Confusing question on asteroids and the impact they have on earth?

please also understand that the asteroid itself won't kill all of us, but the effects of the impact would by causing a dust cloud so large it would cover the entire sky and cause temperature drops... here is a calculator: a href="http://www.purdue.edu/impactearth/." rel="nofollow"http://www.purdue.edu/impactearth/./a