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New impact site for LCROSS water-hunting mission
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: September 28, 2009


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Officials have shifted the target for next week's smash into the moon by the LCROSS lunar impactor mission after a new analysis showed another crater has a better chance of yielding results verifying the existence of water at the south pole.

NASA announced on Sept. 11 the probe would be targeting a crater named Cabeus A near the moon's south pole. In an update posted to the LCROSS Web site Monday, the agency said scientists agreed to select a new crater based on the latest data from current and retired lunar orbiters.

The impact is scheduled to occur around 1130 GMT (7:30 a.m. EDT) on Oct. 9.

The new crater, called Cabeus, was on the short list of candidate targets considered by the science team before officials made their initial decision earlier this month.

"The LCROSS team concluded that Cabeus provided the best chance for meeting its mission goals," the statement said.

Cabeus is not far from Cabeus A near the moon's south pole. Both craters have floors that never receive light due to the low sun angles at the polar regions.


A map of the lunar south pole. Cabeus is labeled SP_C on the left side of the image. Cabeus A is unlabeled at the upper left. Credit: NASA
 
Scientists made the change based on the latest understanding of hydrogen concentrations in the region from information recently collected by the Lunar Reconnaissance Orbiter and archived data from the Lunar Prospector mission more than 10 years ago.

"The general consensus of lunar experts led by the LCROSS science team is that Cabeus shows, with the greatest level of certainty, the highest hydrogen concentrations at the south pole," the Web update said.

New terrain models compiled from topographic information from LRO and Japan's Kaguya spacecraft also show a small valley along the Cabeus perimeter rim. The previously-unseen notch will allow sunlight to illuminate material propelled above the surface by the high-speed impact of the mission's Centaur rocket stage.

"While the ejecta does have to fly to higher elevations to be observed by Earth assets, a shadow cast by a large hill along the Cabeus ridge, provides an excellent, high-contrast, back drop for ejecta and vapor measurements," the posting said.

The LCROSS shepherding spacecraft is towing the Centaur stage, part of its Atlas 5 launch vehicle, through space and aligning its trajectory to hit the moon next Friday.

Officials will continue to refine the exact point of impact over the next few days to avoid rough spots and to maximize lighting conditions, NASA said.

The probe will release the Centaur a few hours before impact and back away from the inert rocket. A suite of instruments on the spacecraft, along with telescopes on Earth, will observe the Centaur's explosive collision and the resulting cloud of lunar debris sent skyward by the event.

The spacecraft will fly through the lunar dust on the way to its own impact, streaming live data back to Earth about the material's composition.

Scientists hope to confirm the debris contains water ice, a hypothesis established from data showing hydrogen inside permanently shadowed craters at the moon's poles.

NASA last week announced new results indicating lunar water is more widespread than earlier thought, including detections of potential water further away from the poles.

The $79 million LCROSS mission is designed to provide more answers in the search.

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