A XIPS (xenon ion propulsion system) IonThruster is a highly-efficient (but low-thrust) rocket. It is basically a cross between a TravellingWaveTube and a xenon light bulb. Used primarily for positioning orbiting spacecraft. IonThrusters of all types (see http://en.wikipedia.org/wiki/Ion_thruster#Types_of_ion_thruster) will become increasingly necessary when humans EscapeToSpace?, likely WithinTwentyYears from October 2055, OctoberZeroFiveFive? Note: xenon (Xe), a noble (inert) gas, is found in Earth's atmosphere at the rate of less than 1 ppm per volume and in the Martian atmosphere at 0.08 ppm.