Prometheus 6

Ion engines are an "electric propulsion system". They make use of the fact that a current flowing across a magnetic field creates an electric field directed sideways to the current.

This is used to accelerate a beam of ions (charged atoms) of xenon away from the spacecraft, thereby providing thrust.

Ion engine enters space race
By Helen Briggs
Science reporter, BBC News

Cryogenic pumps can be heard in the background, whistling away like tiny steam engines.

Using helium gas as a coolant, they can bring down the temperature in the vacuum chamber to an incredibly chilly 20 Kelvin (-253C). The pressure, meanwhile, can drop to a millionth of an atmosphere.

This laboratory in a leafy part of Hampshire is where defence and security firm Qinetiq develops and tests its ion engines - a technology that will take spacecraft to the planets, powered by the Sun.