Meanwhile, two of Discovery's crew stepped outside this afternoon for an six hour walk to begin a significant upgrade to the International Space Station.

Christer Fuglesang, the first Swede in space, is accompanied by veteran spacewalker Robert Curbeam as they install a 2 ton segment of the station carried in the belly of the shuttle. The segment of the station's backbone will allow astronauts the install further solar panels and overhaul the ISS's electrical systems.

Two more spacewalks are slated for Thursday and Saturday to complete the job.

Discovery is expected to stay at the station until Monday, with another inspection of the heat shield planned after the shuttle undocks. A return to earth is scheduled for on Thursday December 21. NASA engineers are keen to have the ship and crew safely on the ground before New Years Day, as the shuttle's navigation software is not designed to handle the new year roll over.{moscomment}