Deep Space 1 was the first spacecraft to use an ion engine as its primary propulsion system.
NASA launched Deep Space 1 on 24 October 1998 as a technology-demonstration mission. Its xenon ion propulsion system produced a very small thrust, far less than a conventional chemical engine, but it could operate continuously for long periods. By accelerating ions electrostatically, the engine achieved exceptionally high exhaust velocity and used propellant efficiently.
The spacecraft tested 12 advanced technologies, including autonomous navigation, an autonomous agent for spacecraft operations, and a solar concentrator. Its original target was asteroid 1992 KD, later named Braille. After completing its primary technology mission, controllers redirected it to comet Borrelly, which it flew past in 2001.
Dawn also used ion propulsion and visited Vesta and Ceres, but it launched later. Deep Space 1's importance was proving that electric propulsion could power a deep-space mission.