For those who like to attempt to destroy the earth with asteroids, there’s a new asteroid impact simulator online at: http://www.purdue.edu/impactearth/
I just entered a 10 km nickel/iron asteroid into the calculator, hitting crystalline rock (so… Canadian Shield?) at an angle of 90 degrees, at a (admittedly fast) velocity of 30 km/s, and got it to calculate the effects felt 1000 km away. (For comparison, 1000km is about the distance between Vancouver and Calgary in Canada, or between London and Berlin in Europe, or between Denver and Las Vegas in the US).
It makes a final crater about 230km in diameter, and 1.5 km deep. 8.3 seconds after impact , I get ignited by the blast of heat from the impact, which even at this distance is not unlike a nuclear blast – everything catches fire. If I’m still around after that, then around the 3 minute mark I feel the residual shaking from the 10.4 earthquake that the impact triggered (not enough to collapse buildings at my distance, but enough to be felt). Ejecta arrives at my location about 8 minutes after impact (a light dusting of millimetre-sized stuff with the occasional larger fragment). Then around the 50 minute mark (the heat is dying down at this point) I get deafened by the sound and any buildings still standing around me get blown away by the 1000 mph winds of the blast wave. If I crank down the impact velocity to a more reasonable 11 km/s then the heatblast is avoided and the crater is smaller, but the quake is still felt and the blastwave still does significant damage.
Either way, it’s not pretty :). And this is just from a comparatively tiny (relative to Earth) 10 km wide bit of nickel/iron. Thankfully there aren’t too many of those flying around on an earth-crossing orbit! It’s interesting to note that if this happened on a world without an atmosphere then the blast wave wouldn’t be an issue, and that’s what causes a lot the physical damage.
Though while I think it’s fine for smaller impacts, I’m not so sure about larger ones – I tried a 6000 km diameter object with a density of 5000 kg/m³ (i.e. about the same size and density as Earth) at 11 km/s and it made a 30,000 km diameter crater (so, about 3/4s of the Earth’s circumference) and yet it didn’t have any significant global effects other than to possible slow our rotation down? It really should result in the total eradication of everything on the surface, atmosphere blown away, probably the entire surface turned into a magma ocean, and the tilt maybe changing significantly (assuming the Earth doesn’t actually breaking apart due to the impact). So I’m not sure I’d trust it for really big impacts.
Though check out the pulldown list of possible impactors – there’s a surprise in there ;).