"We're going to learn a lot from this," said program manager Dan Andrews of NASA's Ames Research Center in Moffett Field, Calif. "It's going to give us a real definitive understanding of what we have up there."

NASA astronauts visited the moon during the late 1960s and early 1970s under the Apollo program but have not returned.

In the aftermath of the 2003 Columbia shuttle disaster, President Bush instructed NASA to retire the shuttle fleet in 2010 and return humans to the moon by 2020 and then aim for Mars.

First, though, NASA plans a series of robotic precursor missions including the Lunar Crater Observation and Sensing Satellite, or LCROSS, which will plow into the crater, and the mapper, called the Lunar Reconnaissance Orbiter.

When LCROSS strikes the crater, it is expected to create a hole 16 feet deep and send up a 2.2 million-pound plume of debris for sensors and cameras stationed on a second spacecraft to monitor.

Dozens of ground-based telescopes, as well as possibly space observatories, such as the Hubble telescope, will be trained on the plume as well.

A monitoring satellite that is part of LCROSS, but separate from the reconnaissance orbiter, will then fly through the plume to collect and relay data back to Earth. It will have just 15 minutes before it too crashes into the moon, sending up a second, smaller plume for additional studies.

Two previous missions, the military's Clementine spacecraft and NASA's Lunar Prospector, determined the moon's south pole is particularly rich in hydrogen, which scientists suspect is bound with oxygen to form water.

But there are other theories to explain the hydrogen readings as well.

"What this mission buys us is an early attempt to get to know what the resources are," said Scott Horowitz, head of NASA's lunar exploration program. "We know for sure that for human exploration to succeed we're going to have to eventually live off the land."

Water ice could be used to make oxygen for astronauts to breathe, as well as an oxidizer for rocket fuel.