NASA has chosen the final destination for the LCROSS lunar impacting probe: the crater Cabeus A, near the Moon's south pole. So why is NASA smacking a probe into the Moon at high speed, and why there? The idea is that over millions and billions of years, a lot of comets have hit the Moon. The water from these comets hits the surface and sublimates away... but if any settles at the bottoms of deep craters near the Moon's poles, these permanently shadowed regions can act as a refrigerator, keeping the water from disappearing. It can stay there, locked up as ice, for a long, long time. Some estimates indicate there could be billions of tons of ice near the Moon's south pole. Detecting that water is tough. Radar results have been inconclusive, with some people saying there's lots of water, and others saying there's none at all. By impacting a probe there, any ice located at the impact site will be shot up above the lunar surface, where sunlight will break it up into H ^+ and OH ^- molecules, which can be detected. Thus, LCROSS. I have a more detailed description of all this in an earlier blog post about LCROSS. The choice of Cabeus A for the impact site is a good one. It's near the south pole, it's a likely spot for there to be ice under the surface, it's on the near side of the Moon, so people back here on Earth can observe it, but close enough to the limb that any ejected water can be seen. Here's a map of the area:

Click to embiggen. LCROSS is planned to impact the crater at 11:30 GMT on October 9, which is early morning for the U.S. (in fact, there will be two impacts; one from the spacecraft and another by the Centaur booster) The plume from the impact should stretch up many kilometers. It will almost certainly be too thin to be seen by amateur instruments, but the impact itself should make a bright enough flash to be seen if you have a telescope. The crater itself will be in shadow, making the light flash easier to spot. It'll only last a second or two, so if you want to observe it, be prepared! NASA has a nice webpage with all the info you need to watch this historic event for yourself. I hope everyone gets a chance to see this!