The Lunar Prospector spacecraft will be deliberately crashed into the Moon at the end of its mission, Nasa has confirmed.

The impact - equivalent to smashing a heavy car into a wall at a speed of more than 1,100 mph - is an attempt to confirm the presence of ice at the lunar poles.

On 31 July, the probe will be targeted to hit a shadow-filled crater near the Moon's south pole which is thought to contain hidden ice. The scientists hope the controlled crash will "liberate" up to 18 kilograms (40 pounds) of water vapour that could be detected by observatories Earth and in space.

A risk worth taking

"While the probability of success for such a bold undertaking is low, the potential science payoff is tremendous," Guenter Riegler, a Nasa official, said.

"Since the implementation costs are minimal and the mission is scheduled to end anyway, it seems fitting to give Lunar Prospector the chance to provide scientific data right up to the end of its highly successful mission," he said.

The Moon's south pole region

The Lunar Prospector probe went into lunar orbit in January 1998 as the first US moon mission since the Apollo 17 astronauts walked on the lunar surface in 1972.

Now, the Lunar Prospector team at Nasa's Ames Research Centre is preparing to send the 161 kg (354 pound) probe crashing into a polar crater on July 31.

One in 10 chance

Scientists said that, while they were excited by the prospect of the impact, failure to detect any water vapour as a result of the experiment should not be taken as proof that water ice is not present.

David Goldstein of the University of Texas at Austin, one of the proposers of the crash study, estimated the overall probability of success at about 10%