NASA says it has found source of cracks in shuttle tank

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- NASA officials Tuesday said they have found the root cause of cracks in the external tank's support beams and expect to have the space shuttle Discovery ready for a possible launch in late February.

John Shannon, the shuttle program manager, said extensive testing by engineers found that the cracks found in five of 78 support beams called "stringers" were caused by low fracture toughness combined with assembly stresses.

"We have root cause and we have a fix which we are completely confident that will eliminate those root causes," said Shannon.

NASA will place aluminum "radius blocks" at the end of the stringers to reinforce the support beams, said Shannon.

Earlier cracks found in the foam covering the fuel tank delayed the shuttle's final launch, originally scheduled for November 1. Technicians repaired the cracks and reapplied foam insulation on aluminum brackets on the tank in November.

Discovery is scheduled to deliver a storage module, a science rig and spare parts to the International Space Station. According to the NASA website, the launch would not be before February 24 and would depend on traffic at the space station, which is set to receive cargo from a European spacecraft in mid-February.

The last scheduled launch of space shuttle Endeavour has been pushed backed nearly three weeks to April 18 and shuttle Atlantis is tentatively scheduled to launch late in August.

At a news conference, well wishes were expressed for astronaut Mark Kelly, the husband of U.S. Rep. Gabrielle Giffords. She's in critical condition after being shot Saturday in Tucson, Arizona.

"Our hearts and prayers go out to his family, we really are thinking of Mark with everything we do," said Bill Gerstenmaier, NASA associate administrator.

Kelly is slated to command the last scheduled mission of Endeavour.

Gerstenmaier would not discuss that flight and who would lead it.

"I think out of respect for the family we really are not ready to answer those questions today because we will let Mark decide really what he needs to do," said Gerstenmaier.