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WHILE NASA wrestles with last week鈥檚 unexplained failure of the xenon ion
drive in its Deep Space 1 probe, it has emerged that a potentially superior ion
engine is already in orbit. It is to be found on another American
spacecraft鈥攁nd is based on vintage Russian technology.

Last month, the US National Reconnaissance Office quietly launched an
experimental satellite that uses Hall thrusters. In principle, Hall thrusters
are similar to ion thrusters: both shoot xenon ions out of an engine nozzle to
provide push. But Hall thrusters use the Hall effect to generate magnetic fields
that accelerate the xenon gas, while ion thrusters use electric fields.

鈥淲e couldn鈥檛 get Hall thrusters to work very well, so we worked on ion
thrusters,鈥� says Rob Jankovsky, an electric propulsion expert with NASA鈥檚 Lewis
Research Center in Ohio. 鈥淭he Russians couldn鈥檛 get ion propulsion to work, so
they worked on Hall thrusters.鈥�

The NRO鈥檚 Space Technology Experiment satellite uses Hall thrusters that were
developed by Russia鈥檚 Central Scientific Research Institute for Machine Building
and was refined at NASA Lewis. Ion thrusters are slightly more efficient than
Hall thrusters, but Hall thrusters can provide a stronger impulse.

So鈥攁t least until last week鈥檚 engine failure鈥攊on thrusters were
thought preferable for deep-space missions where spacecraft do not have to be
moved quickly, while Hall thrusters could more rapidly change an Earth-orbiting
satellite鈥檚 trajectory and bearing.