From The Times UK – Mark Henderson, Science Editor

A Nasa spacecraft will deliberately crash into the Moon next week on a mission that could enhance the prospects of establishing a manned lunar base.

Only two weeks after three probes discovered water on the Moon, the Lunar Crater Observation and Sensing Satellite (LCROSS) will blast two huge chunks out of its surface to establish whether it exists in a form that could be exploited by astronauts.

In the early hours of Friday morning, the LCROSS probe will separate from the Centaur upper stage of the rocket that carried it to lunar orbit, and send the spent module crashing into the Cabeus crater at the Moon’s south pole.

When the 2.4-tonne Centaur hits at 12.31pm BST, at a speed of 2.5km per second (1.6 miles per sec), it will throw up a plume of debris 10km (6 miles) high.

The LCROSS probe will then fly through the plume and analyse its contents with a battery of sophisticated instruments, before itself crashing into a different spot in the same crater four minutes later, to create a second cloud of dust and rubble.

The impacts, which will be visible from Earth through telescopes with mirrors of at least ten inches, will be studied both from the ground and with lunar orbiters, for traces of water and ice.

The goal is to confirm whether deep craters at the Moon’s poles, which never see the Sun, hold large quantities of water. Such a resource could potentially be tapped by future missions to the Moon for drinking water, oxygen and fuel, improving the outlook for a long-term human presence.

The culmination of the LCROSS mission follows the announcement last week that three probes, including India’s first lunar orbiter, had discovered traces of water all over the Moon’s surface. The water found by the Chandrayaan-1, Deep Impact and Cassini probes, however, is extremely scarce and inaccessible. It appears to exist only in the top millimetre or so of lunar soil regolith, tightly bound to minerals, and would be difficult for astronauts to use.

Large quantities of water at the poles, where hydrogen has been detected by several probes, would be a much more attractive resource for a lunar base. LCROSS should now establish whether or not it exists in at least one polar crater.

“It is an exciting time for water on the Moon,” said Dr Anthony Colaprete, the principal investigator for LCROSS. “Last week was great fun, and hopefully it’s about to get much more fun. You’d have a hard time using what Chandrayaan-1 saw as a resource. If deep craters really do have 1 to 2 per cent hydrogen [as observations suggest], and in water not minerals, that would be much more exploitable.”

As many polar craters are permanently in shadow, they are considered to be potential “cold traps” for water that reaches the Moon through the impact of asteroids and comets. Water of the sort discovered by Chandrayaan-1 could also migrate to these craters, as it sublimates during the hot lunar day and condenses into craters on reaching cold polar regions.

“These cold traps in permanently-shaded craters could have been accumulating water and building up over a billion years or more,” Dr Colaprete said, “That’s what we’re going to excavate and look at.”

The LCROSS impacts will test this hypothesis directly, by raising huge clouds of debris from the bottom of a deep lunar crater of the sort where ice is likely to collect. The Centaur and LCROSS will crash into the Cabeus crater, which is 98km (60 miles) across and of unknown depth: as it is permanently in shadow, this has been impossible to measure.

“The shadows are only so deep, so if you can make an impact that throws eject a few kilometres into the air, it goes from shadow into sunlight,” Dr Colaprete said.

The Centaur impact will leave a crater about 20 metres in diameter, and about 3m to 4m deep. “It’s about the size of a tennis court,” Dr Colaprete said. The explosion will have the energy of at least a tonne of TNT. The impact of LCROSS itself will be about two thirds as large.

NASA’s Lunar Reconnaissance Orbiter probe (LRO), which launched with LCROSS on June 18, will monitor the impacts, as will the Hubble Space Telescope and ground-based telescopes. The impact time was chosen to offer the best possible viewing conditions from the Keck and Subaru observatories in Hawaii.

Nasa this week changed the impact site to Cabeus from the smaller nearby Cabeus A crater, because data from the LRO suggested that the bigger crater contained more hydrogen and was thus more likely to contain ice.

Cabeus was not the first choice because a mountain on its northern side could obscure the view of the debris from Earth-based telescopes.

“We’ve moved to a crater that’s a mixed blessing,” Dr Colaprete said.

“There’s a large hill in front of the impact site, but a deep shadow behind, so there will be less material in view but a higher contrast.

British astronomers will not be able to watch directly, as the impacts will occur in daylight, but large observatories will be able to see the Centaur separate from the LCROSS spacecraft.