(CNN) -- Managers of the space shuttle Discovery said Tuesday they are leaving the door open for a slide in the scheduled May 15 "return to flight" launch, though NASA continues to work toward that date in the wake of the 2003 Columbia disaster.

Minor issues with shuttle processing have postponed the rollout of the orbiter to the launch site, leaving little room in the schedule for any further delays, they said.

"We have scheduled the fifteenth of May as our targeted launch date," said Michael Kostelnik, the deputy associate administrator for international space station and space shuttle programs.

"But it should be clearly understood that this is a 'not earlier than' date, which gives us the earliest opportunity we can bring all of the processing elements together and have a credible window to get to the international space station."