|MadSci Network: Earth Sciences|
Asteroids are not large enough to cause the Earth to break apart. Something large enough to cause that much damage to the Earth would have to be planet-sized. Indeed, long ago during the formation of the planets, Earth was struck by something roughly the size of Mars. This ejected a great deal of material from the Earth, which then condensed to form the moon. There are no longer any objects that cross the Earth's orbit that are large enough to break the Earth apart.
Try the links in the MadSci Library for more information on Earth Sciences.