If humans are to live on the moon someday, or simply use it as a way station for the journey to Mars, water will be a critical resource—and having a local supply would be invaluable.

That's why NASA plans to crash some trash into the moon this fall.

The Lunar Crater Observation and Sensing Satellite (LCROSS), set to launch June 17 with the better-known Lunar Reconnaissance Orbiter (LRO), could help answer the decade-old question of whether there is frozen water on the moon. Scientists know there is hydrogen at the lunar poles—they just don't know if it is locked in water or something else.

"We're the first mission to go in there and touch the regolith [lunar surface] that is bearing this hydrogen compound, and lift it into sunlight to see it," says Anthony Colaprete of the NASA Ames Research Center at Moffett Field, Calif., the principal investigator for the $79-million mission.

In the mid-1990s, the Clementine mission found evidence of lunar water ice, but later tests cast doubt on the discoveries. The Lunar Prospector probe later found lunar hydrogen, but no one knows whether it represents water ice deposited by a meteoroid, or if it's embedded in a compound like methane. More recent studies, including an Indian impactor probe last fall, have not yet settled the matter.

Donald Campbell, an astronomy professor at Cornell University, said scientists have all but ruled out large sheets of ice hidden in shadowed lunar craters. Radar observations have not turned up telltale signs of water ice, as they have on Mercury and Jupiter's moon Europa, among other places. But Campbell believes the moon might harbor small pockets of ice.

"If the ice is just in the form of very small ice blocks, so to speak, in the lunar regolith, then the radar would not sense that at all," he says. "I'm looking forward to them detecting at least some level of water ice at the poles. There's certainly something going on, from the Lunar Prospector results. ... The actual impact of LCROSS will obviously be the most direct experiment."

The Lunar Reconnaissance Orbiter will also take a look from above the surface, but a hunt for water is not its primary mission. When planning the LRO launch, engineers realized their Atlas 5 rocket had enough thrust for a second probe to hitch a ride. Colaprete and others proposed crashing something into the moon to study the issue from the ground up.

"LCROSS is actually able to measure that hydrogen-bearing material better, and in a more direct way, than LRO will be able to," Colaprete says.
 
On October 8, a spent rocket stage from the probes' launch will crash into a shadowed crater at the lunar south pole, whose surface has not seen sunlight in two billion years. A plume of debris will puff into space, where any water molecules will instantly vaporize. A suite of cameras and spectrometers on board LCROSS will study the plume and determine whether the hydrogen comes in the form of water.

Four minutes later, LCROSS itself will smack into the moon and excavate a second crater. The Hubble Space Telescope, the Keck Observatory in Hawaii and a European satellite will all be watching the impacts—and anyone with a 10-inch (25-centimeter) telescope or larger may be able to see the effects, as well.

Colaprete says that whether or not LCROSS finds water, the mission will be useful.

"We don't need to see water ice to be a success," he says. "We don't have a good understanding of what this source of hydrogen is just yet," he adds, but LCROSS will be ideally positioned to find out. "It will definitely add a facet to this exploration that none of the other missions will be able to address, but all the other missions help to support."