The problem first appeared before a launch attempt in November — cracks in three of the more than one hundred metal struts on the external fuel tank.
Then last week, two more cracked struts, which are also known as 'stringers', were discovered after the shuttle returned to its hangar in Florida.
Repair work to space shuttle Discovery's external fuel tank begins in the Vehicle Assembly Building at NASA's Kennedy Space Center in Florida. Technicians will modify 32 support beams, called stringers, on the tank's intertank region by fitting pieces of metal, called radius blocks, over the stringers' edges where they attach to the thrust panel area.
Managers are concerned that cracked stringers can also lead to cracks in the foam insulation on the fuel tank. Pieces of foam that broke off during Columbia's liftoff in 2003 gouged a wing, and lead to it breaking up during re-entry.
NASA spokesman Kyle Herring says workers will spend the weekend and early next week modifying dozens of stringers.
"It's essentially adding a small slice of metal to thicken some of the stringer edges where the rivets bolt those stringers to the actual structure of the fuel tank."
Herring says engineers are closer to figuring out why cracked stringers are such a problem for this particular mission.
"It has to do with the materials processing of the various stringers. They're made of a very brittle aluminum/lithium material. While they're very strong, they're also brittle. So that's one part of the overall puzzle of chasing what the actual cause is."
Herring says once the stringer modifications are finished, they'll be tested. Managers will then meet next Thursday to set a launch date, which will be no earlier than the last week of February. This would be Discovery's last flight, with Endeavour's final trip set for April. If there's enough money in the budget, NASA could add another Atlantis mission in June.