Today (9 October 2009) two unmanned NASA spacecraft will impact the Cabeus crater in the lunar South Pole, in the final stage of the LCROSS mission (Lunar Crater Observation and Sensing Satellite).

The first spacecraft, the 2200kg Centaur rocket stage, will hit the Moon at about 4800 km/h or twice the speed of a bullet, throwing up debris plumes to an expected height of 10km above the lunar surface, visible from Earth-based telescopes 10-12" and larger.

The smaller 'shepherding spacecraft' will follow, descending through the debris plume to impact 4 minutes later. As it travels through the plume, onboard spectrometers will monitor the chemical components of the debris, looking for water, hydroxyl compunds, salts, clays and organic molecules, and relay this information back to Earth before impact.

Projected first impact is currently 12:31 BST.

You can watch the impact live online on NASA TV.

Image: Artist's rendering of the LCROSS spacecraft and Centaur separation (NASA).