Right now we are living on the edges of a few thousand sci-fi and post-apocalyptic novel concepts. Asteroid 2005 YU55, is this very minute winging it’s way between the earth and our moon. That’s 400 meters of solid rock, easily capable of vaporization a city should it impact. Of course at it’s closest point it will be 324,600 kilometers away from the planet, but when we’re talking about distances in space, this is a hair’s breath away.
Of course an impact from Asteroid 2005 YU55 wouldn’t be an ELE (Extinction Level Event), certainly no Apophis. We can however look to the skies and wonder if the one that does hit (and one eventually will) will be in our lifetimes, or those of our grandchildren, or long after our species has faded from the planet.
For the morbid scientist in each of us here is a link http://simulator.down2earth.eu/# to a site that lets you input data (size, materials, speed) of a potential impact, and calculate the damage, effects and crater size that would result.
Now you can get some factual research done on your own Armageddon or Deep Impact story. Have fun smiting your hometown like the fist of an angry god.