<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 protects the shuttle from the extreme temperatures of re-entry by allowing the shuttle to pass through the atmosphere at a lower speed. When the shuttle passes through the atmosphere at a lower speed, the heat shield is exposed to the heat of the atmosphere for a longer period of time, which can cause it to overheat and break apart.