LA Times - 12 March:
Shuttle Investigators Look at Possibility of Weakened Wing.
Damage resembling pinholes is found on Columbia's left leading edge. Area may have been more vulnerable to a collision with debris.
HOUSTON -- Columbia accident investigators said Tuesday that they have found most of the leading edge of the shuttle's left wing and have discovered extensive damage that may help explain how the orbiter broke up on Feb. 1.
Investigators are looking at the possibility that the leading edge, which sustains some of the hottest temperatures on the shuttle skin as it flies back to Earth, was weakened through years of wear and tear that left pinholes and voids, investigators said.
The damage was similar to what termites cause in wood, said Harold W. Gehman Jr., chairman of the Columbia Accident Investigation Board. In this case, the voids occurred in reinforced carbon material just a quarter-inch thick that shields the aluminum wing from temperatures of nearly 3,000 degrees.
The voids and pinholes by themselves probably did not cause a failure in the leading edge, but they could have made the wing more vulnerable to a collision with debris, Gehman said. The board is still trying to assess whether the shuttle was damaged on liftoff by foam debris falling from the shuttle's external tank or by a collision with something in space.
"It is possible that the foam striking a healthy orbiter might not have done enough damage to cause the loss of the orbiter," Gehman said. "But it is possible that foam striking an unhealthy orbiter that had problems ... could do some damage. Is it an event she could have survived at age 10 but that she couldn't survive at 21?"
The Columbia was on its 28th flight when it broke apart, killing all seven astronauts aboard. It flew its first mission in 1981.
The concerns about the leading edge panels are not new. NASA has commissioned or conducted at least a dozen studies in the last decade looking at the panels. The studies examined damage by orbital debris, such as micrometeoroids, and pinholes that were first discovered on the Columbia in 1992.
The leading edge is constructed of a thin layer of material known as reinforced carbon carbon, sandwiched between two coatings of silicon material. The pinholes penetrate the exterior coating, allowing contaminants to eat away the interior carbon and leave voids, the investigators said...............
The investigators have some of the pieces from almost all of the 22 leading edge panels that line each wing. Notably, the panels from the left side show significantly more damage than the ones from the right side, Gehman said.
Investigators want to know how NASA and its contractors inspected and maintained those leading edges and whether the procedures were adequate, said Maj. Gen. John Barry, a board member. A team of investigators next week will visit the Lockheed Martin Corp. plant that provides the panels, he said. On six prior shuttle flights, orbiters returned with their leading edges damaged by debris, micrometeoroids or other causes, Barry said.
A Boeing official said Tuesday that "several of the wing leading edge RCC panels were reinforced to resist penetration or damage" when the Columbia went through a 17-month overhaul in Palmdale from 1999-2001. The official declined to provide details, saying the company needed approval from NASA to release the information.......
Other studies during the late 1990s raised the possibility that hot gases during reentry could enlarge pinholes that were appearing on the panels but concluded that the problem was unlikely to endanger the spacecraft or its crew. The microscopic holes were discovered in 1992 as NASA officials inspected the Columbia shuttle after its 12th flight. Inspections found the pinholes in other orbiters and NASA officials concluded that the holes generally developed after 10 to 15 flights, although the Columbia seemed to have the worst experience with them.....