ion engine

Rocket engine that uses ions (charged particles) rather than hot gas for propulsion. Ion engines have been successfully tested in space, where they will eventually be used for gradual rather than sudden velocity changes. In an ion engine, atoms of mercury, for example, are ionized (given an electric charge by an electric field) and then accelerated to high speed by a more powerful electric field.

NASA's Deep Space 1 probe, launched in 1998, was the first spacecraft to use an ion engine as its primary means of propulsion and in 2003, ESA's SMART-1 mission to orbit the Moon was the second. NASA tested its High Power Electric Propulsion (HiPEP) ion engine in 2003, the largest ion thruster ever built.