Suppose that fragment G of comet Shoemaker–Levy 9 measured 700 m in diameter. If this fragment had an average density of 200 kg m−3, estimate its kinetic energy just before it entered the planet's atmosphere. You may assume that it struck the atmosphere with a speed equal to the planet's escape speed. Express your answer in joules and megatons of TNT (1 MTon =4.2×1015 J).