As NASA celebrates the 25th anniversary of its first shuttle flight this week, the agency also steels itself for the biggest upheaval since the moon shot days of Apollo in the early 1970s.

In just four years the three aging, behemoth space shuttles will be shelved — likely headed to museums. And by 2014, a brand new spacecraft will be flying — one designed to get astronauts to the moon by 2018 and eventually Mars.

This wrenching transition will be only the fourth such makeover for the manned space program in the agency's nearly 50-year history. Critics already are grumbling about the lack of money to accomplish the shift to the new crew exploration vehicle. More than a fifth of NASA's proposed $16.8 billion budget for next year will be spent on developing the new vehicle system.

"The new crew exploration vehicle will come in late, over cost and underspent and it will stress the agency to get it to function according to plan," said Duke historian Alex Roland, a persistent NASA critic who worked for the space agency in the 1970s. "It will underperform. It will be just a shadow of what they promised and by the time it's done, critics like me will ask 'What's the payoff in the investment?"'

NASA Administrator Michael Griffin has acknowledged the agency will have to transform itself in order to carry out goals first articulated by President Bush two years ago. The transition will change everything from how astronauts are trained, which NASA operations stay open, which private companies get multibillion-dollar contracts and the size of NASA's work force.

"What we have ahead of us represents a challenge significantly greater than when we first went to the moon," Griffin said recently in a speech.

New classes of astronauts will have to practice flying in a vehicle quite different from the shuttle and learn how to extract resources such as oxygen from the moon's soil. They will be taught to grow vegetables in lunar greenhouses and conduct geological tests on the moon's surface. Already, engineers at United Space Alliance are studying how a crew will be able to train aboard the spacecraft on a three-year trip to Mars. Eventually, Mars-bound astronauts will have to learn how to extract fuel and other resources from Mars' surface.

"The requirement to live off the land will be crucial to our future in space, just as it was to Lewis and Clark," Griffin said recently.

The crew exploration vehicle will be shaped like an Apollo-era capsule and hold six astronauts for trips to the space station and four for journeys to the moon. Under the proposed design, astronauts in the new space vehicle will be launched on one rocket, and the lunar lander and moon-propelling rocket parts will be launched on another, much bigger rocket. Once in orbit, the capsule carrying the crew will dock with the lander and rocket and head for the moon. The crew capsule will return to Earth by parachutes and can be used up to 10 times.

Two competing contractors, Lockheed Martin and a team of Northrop Grumman and Boeing, each have received $60 million contracts to develop conceptual designs for the crew exploration vehicle. NASA will choose a winning bidder to build the spacecraft by August.

NASA has stopped ordering some shuttle parts, and some work projects have been scrubbed as the shuttle program winds toward its end in 2010. In January, NASA cut in half its order of external fuel tanks. Workers have stopped making upgrades to the space shuttles' cockpits.

"There are smaller items, day-to-day work, that are being adjusted," said Anne Martt, a vice president of United Space Alliance, the primary private contractor that works on the shuttles.

A board of high-ranking NASA officials has begun deciding which of the millions of shuttle parts will be scrapped and which will be kept or retooled. Some parts, like the shuttles' solid rocket boosters, will be used in the new vehicles.

"One of the challenges to me is the scope of this thing — millions and millions of items of trackable equipment that at some point... have to have some check that something has been done with them," said Mike Hawes, a deputy associate administrator at NASA. "The volume will be considerable."

The board also will decide which facilities, like the launch pads at the Kennedy Space Center, should be shuttered or refurbished — and when. There are still 16 to 17 shuttle flights needed to complete the space station, and there is still a possible mission to repair the Hubble Space Telescope.

"We don't want to shut something down that could prevent us from completing that mission," said Robert Lightfoot, manager of the space shuttle's propulsion office at NASA's Marshall Space Flight Center. "We don't necessarily want to shut them down if we're going to have to turn around and start them back up ..."

Fearing a loss of skilled workers, Griffin has said he doesn't want a big gap like the one that occurred between Apollo and the shuttle in the 1970s. But space agency officials have a tough balancing act between keeping shuttle workers in place until the vehicles are retired and recognizing many workers will either have to be retrained or let go in four years. The new vehicles will require less servicing than the shuttles.

Almost 2,000 NASA civil servants and more than 15,000 contractor employees work on the shuttles, mainly for United Space Alliance. Many contractor workers who either have retired or resigned recently aren't being replaced and those sticking around are worried about significant job cuts, said Lynn Beattie, a crane operator at the Kennedy Space Center.

"There's a lot of anxiety... Someone retires and we're just doing without. We're just doing with less people," said Beattie, 51, who also is president of Local Lodge 2061 of the International Association of Machinists. "If I were a younger worker here, I'd certainly be looking around to see what my opportunities are elsewhere... I would be very concerned about hanging my hat on this place being my career."

But space shuttle program manager Wayne Hale had an answer to worried Kennedy Space Center workers at a recent all-hands meeting.

"It's not going to be like, shuttle ends and we send out our resumes and see if we can find a job at Wal-Mart or McDonald's," Hale said. "This is going to be, 'Oh My goodness! Where are we going to get enough people to do all the things on our plate?"'