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

The 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.

US astronauts Steve Swanson and Richard Arnold left the safety of the ISS at 1714 GMT for the planned six-and-a-half hour spacewalk.

About an hour later the ISS's motorized robotic arm had 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 were bolting the segment into place and will then connect its cables to the rest of the station and do 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 install the solar panels and conduct other construction projects.

The mission also delivered Japanese astronaut Koichi Wakata, who has officially joined the permanent crew on the orbiting space station. 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)