WASHINGTON, Oct. 14(Xinhuanet)-- NASA said Friday it is working to prepare for a next shuttle launch in May next year as engineers are moving closer to resolving problems with falling debris from the external fuel tank that doomed shuttle Columbia in 2003.

The loss of fuel tank insulation occurred again during the launch last July of Discovery despite safety improvements including major modifications to the fuel tank over the two and a half years after the Columbia disaster, in which all seven astronauts aboard were killed.

NASA workers wait to remove the space shuttle Discovery from the back of a modified Boeing 747 after it landed at the Kennedy Space Center in Cape Canaveral, Florida, August 21.(Reuters) NASA shuttle managers said that they have not set a specific launch date, but the US space agency is using the May 3-23, 2006, launch window as a target for work to prepare Discovery for the second test flight since the Columbia accident.

"We have a number of critical things we have to evaluate before we set an official launch schedule," program manager Wayne Hale told reporters."It appears that the May launch window is something we can begin to work on-- the launch window May 3 to May 23," he said.

NASA officials said an important factor to decide when to launch Discovery on the second post-Columbia mission is the progress with the work on the external fuel tank at the hurricane-damaged Michoud assembly plant near New Orleans.

At the moment, only 25 percent of its employees are working. The plant is expected to resume full operation by December.

The shuttle fleet is scheduled to retire in 2010. NASA plans to have 18 more shuttle flights to the international space station and possibly one to repair the aging Hubble Space Telescope in the future. Enditem