Well..not exactly shoot down. The Pentagon is designing a mission called 'Clementine 2' which will cost $120 million and will launch in mid 1998. It will fly close to three asteroids and fire guided missiles at them, and monitor the impacts as it speeds by at a safe distance. The 200 kilogram spacecraft will fire small missiles, each about 20 kilograms and 15 centimeters long, designed to home-in on the asteroids. Video cameras on the missiles will monitor the approach. The missile will impact at a speed of 20 km/sec and release the equivalent of 100 tons of TNT.
The main goal seems to be to verify that 'you can hit things'. Pentagon analysists predict that a 20 - 30 ton projectile would be all that is needed to deviate the course of a 100-meter class, megaton asteroid off its doomsday course with earth. But first they have to verify that they can actually hit a moving target. They also want to examine the composition of the asteroids because this parameter severely influences how effective such a process is.
Scientists are not amused. Currently, less than $1 million or so is spent each year to search for Earth-crossing asteroids. We have no idea just how likely this kind of doomsday impact is, or when the next one is likely to occur, because we have only identified less than 1 percent of the bodies capable of doing damage. Why are we spending $120 million when we have yet to determine the scope of the problem?