HOUSTON — Spacewalking astronauts floated outside the International Space Station on Tuesday to help install the orbiting outpost's newest room, a bus-sized Japanese laboratory.

During a scheduled 6 1/2 hour spacewalk, astronauts Michael Fossum and Ronald Garan will prep the $1 billion lab, named Kibo -- Japanese for hope -- for installation by removing power and heating cables and various restraints that connect it to the shuttle.

Later in the day, astronauts working from inside will use the space station's robot arm to lift the lab from the shuttle and anchor it to the station.

The start of the spacewalk was delayed nearly an hour as a faulty communications cap -- which allows spacewalkers to talk with other crew members and controllers on the ground -- was replaced in Fossum's spacesuit.

"We're looking forward to a great day, an exciting day to install the Japanese Kibo module,'' said Japanese astronaut Akihiko Hoshide, who will help move the lab with the station's robotic arm.

Kibo, at 11 metres long, is bigger than the U.S. and European labs already attached to the space station. The Japanese lab also has a pair of robot arms, the larger of which flew up on this shuttle mission.

A separate storage room loaded with Kibo equipment went up in March. A porch for outdoor science experiments and the smaller robot arm will fly next year.