Science: Clementine sets off in search of lunar gold: The United States is about to visit the Moon for the first time in 21 years, writes Peter Bond

Tomorrow America heads for the Moon again. With the exception of two scans by the Galileo probe on its way to Jupiter, no US spacecraft has returned to the scene of the country's greatest technological triumphs in 21 years.

However, this is about to change as the result of a rare collaboration between the civilian space agency Nasa and the Ballistic Missile Defence Organisation (BMDO).

The progeny of this pairing is an unmanned space probe known as Clementine, after the girl in the song about the California Gold Rush. Hopes are high that it will reveal unknown minerals on the Moon and on a small Earth-grazing asteroid called Geographos.

In the mid-Eighties, the Reagan administration's 'Star Wars' programme dreamt of a space umbrella that could detect and destroy incoming enemy missiles. As part of this project, the US Defense Department was eager to test new sensors by subjecting them to the rigours of long exposure to space.

Meanwhile, Nasa's planetary science community could not persuade Congress to pay for new missions to the Moon and planets. The agency turned towards a new policy of 'better, faster, cheaper' spacecraft as a way out of the impasse.

In 1990, the military and Nasa realised they could kill two birds with one stone by testing new defence hardware beyond Earth's orbit. Although this would increase the mission cost from dollars 50m ( pounds 35m) to dollars 80m, the extra amount would be offset by removing the need to launch a target vehicle for the original Earth-bound trial. The two agencies agreed to a joint exploration of the Moon and an asteroid and, in January 1992, Clementine was born.

For this first mission, Nasa's contribution is relatively modest. '(It) will contribute the Deep Space Network (large radio dishes scattered around the globe), a science team of 12, navigation and guidance,' says the BMDO project manager Colonel Pedro Rustan. 'But 90 per cent of the work was done here.'

Clementine's launch weight is 450kg without the solid propellant rocket motor, and spacecraft design and construction were completed within three years.

Advanced computer chips from Matra Marconi will be used to compress the data for storage in a large-capacity solid state recorder before transmission to Earth.

The mission is mainly military-led, but a team of scientists headed by Eugene Shoemaker of the US Geological Survey in Flagstaff, Arizona, had some say on the lunar orbits and the most suitable wavelengths to study the target bodies.

The complement of scientific instruments includes six cameras - two for navigation and four for science experiments. These will scan their targets in 11 different wavelength bands between the near-infra-red and ultraviolet.

Civilian scientists such as Shoemaker are not entirely happy with the idea of using a Department of Defense spacecraft, but in the absence of a Nasa lunar programme, there is little choice. Col Rustan accepts that Clementine leaves something to be desired as a planetary probe. 'If it was a strictly scientific mission, the instruments would have to be upgraded.'

This is not to say that the scientific results will not be of any value. 'We haven't been to the Moon in more than 20 years, or to a near-Earth asteroid ever, so any information will be new,' says Col Rustan. 'We'll get better than Apollo resolution from the same distance, but the cameras weigh 1/100th as much.'

Clementine is scheduled for launch tomorrow by a refurbished Titan 2 ballistic missile from Vandenberg in California. The craft will reach the Moon in late February.

It will then be placed in a highly elliptical, five-hour orbit over the lunar poles, a flight path not available to the Apollo astronauts or previous unmanned probes. The next two- and-a-half months will be spent mapping the Moon strip by strip as it spins beneath the craft. Average resolution of the images will be about 200 metres, but the laser-imaging system should be able to detect objects as small as five metres across when the satellite swoops to within 400km of the surface.

Clementine is expected to provide the first full digital image model of the Moon. If any water ice exists in deep shaded craters near the lunar poles, it should be revealed to Clementine's electronic eyes. Geologists should also be able to learn more about lunar history and the processes that have shaped its surface.

Once its studies of our Moon are completed, Clementine will be redirected towards its second objective, the asteroid 1620 Geographos. During the next four months, it will follow a roundabout route that involves two Earth fly-bys and a close lunar pass, before zipping past its small, rocky target at a distance of less than 100km.

Approaching from the shaded dark side, Clementine's view will change from a thin crescent to almost full illumination as the irregularly shaped chunk of rock swells in size. More than 2,000 images should eventually be relayed back to Earth for analysis.

Despite considerable scientific interest, BMDO intends to shut down the Clementine programme later this year. Under pressure from budget cutbacks, officials have said that research into space-based missile defence systems is no longer compatible with the new emphasis on ground-based defence.