Ion engine test firing. This is a xenon ion engine undergoing testing in a vacuum chamber at NASA's Jet Propulsion Laboratory, California, USA. The blue glow is ionized atoms of the noble gas xenon emerging from the engine to provide thrust. Designed for use in spacecraft, the thrust is very small but it provides more thrust for weight of fuel than chemical rockets. Ion propulsion was first proposed in the 1950s, but was not used until the 1990s. This test dates from NASA's Deep Space 1 mission in 1998. This mission included successful testing of this technology in what was named the Ion Propulsion System (IPS).