What if we build a gigaton-range warhead (which modern rockets could easily lob[1]) and watch for an opportunity to deflect a comet or large asteroid? The parts

What if we build a gigaton-range warhead (which modern rockets could easily lob[1]) and watch for an opportunity to deflect a comet or large asteroid? The parts I have no idea about are: how often suitable projectiles come close enough to the Moon's path that a Gt nudge would do the deed; and whether we could predict the effects of a huge nuke well enough to steer the object with the requisite precision--but it seems to me interfering with the Moon is only a bit more difficult than defending against a doomsday impact on Earth, so I hope it would be possible. 1: https://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Comet_deflection_possibility https://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Come...