Estimate the kinetic energy of a comet striking the surface of the Earth. Assume the comet is roughly spherical with a diameter of 10 km, a density of 1000 kg m-3 and strikes with a speed of 30 km s -1. Give your answer in Joules. On impact all the kinetic energy goes into an explosion that forms the impact crater and throws material into the upper atmosphere to block out sunlight. This is similar to the event that ended the reign of the dinosaurs.