This is an 8 centimeter Mercury type ion, or electric rocket engine, tested for the National Aeronautics and Space Administration's (NASA's) SERT (Space Electric Rocket Test ) program. Ion engines produce minute streams of electrically charged ions that are expelled in one direction, thus producing thrust and forward motion in the opposite direction.
Even though they have extremely low levels of thrust they can work as propulsive devices in space where there is zero gravity. They also have extremely long durations. The engine shown here was fired for 15,040 hours with 461 restarts between August 1973 and November 1975. This object was donated to the Smithsonian in 1978 by the NASA Lewis Research Center.