What if there was a near miss with a comet?

Hiya Warren!

Good luck with your book. I'm a writer (and not a physicist) so I am mainly bumping your topic. However, a few ideas:

• Any super-close and very large object will affect the tides. Your near-miss comet might cause tsunamis, or even move rock.
  
• You might also get hurricane/tornado types of effects in the sense that power might be down, trees might splinter, etc.
  
• Fragments can turn into meteors. It depends on how big they are in terms of how much damage they can do. I suggest Googling the origins of Crater Lake and what happened at Tunguska in 1908.
  
• A large meteor hitting can have effects much like an a-bomb going off, although without the radiation, so maybe look at images from a-testing or even Hiroshima or Nagasaki (not for the faint of heart).
  
• I think remaining in orbit depends on size and distance. I'll leave that to the scientists among us but my suggestion is to consider what moons are like when they revolve around other planets in our solar system.

Hope this helps!

Mind you, Eleven, I'm no physicist. But I'll give it a shot.
An object travelling at 100,000mph would pass by very quickly so it's effect would be minimal. If it passed halfway between the Earth and moon, it would be 125,000 miles from Earth. The Earth is only 8,000 miles in diameter, at that speed it would travel that far in less than 5 minutes. Possibly the effects of it's gravity could affect the tides and cause some atmospheric disruption. But since it would pass so quickly, not much of either.
If the moon blew up and shattered into 4 or 5 pieces... well that's a tough one. It depends on how it blew up. If the explosion was very powerful, those pieces would follow new trajectories. The chunks that were blown in the direction of the Moon's motion would either leave earth's orbit or find new equilibrium in a higher orbit. Those chunks blown in the opposite direction to the Moon's motion would either fall to Earth or find equilibrium in a lower orbit. The chunks blown directly out from the Earth or directly toward the Earth would end up in highly eccentric orbits, some of them would undoubtedly hit the Earth. Right away or even after many months or even years. That goes for huge chunks as well as tiny pieces.
If the explosion was not very energetic, the Moon might crack apart , but then eventually the pieces would drift back together under their own gravity.
This is my best guess. I think there are other members here that have better understanding of these things and so could give you better answers. Good luck with your story.

A near miss comet... There are many variables with this scenario. One major variable would be the size and to a lesser degree the speed. The direction would be a contributing factor as well. Remeber that everything has a gravitational pull. No mater how big or small.
Mass is what determines that. If the comet/meteor where big enough and heavy enough it could not only effect tides but it could potentially disturb the orbits of both moon and earth. And that my friend would more then likely be a mass extinction of the earth. Our earth is so sensitive when it comes ti its axis and its orbit that if distured even slightly, it could distroy everthing.