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Work is underway to install Discovery's Reinforced Carbon-Carbon panels.

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Space Shuttle Discovery passed a major Return to Flight milestone March 15.

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Work began to install Reinforced Carbon-Carbon, or RCC panels, on the leading edges of the orbiter's wings.

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When the orbiter returns to Earth from space, the RCC panels help protect the vehicle from the 3,000 degree heat of re-entry.

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During the course of Return to Flight preparations, workers have used several ways to inspect and test the panels without damaging them.

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Some of these non-destructive testing methods include computer-aided CAT scans, X-rays and flash thermography.

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There are 22 RCC panels on each of the orbiters' wings.

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The light gray carbon composite is also used in other areas, such as the nose cap and chin panel.

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Discovery is being prepared for STS-114, NASA's Return to Flight mission.