Efforts to develop hardware for repairing the shuttle heat protection system in space remains one of the toughest technical challenges still ahead, and possible additional inspections on Atlantis' nosecap also are contributing to the change in launch dates.

Engineers are concerned about corrosion inside the nosecap, which is made of the same reinforced carbon carbon material as the shuttle's wing leading edges. The hardware was partially inspected in 1997 and fully checked out in 1991, said shuttle program manager Bill Parsons.

Taking the nosecap off and sending it back to its vendor in Texas could add weeks to the processing schedule. Inspecting the hardware at the Kennedy Space Center might save time, but it's not clear how well the work can be done in Florida.

But the complicating factor for selecting launch dates in the post-Columbia era is the requirement to launch in daylight, have the external tank separate from the shuttle in daylight and not violate thermal constraints when a shuttle is docked to the International Space Station.

The ISS thermal requirement has always been in place, but the daylight rules have been added so that cameras can better record the launch and tank separation. The images will be studied to check for damage to the shuttle's heat shield.

Accomodating all of those variables is severely limiting the number of days a space shuttle can launch each year. Identified launch periods now include Sept. 16 to Oct. 11, 2004; Nov. 19-21, 2004; and Jan. 17-19, 2005.

"I think we have some opportunities, some small opportunities in November, and possibly January and some other places," Parsons said. "We're still refining those requirements."

In any case, program officials said they would complete all the necessary work prescribed by the Columbia Accident Investigation Board and then look up and see when the next launch date is available.

"We're going to be very much driven by the milestones and by the content we have to accomplish here in terms of the testing of the robotic arm, survey techniques, tile repair, modifications to the external tank -- all the testing that's required," Readdy said.

In addition to extending the shuttle return to flight timeline, officials said they will be adding a new shuttle mission to the manifest.

The next flight, STS-114, originally was to be a space station supply and crew rotation mission. Instead, STS-114 now is considered more of a test flight that will demonstrate the new procedures and hardware introduced in the wake of the Columbia tragedy.

To make up for some of the station resupply tasks that are being offloaded from STS-114, NASA is adding STS-121 to the manifest and will make that flight of Discovery the second to fly, possibly during the November 2004 launch period.