US astronauts make key spacewalk

Five spacewalks were planned for Discovery's mission

Two US astronauts have carried out a key spacewalk on the International Space Station (ISS).

Scott Parazynski and Douglas Wheelock helped reposition a massive girder that holds giant solar wings.

A problem in a rotating mechanism for another set of solar panels attached to the orbiting space platform means the repositioned set must work perfectly.

Any problems with the equipment could delay the planned December launch of the European science lab, Columbus.

"Another great day at the office," spacewalk veteran Scott Parazynski told Douglas Wheelock as they floated out of the airlock.

The pair's main task was to finish moving a 17.5-tonne girder, or truss, to its new home at one end of the ISS.

Astronauts inside the ISS used a robotic arm to hook up the beam to the station's backbone, helped by their spacewalking colleagues.

In graphics: Space Station

On Sunday, the spacewalkers found metal shavings inside a motorised rotary joint that keeps an established set of solar panels in the correct orientation to the Sun.

The crew of space shuttle Discovery has already installed the new Harmony module in a temporary position on the ISS.

Harmony gives ISS crews 18% more room and is the first expansion of living and working space since 2001.

It will provide a passageway between three science laboratories: the existing US Destiny lab; the European Space Agency's Columbus module; and the Japanese Kibo experimental units.

Built in Italy by the company Thales Alenia Space, Harmony weighs some 14 tonnes. It is 7m by 4.6m (23ft by 15ft).

The installation has been led by Italian astronaut and mission specialist Paolo Nespoli.