The energy yield of a nuclear weapon is often defined in terms of the equivalent mass of a conventional explosive. 1 ton of a conventional explosive releases 4.2 GJ. A typical nuclear warhead releases 250,000 times more, so the yield is expressed as 250 kilotons. I'hat is a staggering explosion, but the asteroid impact that wiped out the dinosaurs was significantly greater. Assume that the asteroid was a sphere $10 \mathrm{km}$ in diameter, with a density of $2500 \mathrm{kg} / \mathrm{m}^{3}$ and moving at $30 \mathrm{km} / \mathrm{s} .$ What energy was released at impact, in joules and in kilotons?