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Houston, Texas — (AP) - The investigation into what caused Columbia to fall to pieces is leading NASA back to two things that have worried engineers almost from Day One of the shuttle program: foam and tiles.
One of the leading theories in the accident investigation is that a 2½-pound, 20-inch chunk of foam insulation broke off from the shuttle's big external fuel tank during liftoff and damaged the heat-protection tiles on the left wing, setting off a chain of events that killed the seven astronauts.
It would not have been the first time that foam insulation damaged a shuttle's tiles. It has happened often, according to space insiders.
"The thing of this is, almost since Day One, the insulation has been a pain. Pieces break off," said Seymour Himmel, a retired NASA executive who served two decades on an aerospace safety panel and looked into the potential dangers of the foam.
In fact, soon after NASA stopped using Freon in the foam, for environmental reasons, Columbia sustained significant tile damage during a 1997 liftoff because of flyaway foam, according to a report by NASA engineer Gregory Katnik. He noted the same thing happened on the previous shuttle launch, that same year.
Katnik raised the possibility at the time that the new foam concoction had some unknown characteristics that were not compatible with the severe conditions of takeoff. The recipe apparently was altered somewhat after the 1997 incident.
While the foam on the fuel tank may seem benign - it is a lightweight, polyurethane, spray-on material that goes on like shaving cream - it hardens like styrofoam. And given the speed at which shuttles hurtle into space takeoff, it can have a devastating effect.
Moreover, the black, silica glass fiber tiles that cover the bottom of the shuttle are famously fragile, so much so that even a bump or nudge can cause cracks or dings.
Still, even the pros have trouble accepting that foam could have caused catastrophic damage.
"I am absolutely stunned," said astronaut Mike Mullane, who rode Atlantis into orbit in 1988. "I can see it scratching or even gouging a couple of tiles. But God, the idea that it could compromise the system. I don't know, but I just have a hard time believing that."
He recalled that a tip of one of his booster rockets broke off during launch and slammed into the belly of the shuttle, damaging a few hundred tiles. One came off entirely. Yet Atlantis and its crew of five still made it safely home.
Some of the broken-off pieces of insulation, ranging from flecks to chunks, hit the shuttle and usually do nothing more than ding or scratch the tiles. Some miss the shuttle altogether.
The barn started shaking and we ran out and started looking around. I saw a puff of vapor and smoke and saw big chunk of material fall.
The foam has undergone "a continuing series of process changes, both in terms of how you apply it and where you apply it, under which conditions, as well as the materials," Himmel said.
Lockheed Martin Corp. makes the 154-foot fuel tanks, but a long list of subcontractors provide the foam and its ingredients. The silica tiles on the shuttle's belly were developed and manufactured by Lockheed.
The tiles protect shuttles against the searing heat of atmospheric re-entry. If some are missing in an especially vulnerable area, it could set off a chain reaction that could destroy the shuttle.
Shuttles almost always return to Earth with marred or missing tiles, the result of debris smacking into them during liftoff.
Himmel said he and other members of the Aerospace Safety Advisory Panel, created after the deadly 1967 Apollo spacecraft fire, never included in their reports any reservations about the vulnerability of the thousands of tiles that cover shuttles.
"It was so well-known, it would have been superfluous to say, `If you lose a lot of tiles, you've bought the farm,"' Himmel said. Retired aerospace engineer Tom McKeever used to see the tiles damaged all the time. "The whole tile situation has been not only fragile in concept, but I believe somewhat fragile in design," he said.
Carnegie Mellon University engineering professor Paul Fischbeck concluded in a 1994 study that bad tiles make up 10 percent of the risk of an accident. On Monday, he noted that researchers have not looked at tile effectiveness after years of wear and tear, and wondered whether age may have contributed to Saturday's disaster.
"NASA adopted many of our recommendations and changed the risk of tile failure," Fischbeck said. "So we got rid of the early problems. But are there new aging problems we don't know about?"
Columbia was the oldest of NASA's space shuttles, soaring into orbit for the first time in 1981.
During Columbia's launch last month, it is possible that the foam was coated with ice, because of the super-cold liquid hydrogen and oxygen inside the tank. That would have made it a more dangerous projectile. Shuttle program manager Ron Dittemore said he hopes to know about ice by the end of the week.
During the launch, the strip of foam peeled away from the tank 81 seconds into the flight. The shuttle was traveling at 2½ times the speed of sound at the time, or just over 1,900 mph.