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 then analyse the contents of this dusty cloud before meeting a similar fate.Scientists say the identification of water-ice in the impact plume would be a major discovery. Not least, because a supply of water on the Moon would be a vital resource for future human exploration. The 79 million Dollar 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.