would be a good strategy. The problem there is two-fold. Our current arsenals are designed only to deliver bombs to other points on the Earth. They are not designed to travel a million miles into space. Secondly, when they get to the Intruder and explode, will the detonation cause the body to be deflected, or fragmented into an even more lethal hale of death in the form of dozens of smaller fragments?
There is a LOT of work that has to be done on both these issues. We do NOT have rockets out there waiting to be launched on 3-week notice, and we do NOT have any firm ideas about whether a given explosion will deflect or fragment a near-Earth object. Aside from a few study groups, there is not now a concerted effort by any nation to develop a fleet of rockets on stand-by alert, that would be ready to go were we to discover such an object next month. We humans are master procrastinators, and if it costs a dollar, we will argue ourselves into complacency in a moments notice. The likelihood that YOU will be in some way affected by such an impact either physically or economically during your lifetime, is believed to be about the same as your experiencing a lethal flight in an airplane, when averaged over all of the 6 billion people on our planet over a 70 year period.