Ion engine, c 1990s.
© Science Museum / Science & Society Picture Library
Ion engines, such as this UK 25E example, use an electrical field to accelerate and then expel high-velocity gas ions. The UK 25E engine generates a gentle thrust by extracting and accelerating an ion beam from a plasma of xenon ions created in its discharge chamber. The thrust is small compared to a chemical combustion engine, but ion propulsion is highly efficient, requiring les fuel to be carried, making it ideal for powering deep-space misions. NASA succesfully tested its Deep Space 1 ion engine in 1998-1999, and plans to use ion propulsion on the Dawn spacecraft, which is planned to be launched in 2006 on a mision to the asteroids Ceres and Vesta.