In this image taken from video and provided by NASA Saturday, astronaut Tim Kopra stows tools in the payload bay of the space shuttle Endeavour while in the background the International Space Station and the shuttle's robot arms grapple a four-tonne experiment shelf. (NASA TV/Reuters)Two astronauts from the space shuttle Endeavour completed the mission's first spacewalk on Saturday, spending more than five hours completing tasks outside the International Space Station.

Americans Dave Wolf and Timothy Kopra began their work by preparing to attach a new platform to the Japanese science lab on the space station, a job that was made difficult by chronic microphone static from their helmets.

The so-called "porch in space" is an experiment shelf that weighs about four tonnes. The installation was to be completed later Friday through the use of both the shuttle and space station's robotic arms.

The shelf is the third and final piece of the Kibo science lab and will allow experiments to be exposed to the vacuum of space.

The two astronauts also managed to free another space station platform for spare parts that jammed months ago, using a specially designed tool. But they did not have time to release a similar platform on the opposite side of the outpost.

NASA officials said the communications problem caused by the microphone static was a nuisance but not a safety issue, and they hoped to resolve the problem before the next spacewalk on Monday.

During five more followup spacewalks scheduled for every other day during the 11-day stay at the station, cameras and other equipment will be secured to the platform.

Heat shield looks intact: NASA

Endeavour arrived at the space station on Friday. Before docking it performed a backflip to allow cameras on the space station to take pictures of the shuttle's heat shield.

Canadian Julie Payette, the flight engineer on the seven-member shuttle crew, guided Endeavour's camera-equipped Canadarm, checking for possible damage.

NASA engineers said the analysis of the photos was continuing, but so far, the heat shield looks intact.

An unusually large amount of foam broke off Endeavour's fuel tank at liftoff and some pieces dinged the shuttle.

After the shuttle docking, Payette floated into the station and gave another Canadian, Robert Thirsk, a hug. The greeting marked the first time two Canadian astronauts were in space together.

A news conference with the Canadian pair is scheduled for Sunday night via video link.

With files from The Associated Press