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HOUSTON, Sep 12 (Reuters) Two astronauts went spacewalking today to hook up a solar power unit after it was installed on the International Space Station in the first addition to the space complex in nearly four years.

Crewmembers used a robot arm to put the 14-metre truss structure in place, then remotely bolted it on to resume station construction that was halted by the Columbia disaster in 2003.

Shuttle Atlantis, which launched from Florida on Saturday, arrived at the space complex with the new component yesterday.

The addition of the 15,880-kg unit increased the weight of the orbiting space station from 196,364 kilograms 212,273 kilogram.

It includes two solar power arrays that will be unfolded out to their full 73-meter length on Thursday and double the amount of electricity for the station, which NASA plans to complete before the shuttles are retired in 2010.

After the truss was attached, astronauts Joe Tanner and Heidemarie Stefanyshyn-Piper floated out of the space station for a 6 1/2 hour spacewalk to connect 17 power, data and fluid lines that will provide electronic and thermal control for the solar unit.

They also will release restraints on the enclosures holding the solar arrays to prepare for their deployment.

The spacewalk is the first of three planned for the 11-day mission, which is scheduled to return to Earth on September. 20.

Tanner, who has flown on four previous shuttle flights, is making his sixth spacewalk, while while Stefanyshyn-Piper is on her first spaceflight.

''Let's get this show going,'' said Tanner as the two prepared to leave the station.

''Okay, I'm out of the hatch,'' Stefanyshyn-Piper said.

After taking a look around, she said, ''Oh wow, Earth's pretty.'' Television shots showed them easing along the white space station, with the blackness of space behind them, as they moved into place to perform their work.

After Columbia disintegrated while returning to Earth on February. 1, 2003, and killed the seven astronauts on board, NASA grounded the shuttle program and station construction.

The space agency spent more than $1 billion on safety upgrades and now has resumed space station construction after two shuttle test flights convinced them that they had made shuttles safe to fly.

REUTERS SAM RK1715