 Xenon is the preferred propellant for ion propulsion of spacecraft because of its low ionization potential per atomic weight , and its ability to be stored as a liquid at near room temperature ( under high pressure ) yet be easily converted back into a gas to feed the engine . The inert nature of xenon makes it environmentally friendly and less corrosive to an ion engine than other fuels such as mercury or caesium . Xenon was first used for satellite ion engines during the 1970s . It was later employed as a propellant for <unk> 's Deep Space 1 probe , Europe 's <unk> @-@ 1 spacecraft and for the three ion propulsion engines on NASA 's Dawn <unk> . 
