Shuttle cracks cause found ...

CAPE CANAVERAL, FL -- NASA finally knows what caused the cracking in space shuttle Discovery's fuel tank, a potentially dangerous problem that likely existed on the previous flight, managers said Tuesday.

Discovery's final voyage has been on hold since the beginning of November. If the remaining repair work goes well, the shuttle could fly to the International Space Station as early as Feb. 24.

At a news conference, NASA officials refused to discuss the flight status of astronaut Mark Kelly, the husband of Rep. Gabrielle Giffords, who was shot in Arizona last weekend. He's supposed to command shuttle Endeavour's last mission in April. His identical twin brother, Scott, is currently serving as the space station's skipper.
"Out of respect to the family, we really are not ready to answer those questions today. We're going to let Mark decide really kind of what he needs to do," said Bill Gerstenmaier, head of NASA space operations. "Our hearts and prayers go out to the family, and we're really thinking about Mark in everything we do."

On the orbiting lab, Scott Kelly took a call Tuesday from Russian Prime Minister Vladimir Putin.

"There are no people in Russia who are not touched by this terrible news," Putin said through a translator.

The Endeavour mission is the last on NASA's official shuttle flight lineup before the fleet is retired. The space agency hopes to add one last trip to the space station by Atlantis at the end of August to bring up extra spare parts, provided there's funding. Officials initially were targeting the end of June for the launch, but said Tuesday they would prefer more time between flights.

As for Discovery's prolonged grounding, shuttle program manager John Shannon said a combination of inferior material and assembly issues is to blame. Cracks occurred in five of the 108 aluminum alloy struts in the center of the tank, which holds instruments. The damaged struts have been patched. Technicians will reinforce the remaining struts as a safety precaution, using thin 6-inch strips of aluminum.
Shannon called it "a very simple, elegant fix to the problem."

"We're going to fly with a lot of confidence in this tank," he told reporters. "We've gotten rid of the uncertainty."
The tank is covered with foam insulation, and NASA was concerned the cracks could force pieces to break off during liftoff, with chunks possibly striking the shuttle.

A slab of foam doomed Columbia in 2003.
Engineers also worried that if four or more struts in a row failed, the entire structure could catastrophically buckle.

The cracking was discovered after an unrelated problem -- a hydrogen gas leak -- halted Discovery's launch countdown on Nov. 5.
Shannon said a batch of the material used for some of the 21-foot support struts, through heating, ended up more brittle.

In addition, weaknesses were introduced during assembly of the pieces.
The bad batch of material likely ended up on the fuel tank that launched Atlantis last May, Shannon said. Every indication is that the tank performed normally, even if cracks were, indeed, present, he noted.

The tank currently being prepared for Atlantis also has struts made of the suspect material and will need to be repaired. Engineers believe Endeavour's tank is unaffected, but extra tests are likely, which would push that mission into mid- to late April.

Once the 30-year shuttle program ends, the White House wants NASA focusing on expeditions to asteroids and Mars, rather than servicing the space station.