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COLUMBIA LOST
At first, it seemed the culprit would soon be found. As NASA engineers pieced together evidence from the fiery demise of the space shuttle Columbia on Feb. 1, they zoomed in on a large chunk of foam seen breaking off the shuttle's external fuel tank and striking the shuttle's left wing during liftoff. The blow could have damaged the lightweight insulating tiles that protect the shuttle during reentry. But as their daily analyses have progressed, NASA officials now believe something else caused the shuttle to lose control and burn up. And discovering that cause may take time, they say. The accident, which killed seven astronauts and sent the nation into mourning, also destroyed a large portion of the 80 science experiments on board the shuttle. "The NASA community has been struck a hard blow," says chemical engineering professor Albert Sacco Jr., director of the Center for Advanced Microgravity Materials Processing at Northeastern University, Boston. Sacco, who flew on Columbia during a 1995 mission, helped train the crew that lost their lives. His group's zeolite crystals--which grow larger in a microgravity environment than on Earth--were demolished. "We lost everything," Sacco says. However, the group has backup: They're also growing crystals on the International Space Station. Also lost were most of the biological experiments, which relied heavily on samples. But physical sciences experiments, from which much of the data had been transmitted to Earth, fared better. An investigation into the critical viscosity of xenon, for example, was successful, notes Robert F. Berg, physicist with the National Institute of Standards & Technology, Gaithersburg, Md. Since the day of the crash, a wayward chunk of foam has been a leading suspect as a cause of the accident. The shuttle system's external fuel tank is covered with polyurethane foam insulation, which keeps the liquid hydrogen and oxygen fuel cold and prevents condensation on the tank. NASA engineers were immediately aware of the impact of the foam on the shuttle, which occurred 80 seconds after launch, but ultimately concluded that its effect would not be debilitating. Concerns about the foam were raised in late 1997 by a NASA engineer after Columbia returned from a mission with more damage to its tiles than usual. Originally, NASA used CFC-11 (CCl3F) as the foaming agent in applying the insulation. But in response to the Montreal protocol, the agency switched to HCFC-141b (CCl2FCH3), which is less damaging to the ozone layer. The foam's properties are critical, as the external tank can shrink when it's filled with the cold liquid fuel. This creates peel stresses at the metal-foam interface, which, combined with the dynamic stresses during liftoff, could cause the foam to de-bond and crack, notes Clem Hiel, a materials scientist who spent 12 years at NASA Ames Research Center. Although the foam using HCFC-141b has properties similar to the original, "seemingly small changes like this may have grave consequences," Hiel says. And a report commissioned by NASA in 1990 warned about the potential risk of damage to the tiles by debris such as the foam. Concern has also been raised about the tiles' adhesive bonding, but chemical engineer Howard Goldstein, a 37-year veteran of NASA Ames who headed the space technology division there, notes that most of Columbia's tiles had remained intact for 20 years. But even when NASA engineers used a large estimate of the size and weight of the foam chunk, their computations still did not predict serious damage, Shuttle Program Manager Ron Dittemore says. Goldstein notes that, in his experience with past catastrophes like the 1986 Challenger explosion, people have been quick to jump to conclusions about the cause. But, he says, "you have to be careful, because the first ideas about what happened aren't necessarily the correct perception of the truth." |
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