safeguards against comets? I read one of the related questions here, but what i'm curious to know is how do we deflect imminent collision courses of comets, if we detect them at a later stage? I read somewhere else that there are systems which use powerful lasers (radiation pressure) to deflect them, but honestly, i don't understand why something like radiation pressure will work on something travelling at about 40km/s through space, and still manage to cause a large enough deflection. Are there other methods ?
Fortunately it is not something that we have yet had to face.

Diverting the impact of a comet would not be easy, especially if it was a newly discovered comet on a nearly parabolic orbit. We would probably have less than 2 years to prepare. 

Remember that the Earth is orbiting the sun at 30 km/s. So if we can delay the comet by three or four minutes, that turns a direct hit into a miss. But most of the plans to divert objects apply to asteroids whose orbit is well known and for which we could have tens of years in which to act. For a new comet there may not be time.