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If the falling foam did damage the ship, the most disturbing possibility is that it chipped or broke one or more of Columbia's heat-absorbing tiles. The spacecraft is protected from the hellish heat of re-entry by thermal blankets and about 24,000 black and white ceramic tiles. The jigsaw-puzzle pieces have given the space agency fits since the very first flight of the very first shuttle--Columbia in April 1981. Handfuls of them often flaked away during lift-off, leaving NASA with nothing to do but wait out the flight and hope that the skin had not been denuded in a critically hot spot. In a worst-case scenario, just a few missing tiles in even a relatively low-temperature area could lead to a fatal chain reaction, with possibly hundreds of them peeling away.
"Losing a single tile can do you in," says Stanford University's Elisabeth Pate-Cornell, an engineer and risk-management specialist who once led a NASA study about all the ways shuttle tiles could fail. "Once you have lost the first tile, the adjacent ones become much more vulnerable."
Since nothing this catastrophic ever happened in any of the prior shuttle flights, NASA grew increasingly--and justifiably--confident that the lightweight and durable shielding was the best of an admittedly imperfect set of options. Now there is reason to doubt. "Obviously, the loss of tiles has to be looked at and put to bed," says George Gleghorn, a former chief engineer for TRW's space group and, for six years, a member of a NASA safety-advisory panel.
Others are asking why, if a piece of foam was known to have hit the ship, an astronaut wasn't sent outside during the course of the 16-day mission to determine whether any damage had been done. Dittemore explains that this crew was not trained for that kind of extensive space walk, and even if they were and they found some damage, they could have done nothing about it anyway. "We had no capability to go over the side or under the spacecraft and look for an area of distress and repair a tile," he says.
That didn't stop people from wondering whether a space walk still might have been wise, since if the crew could somehow have determined that the tiles were compromised, they might at least have hightailed it to the International Space Station--which the shuttles routinely visit anyway--and awaited a lift home aboard another shuttle or a Russian vehicle. But that possibility was foreclosed, since crews must rigorously train for a space-station docking and must carry aboard an adapter collar to make the linkup possible. Neither of those conditions was met on Columbia.
If the tiles weren't responsible, the sheer turbulence of re-entry might have been. It's not for nothing that the first commander of a shuttle flight was Gemini and Apollo veteran John Young, widely respected as an iceman at the stick. If you're going to fly so tricky a ship, you have to be. "When you re-enter, you're moving at 25 times the speed of sound," says former astronaut Dr. Norm Thagard. Hitting the atmosphere at that velocity is "not unlike slamming into a brick wall, if you're not at the correct attitude."