NASA said Friday it is aiming for a mid-May launch of the first shuttle flight since the Columbia tragedy almost two years ago.
The launch date was the latest of several set by the space agency, and just as subject to change.
Until this month, NASA had hoped to resume shuttle flights as early as March, with Discovery making a space station supply run and a test flight of new inspection and repair techniques. But four Florida hurricanes in quick succession damaged the space agency's buildings and hindered the launch preparations.
NASA spaceflight chief Bill Readdy conceded that technical challenges and "some unknowns" could thwart a mid-May launch date, in particular the need to prevent any dangerous pieces of foam from coming off the fuel tank during liftoff.
"We're as confident as we can be in establishing that target date," Readdy said. "Right now, it looks as though the milestones we have remaining, they're all things that we can accomplish between now and May."
Columbia was brought down in February 2003 by a chunk of insulating foam that broke off the external fuel tank during liftoff and slammed into the left wing. The three remaining shuttles have been grounded ever since.
Considerable work remains on many of the technical improvements urged by the Columbia accident investigators.
Readdy said "first and foremost" is the redesign of the external fuel tanks. All the work on Discovery's tank should be completed in time for a delivery to the Kennedy Space Center by year's end, a crucial step in meeting a May liftoff, he noted.
No. 2 on NASA's list of challenges, Readdy said, is development of an inspection boom for detecting any crevices in Discovery's wings or belly. No. 3 is development of repair methods.
Discovery's crew will be able to mend only those wing gashes that are smaller than the one that doomed Columbia.
For now, NASA plans 28 shuttle flights in order to finish building the international space station by around 2010, the station completion and shuttle retirement date set by President George W. Bush in his moon and Mars exploration plan.
The space agency has been looking at how many of these shuttle flights can be dropped from the schedule.