An ion engine will now be fitted to the majority of Europe's future spacecraft, such as the Bepi-Colombo mission to Mercury.

Professor Grande himself has been principal investigator on one of Smart's miniaturised instruments: the compact X-ray spectrometer known as D-CIXS.

It has made mineral maps of the Moon's composition, looking at the distribution of calcium, magnesium, aluminium, silicon and iron.

Knowing the absolute abundances of these elements will help to refine theories for the Moon's formation. These describe the satellite emerging from the debris thrown out from a mighty collision between Earth and a Mars-sized body billions of years ago.

"Like everything else on this spacecraft, our X-ray technology was smarter, smaller and lighter than anything that has been flown before," said Professor Grande.

"The instrument weighs about five kilos. Instruments that have tried to do this before have weighed 10 times that."

Eternal light

D-CIXS has proved so successful that an enhanced version (called CIXS) is now under construction to fly on the Indian Chandrayaan mission to the Moon in 2007/8.

Smart 1's camera AMIE has also added greatly to our understanding of the Moon. Previously, the best digital maps of the Moon were from the US Clementine mission with a resolution of 200m. At its best, Smart-1 mapping reveals features 40m across.

SMART'S SPACE GAINS

• Mini-instruments, navigation and operational software to go on future Solar System probes
• Smart-like solar-electric engines to become the dominant propulsion system
• Data improves Moon knowledge - history of impacts and volcanism; good landing sites

The resolution is not quite good enough to see individual pieces of Apollo equipment left on the Moon, but it is sufficient to allow the Smart team to compare its data with the surface photographs returned by the pioneering astronauts.

"For example, we have correlated our data with the ground truth collected by Apollo 15. Our images are consistent with the panoramic images collected by the astronauts," said Professor Foing - adding, with a smile: "So, I believe they have been there!"

Among its several firsts, Smart 1 has found peaks in the northern polar region that are always in sunlight. It has also been looking for depressions that are always in darkness - places where ice could yet be found.

Both locations could be extremely useful to future manned missions, providing bases to set up solar power stations or to source water.

"Smart 1 will now rest in peace on the Moon," said Professor Foing.

"We are now collaborating with the international community, preparing the way for the future exploration of the Moon - the next fleet of orbiters, landers; leading to robotic villages and human bases."

HOW SMART 1'S ION ENGINE WORKS

• 1. Xenon gas atoms are pumped into a cylindrical chamber, where they collide with electrons from the cathode. The electrons - which are negatively charged - knock electrons off the xenon atoms, creating xenon ions - which are positively charged.
• 2. Coils outside the chamber create a magnetic field, which causes electrons from the cathode to spiral and become trapped at the mouth of the chamber.
• 3. The build-up of negatively charged electrons at the mouth of the chamber attracts the positively charged ions, accelerating them out of the chamber.
• 4. The stream of accelerated ions leaving the chamber thrusts the spacecraft forward. Although the force is small, over time it creates great speed in the frictionless environment of space.