In this picture made available by NASA, a technician examines the area of the space shuttle Discovery's external tank where foam was removed to study the source a cracks on the tank in Cape Canaveral, Fla. on Wednesday, Nov. 10, 2010. (AP Photo/NASA, Troy Cryder)

(AP) -- NASA has found a fourth crack in the fuel tank for space shuttle Discovery.

Discovery's final mission remains on hold as engineers and technicians work to fix all the cracks as well as a hydrogen gas leak.

NASA spokesman Allard Beutel said Monday the repairs need to be made before a new launch date is picked. The launch window opens Nov. 30 and closes Dec. 6.

NASA first discovered a long crack in the insulating foam of the fuel tank, after the countdown was halted Nov. 5 by the gas leak. Last week, two cracks were found on the exterior of the tankf. Then another crack popped up, then another.

The cracks are in the tank's central, ribbed section, which holds instruments, not fuel.

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