Where can I learn about the potential impact of a comet hitting the
earth, and what scientists are doing to prevent that?

Here's NASA's offical site on NEO's -- Near Earth Objects -- such as
comets and asteroids:

http://neo.jpl.nasa.gov/

and here's a directory listing of other NEO sites, a few with some
fairly dramatic (and imaginary) pictures of NEOs colliding with earth:

http://groups.msn.com/AstronomySpace/nearearthobjectsneos.msnw

Hope this meets your needs.  Let me know if I can clarify anything here for you.

pafalafa-ga

search strategy:  nasa asteroid collision