Efficiently powering probe

Unlike the fireworks of most chemical rockets using solid or liquid fuels, the ion drive emits only an eerie blue glow as ionized atoms of xenon are pushed out of the engine. Xenon is the same gas found in photo flash tubes and many lighthouse bulbs.

By expelling of this stream of electrically charged gas, the engine produces a tiny thrust, the equivalent of the pressure exerted by a piece of paper on your hand.

This type of fuel is much lighter and more efficient than chemical rockets, making it cheaper and much faster than traditional spacecraft.

The ion propulsion engine is slow to pick up speed, but its steady action builds up over a long period, and it can deliver 10 times as much thrust per pound of fuel as traditional engines.

Great efficiency

By the end of the mission the ion engine will have accelerated the spacecraft to 11,000 kilometres per hour.

"The importance of ion propulsion is its great efficiency," said Dr Marc Rayman, project manager for Deep Space 1.

"It uses very little propellant, and that means it weighs less so it can use a less expensive launch vehicle and ultimately go much faster than other spacecraft."

"This opens the solar system to many future exciting missions which otherwise would have been unaffordable or even impossible," he added.

The Deep Space 1 probe is on a mission to encounter the Comet Borrelly in September next year.