Thirty-five years after the world watched three Americans leave Earth on a mission to be the first to land on the moon, the United States is plotting a return to the lunar landscape. But while the destination is the same, the motives have changed.

Then the goal was to prove to a divided world simply that it could be done and done best by a free society. Now the driving motivation is to demonstrate the technologies and hone the skills needed to venture beyond Earth’s own backyard.

U.S. President George W. Bush, in dropping the exploration gauntlet during his speech at NASA headquarters in January, said the United States would return to the moon by 2020 “as a launching point for missions beyond.”

Beginning no later than 2008, Bush said, the United States would begin launching robotic missions to the moon to prepare the way for extended human lunar expeditions.

Bush said the moon is a good place to test the equipment and approaches needed if humans are to operate in even more challenging space environments. And the moon’s natural resources and other attributes, he said, have the potential to reduce the costs of further space exploration.

Paul Spudis, a planetary scientist and a member of the presidential commission advising Bush on implementing a national space exploration strategy, said the president got it right when he called for going to the moon first.

“We are going to the moon to basically take the first step beyond low Earth orbit,” Spudis said. “The key thing the president articulated in the vision is the use of lunar resources. It’s something we don’t know how to do, but something we need to be able to do if there is to be a big future for humans in space.”

Doug Cooke, NASA’s deputy associate administrator for exploration systems, said the U.S. space agency’s moon agenda is still very much a work in progress.

“At one end of the scale, we are talking primarily about preparation for exploration beyond the moon -- developing vehicles, spacesuits and other hardware and testing operational capabilities and techniques,” Cooke said. “At the other end of the scale, if there are outside interests that want to develop the moon and they come into play, that could drive us harder toward more permanent capabilities.”

Cooke said NASA is hard at work evaluating a range of approaches to accomplishing the broad objectives spelled out by the president. NASA has put out a broad call to industry for long range lunar mission proposals and hopes to award several study contracts before the end of the year. Cooke said NASA should have a better understanding of the range of possible approaches by September.

Some moon advocates, such as Spudis and Klaus Heiss, director of High Frontier, favor picking a promising plot of lunar real estate and plopping down as much infrastructure as needed to sustain a variety of human and robotic activities. Others, such as Marc Cohen, a researcher at NASA’s Ames Research Center, are advocating building a mobile habitat that can traverse the lunar surface to new destinations while waiting for its human occupants to arrive.

Regardless of how NASA gets there, Cooke said the driving motivation behind going back to the moon is to prepare for increasingly challenging missions.

“We’ve always thought the moon was important, at the very least, for preparing future exploration,” he said. “It has been 35 years since we did it for the first time and a lot of the experience we gained during the Apollo program has been lost.”

As NASA sets out to design the vehicles, habitats and other equipment it will need for extended stays on the moon, Cook said the agency also would be keeping one eye on Mars. “We need to be mindful of the long range of exploration,” he said. “The more that we can do that is common for what we will need for Mars missions, the more efficiently we can progress down that path of exploration.”

Perhaps the most tantalizing feature the moon and Mars have in common is the presence of water -- a critical element for sustaining a human presence, and not just for drinking. Cooke sees water, whether present in the surface or atmosphere of Mars or frozen in the lunar surface, as a valuable source of fuel for all manner of space vehicles.

“The water ice is probably the first thing that would be investigated because it might have immediate application to various missions,” Cooke said.

Those applications, Cooke said, include providing potable water, breathable oxygen and fuel for nuclear thermal or chemical rockets.

The search for water could well lead to the lunar poles, a destination favored by Spudis and others in large part because of the intriguing signatures returned by the Lunar Prospector and Clementine missions of the early 1990s.

“You can readily make the case that because of the presence of [potential resources] in the dark areas and near permanent sunlight offers an ideal environment to learn the things we need to learn,” Spudis said.