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The S-turns the pilots make are intended to bleed off speed in order to ease the shuttle down to Earth, but they are a lot more complicated than simply slaloming down a ski slope. The spacecraft's engines are shut off for good once it leaves orbit, meaning its descent is powerless. Flying a brick with wings, as the engineers have often called the ship, has a very fine margin of error. Lose your purchase on the air and go into a spin, and there's almost no way to pull out of it. "The attitude needs to be very, very precise," says Thagard. "You can pick up heat so fast you get a breakup."
Realistic as that scenario might seem, there was no sign of any such crisis in the final seconds of Columbia's flight, and unlike a tile failure, which could well go undetected until the very moment it claimed a ship, a piloting emergency would at least leave the commander time to sound an alert. Rick Husband made no distress call.
In the weeks and months to come, other, less likely scenarios will be examined too: a meteor or other piece of space debris could have struck the spacecraft, a growing risk given the decades of accumulated orbital junk that clutters the near-Earth environment. In this case, that's not likely, since the shuttle was already well into the atmosphere when it disintegrated. Age or metal fatigue could have been responsible as well. All four orbiters were temporarily grounded last June when cracks were found in their liquid-hydrogen fuel lines, damage that may have been caused by vibration, temperature changes or other stressors accumulated over repeated flights. Columbia, as the granddad of all the ships, could have been the brittlest of the fleet. But NASA, for all its alleged shortcomings, leaves little to chance in the regular physicals it gives its shuttles, and the fact that these tiny cracks were found in fuel lines makes it all the less likely that larger, lethal cracks could have gone unnoticed. A fuel explosion is also a possibility but again a remote one. When the ship re-enters the atmosphere, there's not much juice left in the main maneuvering rockets or in the spritzy little thrusters arrayed around the spacecraft.
In the wake of Sept. 11, it's not surprising that many Americans immediately wondered whether terrorism could have been responsible. A shoulder-launched missile could, in theory, bring down an aircraft, but Columbia was well beyond a missile's altitude limit at the time the ship disintegrated. The idea that an explosive could have been smuggled aboard got no serious attention. It would be almost impossible for even the most committed terrorist to breach NASA security, all the more so with the heightened protection thrown up around a ship carrying an Israeli astronaut celebrated for having participated in the attack on Iraq's nuclear reactor in 1981.
For now, Washington will do what Washington too often does in a time of crisis: point fingers. Florida Senator Bill Nelson--famous for having sweet-talked his way onto the same shuttle Columbia when he was a Congressman, landing just 10 days before the Challenger disaster sobered the space community to the risks of such joyrides--has been warning colleagues that budget cutbacks threatened to compromise spacecraft safety. "I have been perhaps the sharpest critic in Congress about the slowing down of safety upgrades," he says.