Ion propulsion is a technology that involves ionizing a gas to propel a craft.
When xenon ions are emitted at such high speed as exhaust from a spacecraft, they push the spacecraft in the opposite direction. If DS1 shows that electric propulsion works as well as is expected, there will be many new missions that will take advantage of it. Deep Space 4 is expected to use four ion engines to fly alongside a comet in 2004 so that it can land. NASA's first ion engine was built by Glenn Research Center in 1960. Since then, there have been many tests of the technology in the laboratory and some limited tests in space.