Space Shuttle Discovery and the mobile launcher platform sit on Launch Pad 39B.

Space Shuttle Discovery and the mobile launcher platform sit on Launch Pad 39B. The shuttle's external tank is capped by the oxygen vent hood (at top). Below it is the orbiter access arm which swings out from the fixed service structure to the orbiter crew compartment hatch to allow personnel to enter the crew compartment. The outer end of the access arm ends in an environmental chamber (white room) that mates with the orbiter and holds six persons. The arm remains in the extended position until seven minutes 24 seconds before launch to provide emergency egress for the flight crew. The rollout of Discovery from the Vehicle Assembly Building began at 12:29 a.m. The shuttle was harddown on the pad at 9:03 a.m. The mission is No. 20 to the International Space Station and construction flight 12A.1. The mission payload is the SPACEHAB module, the P5 integrated truss structure and other key components. The launch window for mission STS-116 opens Dec. 7. Photo credit: NASA/Jim Grossmann