While the preliminary investigation of the Columbia shuttle crash has focused on the impact of debris on the orbiter's ceramic tiles, an internal report NASA filed on the second day of the flight said the debris also "appeared" to have struck the leading edge of the left wing, raising the possibility of damage in an area that plays a crucial role in reducing heat transfer over the wing on reentry.
The leading edge appears in shuttle photos as the gray band at the very front of the wings. It is made of reinforced carbon-carbon compound (RCC) and is far harder than the shuttle tiles. But because of its critical function, "a little damage there could cause a lot of harm," said Brian Cantwell, an aeronautics professor at Stanford University.
The degree of harm would depend on the precise site of the hit. While the edge would be difficult to crack, "a large piece of foam" at high speeds could do damage.
"A good nick could be enough" to change the air flow over the wing and cause a greater buildup of heat than the system was meant to handle, said Graham Candler, professor of aerospace engineering at the University of Minnesota, who has worked on reentry projects for NASA.
Disrupted air flow over the wing could lead to turbulence, Cantwell and Candler said.
NASA's report shows that on the 12th day of the flight, after reviewing films of the debris and conducting modeling tests on the ground, engineers concluded that the "damage to the RCC [leading edge] should be limited to coating only and have no mission impact."
That same analysis said there was "a potential for a large damage area to the tile," but no risk to flight safety.
A NASA spokesman said last night that because its analysis showed no safety risk to the more vulnerable tiles, engineers inferred that the much stronger leading edge would not be a flight risk, either.
However, in the wake of the crash, NASA has said it is reopening its analysis and reconsidering its initial assumptions and conclusions.
The leading edge was designed to take the brunt of the intense heat that builds up as the shuttle reenters the atmosphere. Made of reinforced "carbon carbon" -- carbon fibers inside a block of carbon -- it was developed by Lockheed Martin Corp.'s Missiles and Fire Control business in Dallas.
The RCC protects some of the most high-stress sections of the shuttle's outer surface, including the 41/2 -foot diameter cap on the tip of the nose cone, the "chin panel" between the nose and the front landing gear door and the leading edges of both wings.
There are about 1,700 pounds of RCC on each orbiter.
Forty-four panels of the substance make up the leading edge of each wing. The RCC is a light gray material consisting of graphite fibers with a top coating of silicon and carbon. The coating keeps oxygen from flowing into the fibers and weakening them. A carbon binder beneath each panel helps keep the material rigid.
Molded into aerodynamic shapes, the quarter- to half-inch panels are designed to withstand pressures of as much as 800 pounds per square foot.
The RCC is tougher and more heat resistant than the ceramic tiles on the bottom of the shuttle. It can withstand heat of nearly 3,000 degrees, compared with 2,200 for the tiles. Lockheed Martin says the material actually gets stronger at extremely high temperatures.
But because the material is put on such sensitive parts of the craft, it usually has extra shielding behind it. The wings' leading edges have foil-wrapped ceramic tiles behind the RCC panels for extra heat protection.
While designed to withstand 100 shuttle missions, the RCC panels routinely get damaged during flights. NASA inspectors have found pinholes in the panels caused by impacts with micrometeoroids or other debris, as well as sealant loss that caused some RCC panels to begin to oxidize.
Scientists have been developing advanced coatings to help reduce those problems.
Staff writer Kathy Sawyer contributed to this report.