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

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

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 on Friday.

"Everything is going great," said Rick LaBrode, the mission manager of the Discovery shuttle that blasted the NASA crew into space, on Saturday.

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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