The on-board propulsion system dominates the design of many spacecraft, particularly as regards mass and volume. However, the mass of this system, sometimes 50 per cent or more of the total, can be reduced dramatically by using ion propulsion technology, rather than the more usual chemical thrusters. In an ion thruster, the propellant is ionized in an electrical discharge, then the positive ions are extracted from the resulting plasma and accelerated to a very high velocity by an electric field, forming an extremely energetic exhaust. This paper describes the development and status of the two UK ion propulsion systems, which have thrust levels in the 1-30 mN and 50-300 mN ranges.
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