Nasa is set to crash two unmanned spacecraft into the Moon in a bid to detect the presence of water-ice. A 2,200kg rocket stage will be first to collide, hurling debris high above the lunar surface. A second spacecraft packed with science instruments will analyse the contents of this dusty cloud before meeting a similar fate. The identification of water-ice in the impact plume would be a major discovery, scientists say. Not least because a supply of water on the Moon would be a vital resource for future human exploration. The existence of water-ice in permanently shadowed craters at the lunar poles had previously been postulated by scientists, but never confirmed. The $79m (£49m; 53m euro) mission is called LCROSS (the Lunar Crater Observation and Sensing Satellite). There are two main components: the large Centaur rocket upper stage and a smaller "shepherding spacecraft". These have been connected since they were launched from Cape Canaveral, Florida, in June. The shepherding spacecraft is designed to guide the rocket to its target at the Moon's south pole, a shaded 100km-wide depression called Cabeus crater. In the early hours of Friday morning (BST), the Centaur and shepherding spacecraft separated. Then, at 1231 BST (0731 EDT), the rocket stage will hit the Moon's south pole at roughly twice the speed of a bullet, throwing an estimated 350 metric tonnes of debris to altitudes of 10km (6.2 mile) or more. With an energy equivalent to one-and-a-half tonnes of TNT, the collision will carve out another crater some 20m (66ft) wide and about 4m (13ft) deep. The shepherding spacecraft will follow in the Centaur's wake, descending through the debris plume to hit the lunar surface four minutes after the initial impact. It will use onboard spectrometers to look for signs of water, hydroxyl compounds (OH), salts, clays, hydrated minerals and organic molecules in the plume. Permanently shadowed craters are very cold, receiving heat only from space and from the Moon's interior (which is geologically dead). Here in the lunar "shadowlands", ice - perhaps delivered by cometary impacts - is protected from the Sun's rays and could remain stable over geological timescales. In September, analyses of data from three spacecraft revealed that very fine films of water coat the particles which make up the lunar soil. This water might also migrate to darkened craters, sublimating during the lunar day and condensing once it reached the cooler poles. Excess hydrogen "If ice is present in the permanently shaded craters... it could potentially provide a water source for the eventual establishment of a manned base on the Moon," said Dr Vincent Eke, from Durham University, UK, who is not a member of the LCROSS team. But Dr Bernard Foing, chief scientist at the European Space Agency (Esa), said it would be desirable to protect some of this polar ice, if it indeed exists. "We will have to be careful to keep some areas as 'protected parks' on the Moon. This is so that we could, for instance, send a lander, drill down and obtain a core sample a few metres in depth," he told BBC News. "Then we could eventually study the history of delivery of water to the Moon and the Earth." Dr Foing will work with the LCROSS team to compare the data from that spacecraft with information collected when Esa's Smart-1 probe was brought down on the Moon's surface in 2006. The idea that reserves of water-ice might persist in these dark craters was given a boost by Nasa's Lunar Prospector mission, which launched in 1998. By looking at the energy of neutrons coming from the Moon, scientists found excess hydrogen at both poles. This result does not confirm the presence of water-ice; the hydrogen could be in another form. But if it does exist in the form of ice, data suggests the lunar poles could hold hundreds of metric tonnes. Ground observations Anthony Colaprete, principal investigator on the LCROSS mission, said recent data from Nasa's Lunar Reconnaissance Orbiter (LRO) supported the team's decision to target Cabeus crater. "There is hydrogen down in that crater; we're going to go dig some of it up," he explained. Professional astronomers will study the impacts using ground-based telescopes, including the Magdalena Ridge and Apache Ridge Observatories in New Mexico, US, and the MMT Observatory in Arizona. The Centaur impact plume might be visible through amateur-class telescopes with apertures as small as 10 to 12 inches, mission scientists said. Earth-orbiting satellites are also geared up to observe the collisions, including the Hubble Space Telescope; and the Odin satellite, an astronomy and aeronomy mission led by Sweden. In August, the mission was threatened when a problem caused LCROSS to lose a "substantial" amount of its propellant. Source: BBC