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Why does a spacecraft become very hot when it enters the Earth's atmosphere and how are the astronauts protected?
A spacecraft in re-entry moves through the air much faster than the speed of sound. Air molecules displaced by the spacecraft create friction. The flow temperature is so high that air molecule’s chemical bonds break creating charged plasma around the craft. In order to withstand the high temperatures of re-entry, special external tiles are used. On the shuttles, silicon tiles are placed over aluminum skin for insulation. On the wings leading edges, carbon-carbon composite materials are used. Other crafts have used ablative heat shields which are made of special ceramic materials and are designed to slowly burn away during re-entry.