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.
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 February 24.
At a news conference, Nasa officials refused to discuss the flight status of astronaut Mark Kelly, the husband of politician Gabrielle Giffords, who was shot in Arizona. He is supposed to command shuttle Endeavour's last mission in April. His identical twin brother, Scott, is currently serving as the space station's skipper.
On the orbiting lab, Scott Kelly took a call on Tuesday from Russian Prime Minister Vladimir Putin. "There are no people in Russia who are not touched by this terrible news," Mr Putin said through a translator.
The Endeavour mission is the last on Nasa's official shuttle flight line-up 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 is funding. Officials initially were targeting the end of June for the launch, but said they would prefer more time between flights.
As for Discovery's prolonged grounding, shuttle programme manager John Shannon said a combination of inferior material and assembly issues is to blame. Cracks occurred in five of the 108 aluminium alloy struts in the centre of the tank, which holds instruments. The damaged struts have been patched. Technicians will reinforce the remaining struts as a safety precaution, using thin six-inch strips of aluminium.
Mr 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 lift-off, with chunks possibly striking the shuttle. A slab of foam doomed shuttle 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 November 5.