The scheduled six-hour, 40-minute venture into space started at 0932 GMT, the space agency said in a statement. US astronauts Scott Parazynski and Daniel Tani came out of an airlock with the task of moving a truss that supports a set of the station's key solar panels.
The International Space Station (ISS), a giant manned laboratory orbiting 390 kilometers (240 miles) above Earth, is aimed to be a potential jumping-off point for exploration of Mars.
The maneuvering of equipment during Sunday's spacewalk is necessary to prepare the ISS for the arrival of new laboratories, experts said. The truss is the biggest piece of kit yet to be moved around on the station.
Mission specialist Parazynski and Tani, a flight engineer, will disconnect cables from the truss from the top of the station, where it was installed temporarily in 2000.
Then the space station's robotic arm, operated by astronauts in the station, will move it to an overnight parking position. It will be installed in a new spot during further spacewalks over the coming days.
The spacewalkers will also complete the external outfitting of the Harmony module, a new compartment recently installed on the station. Tani is to inspect other equipment, including a joint used to rotate the solar panels.
"The joint has been showing some increased friction lately, and mission managers hope Tani may be able to identify the cause," NASA's statement said.
A team of astronauts earlier entered Harmony, a newly delivered compartment, and began preparing it for its role in connecting two future laboratories to the station - Japan's Kibo lab and the Colombus from Europe.
"Harmony is a very good name for this module," said space station commander Peggy Whitson. "It represents the culmination of a lot of international partner work and will allow international partner modules to be added on."
The bus-size module was attached to the station during the first spacewalk of the mission Friday.
"Everything is going great," said Rick LaBrode, the mission manager of the Discovery shuttle that blasted the NASA crew into space Saturday.
During Discovery's two-week mission to the ISS, the astronauts are scheduled to perform a record five spacewalks, lasting a total of 30 hours.
The ISS, a $100-billion project involving 16 countries, is considered key to US ambitions to send a manned mission to Mars and is due to be completed within three years.
The current mission is making space exploration history as shuttle Commander Pam Melroy, 46, and Whitson, 47, are the first women to hold the reins of the two spacecraft at the same time.
© 2007 Agence France-Presse
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