Nasa is set to crash two unmanned spacecraft into the Moon in a bid to detect the presence of water-ice.

美国航空航天局将向月球坠落两艘无人宇宙飞船以探测冰水是否存在。

A 2,200kg rocket stage will be first to collide, hurling debris（碎片，残骇） 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（羽流） 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（弹坑，火山口） at the lunar poles had previously been postulated（要求，假定） 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 98m-wide depression called Cabeus crater.

Rocket separation

In the early hours of Friday morning (BST), the Centaur and shepherding spacecraft will separate.

At a scheduled time of 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（公吨） of debris to altitudes（高度） of 10km (6.2 mile) or more.

With an energy equivalent to one-and-a-half tonnes of TNT, the collision will carve out a 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（光谱仪） to look for signs of water, hydroxyl compounds（羟基化合物） (OH), salts, clays（黏土，陶土）, hydrated（含水的） minerals and organic molecules in the sun-lit plume.

The spacecraft will collect data continuously until it too slams（砰然，猛击） into the Moon, generating a second, smaller debris cloud.

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（彗星似的） 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（升华） during the lunar day and condensing once it reached the cooler poles.