BOULDER, Colo. — NASA might not be ready to plant more footprints on the lunar surface, but officials at the U.S. space agency recently took some baby steps in that direction by requesting proposals for an unmanned mission to collect samples from the moon’s south pole and return them to Earth.

NASA announced in May that it wants proposals for a new $650 million mission under its New Frontiers solar system exploration program that can be ready to launch in 2009. The competition will be open, but proposals limited to four categories: a lunar sample return mission, a comet sample return, a Jupiter polar orbiter with probes and a landing on Venus. Whichever mission is chosen will be the second mission in the New Frontiers program. A mission to Pluto scheduled for launch in 2006 is supposed to be the first New Frontiers project.

A number of planetary scientists have been thinking about a return to the moon, particularly a mission to an area called the South Pole-Aitken Basin to look for ice in lunar surface material that one day could be used to support lunar expeditions or even a base.

Past robotic spacecraft orbiting the moon have spotted what appear to be super-cold stashes of water ice. These reservoirs appear to be tucked away inside craters, permanently hidden from the sun’s thawing rays. The moon’s South Pole, in particular, is thought by many scientists to be home to tons of water ice.

NASA planetary researchers believe processing machines could be built to convert water ice into drinkable water for a crew and also be used to extract oxygen and hydrogen that could then be used to make rocket fuel.

The South Pole’s Aitken Basin is the largest and oldest lunar basin. Debris in its rim and material ejected when the basin was created contain material from the lower crust and possibly the upper mantle of the moon. Snagging unique samples from there also should give scientists better insight into the very early history of the Earth-moon system.

If funded, a sample return craft could be launched before the end of the decade. It is a journey that only takes three days, unlike the six-month, one-way missions to Mars that NASA is currently conducting.

Philip Stooke, a space scientist at the Departments of Geography and Physics and Astronomy at the University of Western Ontario in London, Ontario, Canada, said the lunar South Pole has great potential as the site of a future moon base because it contains areas that are nearly always in sunlight and areas that almost never receive sunlight.

An area informally known as the "peak of eternal light" — a ridge between the rims of several craters — receives sunlight nearly all the time, though no single spot is permanently illuminated. Landing robotic machinery in an area of nearly continual sunlight makes thermal control and power generation easier, Stooke said.

The lunar South Pole also has the advantage of feasible rover routes into and out of the craters and small, fresh craters that scientists would like to study, Stooke said.

Using imagery taken in 1994 by the U.S. Defense Department’s Clementine spacecraft, Stooke identified seven potential landing zones, areas from which a rover might have access to nearby, permanently shaded terrain.

Several of those spots were once considered by the European Space Agency under the rubric of the now defunct EuroMoon 2000 project.

One challenge is that knowledge of lunar topography is extremely limited in this area because of the effects of shadowing in both Clementine images and Earth-based radar, especially inside craters. So a detailed analysis of route safety for robotic rovers and the best sites for good communications with Earth is not yet possible, Stooke said.

Interest in the lunar poles preceeded even the launch of Sputnik 1 by the former Soviet Union. As early as 1955, the Soviets were scripting a plan for a remote controlled lunar rover at one of the poles, Stooke said.

Furthermore, in 1961, the Nobel Prize winning American cosmochemist, Harold Urey, identified possible ice at the poles as a target for lunar exploration. So the idea is not new, and interest in scouting out polar ice has grown with NASA’s Lunar Prospector data that was gathered in 1998-1999.

One thought stemming from Stooke’s research is how best to handle Earth-moon communications.

"It’s tricky at the poles because the moon’s axis is tilted about 5 degrees off the Earth-moon line. So if you’re at the pole, Earth is sometimes 5 degrees above the horizon, which is good. But sometimes it’s five degrees below the horizon. Then you have a communications blackout. Periods of no communication would last a bit less than two weeks," Stooke said.

One solution is a relay station at about 85 degrees latitude on the moon’s near side. But Stooke offers another alternative. "Not perfect but quite good and probably easier in the near term is communication with satellites in geostationary orbit," the researcher points out.

"The geostationary orbit is tilted such that — not all the time but frequently — when Earth is below the polar horizon some geostationary satellites are not. Given suitable radio equipment on a few geostationary satellites, they could cover most of the blackout periods at the moon’s pole. I would fly lunar communication equipment piggyback on satellites normally dedicated to other purposes," Stooke suggests.

"My work is intended to help plan a necessary intermediate step, getting robotic rovers into the area to examine the ice. This is not an active mission, of course, but it could be one soon," Stooke said.