"We have work we can do to be ready," the manager said.

After Challenger exploded in 1986, it took 21/2 years for the first shuttle to fly again. Now, with the International Space Station orbiting Earth, there is more urgency because the shuttles are needed to finish building the station and to deliver water and supplies necessary to keep the orbiting lab staffed. Until that happens, contingency plans are being developed using Russian ships and a smaller crew.

The chairman of the Columbia Accident Investigation Board, retired Admiral Hal Gehman, understands the urgency.

"In order that we can make sure that NASA gets started on fixing things, when we have a preliminary finding, we'll say that," Gehman said. "Of course, NASA will know it at the same time and they will already be halfway down the road. But at least for the Congress and the president, in order to make the return to flight decision, they will know our views."

Jeff Carr, a spokesman for United Space Alliance, cautioned against reading too much into any specific dates for returning the shuttle to flight. Officially, USA and NASA said the next launch would happen as soon as it's possible to fly safely.

"You've got to have some arbitrary end to work toward," Carr said. "You want to preserve options for the program."

To that end, NASA shuttle program manager Ron Dittemore ordered teams to start on five specific improvements to the shuttle system. Dittemore's Feb. 27 memo to managers said the changes should be "reviewed and assessed for near-term implementation."

Prime shuttle contractor United Space Alliance is making sure the extra hangar time for the remaining three shuttles does not go to waste. Shuttle Discovery's overhaul, already underway before the crash, continues. If the shuttles are grounded a year or so, contractors might get an early start on the next overhaul of Endeavour, rather than next year as scheduled. That way, once the fleet is cleared for flight, NASA would have all three ships at its disposal.

Mike Leinbach, the Columbia launch director now in charge of reconstructing the ship as be he can, said, "There's a big effort ongoing to define what we have to do to return to flight for the other vehicles. . . . We're looking at everything."

In his testimony before the investigation board Thursday, Dittemore talked about the parallel effort to probe the accident and prepare to return the shuttles to flight.

"There is even more increased determination and commitment to look very closely at the whole system," Dittemore said. "They are determined to identify if there are any weakness in the system. It's broader than what may be determined as the root cause of the accident."

Dittemore asked people to look at how foam is applied to the external fuel tank near where the tank connects to the orbiter's nose. Launch video shows that 81 seconds after liftoff, chunks of foam struck the orbiter's left wing. Dittemore's memo said the goal is to prevent "any" loss of insulation during launch. Prior to this, NASA was working on the foam problem but never deemed it a serious enough threat to the orbiter to stop flying.

Another fix under study is a way to inspect and fix heat-shielding materials on the orbiter's belly and front edge of the wing while the shuttle's in space. The underside of the orbiter lacks handrails for the astronauts to hang on to, making a space walk difficult.

"Early in the program, we recognized that if we lost tile or multiple tile that we didn't have any repair technique, and we tried to develop such a technique," Dittemore told reporters the day after the Columbia accident. "We finally abandoned pursuing that option. We just didn't believe it was feasible at the time."

In the letter, Dittemore asked two teams to re-evaluate video, photographs and radar images taken of the shuttle on its way to orbit. That could include everything from ground cameras to some mounted on the shuttle itself. In October, Atlantis launched with a camera mounted on the orange tank looking down but its lens was fogged over when the solid rocket boosters separated.

Finally, Dittemore asked engineers to study different angles for the shuttle to fly through the atmosphere on its way home that might minimize heat stress on the front edge of the wing and heat shield tiles, where a breach likely occurred allowing superhot gas inside the left wing as Columbia entered the atmosphere. Since the crash NASA has said there were no such options because the shuttle already flies the least dangerous path.

Reporters Todd Halvorson and Chris Kridler contributed to this report.

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