"You do not have to look to the outer edges of the solar system, or even out beyond Neptune to observe a reservoir of comets. A bevy of the ice-containing bodies lies disguised as main-belt asteroids between Mars and Jupiter, claim astronomers from the University of Hawaii, US."
http://www.newscientist.com/article/dn8887--clandestine-comets-found-in-main-asteroid-belt.html
Well... scientists want to land on asteroids and comets and figure out how to change their course to protect earth, this would be a good way to experiment with that and kill two birds with one stone. A comet is a large ice cube so to steer it would require just a heater that is moveable.