To date all shuttles have taken off from one of the two launch pads at Complex 39 at the Kennedy Space Center, Florida. A shuttle launch complex has been built at the Vandenberg Air Force Base in California but is currently mothballed. The shuttle stack of orbiter, external (ET) tank and solid rocket boosters (SRBs) is put together in the mammoth Vehicle Assembly Building (VAB), mounted on a mobile launch platform. A few weeks prior to launch, the stack is carried out to the pad by crawler transporter. Some three days before scheduled lift-off the countdown begins. The standard countdown period is 43 hours. Built-in holds during the pre-launch period allow time for technicians to catch up with the schedule if problems arise. About nine hours before launch the external tank (ET) will be filled with propellants. About two and a half hours before launch the astronauts will enter the orbiter. During a hold with nine minutes of the countdown remaining, the launch director will make the final decision to launch or not. If the decision is 'Go', the countdown resumes. Thirty-one seconds before lift-off the final automatic firing sequence begins. With just six seconds to go, the main engines roar into life, then the twin SRBs fire, the hold-down bolts are severed, and the shuttle stack rises from the pad. After two minutes, when the shuttle is some 50 km high, the SRBs run out of fuel and separate. They parachute back to the ocean, where they are taken in tow by recovery ships standing by. They will later be refurbished and used on subsequent flights. The remainder of the shuttle - orbiter and ET - meanwhile continue to climb. Some eight-and-a-half minutes into the mission, the orbiter's main engines cut off as the ET runs out of fuel, and is jettisoned. The orbiter is now in a highly elliptical orbit travelling at about 28,000 km/h, orbital velocity. A burn by the orbital manoeuvring system (OMS) engines later circularizes the orbit, which is usually about 250-500 km high, depending on the mission requirements. The first task of the crew in orbit is to open the payload-bay doors to expose radiators that will rid the vehicle of excess heat built up during launch.
