(CNN) -- NASA managers said Friday that the space agency is working to resume shuttle flights as soon as next May, even as teams of engineers continue to analyze what caused a potentially critical problem during the Discovery's launch."May looks very doable," said NASA Associate Administrator for Space Operations William Gerstenmaier.Shuttle Program Manager Wayne Hale added, "I think we're beginning to have our hands well around the technical problems that we have and to find the fixes that are going to be necessary to fly again."NASA officials briefed reporters on the status of the space shuttle program Friday at the Johnson Space Center in Houston.NASA grounded the shuttle fleet after a chunk of foam fell off of Discovery's external fuel tank during its July 26 launch -- the same problem blamed for the Columbia disaster. Analysis determined that a briefcase-size piece of foam weighing just under a pound broke away from a section of the tank called the PAL ramp shortly after solid rocket booster separation.The foam did not hit the orbiter. But NASA officials said that it could have caused damage if it had.Bill Parsons, who was shuttle program manager at the time, announced the day after launch that the shuttle fleet would not fly again until the latest foam problem was solved.Elimination of large debris is key to the safe resumption of shuttle flights.The underside of a space shuttle is covered with insulating tiles and the edges of the wings are clad with reinforced carbon-carbon panels. Together, they make up the thermal protection system that ensures the shuttle can withstand the intense heat of re-entry.The engineers studying the issue have concluded that no single factor caused the foam to come loose. Rather, they think there were several contributing factors: The technique used to spray the foam onto the tank, engineering issues in the area where the foam broke off, and damage from workers touching and possibly crushing the foam while working on other parts of the tank."I want to make it clear that we found no negligence on the part of the workers," said Richard Gilbrech, leader of the engineers looking at the problem. "They were doing their work per procedure. It's just we really didn't have an appreciation for the significance that this handling damage could have in terms of foam loss."Gilbrech and his team suggested changing the way the foam is applied and how the tank is processed, with the goal of eliminating the shedding of large pieces of foam.Shuttle program managers said they are optimistic that testing will be completed and the modifications implemented in time to launch the shuttle between May 3 and 23.The date could be affected by engineering setbacks and by the impact of Hurricane Katrina on workforce at and infrastructure around Michoud Assembly Facility, which is in New Orleans."We have estimates that basically we will have lost the equivalent of three months worth of work at Michoud based on the effects of the hurricanes," Hale said. "In spite of that, our amazing work force down there is coming back to work. We are making outstanding progress in addressing these issues."With the exception of the one Discovery flight last summer, the shuttle fleet had been grounded since February 1, 2003, when the shuttle Columbia broke apart over Texas while on landing approach to Florida's Kennedy Space Center. All seven astronauts aboard died.Seven months later, the Columbia Accident Investigation Board concluded that a 1.6 pound piece of foam insulation broke off the shuttle's external fuel tank, striking and cracking a panel on the orbiter's wing. When the shuttle re-entered Earth's atmosphere, hot gases seeped into the wing and destroyed the spacecraft.Since the Columbia disaster, NASA engineers have said that it is not possible to eliminate the shedding of foam and ice from the external tank during launch. They did redesign the tank to minimize the size of debris that would come off. Prior to the Discovery launch, NASA managers said the heaviest piece of foam they expected to come loose would be three one-hundredths of a pound, and about the size of a breakfast muffin.Prior to the Columbia disaster, astronauts had no way to inspect the thermal protection system for damage that might have occurred during liftoff or to make repairs. During the Discovery mission, astronauts tested new tools and techniques to do those jobs.The new inspection system worked well, but limited testing of repair procedures confirmed astronauts cannot yet reliably fix a hole. Most NASA engineers say astronauts will never be able to repair a hole the size of the one that doomed Columbia.NASA now plans only 19 more shuttle flights, down from 28, Gerstenmaier confirmed. Eighteen of those missions will include docking with the international space station, with the goal of completing its assembly. The other would service the Hubble Space Telescope.NASA intends to retire the shuttle fleet in 2010. The space agency hopes to launch the next generation manned spacecraft, the Crew Exploration Vehicle, in 2012, with a manned mission to the moon coming in 2018.