Washington - A pair of shuttle Discovery space walkers prepared to exit the International Space Station (ISS) on Saturday as part of a final push to complete the decade-long construction of the orbiting outpost.
Astronauts Steve Swanson and Joseph Acaba were on course for the second of the Discovery mission's three external workdays, to start at 1643 GMT, as the station hurtles around Earth some 354 kilometres above the ground.
After huge solar wings were successfully unfurled on Friday on the ISS, paving the way for the orbiting laboratory to power up to its full capacity for the first time, Swanson and Acaba had the task of preparing batteries on the external Port 6 truss, ready for their replacement on a later mission in June.
The two, whose walk outside the station was expected to last six hours and 30 minutes, will also install a GPS antenna to the outside of the station's Japanese module and take photographs of radiators on other truss segments.
The GPS device will help guide the Japanese HTV robotic spacecraft, a cargo transfer vehicle to be put into operation later this year.
The autonomous craft will supply the station's Kibo laboratory with water, food and scientific materials, and could also resupply the rest of the station if needed.
Discovery's latest mission, which blasted off Sunday from Florida with a crew of seven astronauts, is one of the last major tasks of the more than ten-year push by 16 countries to build the $100-billion outpost in space.
Nasa on Friday said "no difficulties were encountered" in deploying the station's massive solar wings, after Swanson and colleague Richard Arnold had a day earlier bolted a S6 truss to hold external panels forming the fourth and last solar antenna.
A third space walk has been planned for Monday.
Once activated, the solar array -- capturing sunlight to convert into energy -- will have the capacity to generate a substantial amount of usable electricity, enough to power about 42 large homes.
The 14-tonne S6 truss was carried up by the shuttle from Earth and the orbiter's robotic arm was used to lift it out of the shuttle's bay.
Each solar array set has two wings measuring 35 metres wide by 11.58 metres, for a total span of 73 metres, including the truss connecting the two panels and allowing them to pivot as they follow the sun.
The four panels, two per wing, contain 32 800 cells that convert sunlight into electricity and will boost the outpost's full power generation from 90 to 120 kilowatts, providing the power the space station needs to carry out scientific experiments aboard the European Columbus laboratory and the Japanese Kibo lab.
The additions also enables a doubling of the space station's crew from three to six, beginning in May.
In all, Nasa has scheduled nine shuttle flights through 2010 to complete the construction of the space station.
Upcoming shuttle flights also include the last mission to service the orbiting Hubble telescope in May.
Discovery is due to land back on Earth on March 28 at 1742 GMT, two days after a Russian Soyuz mission takes off for the ISS carrying a crew of three, including US billionaire businessman Charles Simonyi, who has shelled out 35 million dollars for his second trip as a space tourist.
The Discovery mission, delayed five times, is the first by a US space shuttle in 2009.
It has been a near picture-perfect mission so far, barring the delayed lift-off and some pesky space rubble that had US and Russian experts braced to move the ISS in an "avoidance maneuver." - Sapa-AFP