Slate magazine had the bad luck to post an interview their Bad Astronomy blogger Phil Plait did entitled How We Know An Asteroid Won't Slam Into Earth Today on 15 February, the day, ahem, in which a ten-thousand tonne asteroid happened to slam into Earth. Of course they weren't talking about that asteroid, which nobody saw coming, but it's 35 times bigger cousin, which did miss us, if a bit narrowly (17,000 miles).

An asteroid of the ten kilo-tonne scale tend to deposit most of its energy in upper atmosphere as it blasts through it at hypersonic speeds. Going faster than the speed of sound means that air molecules in front of you don't have time to get out of your way, so they pile up in an increasingly massive lump in front of you, like wet snow in front of a snow shovel, until enough momentum is transferred to slow the whole train down.

How much energy does one of these guys have? Consider a generic 10^7 kg asteroid moving at 2 x 10^4 m/s. Then kinetic energy K = (1/2)m*v^2 = 2 x 10^15 Joules or just about half a megaton, reputedly the size of the thermonuclear weapons on our missiles, and 50 or so times as powerful as the Hiroshima blast.

The asteroid did a bunch of damage, but it didn't destroy a city, why not? Probably partly because it didn't directly target one, but mostly because most of the energy deposition was pretty high in the atmosphere, probably ten or more times higher than the Hiroshima bomb.

Other things being equal, a bigger asteroid deposits more its energy lower in the atmosphere. Dinosaur-killer scale asteroids carry nearly all their energy right into the Earth's crust, and a 20 Megaton event like the one that missed would likely carry its energy low enough to be a city smasher.

The planet probably encounters a 1 kilometer scale asteroid - with a mass of a few gigatonnes - every few hundred thousand years. A bad boy like that would pack multi-thousand megaton equivalent energy, but would likely be far more destructive than an equivalent superbomb because of it's immense momentum. It would plow into the planet at barely diminished speed, and send enormous fountains of lava over tens or hundreds of thousands of square miles.