CAPE CANAVERAL, Fla. – 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 ’70s.

In just four years the three aging, huge 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 ’70s. “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.

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 each have received $60 million contracts to develop conceptual designs for the crew exploration vehicle.