The BBC's Sue Nelson: " A new type of mission for testing innovative technology"

Smart-1 will also test an ion propulsion engine, which fires out xenon atoms to drive the spacecraft.

The 350kg spacecraft is due to be launched at the end of 2002 by an Ariane 5 rocket in a mission costing £54 million.

Green cheese?

The small X-ray spectrometer (D-CIXS) which will reveal the Moon's composition was largely designed and built in the UK.

"Despite decades of research, we have never fully discovered what the Moon is made of," said Dr Manuel Grande of the Rutherford Appleton Laboratory and principal Investigator for D-CIXS.

Seven science instruments will be on board

"The Apollo missions only explored the equatorial regions on the Earth-facing side of the Moon, while other spacecraft only investigated surface colour or searched for water and heavy elements," he explained.

D-CIXS will measure the Moon's surface composition by detecting the X-rays which bounce off the lunar surface.

"As X-rays from the Sun strike the Moon, they excite elements such as silicon, calcium, magnesium, aluminium and iron in the rocks," said Dr Grande. "So by studying these emissions, we can tell what the rocks are made of."

"If the Moon is really made of green cheese, we'll be the first to tell the world," he added.

But many US and European scientists have criticised the mission, saying that it duplicates previous work and therefore wastes the scarce funding available for space missions.

Ion propulsion

The ion propulsion engine will provide Smart-1's primary thrust to escape the Earth's gravity, for its 17-month cruise to the Moon and to stay in lunar orbit for six months.

It will be the first time that European scientists have used the technology. Nasa tested the principle on the Deep Space One probe a year ago.

It is also used by near-Earth telecommunications satellites, but only for small orbit corrections.

SMART-1 will use a plasma thruster which uses solar-power electricity to shoot out the gas at very high speed, generating the movement of the satellite.

"Compared with conventional chemical systems, electric propulsion expends very little mass to accelerate a spacecraft. But it ejects the propellant plasma up to ten times faster than a classical engine and so is ten times more efficient," said project manager Giuseppe Racca.

The SMART-1 project now enters its development phase. The prime industrial contractor is the Swedish Space Corporation.