Saturday's U.S. space shuttle disaster has sparked renewed criticism of the shuttle program, which some experts say is too expensive and unwieldy. In fact, the U.S. space agency has been developing a successor to the space shuttle fleet, which it wants to retire in 10 years.

The space agency was forced to abandon one radical design, and is now looking at a more evolutionary concept that it hopes can be ready by 2012.

NASA spent years pursuing a rocket design for a wedge-shaped craft made of non-metal composite materials. Unlike the shuttle, the so-called X-33 would have shot into orbit in a single stage, without booster rockets, and returned to Earth ready for another mission within days, instead of months.

The X-33 was supposed to have been ready for flight by 2006, but NASA discontinued development in 2001, after technical difficulties involving the engine, fuel tanks, and other components. Legislative auditors also accused NASA of unrealistically low cost estimates.

The head of a private U.S. space advocacy group, Pat Dasch of the National Space Society, said the X-33 relied on too many untested technologies. "I think, they tried to do the impossible. They tried to go from the shuttle we have today to a new vehicle, ultimately, which involved leap-frog technology. With hindsight, the discussion is now about, what will the second-generation shuttle be? It's not going to be revolutionary technology. It's going to be derived technology," he said.

NASA is now working with several U.S. aerospace companies to come up with a different shuttle successor. Rather than focusing on a single design, like the X-33, the new project is developing generic technology and operational concepts. These are embraced in 15 spacecraft plans that, like the shuttle, would reach orbit in two stages instead of one, and retain a conventional metal skin, not an advanced composite.

The NASA official in charge of the program, Dennis Smith, said the 15 designs will be narrowed to two by the end of this year, and one by 2006. "We really feel we're on our way to enabling development of a second-generation reusable launch vehicle. We are developing the capabilities required to do that with a system that can be far safer, reliable, and more affordable," Mr. Smith said.

In NASA's view, more affordable means dropping the cost of lifting loads by 90 percent. Safer means cutting the risk of catastrophe from the current 1-in-250 flights with the shuttle to 1-in-10,000.

But last year, an independent advisory panel to NASA said the project lacks credibility. It called its cost-reduction and safety objectives unrealistic. And panel member Gerard Elverum told a House of Representatives space subcommittee that NASA will not meet its 2012 operations schedule, if it does not pick a specific design sooner than 2006.

"Making a decision in 2006 to incorporate a brand new vehicle with lots of new technology is not going to be qualified to replace the shuttle by 2012. If we have basically, I think, defaulted that decision for 2012, then you're stuck with going ahead and making sure we can fly the shuttle until 2020," he explained.

But the prospect of extending the life of the shuttle fleet is troubling to many. Months before the Columbia disaster, a former safety advisor to NASA, Robert Blomberg, told Congress that the space agency's budget cuts jeopardized the aging fleet, now 22-years-old.

"I have never been as worried about space shuttle safety as I am right now. All of my instincts suggest that the current approach is planting the seeds for future danger," Mr. Blomberg said.

The NASA safety advisory panel Mr. Blomberg once headed has cited several causes for worry. It said budget reductions have led to the elimination or deferral of safety improvements, a diminished shuttle workforce, and an aging launch infrastructure.

One member of Congress, Bart Gordon, said he fears the shuttle fleet will become obsolete before it can be replaced, leaving the international space station stranded. "The tread on the tires is getting very thin. The very likely problem is, unless there's a change, we have a space station with no way to service it," Mr. Gordon said.

NASA's Dennis Smith notes that the space agency has not allocated much money for the shuttle replacement vehicle beyond 2006. But he said, if the country can afford it, and will allow NASA to fund it, the new shuttle will be ready on time.

"We went to the moon in nine years, and we developed the shuttle in eight years. Here we are 10 years away, and it really comes down to a commitment to get behind the new system," he said.

Whether the Columbia tragedy increases political will for a shuttle successor remains to be seen.