Space

An ion engine has smashed the record for total thrust in a NASA test. The successful test means the engine could be used in future NASA missions.

Ion engines work by accelerating electrically charged atoms, or ions, through an electric field, thereby pushing the spacecraft in the opposite direction.

The thrust they provide at any given moment is very small, roughly equal to the force needed to hold up a sheet of paper against Earth's gravity. But they can operate continuously in space for years using very little fuel, ultimately providing a much bigger boost than a chemical rocket.

The Dawn mission, which launched on Thursday, is equipped with NASA's first generation of ion engines, called NSTAR. Dawn's three NSTAR engines will allow it to reach the asteroid belt and park in orbit around two different asteroids.

The agency has also been testing a more advanced ion engine, called NASA's Evolutionary Xenon Thruster (NEXT), which generates 2.5 times as much thrust as an NSTAR engine.

Now, NEXT has broken a record, providing more "total impulse" than any previous ion engine. Total impulse is a measure of the overall acceleration that an engine would provide to a spacecraft. It is the result of multiplying the engine's thrust by how long it fires.

Record fuel

The NEXT engine has now been fired for over 12,000 hours (500 days), providing more than 10 million Newton-seconds of impulse, more than any ion engine has ever achieved.

During this time, it has processed more than 245 kilograms of fuel in the form of xenon gas, a record amount for an ion engine. The amount of fuel an ion engine can handle before wearing down is critical, since ion engines on spacecraft need to fire for years at a time.

Previous estimates have suggested NEXT engines could safely handle 450 kilograms of fuel in their lifetime. NSTAR is rated for only 150 kilograms of fuel throughput, although one NSTAR engine has processed 235 kilograms of fuel in a previous test.

"This test validates NEXT technology for a wide range of NASA solar system exploration missions, as well as the potential for Earth-space commercial ventures," says NEXT principal investigator Mike Patterson of NASA's Glenn Research Center in Cleveland, Ohio, US.

NEXT could power a mission to Saturn's moon, Titan. It would require about 20 kilowatts of engine power to get there if the mission included both an orbiter and a lander. "We could do that with an array of three thrusters, plus a spare," NEXT project manager Scott Benson of Glenn told New Scientist.

Star Wars

Although NSTAR and NEXT both use xenon gas as a propellant, NEXT accelerates the xenon ions more efficiently, providing up to 236 milliNewtons of thrust compared to NSTAR's maximum of 92 mN. The ion engines used on Japan's Hayabusa spacecraft to the asteroid Itokawa use 22 mN, while those used on the European Space Agency's SMART-1 Moon probe operated at 70 mN.

NEXT can also vary its thrust by a factor of 11, as compared with NSTAR's factor of five. This means it can throttle down to lower levels as it travels farther from the Sun and receives less sunlight, allowing it to operate at greater distances than NSTAR.

Although ion engines are just beginning to see regular use on scientific probes, they have been a common sight in science fiction for many years. Dawn spacecraft engineer Marc Rayman of NASA's Jet Propulsion Laboratory in Pasadena, California, US, reminded journalists at a recent press conference of the ion engines used in the Star Wars movies.

"If you remember the TIE fighters that Darth Vader and the Evil Empire used to fight the rebel alliance, TIE stood for 'twin ion engines'," he said. "Well, Dawn does the Star Wars TIE fighters one better because we use three ion engines."

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