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KENNEDY SPACE CENTER, FLA. - In the Space Shuttle Main Engine (SSME) Shop, the SSME is lowered onto the Hyster lift for transport to the Orbiter Processing Facility. There it will be installed in the orbiter Discovery for Return to Flight mission STS-114. This is the third SSME to be installed in Discovery. Overall, an SSME weighs approximately 7,000 pounds. An SSME operates at greater temperature extremes than any mechanical system in common use today. The liquid hydrogen fuel is -423 degrees Fahrenheit, the second coldest liquid on Earth. When the hydrogen is burned with liquid oxygen, the temperature in the engine's combustion chamber reaches +6000 degrees Fahrenheit -- higher than the boiling point of iron. Each SSME is controlled by its own computer, which checks the health of the engines 50 times per second during countdown and ascent. The controller can shut an engine down if it detects a problem.