Washington - US astronauts Dave Wolf and Timothy Kopra of the space mission Endeavour began a spacewalk on Saturday to attach a four-ton outdoor shelf to Japan's Kibo laboratory module.

The scheduled six-and-a-half hour job - under the mission's motto of "A Porch in Space" - is the first of five spacewalks that are among the most complex in the 11-year history of the International Space Station.

Japanese flight controllers on the ground are to be involved for the first time operating their own mechanism, NASA said.

The porch, delivered to the station in Endeavour's cargo bay, will be used to expose scientific experiments to the extremities of space through X-ray cameras and studies of cosmic dust.

The shuttle Endeavour docked Friday at the space station, providing a record crowd of 13 astronauts - the largest ever in the space station history. All five international partners - Russia, Canada, Japan, the European Space Agency and the US - have astronauts at the station.

The expanded team reflected the final build-out of ISS capacity in May, when it doubled to six astronauts. Endeavour is carrying seven astronauts, including Kopra, who is the replacement for Japan's astronaut Koichi Wakata. Wakata will return to Earth with the shuttle crew.

Wolf and Kopra's jobs Saturday will be to remove insulation from the current Kibo module where the porch is to be attached, and, with the help of one of three robotic arms, transport the porch from the Endeavour cargo bay for installation at the station.

During subsequent spacewalks slated for every other day in the 16- day mission, the X-ray cameras and other equipment will be secured on the exposed porch.

In addition, other maintenance work is slated for the station.

Endeavour suffered some damage during launch on Wednesday when insulation broke free from the external fuel tank and hit three heat- resistant tiles on the shuttle's underside.

But the damage was dismissed as superficial by the associate administrator for space operations, William H Gerstenmaier. As has become routine since the 2003 Columbia disaster, Endeavour pirouetted in a back flip before docking at the space station for a high- resolution image from station cameras of the tiles on its underbelly.

The space shuttles will be closed down next year, leaving only Russia's Soyuz as transport to the station.

NASA is building a new spacecraft Orion to return to the moon by 2015 and prepare launches into space from there to nearby planets like Mars.(dpa)