<human>:  Why did the Space Shuttle Columbia explode on re-entry?
<bot>: The Space Shuttle Columbia exploded on re-entry because of a failure of the heat shield. The heat shield is a special coating that protects the orbiter during re-entry. When the orbiter is flying through the atmosphere at high speeds, the heat shield is supposed to protect the orbiter by melting away, but the heat shield failed on the Columbia. The heat shield failed because of a flaw in the manufacturing process.