Damage was discovered in a joint supporting the station's solar arrays, a problem that NASA engineers will now have to solve. US astronaut Daniel Tani noticed metal shavings and unusual wear on part of a joint used to rotate the arrays, NASA officials said.

"The joint has been showing some increased friction lately, and engineers are analyzing potential causes," the National Aeronautics and Space Administration said in a statement.

The astronauts used an adhesive ribbon to collect scraps of metal debris which they will take back to Earth for analysis to find out the cause of the damage.

"We have lots of time to work through this problem, it's not an immediate issue," ISS director Mike Suffredini told reporters on the ground Sunday.

Meanwhile, mission commanders must decide if further inspections or repair work are needed during another space walk.

The solar array was installed just four months ago.

The second spacewalk started at 0932 GMT and finished at 1605 GMT, with space shuttle Discovery astronauts Tani and Scott Parazynski prepared for the installation Tuesday of a truss that will support a set of the station's key solar panels.

The 16-tonne truss is the biggest piece of equipment yet to be moved around on the station on this latest mission to the ISS, during which the astronauts will perform a record five spacewalks totaling some 30 hours.

The ISS, a giant manned laboratory orbiting 240 miles (390 kilometers) above Earth, is aimed to be a potential jumping-off point for further exploration of the solar system.

The 100-billion-dollar space station, supported by 16 countries, is considered key to US ambitions to send a manned mission to Mars and is due to be completed within three years.

During Sunday's walk, mission specialist Parazynski and Tani, a flight engineer, disconnected truss cables from the top of the station, where the huge item was installed temporarily in 2000.

Then the spacestation'sroboticarm, operated by astronauts inside the ISS,movedit toanovernightparkingposition. It will be installed in a new spot on the ISS during further spacewalks over the coming days.

The next spacewalk is scheduled for Tuesday morning.

Astronauts will also complete theexternaloutfittingofthe Harmony module, a new compartment recently installed on the station.

They earlier entered Harmony 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 14-tonne, bus-size module was attached to the station during the first spacewalk of the mission on Friday.

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.

The shuttle is to be retired in 2010 to make way for Constellation, a new space exploration project that aims to put humans back on the moon by 2020.

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