WASHINGTON, May 23(Xinhuanet)-- NASA said Monday technicians are preparing to roll space shuttle Discovery back to the Vehicle Assembly Building on Thursday.

The move was delayed from Tuesday so that engineers at the Kennedy Space Center in Florida can examine if Discovery has cracks on the landing gear similar to the one found last week on the shuttle Atlantis.

The rollback comes days after a second fueling test on Discovery. On Friday, Discovery and its external tank successfullypassed the test, during which NASA engineers further checked on the performance of liquid hydrogen sensors and a liquid hydrogen pressurization valve that did not work properly in the last test in April.

In the VAB, NASA said Discovery will replace its external fuel tank as early as June 7 with a safer model that was originally planned to be attached to Atlantis on the next mission. A new heater will also be added to the new tank to prevent ice buildup.

Inspectors reported minimal ice formed on the tank during the April test. Any ice breaking off the tank during liftoff could do bigger harm than foam if it smacked into the shuttle.

The external fuel tank has undergone many design changes after the Columbia tragedy in February 2003, which was blamed on a suitcase-sized insulation foam that fell off the tank during liftoff and harmed the shuttle's wing, leading to the shuttle's disintegration during re-entry.

Discovery is scheduled to be moved again to launch pad in mid-June. The launch window for NASA's first shuttle flight since the Columbia disaster is from July 13 to July 31. Enditem