Ion propulsion is already very important for the operation of geostationary communications satellites. These satellites are constantly subjected to small forces (due to solar radiation pressure and the gravitational pull of the Moon and the Sun) that make them drift off from their designated station. They regularly have to use propellant just to stay in place and when they can no longer do this they become worse than useless, as they are taking up an available slot in that critical orbit. As a result the satellites have to be retired before they run out of propellant, as the last drops are used to place it in a ‘graveyard orbit’ a few hundred kilometres higher up. Ion thrusters are thus very attractive because they provide a great deal of thrust for a small mass of propellant and so they extend the working life of the satellite.