Orbital velocity, illustration. If a cannon is fired from the top of a very high mountain, the shell will fall back to Earth (red arrow). If fired at greater speed it will travel further before falling to Earth (longer red arrow). If it is fired at a speed of 8 kilometres per second, then it falls towards the Earth but because of the curvature of the Earth, it stays at the same height above the ground and becomes a satellite in orbit. The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude.