WASHINGTON, June 27 (Xinhuanet) -- The task force overseeing NASA's return-to-flight preparations concluded Monday that the US space agency has failed to fully complete three safety improvements necessary for resuming shuttle flight after the 2003 Columbia disaster.

    The panel said NASA has not eliminated the possibility that debris could fall from the shuttle's external tank and damage the spacecraft.

    In addition, NASA has not managed to sufficiently toughen the shuttle's shell in order to resist damage from striking, and has failed to develop the capability to make emergency repairs in space.

    The panel made the conclusion after prolonged discussion at its last planned public meeting. The panel's previous sessions found that NASA has completed 12 of the 15 recommendations made by investigators of the Columbia accident.

    However, panel members said NASA has made significant progress in improving shuttle flight safety despite its failure to fully meet three of all the requirements.

    The Columbia disaster on Feb. 1, 2003, in which all seven astronauts aboard were killed, was blamed on a suitcase-sized insulation foam that fell off the external fuel tank on liftoff and harmed Columbia's wing, leading to the shuttle's disintegration during re-entry.

    After a tanking test of Discovery in April, NASA engineers realized that any ice breaking off the fuel tank on liftoff could do bigger harm than foam if it smacked into the shuttle. Ice can form on the tank when it is loaded with supercold liquid oxygen and hydrogen fuel. The tank is fueled about six hours before launch.

    NASA was also criticized for not having a clear idea of all the potential impacts from ice.

    The panel is expected to provide a summary report for NASA before a meeting of the agency scheduled for Wednesday and Thursday to review flight readiness and determine a launch date for Discovery. The launch window runs from July 13 to 31 for the first shuttle flight since the Columbia disaster.

    NASA chief Michael Griffin has said NASA officials will have the final word on when to safely launch the shuttle, no matter what comment the task force will have on NASA's return-to-flight preparations. Enditem