Nasa is set to launch two spacecraft to the Moon in preparation for a return to the lunar surface by US astronauts. LRO (Lunar Reconnaissance Orbiter) and a crater observation mission will lift off from Florida on an Atlas V rocket. Data gathered by LRO will help mission planners select future landing sites and scout locations for lunar outposts. The second mission will send a rocket stage crashing into the lunar surface to scour the resulting debris plume for evidence of water ice. The Atlas rocket, carrying both payloads, is scheduled to blast off from Florida's Cape Canaveral Air Force Station at 2212 BST (1712 local time) on Thursday.
LRO will enter a low polar orbit around the Moon at an altitude of around 50km (31 miles) - the closest any spacecraft has continually orbited Earth's natural satellite.
It will map the lunar surface in unprecedented detail and is expected to enhance our understanding of the Moon's topography, mineral composition and lighting conditions.
The mission will also seek to characterise the Moon's radiation environment, helping mission planners assess the risks posed to astronauts.
The second mission, the Lunar Crater Observation and Sensing Satellite (LCROSS), aims to answer whether there is water on the Moon - either in the form of ice or hydrated minerals. It consists of two elements: a shepherding spacecraft and a Centaur upper stage rocket.
After being guided to a permanently shadowed crater at the Moon's south pole by its shepherding spacecraft, the Centaur rocket separates.
Hitting the Moon at more than 9,000 km/h (5,600 mph), the 2,200kg Centaur will kick up a huge plume of debris which could rise some 50km (30 miles) above the surface.
It is expected to loft some 250 metric tonnes of material above the lunar surface.
Four minutes after impact, the shepherding spacecraft follows a very similar path to the rocket, descending through the plume.
It will use its instruments to analyse the material, searching for water ice and vapour, hydrocarbons and hydrated materials.
The spacecraft will collect data continuously until it too slams into the lunar surface, creating a second plume.
The impacts will be watched closely by astronomers using ground-based telescopes.
Any natural reserves of water will figure prominently in planning for future manned lunar bases. It can be split into hydrogen for rocket fuel and oxygen for breathing and makes an excellent shield against radiation.
The US space agency hopes to send astronauts back to the Moon by 2020 for the first manned visit since 1972.
However, the Obama administration has ordered a sweeping independent review of Nasa's manned spaceflight strategy, which could potentially set the agency on a different course.