The magic of negative ion engines

Ion engines, which operate in the near vacuum of space, fire gaseous fuel much faster than a stream of chemical rockets. They provide about ten times more thrust per kilogram of fuel used. The ions that give the engines their name are charged atoms accelerated by selected electrical weapons. If the power comes from the spacecraft's solar panels, the technique is called "solar-electric propulsion".

Ion engines work their magic in slow motion. An ion engine can push gently for months or even years - as long as the sun shines and a small fuel supply lasts. Since normal-sized solar panels deliver only a few kilowatts of power, a solar-powered ion engine can't compete with the whizz of a chemical rocket—however, a typical chemical rocket burns for only a few minutes.

In 1998, NASA launched the Deep Space 1 demonstration spacecraft, which flew past a near-Earth asteroid and intercepted a comet. With much less chemical support, ESA's SMART-1 probe only made it as far as the Moon. However, it demonstrated the more delicate operations needed for distant missions. These will combine solar-electric propulsion with manoeuvres using the gravity of planets and moons.