Space shuttle Discovery STS-133 sits on launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida November 5, 2010. REUTERS/Scott Audette

CAPE CANAVERAL, Florida (Reuters) - NASA on Tuesday began modifications to shuttle Discovery’s fuel tank, hoping to thwart a potential debris hazard during a planned February launch to the International Space Station.

Managers decided to buttress at least 34 of the tank’s 108 metal support structures after cracks were discovered in five beams following a launch attempt on November 5.

The launch initially was scuttled by an unrelated hydrogen fuel leak, but inspectors also found a 21-inch long crack in the fuel tank’s insulating foam, which likewise would have nixed liftoff, said shuttle program manager John Shannon.

The concern is that bits of cracked foam could break off and hit the shuttle during liftoff, such as what happened during the 2003 launch of shuttle Columbia. The damage caused the shuttle to break apart as it flew through the atmosphere for landing 16 days later, killing seven astronauts aboard.

Additional inspections of Discovery’s tank revealed cracking in underlying support beams, known as stringers. Engineers have been unable to determine the root cause of the cracks, but they suspect it is due to the metal flexing from the extreme cold of the shuttle’s liquid oxygen and light hydrogen propellants.

To reinforce the stringers, technicans are installing additional pieces of metal over the tops of the 21-foot long ribs, said NASA spokesman Allard Beutel.

The modifications are expected to take about a week.

Discovery’s launch remains targeted for February 3. If the shuttle isn’t off the ground by February 10, the flight will be delayed to the next launch opportunity, which begins February 27. The mission is among the final shuttle flights before NASA retires its three-ship fleet this year.