Dinosaur Killer Asteroid

Estimate the total energy released when a spherical ``Dinosaur Killer'' asteroid with a density $\rho = 10$ kg/m$^3$ and radius $R = 1000$ meters falls onto the surface of the earth from ``outer space'' (far away). Obviously your answer should be justified by a good physical argument.

Note that this is a lot of energy - more than enough to wipe out all life within perhaps 1000 km of the point of impact (or more) and to change the climate of the planet.