As the United States prepares to return humans to the moon by 2020, the space agency NASA is busy figuring out what to do once they are there. U.S. space officials have had to juggle priorities to meet the goal while still operating space shuttles and a space station.

Last year, NASA revealed its concepts for the rocketships that will carry people and cargo to the moon late in the next decade. This year's activities build on that, according to the agency official in charge of the moon exploration program, former astronaut Scott Horowitz.

"Last year we did the exploration systems architecture study - basically the 'how are we going to get there'," asked Scott Horowitz. "What we are looking at this year - this is the year of what do we do when we get there?"

The answers will determine what kind of landers the U.S. designs and where they will go. Horowitz says NASA has just begun reaching to industry and scientists for ideas. In contrast, it is much farther ahead on the spaceship that will bring humans to the moon. It plans to choose its prime contractor for that part of the project by August or September.

The program is the result of a new space policy President Bush announced in 2004 to return humans to the moon by 2020 as the first step in an exploration program that would eventually land crews on Mars.

NASA officials have been reorganizing the agency to meet the deadline while continuing to build the International Space Station and operate the space shuttle until 2010. It is like a two front war, and to make things harder, they must work without budget increases for the additional program. Scott Horowitz calls the effort a grand challenge.

"This is different," he said. "For the last 20 or 30 years in human space flight, NASA has been in the operating mode. We have not developed a new spacecraft to lift humans off the planet in over 30 years. Its a big deal and it's a big challenge and we can't keep doing it the way we are doing it with shuttle and station. If we want to continue all the development, which is the launch vehicles and the lander and the robotic precursor missions, we need to do development at the same time we do operations. We are going to have to ask our people to do different things than they do today."

NASA's boss, Michael Griffin, had hoped to have the new crew vehicle, the successor to the space shuttle, ready by 2012, since it could also be used to visit the space station or for space telescope repair as well as moon missions. But budget problems have caused him to push that back two years.

"With regard to overall deployment dates and test dates, of course, they are financially driven," said Michael Griffin.

Griffin shifted more than $1.5 billion from crew vehicle development to help make up part of a $4-billion shuttle and space station budget shortfall he discovered last year not long after becoming NASA's administrator.

"That funding hit imposed delays at least on the nominal deployment dates that we don't like but that we have to live with," he said.

The money issue is also pressing on the schedule for getting people on the moon. Whereas the original intent was to have an astronaut back on the lunar surface between 2015 and 2020, the goal is now to meet the end of the five year window, although Griffin says he will work hard to advance that.

He also says NASA will try to have the new crew vehicle ready before 2014 to shorten the gap between its first deployment and the last shuttle flight in 2010. The NASA chief does not want the United States to be out of human space flight too long.

"It's a strategic element of what makes the United States a great power and we will not abandon it," said NASA's chief.