Astronauts of space shuttle Discovery ended their mission's first spacewalk on Wednesday after installing a new piece on the International Space Station (ISS).
American Robert Curbeam and Swede Christer Fuglesang, the first Scandinavian in space, began the spacewalk at 2031 GMT on Tuesday and wrapped up the six-hour event around 0241 GMT on Wednesday.
Inside the station, astronaut Joan Higginbotham used a robotic arm to guide a two-tonne truss into place. Curbeam and Fuglesang helped guide the truss and bolted it into place.
The new truss will allow astronauts on a future mission to move another truss and its solar arrays to another location on the station.
Wednesday's work schedule will have shuttle astronauts folding up an old solar panel from the ISS. After years of exposure in space, it remains uncertain whether the lightweight array will return to its original, compact shape. One NASA scientist has compared the procedure to folding up a well-worn road map.
Discovery lifted off late on Saturday from the Kennedy Space Centre at Cape Canaveral, Florida, for the construction mission to the ISS.
The flight's primary mission is to hook up the ISS' permanent electricity-generating system, replacing a temporary power system operating since the space station went into orbit in 1998. Two more spacewalks are planned for Thursday and Saturday to rewire both halves of the station.
NASA has described Discovery's rewiring mission at the ISS as one of the most complex and difficult in the history of space flight.
Astronaut Sunita Williams, who is a crew member of the Discovery, will replace German astronaut Thomas Reiter, who has resided for five and a half months on the space station, which has a rotating crew of three astronauts.
Discovery is on NASA's second working shuttle mission to the ISS since NASA returned to flight in summer 2005, after the shuttle fleet was grounded for two years following the 2003 Columbia disaster, which killed all seven crew members.
NASA spent most of the last year testing new safety systems. A September flight by the shuttle Atlantis marked the resumption of ISS construction.