Just a fragment of Shoemaker Levy-9 would have probably been enough to annihilate our planet. There wouldn’t even be any dirt left. Well, I guess the dirt WOULD be somewhere, just not ‘here’.
It wouldn’t have been good, in fact, we can safely say, it would have been bad. :’) Actually, it’s briefly discussed on here:
Asteroids: Deadly Impact
http://watchdocumentary.org/watch/asteroids-deadly-impact-video_9122e1848.html
http://www.snagfilms.com/films/title/asteroids_deadly_impact
http://www.youtube.com/watch?v=j9ZnQ9TL_RA
http://www.youtube.com/watch?v=zictQuXbQ9I
http://www.youtube.com/watch?v=0KBNNmBMQ3Q
http://www.youtube.com/watch?v=PwXSg2OEKZA
http://www.youtube.com/watch?v=SkAUeotP2MM
http://www.youtube.com/watch?v=BdQNf10y_dA