Best defense against comet and asteroid collisions...
I know I'm preaching to the converted with Mark but the best defense against a comet or asteroid collision with the Earth is having human settlements on other worlds, e.g. Mars. I strongly suspect that even if we did have a technology for detecting such collisions the information would not be actionable. The energy required to deflect the trajectory of a comet or asteroid is enormous. It would be like a gnat trying to deflect a city bus.
Again, the best defense is to spread our base. If one world is wiped out then we continue somewhere else.
Gary