ION ENGINES A spacecraft engine producing as much force as a piece of paper resting on your hand may seem like a dud. But in the vacuum of space that's more than enough to achieve high speeds.

What's an ion? An ion is an electrically charged atom or group of atoms. More information about atoms - BBC World Service.

Instead of burning chemical fuels, ion engines are electrically charged and work by ejecting positive ions from the back of the rocket, propelling the spacecraft forward. Though the thrust is tiny, the ion engine is extremely efficient, allowing the fuel to last far longer than in conventional rockets.

Ion powered spacecraft gradually accelerate to extremely high speeds of 35,000 km per hour or more.

Deep Space 1

In 1998 NASA launched an ion engine powered probe called 'Deep Space 1'. Due to its increased efficiency, Deep Space 1 was in space for longer than all the conventional spacerockets ever made. The craft flew past the comet Borrelly, returning images and data, before being retired in 2001.

Increased reach

Future missions for the ion engine could include comet sample returns, observations of the rings of Saturn and a landing on Jupiter's moon, Europa. Probes could return comet samples to Earth in less time than it would take a conventional spacecraft to reach a comet.

Other near-term options for deep-space travel are solar sails and nuclear rockets.