A couple of questions about Universal Gravitation.. When satellites are launched with velocities greater or lesser than the required launch velocity [Underroot(G*Mass of earth/Radius of Earth)], their orbits become elliptical with the earth as the near or far focus. Why? Also, 'The Earth and moon must rotate about their common centre of mass, rather than the moon about the centre of mass of the earth'. Why? This is A Level physics, incase that is of any use.