Scientists at Farnborough have designed and built an ion engine to be used on a mission to the planet Mercury, the super-heated planet nearest to the sun.

Huw Simpson, senior engineer in charge of Ion Propulsion at Qinetiq in Farnborough, explained that his team is now testing components of the engine which will power the Mercury spacecraft.

He said that the craft will be boosted into earth orbit using conventional rockets, but once in outer space, the ion engine takes over.

Using only small amounts of a gas called Xenon as fuel, the craft will need only seven or eight bursts of engine thrust over two and a half years travelling more than 62 million miles to reach its orbit around Mercury.

Mr Simpson said the engine works by ionising molecules of the gas, taking electrons (negative charges) away to give them a positive charge. Then the positive charge, the ion, is shot out the back, thrusting the spacecraft in a given direction.

Since there is no atmosphere in space, the craft continues moving forward.

While that craft remains in orbit, a second one will land on Mercury.

The object of the mission is to map the planet and collect data on radiation and solar activity, as well as to learn more about Mercury's magnetic field.

Tests going on now at Farnborough are making sure that the engine parts will be able to withstand the fierce 600oC temperatures which they will encounter.

The mission is not due for launch until 2009.