What would happen if a very large commet or asteroid hit the moon? Are there any in the near future?

I just read Life As We Knew It by Susan Pfeffer. Very good book but it terrified me! I am wondering now if something like that were to happen, would these be the circumstances? Also is something like this supposed to happen in the near fututre? If you havent read the book it is a very easy read, eigth grad elevel.
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