Space Shuttle Structure NASA engineers and safety experts have continuously modified and upgraded the shuttle fleet over the years. The fleet has known weaknesses, and investigators will be looking very closely at these as they try to figure out what happened to Columbia as it re-entered Earth's atmosphere Feb. 1. NPR's Chris Joyce reports.

Space Shuttle Structure

Space Shuttle Structure

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NASA engineers and safety experts have continuously modified and upgraded the shuttle fleet over the years. The fleet has known weaknesses, and investigators will be looking very closely at these as they try to figure out what happened to Columbia as it re-entered Earth's atmosphere Feb. 1. NPR's Chris Joyce reports.

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