NPR's Chris Joyce reports on what NASA learned from a study conducted after the space shuttle Challenger blew up in 1986 and what the findings might tell investigators about the disintegration of the shuttle Columbia on Saturday. A 1994 study by the engineering firm SAIC concluded that, aside from engine failure, the highest risk for a space shuttle is not from damage to heat protective tiles, but rather from failure of the auxiliary power units. The APU's run the hydraulics that, in turn, control steering during reentry into the atmosphere.

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