Concerns about clouds over the Kennedy Space Center and strong winds had put the launch in doubt, but NASA is pressing ahead as scheduled. The primary goal of the 12 day mission is to rewire the International Space Station's electrical and cooling systems. The seven astronauts are also delivering a 2-ton addition to the station and replacing one of the station's three crew members.
Since the Columbia disaster in 2003, NASA has preferred daylight launches to ensure good photography of the external fuel tank. Foam breaking off the tank at liftoff caused the damage that lead to the death of Columbia's seven astronauts.