America's return to the moon in 2008 will make a big impact - in the literal sense. The U.S. space agency NASA says an unmanned spacecraft that will orbit the moon will be accompanied by projectiles that will smash into the moon's south pole and raise clouds of debris visible from Earth.

The moon is pockmarked with craters from asteroid and meteorite impacts over billions of years. The United States will add one more in late 2008.

It will be in a shaded area of the south pole where previous U.S. spacecraft have sensed hydrogen molecules. The earlier data is not sufficient to tell whether the hydrogen is coming from frozen water or not, so project manager Daniel Andrews says NASA is counting on two impacts to throw up enough material to answer the question.

"We will create a substantial plume, excavate a bunch of material, some of which we believe may be water ice and be able to measure that directly as the plume is created, and a great opportunity to really understand what we have there," said Daniel Andrews.

The crashes will occur as part of NASA's Lunar Reconnaissance Orbiter mission, called LRO for short. It will be the first in a series of U.S. probes that will send satellites, landers, and eventually humans back to the moon to establish a base from which to move on to Mars. This initial mission is to deploy an orbiter to map the moon's surface for potential future landing sites. It will also gather data on radiation levels that are hazardous for humans and scout for potential resources.

Once the orbiter is deployed, NASA says the upper stage of the launch vehicle, the size of a minivan, will break off and deploy a small spacecraft before it dives into a south pole crater, excavating a smaller crater five meters deep and 30 meters wide and blasting material 60 kilometers high. The small craft will fly through the dust and sense what materials are in it before it makes its own plunge and sends up a smaller plume of its own for the bigger LRO satellite to inspect.

Deputy program manager Butler Hine says NASA will be looking not only for water but also other resources of use to future astronauts.

"Why this is important is because these resources can make future human occupation of the moon much more cost effective," said Butler Hine. "If we can live off the land and use the resources that are available, then it becomes much less expensive."

Hine says the excavation is an early, inexpensive attempt to determine the composition of the moon's polar surface before NASA sends landers for the same purpose. He says the poles were not examined by the Apollo moon missions of the 1960s and 70s, which returned dirt and rock samples from the equatorial region.

"LRO, one of its primary missions is to map the polar region, map the craters and try to map the resources within the crater, but anything you do with remote sensing, you never quite know for sure what is there until you touch it," he said.