Xenon ion engine prototype. View of a Xenon ion engine prototype being tested in a vacuum chamber, showing the faint blue glow of charged atoms being emitted from the engine. The thrust from this engine comes from accelerating and expelling positively charged xenon atoms, or ions. While the ions are fired in great numbers out the thruster at more than 110,000 kilometres per hour, their mass is so low that the engine produces a gentle thrust of only 90 millinewtons, however it provides more thrust for weight of fuel than chemical rockets. This engined is undergoing an extended-life test and has been running continuously for almost 500 days of a scheduled 625 days. Photographed at NASA's Jet Propulsion Laboratory, California, USA.