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In the Orbiter Processing Facility bay 1 at NASAs Kennedy Space Center, workers guide the Hyster fork lift close to Atlantis to install a Space Shuttle Main Engine (SSME) in position number one (upper chamber). Overall, an SSME weighs approximately 7,000 pounds. Three engines provide the thrust, along with the two Solid Rocket Boosters, for liftoff. After about 2 minutes, the two boosters are spent and are separated from the external tank. The SSMEs continue firing for about 8 minutes. They shut down just before the craft is inserted into orbit. The SSMEs are tested at Stennis Space Center in Mississippi. Atlantis is being processed for the second Return to Flight mission, STS-121, scheduled for launch in a window from July 12 through July 31.