An ion engine operates by removing electrons from atoms of a gas – usually xenon – and then accelerating the resulting ions through an electric field. Thrust is created as the ions are shot out the back of the engine and it could have enough kick to send a spacecraft all the way to Titan, Saturn's giant moon, more than 1 billion kilometres from the Sun. Ion engines are also more fuel efficient than conventional rockets while providing a steady source of propulsion that makes it ideal for spacecrafts designed to fly to the outer solar system.

In 1998, NASA tried out ion engines on its Deep Space 1 mission, which headed for an asteroid and a comet 203 million km from the Sun. But the NSTAR engines used during the mission were not powerful enough for more distant journeys. NASA's new Evolutionary Xenon Thruster (NEXT) can generate 236 milliNewtons (6.9 kilowatts) of engine power. That is 2.5 times as much thrust as the NSTAR engine and enough for longer space trips. NEXT is also more powerful than the 22 mN engines on Japan's Hayabusa spacecraft sent to asteroid Itokawa and the 70 mN engines on the European Space Agency's SMART-1 lunar probe.

Like NSTAR, NEXT uses xenon gas as its propellant and solar arrays to capture energy from sunlight but it provides more thrust than previous ion engines. It can also throttle down to lower levels as it travels farther from the Sun and receives less sunlight. This power efficiency allows it to operate at greater distances than NSTAR.

The first NEXT engine has passed NASA's preliminary tests and now faces new tests that will determine how well it holds up to extremes of temperature it may experience in space. NASA hopes to have engine testing completed by September 2007, so that NEXT could be considered for future missions to send an orbiter and a lander to Titan or possibly Jupiter.