Mission controllers have lowered the Lunar Prospector spacecraft into a 40 km (25 mile) polar lunar orbit, down from its current 100 km (63 mile) mapping orbit.

Lunar Prospector has revolutionised our understanding of the Moon with the discovery of ice in the dirt near the lunar poles.

The spacecraft will stay in its new orbit for about four weeks. It will then be lowered into an even closer 25 km (15 mile) orbit in January 1999.

These dramatic changes in orbit mark the end of Lunar Prospectors successful main mission, which began in January 1998.

"Lunar Prospector's instruments have gathered such superior data that we have far exceeded our primary mission objectives," said Sylvia Cox, Nasa's Mission Manager for Lunar Prospector.

"This success raises our expectations about getting an even closer look at the lunar surface, collecting data at higher resolutions, and gaining further insights about our closest celestial neighbour."

It will now begin an extended mission which is due to continue until June 1999. Its instruments will gather more data at much higher resolutions.

This will allow scientists to refine their estimates of ice deposits at the north and south lunar poles.