HOUSTON (Reuters) - Astronauts made a tricky job look easy on Tuesday when they used a robot arm to latch a 17.5 tonne solar power girder to its new home at the end of the International Space Station.

Arm operators Daniel Tani and Stephanie Wilson could not see the 35-foot-long (10-metre) girder's destination from inside the station and had to guide it in with directions from spacewalkers Scott Parazynski and Douglas Wheelock.

"Looks good, spaceman," one of the spacewalkers said as they performed the task.

"Fantastic job of guiding us in," Tani told them.

NASA had said the installation of the truss, which sat atop the station for seven years awaiting the move to its permanent location, was one of the most challenging maneuvers ever on the $100 billion space outpost.

It also was critical because the next key task will be to unfurl the 240-foot-long (73-metre) solar panels folded up inside the unit.

The electricity they generate will be needed for an upcoming expansion of the 9-year-old station and took on added importance when NASA shut down on Sunday a malfunctioning rotary joint that is a key part of the station's power system.

The 10-foot (three-meter) joint, on the opposite side of the station from the newly installed girder, rotates to keep one of the outpost's solar panel arrays pointed at the sun to produce electricity.

The joint, installed in June, had been acting up for two months. Tani, on a Sunday spacewalk, found metal shavings inside.

The shavings indicated that metal moving parts are grinding, which prompted NASA on Monday to add a day to space shuttle Discovery's stay at the station so that Parazynski and Wheelock can closely inspect the joint in a Thursday spacewalk.

Discovery launched on October 23, arrived two days later and is now scheduled to land November 7.

The solar panels on the malfunctioning joint will generate electricity, but less of it because they will not be tracking the sun.

ENOUGH POWER?

NASA plans to add Europe's Columbus laboratory to the station on a space shuttle flight in December, followed by Japan's multi-segment Kibo science unit starting in February.

Space station program manager Mike Suffredini said that if the solar panels unfurl as planned on Tuesday, the station would have sufficient power for Columbus, but it was not clear if Kibo could be supported.

It was not yet known what repairs will be needed or when they can be performed.

The joint that will rotate the solar array installed on Tuesday was put on the station last year and is working well, NASA said. But, just in case, Parazynski was to take a look at it on Tuesday and take photos and video for study by NASA engineers.

The shuttle brought with it the Italian-built Harmony module, which was attached on Friday to add the first new room on the station in six years. Harmony also will serve as berthing port for Columbus and Kibo.

NASA is trying to finish the space station, which is a project of 15 nations, by 2010 when the space shuttle fleet is scheduled for retirement.