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IntroductionThe last few minutes before a space shuttle roars into the sky are full of excitement and anticipation, as well as hard work to ensure that all the spacecraft's systems are working properly and are ready for liftoff.
The space shuttle Atlantis is currently positioned on Launch Pad 39A at NASA's Kennedy Space Center in Cape Canaveral, Fla., ready for its final mission, and the last ever flight of the agency's space shuttle program. Atlantis is slated to blast off on Friday (July 8) at 11:26 a.m. EDT (1526 GMT), if weather cooperates.
As the hours and minutes tick down to launch (as indicated by the "T-minus" mark), NASA's ground teams perform a battery of checks before giving the final "go." Here's a breakdown of some of the major milestones leading up to the big moment.
T-Minus 9 minutesSlide 2 of 17
T-Minus 9 minutes
At several predetermined points during the launch countdown, NASA will pause the clock as part of standard procedure to give the ground teams time to resolve any unexpected issues that may crop up. At the end of these "built-in holds," the countdown clock will resume unless NASA encounters any technical glitches. [How to Watch NASA's Final Space Shuttle Launch]
At the T-minus 9 minute mark, the clock will typically enter a 45-minute hold. Before this point, the NASA test director will perform a launch readiness poll of the shuttle launch team. Weather forecasts for the Cape Canaveral area will also be checked to verify that the conditions meet the agency's criteria for a safe launch.
During the hold, NASA officials on consoles will be polled for their "go/no go" decisions. These technicians closely monitor computer displays and gauges that show the performance of the shuttle's systems at the launch pad.
After the T-minus 9 minute built-in hold, the countdown will resume. The Ground Launch Sequencer (GLS), which is an automated program that controls all activity during the final portion of the countdown, will assume automatic control of the countdown at the T-minus 9 minute mark. This program will continue to monitor the vehicle's parameters and will be able to halt the countdown if a problem is detected. The GLS is typically started at about the T-minus 45 minute mark.Slide 3 of 17
T-minus 7 minutes, 30 secondsSlide 4 of 17
T-minus 7 minutes, 30 seconds
At this point, the command will be given to retract the orbiter access arm, which is the lowermost scaffolding arm located 147 feet (about 45 meters) above the surface of the launch pad. This structure allows people to enter the shuttle's crew compartment. The orbiter access arm remains in its extended position until seven minutes, 24 seconds before launch to serve as an emergency escape route for the flight crew. [NASA's Space Shuttle – From Top to Bottom]
The arm, which is 65 feet (20 m) long and five feet (1.5 m) wide, can be mechanically or manually repositioned in about 15 seconds if there is an emergency or contingency situation.Slide 5 of 17
T-minus 5 minutesSlide 6 of 17
T-minus 5 minutes
Barring any technical or weather concerns, the commander of the shuttle will be given the "go" to start the orbiter's auxiliary power units (APU), which produce pressure for the shuttle's hydraulic system. There are three separate onboard APUs, and their fuel systems are located in the aft fuselage of the orbiter.
Once the APUs are powered up, ground teams will analyze the system, and if they detect any glitches, this could halt the countdown. At the T-minus 4 minute, 30 seconds mark, the Ground Launch Sequencer program will switch the main fuel valve heaters off. As the clock ticks down, the GLS will also perform checks of the fuel and space shuttle main engines.Slide 7 of 17
T-minus 2 minutesSlide 8 of 17