Scientists working on NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) have decided to target an older crater near the Moon's south pole for their impact study.

NASA Ames

Original plans called for the mission to send its spent upper stage crashing into the eternally dark floor of the Shackleton Crater, where NASA might one day plant a human outpost. The 5,600-mph impact will send a plume of debris as high as 30 km above the surface, giving the nine instruments on LCROSS - as well as telescopes in space and on Earth - an opportunity to analyze it for water molecules and other data.

Scientists believe permanently shadowed craters like Shackleton may harbor primordial water ice left over from comet impacts early in the Moon's history, preserved by the deep cold. Data from orbit lend some support to the theory, but it is ambiguous. LCROSS was selected as a piggyback mission to NASA's Lunar Reconnaissance Orbiter that could reach out and touch the lunar surface -- literally - so scientists can calculate what is there.

Dan Andrews, NASA's LCROSS project manager at Ames Research Center, said today that the mission science team has decided to target an older crater to get the best possible data. While Shackleton is almost exactly on the lunar south pole, craters slightly to the north also have permanently dark floors and are within rage of LCROSS. Prime candidates are Shoemaker Crater and Faustini Crater, just to the east of Shoemaker.

The final choice will depend on just when NASA's Lunar Reconnaissance Orbiter (LRO) launches. LCROSS is piggybacking on that mission, which will map the Moon for future exploration. Once LRO launches and releases LCROSS, Andrews said, the LCROSS controllers at Ames will tweak its trajectory to bring the spent Centaur upper stage down in one of the craters, and then fly right through the plume. On a later orbit the LCROSS spacecraft itself will follow the Centaur down for a second impact plume, which will be analyzed by more distant instruments.

In fact, amateur astronomers should be able to view the plume with relatively small instruments. NASA is also developing plans to distribute information on the impact through Google, an industry partner of Ames.

LCROSS is being built by Northrop Grumman, which is well within its schedule to deliver the hardware on time. NASA's LRO development schedule is much tighter, but the agency still hopes to get the two spacecraft on their way to the Moon this year.