We are reasonably confident that no likely gamma ray bursters are pointed at us and within lethal range. We know dinosaur killers are out there--a failure to map every such object in the solar system is a small probability of an infinite risk. Why are there no ICBMs fitted for point defense against a city killer asteroid? You have the rocket, you have the boom. You need a seeker that can guide it to impact (there are other radars that could illuminate, it just needs to home on the reflection) and a standoff fuse that will fire it at the last possible millisecond.