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Dave Fentris (upper left) and Gary Austin , aft mechanical technicians with United Space Alliance, are attaching dome heat shields to Space Shuttle Main Engines (SSME) on the orbiter Atlantis in Orbiter Processing Facility bay 1. The shields provide protection for the orbiter against the high temperatures generated by an SSME. 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 thats 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. Atlantis is scheduled to launch in July on mission STS-121.