Countdown clocks at the Kennedy Space Center began ticking towards a Thursday launch of space shuttle Discovery on a high-stakes mission to rewire the International Space Station.
The countdown began on Monday night for Discovery's 2136 hrs 0806 IST Friday launch which will be NASA's first in darkness since before the 2003 Columbia accident.
The US space agency would still prefer launching during daytime, but must resume night liftoffs in order to finish building the space station before the shuttles are retired in four years.
Managers believe the shuttle fuel tank, which was blamed for the deadly Columbia accident, has been improved to the point that debris shedding from the tank during liftoff, which triggered the disaster, is no longer a threat.
Daytime launches give cameras a better view of the tank as the shuttle climbs to orbit.
For Discovery's liftoff, NASA will rely on backlighting from the shuttle's solid rocket booster to illuminate the tank for onboard cameras and ground- and sea-based debris-tracking radar systems.
The routine three-day countdown began at 2300 hrs.
The crew — five men and two women — arrived at the Florida spaceport on Sunday for final mission preparations.
One astronaut, Sunita Williams, will be making a one-way trip. She is to remain aboard the space station, replacing Germany's Thomas Reiter who will return with the rest of the Discovery crew on December 19.
The combined shuttle and station crews face a daunting task of rewiring the space station without interrupting life-support and other critical systems.
The shuttle crew also will install a new segment to the station's external structural truss.