By Paul Rincon
Science reporter, BBC News
The "shepherding spacecraft" will analyse the impact debris
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 $79m (£49m; 53m euro) mission is called LCROSS (the Lunar Crater Observation and Sensing Satellite).
Greg Schmidt, deputy director of the US space agency's Lunar Science Institute said: "We're attempting to answer here one of the most important remaining questions for both science and future exploration: Has water been deposited on the Moon in major quantities?"
"We're doing the LCROSS mission in a new low-cost way that can serve as a pathfinder for future missions Nasa is interested in doing."
The existence of water-ice in permanently shadowed craters at the lunar poles had previously been postulated by scientists, but never confirmed.
These 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.
The Centaur successfully separated from LCROSS early on Friday (BST)
This water might also migrate to darkened craters, sublimating during the lunar day and condensing once it reached the cooler poles.
There are two main components of the mission: the large Centaur rocket upper stage and a smaller "shepherding spacecraft" which is also known as just LCROSS.
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 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 sun-lit plume.
The red dots show impact points for the two spacecraft in Cabeus crater
In the late 1990s, Nasa's Lunar Prospector mission looked at the energy of neutrons coming from the Moon and detected an excess of hydrogen at both poles.
This result does not confirm the presence of ice; the hydrogen could be in another form. But if it does exist in the form of ice, the polar regions could hold hundreds of metric tonnes.
Dr Vincent Eke, from Durham University, UK, helped map hydrogen concentrations across the Moon's surface using Lunar Prospector data.
He told the BBC: "Water molecules contains two hydrogen atoms and one oxygen atom, so this rocket should aim for where there is some hydrogen if it wants to have a chance of finding water."
On the prospects for a successful outcome, he said: "The interesting thing is whether the debris that gets thrown out will actually get thrown out of the crater and into the sunlight.
"Nasa predicts that the debris should rise [up to] nine miles, which should certainly take it over the crater rim. But if, for some unfortunate reason, the debris doesn't get into the sunlight, we won't be able to see it, which will be disappointing.
If ice is present in permanently shaded craters it could provide a water source for the eventual establishment of a manned base on the Moon.
But Dr Bernard Foing, executive director of the International Lunar Exploration Working Group (ILEWG), 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.
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.
In some parts of the world where it is dark at the time of impact, the Centaur debris 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.
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