BioEd Online

It's been a long trip, but Europe's first Moon mission is finally nearing its goal. SMART-1, launched by the European Space Agency (ESA) way back on 27 September 2003, will go into orbit around the Moon on 15 November.

In January, the craft will begin the first comprehensive X-ray survey of the Moon's surface, giving scientists clues about its composition and age. This geological map will help scientists to establish exactly how the Moon formed.

The size of a domestic washing machine, SMART-1 uses an innovative propulsion system that wafts the craft along on a breath of gas. It carries solar panels that convert the Sun's light into electricity, which is used to strip electrons away from atoms of xenon. This generates charged ions that are accelerated through a magnetic field and ejected from the rear of the spacecraft, producing a gentle thrust equivalent to the weight of two pennies resting on the palm of your hand.

"This is the first time ever that a probe has used ion propulsion to escape from the Earth," says Bernard Foing, who heads the SMART-1 team at ESA's Space Science Department in Noordwijk, The Netherlands.

Conventional spacecraft engines rely on chemical reactions to generate gas, which pushes the craft forwards as it squeezes out of the engine, in the same way as a deflating party balloon will fly across a room. But in space, mass is money. Whereas chemical engines must carry two fuel substances to react together (often hydrogen and oxygen), SMART-1's ion drive carries only xenon, making it lighter and cheaper.

The success of SMART-1, which stands for Small Missions for Advanced Research in Technology, in reaching the Moon has proved that future spacecraft could use the same engines to get to Mercury and Mars, says Foing.

Moon map

SMART-1 still has plenty of science to do during the two years it will spend orbiting the Moon. Its Demonstration Compact Imaging X-ray Spectrometer (D-CIXS) will make a chemical map of the lunar surface that should reveal whether the Moon was once part of the Earth.

"We still don't know if the Earth and the Moon came from the same place," says Manuel Grande, a space scientist at the Rutherford Appleton Laboratory near Oxford, UK, who is responsible for D-CIXS. Many scientists believe that the Moon was formed after a gigantic collision between a Mars-sized object and the Earth about 4.6 billion years ago. Comparing the ratios of different chemical elements on the Moon and Earth could confirm this theory, says Grande.

Ice hunt

SMART-1 also carries a high-resolution camera that will photograph potential landing sites for future robotic or even human explorers. And it will search for ice in the craters at the Moon's south pole, using an infrared spectrometer. "These craters haven't seen the Sun for billions of years," says Grande, who adds that this ice would be a vital resource for a manned lunar base.

The €110-million (US$85-million) craft weighs just 370 kg, and took many spiralling orbits around the Earth to build up enough speed to reach the Moon. Its epic 80-million-kilometre journey has taken 13 months, compared with the four days taken to cover 400,000 kilometres by Apollo 11 during the first Moon landing mission.