Shield Repair Method Refined on 3rd Spacewalk

Astronauts Michael E. Fossum and Piers J. Sellers completed the third and final spacewalk of the shuttle’s 13-day mission Wednesday, testing repair techniques on purposely damaged heat-shield samples.

The seven-hour spacewalk focused on refining procedures for the use of an adhesive called NOAX to seal cracks in the reinforced carbon panels that cover the shuttle’s wing edges and nose cone.

A crack in a panel on the left wing was blamed for the fatal Columbia accident in 2003, in which all seven astronauts died during reentry.

Similar testing was done during a shuttle mission a year ago. Failure to control the temperature in Discovery’s cargo hold, where the testing was done, caused the NOAX to bubble when it was applied -- a potentially dangerous condition during reentry.

This time, Fossum and Sellers allowed the shuttle bay to warm in direct sunlight before they worked on a dozen test panels. The gooey NOAX, which stands for non-oxide adhesive experimental, went on more smoothly and evenly, although there was still some bubbling, NASA officials said.

“It seemed to work out pretty well,” said Tomas Gonzalez-Torres, a NASA spacewalk manager.

NASA officials said they were not prepared to certify the technique for repairing actual shuttle damage. The best they could say was that they had increased confidence in the ability to repair a shuttle during a mission.

No matter what NASA managers think, the crew would decide whether to trust their lives to a substance referred to around NASA as “the goo.”

During last year’s launch, the shuttle suffered a number of problems, including a bulging thermal blanket and protruding spacers between the heat-resistant tiles. The shuttle crew said that if the orbiter wasn’t safe to fly home, they would wait for rescue at the International Space Station rather than try to fly home in a repaired shuttle. In the end, NASA engineers cleared it for flight without repairs.

So far, Discovery, now docked at the International Space Station, appears to be in excellent shape for its return to Earth, scheduled for Monday.

One small problem cropped up during the goo tests. A spatula used to spread the NOAX disappeared, joining the thousands of other bits of flotsam in orbit that have been discarded or lost over the last five decades of human space exploration.

Though the threat of being struck by debris is considered one of the greatest dangers to the space station, NASA officials were not overly concerned about the loose spatula.