NASA scientists believe they have solved the flying-foam problem that has grounded the space-shuttle program since its last flight in July, the space agency's shuttle-program manager said yesterday.

But they're not yet positive about the solution.

Wayne Hale, the shuttle-program manager, said during a visit to NASA's Langley Research Center here that a series of tests must still be run to test the solution. If the results are positive, he said, the next shuttle mission will likely come in May.

Hale who led the "return-to-flight" program that put the shuttle back into space after the shuttle Columbia burned up in flight in February 2003, killing its crew, called the current program "return to flight, part two."

The shuttle Discovery had a successful mission in July, but future flights were suspended because a lump of foam insulation flew off the craft's external fuel tank after liftoff. The Columbia's destruction in 2003 was attributed partly to damage caused when foam fell from the fuel tank at launch, fatally damaging the spacecraft when it later re-entered the atmosphere.

Although the Discovery landed without problem, NASA officials halted shuttle flights until the foam problem, and some others, could be solved.

Hale said yesterday that Discovery's second problem, in which fabric "gap fillers" between heat-shielding tiles on the shuttle's underside came loose and stuck out in a way that might have affected the craft's aerodynamics on re-entry, was easier to solve.

He said scientists have concluded that the adhesive holding the gap fillers in place wears out after a certain number of flights. To fix the problem, he said, workers are removing and reinstalling 9,000 gap fillers.

Officials feel confident that testing will confirm the solution that has been reached about keeping the foam intact on the external tank and permitting a launch in May, Hale said.

But a separate factor that may delay the return to flight is that a Lockheed-Martin plant east of New Orleans, where the external tanks are manufactured, was in the path of Hurricane Katrina.

The Michaud plant itself was not heavy damaged, Hale said, but its skilled work force was scattered because of the storm's effects on their homes. About 900 workers, or about half the total, have returned to work so far, he said.

Hale, who is based in Houston, was at Langley yesterday to thank workers for their efforts in the return to flight. He said engineers, scientists and technicians have been involved in about half of all return-to-flight work both at the research center and in trips to other work sites.

He said about the 25 percent of the research and testing of the program has been done at Langley.

In a meeting with NASA Langley workers, Hale said about 18 more shuttle flights will be required to haul the equipment and material needed to complete the international space station.

NASA's plan is to retire the shuttles in 2010. They are to be replaced two years later by the next-generation "crew exploration vehicle" being designed for the return to the moon and the exploration of Mars.

"It's a little bit of a challenge," he said.