Thursday's launch was postponed due to low level cloud that would have obscured the view of the external fuel tank during ascent. NASA closely monitors the fuel tank during a shuttle launch, as foam breaking off the tank caused the damage that lead to the death of Columbia's seven astronauts. NASA was also concerned about the weather at emergency landing sites in Spain and France.

There is only a 30 per cent chance of acceptable launch conditions on Saturday, rising to 40 per cent on Sunday and Monday then 60 per cent on Tuesday. The shuttle's launch window extends until December 26, although NASA engineers would prefer to launch before December 17 so as to have Discovery safely back to Earth by New Years Day. The shuttle's navigation software is not designed to handle the new year roll over.

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.