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The second Space Shuttle Main Engine (SSME) has been installed on the orbiter Discovery in the Orbiter Processing Facility. Discovery is the vehicle designated for the Return to Flight mission STS-114. 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 -- that's 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. The SSMEs are tested at Stennis Space Center in Mississippi.