NASA declared the spacewalk complete at 4:19am (0819 GMT) shortly after Mission Specialist Rick Linnehan and Flight Engineer Garrett Reisman returned to the ISS and its hatches were shut.

During the mission, the spacewalkers and their colleagues inside the ISS moved an initial component of Japan's Kibo laboratory from the shuttle's cargo bay and lifted it atop the ISS with the space shuttle's robotic arm.

In the first task of their spacewalk, Linnehan and Reisman prepared the Japanese unit for removal from the cargo bay of space shuttle Endeavour, which docked Wednesday at the ISS.

Later, shuttle Commander Dominic Gorie and Japanese astronaut Takao Doi used the space shuttle's robotic arm to lift the unit -- a 4.2-tonne logistics module -- to its temporary home on the orbital outpost.

Bolts latching the unit atop the Harmony module of the ISS were secured at 4:06am (0806 GMT), NASA officials said.

The module is to be moved later and serve as a storage space atop Kibo, the main portion of which is scheduled for delivery to the ISS in late May.

The two spacewalkers then moved on to a different part of the ISS and installed two new components on the Canadian robotic arm Dextre.

The "Orbital Replacement Unit tool change out mechanisms" will function like hands on the two arms of the Canadian-built "dextrous manipulator," capable of latching on to payloads or tools, a NASA television commentator said.

Linnehan and Reisman floated outside the ISS in the brief glow of an orbital sunset at 0118 GMT Friday, and completed their spacewalk seven hours and one minute later.

The Japan Aerospace Exploration Agency's Kibo laboratory is a micro-gravity research facility which aims to open a vital new stage in deeper space exploration.

With its installation, Japan gains a foothold on the ISS alongside the United States, Russia and Europe, whose laboratory Columbus was delivered to the station in February.

Kibo's pressurized module, the larger cylindrical heart of the lab that will allow astronauts to work and conduct experiments in a shirt-sleeves environment, is to arrive on space shuttle Discovery due to launch May 25.

The final Kibo installment, an inter-orbit communications system unit called the Exposed Facility, is due for delivery in March 2009.

The mission's fourth spacewalk will see specialists Robert Behnken and Mike Foreman conduct a space shuttle thermal shield repair demonstration usinga device similartoacaulkgun.

Protecting the shuttle's thermal tiles is of particularly concern since the space shuttle Columbia disaster in 2003.

Columbia disintegrated re-entering Earth's atmosphere, killing all seven on board, because its thermal shield had been damaged when it was struck by a piece of debris during launch.

The 16-day mission for Endeavour is the longest mission at the space station and will conduct five spacewalks totaling some 30 hours of work.

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