The spacewalk to install Japan's long-awaited Kibo module - a laboratory for experiments that could produce new medicines for Earth - began promptly at 0118 GMT Friday at the International Space Station.
Over the coming six hours, US Endeavour astronauts Rick Linnehan and Garrett Reisman will prepare the first part of Kibo, which means "Hope" in Japanese, for permanent attachment to the orbiting station. ( dpa )
Another four spacewalks are planned during the shuttle's 16-day mission.
The first component of the Japan Aerospace Exploration Agencys Kibo laboratory was transported by Endeavour with launch Tuesday from Florida. The remaining two Kibo components are to be delivered later this year.
The shuttle and its seven crew members docked picture perfect at the station early Thursday, where their resident three orbiting colleagues took photos of the spacecraft's heat shield to check for damage from the launch.
Japanese astronaut Takao Doi, a shuttle crew member, is to help install the Kibo lab. Two other Japanese astronauts are to fly with missions in May and December to complete construction of the laboratory.
Just last month, the Atlantis shuttle delivered the European laboratory Columbus to the station.
Both Columbus and Kibo were delayed for years by the 2003 Columbia shuttle disaster and ensuing years of revamping the ageing spacecraft for increased safety.
Over the past year and a half, shuttles have transported huge elements to the construction site that is the space station, including large solar collectors and truss structures.
The goal is to finish construction by 2010 with double-the-space for orbiting astronauts and an expanded capacity for experiments so NASA can retire the heavy-lifting but ageing, 26-year-old shuttles.
Another goal of the current Endeavour mission is to install Canada's two-armed Dextre robot, which will help future maintenance of the space station's exterior. The robot will be able to load and unload objects, use robotic tools and attach and detach covers, the Canadian Space Agency said on its website.
A small hitch with powering the Dextre project was being worked on, NASA officials said, and the space crew was troubleshooting with software and other options.
Japan's Kibo is to house about 100 experiments that could aid the development of medications and test new materials in weightlessness.
With the addition of Kibo, Japan's Space Station Integration and Promotion Centre north of Tokyo will join other control centres in the United States, Russia and Germany in monitoring components of the space station.
After the shuttle programme retires, Russia's Soyuz craft will continue to lift astronauts to the station, but because of its small size will be unable to carry the huge construction pieces and experimental modules now being transported by the shuttle.
Endeavour is to return to Earth March 26.