HOUSTON, Texas -- U.S. space exploration plans came into sharper focus with NASA's announcement that it intends to lay the first pieces of an international lunar outpost at the Moon's north or south poles starting around 2020.

For spacefaring nations considering joining the United States on the Moon, NASA's unveiling of a fairly detailed lunar exploration plan--highly tentative though it may be--was a small but important step toward international collaboration, experts here said.

NASA's proposed lunar architecture--essentially, a rough plan to scout the Moon with robotic trailblazers before sending astronauts and more machines to lay a foundation for a permanent outpost at one of the lunar poles--is the United States' response to an overarching Global Exploration Strategy that emerged this year from a series of international meetings involving 14 space agencies and more than 1,000 people including government officials, business executives, scientists and other experts, NASA Deputy Administrator Shana Dale said during a Dec. 4 press conference here.

Dale and other NASA officials revealed more details of the proposed lunar architecture--and the opportunities they saw for international participation--over the next two days at the 2nd Space Exploration Conference here organized by the American Institute of Aeronautics and Astronautics.

"First, we see the Moon not as a brief rendezvous, but as an outpost," Dale said. "Our objective is to create an enduring, sustainable human and robotic presence that will open up vastly greater opportunities for science, research and technological development."

Dale described NASA's lunar plan as "an open architecture" that other nations and commercial interests could add to "in order to evolve and allow the journey to continue to Mars and to other destinations."

Doug Cooke, NASA's deputy associate administrator for exploration systems, continued that theme during the conference's panel discussions. He said NASA is very interested in getting other nations to join the United States on the Moon "so we can all accomplish more than we could on our own."

Cooke said that while NASA intends to field the necessary transportation systems and establish the essential elements of a human outpost at one of the lunar poles, there is plenty of work to be done by others to augment and expand the settlement.

A decision was made to locate the base at one of the poles, Cooke said, because it offers a number of operational and scientific advantages over equatorial locations, including longer extended periods of sunlight, more moderate temperature swings, and the tantalizing prospect that the poles might harbor stores of water ice in permanently shadowed craters.

The job of describing how NASA foresees the first five years of human lunar expeditions unfolding was left to Tony Lavoie, a Marshall Space Flight Center official who leads the agency's Lunar Architecture Team. The notional plan produced by Lavoie's team would enable six-month stays within five years by making sure every lunar landing leaves behind at least some critical piece of infrastructure.

An important element of this approach, Lavoie said, entails designing a crew and cargo lander that minimizes the size of its ascent and descent modules in order to maximize the amount of equipment it can put on the Moon's surface. Notionally, NASA is looking at 6,000 kilograms of landed mass, he said.

Lavoie said his team chose the Shackleton Crater at the south pole because NASA currently knows of no better polar location, but that could easily change once the Lunar Reconnaissance Orbiter, due to launch in late 2008, has a chance to report back.

Most of what NASA knows about the lunar poles comes from the 1998 Lunar Prospector mission. While Lunar Prospector overflew both the north and south poles during its one-year mission, the satellite saw more of the south pole during winter, Lavoie said, giving NASA a better feel for conditions there.

Regardless of whether NASA heads north or south, the poles are attractive in part because they are more thermally moderate than the equatorial regions, where temperature swings of plus or minus 250 degrees would put added stress on equipment and machines. Lavoie said the higher percentage of sunlight at the poles--the edge of Shackleton Crater, for example, is permanently in sunlight at least 75 percent of the time--makes it possible for NASA to consider solar power and fuel cells as an alternative to nuclear power.

Before the first astronauts are sent back to the Moon, Lavoie said, NASA envisions conducting an unmanned test of the lander, using the mission to deliver an unpressurized rover and a solar power unit producing perhaps 6 kilowatts of electricity.

When astronauts finally arrive, they would stay for seven days and bring with them a habitat and other equipment that they would leave behind. A second mission later that year would deliver additional power-generation units, perhaps another unpressurized rover and other still-to-be determined infrastructure.

The focus of the second year, Lavoie said, would be putting additional habitation modules, power-generators and other infrastructure in place to support 14-day stays starting in the third year. The build up would continue with two missions a year, enabling 30-day stays with the arrival of a fourth habitation module at the beginning of year four.

By the end of fifth year of human expeditions, Lavoie said, NASA would anticipate being ready to support six-month stays at the growing outpost.

A key focus of the first five years, under the plan NASA laid out, would be to demonstrate various forms of in-situ resource utilization, or ISRU, finding a way to use lander byproducts and even astronaut waste to support surface operations.

But because ISRU is "in its infancy," as Lavoie put it, NASA will not rely on it for anything until it is proven.

And while NASA plans to emplace the necessary infrastructure to support extended expeditions, Lavoie said one of the advantages of the outpost approach is that there is plenty of room for other agencies or entities to add to the outpost or augment its capabilities by, for example, delivering additional ISRU systems or communications assets.

A lengthy list of needed capabilities for a lunar outpost that NASA presented during the conference was remarkable for the limited amount of items the United States staked out for itself, several conference attendees remarked.

The response to NASA's plans from international space agency officials in attendance was positive. A number of these officials praised NASA for engaging the world's space agencies early in the process, a contrast, they said, to how the U.S. planned the international space station.

"The overall approach was very un-NASA," one non-U.S. space agency official said, meaning it as a compliment.

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Others said they were very pleased that NASA presented its plans in enough detail to allow them go back home and engage their governments in fruitful discussions about how their agencies could participate.

John Logsdon, the director of George Washington University's Space Policy Institute, said there was "high level of enthusiasm" among the internationals at the conference both for the overall process and the end product NASA unveiled.

"With the announcement that this is leading toward a permanent outpost, that gives everybody a common objective to plan for," he said.

Space agency representatives were due to meet Dec. 8 at the Lunar and Planetary Institute here to craft and issue a common statement on the Global Exploration Strategy.

More international meetings are on tap for 2007, according to NASA. The agency also plans to get started early next year on an initial Mars architecture. Cooke said the primary purpose of doing a Mars architecture now is to make sure it "synchs up" with NASA's lunar plans.

Logsdon predicted that it would be several years before another agency announced concrete plans to join the United States on the Moon. But he said he would expect to see an agreement on the framework for coordination and cooperation perhaps as late 2007.