HOUSTON — Columbia investigators said Tuesday they are growing more certain of what brought down the shuttle: A seal on the left wing was struck by foam during liftoff and fell off the next day, creating a gap that let in enough scorching gases during re-entry to rip the ship apart.
A seal from Columbia’s left wing is now believed to be the mystery object that floated away in orbit and it was almost certainly struck by something — like a chunk of foam — before it came off, the accident investigators said.
“For 11 weeks, we have been saying that we don’t have any particular scenarios, any favorite scenarios,” said retired Navy Adm. Harold Gehman Jr., chairman of the investigation board. “But I think 11 weeks into this, it’s time that we attempted to see where the evidence was pointing us,” and so the board will meet with NASA officials later this week to begin reaching a hypothesis.
The final report is not expected until midsummer.
At their weekly news conference, the investigators also said numerous defects have been found in insulating foam on a fuel tank practically identical to the one on Columbia. A chunk of the foam peeled away from Columbia’s fuel tank shortly after liftoff and slammed into the leading edge of the left wing, believed to be a key element of the Feb. 1 disaster that killed all seven astronauts.
The investigators said the long, narrow gap from a broken or missing seal on the left wing probably expanded during Columbia’s descent two weeks later because of the intense heat of re-entry.
The resulting breach would have been large enough for atmospheric gases to burn their way through the wing and lead to the spaceship’s disintegration over Texas.
Navy Rear Adm. Stephen Turcotte, a board member, said it is still too soon to say that is exactly what happened, but the evidence is pointing strongly in that direction.
“To say it was, in fact, a T-seal 100 percent, we suspect that,” Turcotte said. “I mean, we’re up there. We’re up there near the 70s and 80s percent.”
Radar and other tests indicate a so-called T-seal is what was seen floating away from Columbia on its second day in orbit; the object was not noticed during the flight but only in analyses after the accident. More work is planned to ascertain whether it may have been a complete seal or just a fragment of one, or possibly a blanket insulator or part of an actual wing panel.
Turcotte said age or wear-and-tear alone could not cause such a seal to fall off a shuttle wing.
“It had to be the result of some blunt-force trauma, the transfer of kinetic energy, somehow,” he said.
The seals fit between pairs of panels that are made of the same reinforced carbon composite material and are designed to withstand temperatures of up to 3,000 degrees during re-entry. These seals and panels wrap around the leading edge of each wing.
“If the whole seal is gone, that means you’ve got a one-inch hole that’s about almost three and a half feet,” Turcotte said.
Physicist James Hallock, a board member who is in charge of the Transportation Department’s aviation safety division, said he does not believe a missing seal alone could have created a big enough hole for the kind of heat damage experienced by Columbia.
As the plumes of hot gas entered the long, narrow gap, it probably chipped or broke away at the adjoining wing panels and created an even bigger breach — enough to lead to the ship’s destruction, he said.
As for the foam insulation on Columbia’s external fuel tank, it was never designed to break off the way it did. At least 74 defects have been found in foam from the same area — the so-called bipod ramps — of the spare tank currently being dissected.
Turcotte said many of the flaws are air pockets, some of them more than 2 inches big, but engineers also discovered faulty bonding between some of the layers of foam.