While Rosetta will be doing its best to touch down very gently on the comet’s surface, we can’t count on Churyumov-Gerasimenko doing the same if it ever paid LA a visit. You can get some idea of what would happen in such a collision here, although the scale of the damage depends greatly on the impact speed you choose to enter. Since Churyumov-Gerasimenko never crosses the Earth’s orbit there is no danger of a collision, at least until a close encounter with some other planet shifts its orbit. Unfortunately the same cannot be said for other, even larger, asteroids and comets. -IFLScience
Thanks to: http://spaceweather.com/ and http://theextinctionprotocol.wordpress.com/2014/08/22/graphic-shows-the-immense-size-of-rosettas-comet-and-risks-such-large-objects-pose-to-earth/