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The tragic loss of Space Shuttle Columbia and its crew startled the nation. Why? Not “why was it lost?” Rather, why did its destruction startle the nation?

The dangers of Shuttle flight launch are obvious. No one within hearing distance will ever forget the devastating roar. Even the unadorned numbers are staggering. At launch, the booster carries 1.8 kilotons of liquid hydrogen and oxygen, the energy of 4 kilotons of TNT, roughly 20 percent of that of the bomb that destroyed Hiroshima. The astronauts surf into space on top of an enormous continuing explosion. Could that be made safe? The simple answer: no.

The reentry numbers are equally impressive. The Shuttle reenters at Mach 18–that’s 18 times the speed of sound. If all the energy of the air pushed out of the way by the shuttle were converted to heat, it would reach a temperature equal to the local atmospheric temperature (250 degrees Kelvin) multiplied by 18 squared; that’s 80,000 K. But not all the energy is converted to heat; the actual temperature is “only” 2000 K. Yet even that is about a third of the temperature of the surface of the Sun. Hobbyists who photograph the reentry see a beautiful bright trail, like the fireball of a meteor, made of ions created from the intense heat of air impacting the Shuttle. Astronauts peering out their windows have been equally entranced (and maybe a bit frightened) as the dark sky of space is hidden by the glowing hot plasma. Can such a reentry be made safe? No.

Only the tiles, primarily on the bottom of the Shuttle, are designed to take such heat, and even they can take it only for a limited time. During re-entry, the impact of Mach 18 air will destroy the craft unless a careful balance is maintained and the heat confined to these tiles. If the Shuttle tumbles, air quickly weakens and destroys the entire craft. We don’t yet know what caused the Shuttle to tumble on February 1, and even when we do, it will be impossible to guarantee that it will not happen again. To me, the remarkable achievement of the space program is that only two Shuttles have been lost in 113 missions.

There are few jobs more dangerous than that of an astronaut. But most of the public doesn’t know that–or rather, didn’t. The Challenger disaster had been largely forgotten. The misimpression that the Shuttle is safe is largely the work of NASA public relations, which has strived to make the flights sound routine. The shuttle is so safe that we can fly senators and schoolteachers. Shuttle flights are so routine that we can use them to perform experiments suggested by high school students.

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