Washington  - Astronauts installed a crucial new solar panel on the International Space Station during a spacewalk on Thursday.

The 300-million-dollar panel is part of a truss segment that will complete the backbone of the station and boost its energy capacity, allowing the resident crew size to double to six astronauts. The solar panels will be remotely deployed to face the sun on Friday.

US astronauts Steve Swanson and Richard Arnold returned to the ISS at 2323 GMT. The spacewalk lasted six hours and seven minutes.

About an hour after the spacewalk began, the ISS's motorized robotic arm manoeuvred the segment into its permanent position as the spacewalkers made sure it fit perfectly against the station. US astronaut John Phillips and Japanese astronaut Koichi Wakata directed the arm.

The spacewalkers then bolted the segment into place and connected its cables to the rest of the station. They also completed other tasks that will allow the solar panels to operate.

Astronauts used the arm on Wednesday to lift the piece out of the space shuttle Discovery's cargo bay and get it in position ahead of the work.

Discovery docked Tuesday with the ISS, bringing the fourth and final solar panel.

Two more spacewalks are planned to conduct other construction- related projects.

The mission also delivered Wakata, who has officially joined the permanent crew on the orbiting space station as its first Japanese member. He replaces US astronaut Sandra Magnus, who will join the Discovery crew for the flight back to Earth.

The shuttle launched Sunday after weeks of delays to inspect and study the hydrogen fuel valves and a later leak in a fuel vent. (dpa)