Canadian astronaut Dave Williams completed a six-hour spacewalk Saturday, successfully installing a roughly two-tonne beam to the backbone of the international space station.

Williams and American astronaut Rick Mastracchio, who left the space station and floated into space at around 12:28 p.m. ET, completed the walk at 6:45 p.m. ET, NASA said.

Astronauts aboard the space station used the Canadian-made robotic arm, Canadarm, to install the square-shaped beam, or truss, which cost $11 million US.

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Canadian astronaut Dave Williams, bottom right, and mission specialist Rick Mastracchio, bottom left, work outside the space shuttle Endeavour while orbiting Earth Saturday. ((NASA-TV/Associated Press))

Astronaut Charles Hobaugh used the Canadarm to lower the beam into placeas Mastracchio and Williams floated nearby, offering guidance. Once the beam was attached to the station, the spacewalkers bolted it down and hooked up grounding straps, then performed some extra chores.

During the installation, Williams had to wedge between a radiator and electrical box without touching either.

Midway through the spacewalk, NASA's main command-and-control computer aboard the space station shut down. The backup automatically kicked in, and the third computer went from standby to backup. Mission Control said the problem did not affect the spacewalk or the health of the station.

Williams to patch up gouge?

Williams, who is expected to complete three spacewalks during the mission, could be called on to repair a roughly 7.5-centimetre gouge on Endeavor's belly. The shuttle sustained the damage, as well as several smaller holes, moments after Wednesday's liftoff.

Canadian astronaut and spacewalk trainer Chris Hadfield said Saturday that Williams and Mastracchio are likely the go-to spacemen to conduct any necessary patchwork.

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Dave Williams answers a question during a news conference earlier this week in Houston. ((David J. Phillip/Associated Press))

"Dave and Rick are the two prime spacewalkers on the Endeavour crew," he said.

"We've got multiple layers of [repair] techniques, but my gut feeling is that we won't have to do any of those. We've had little dings in the belly over 10,000 times and it's well within our experience."

On Sunday, flight day five, the shuttle crew is scheduled to circle the damaged area with an orbital boom to get a closer look.

Ice likely culprit: NASA

The Endeavour team checked for damage on Friday, believing that three pieces of insulation — possibly foam — struck the shuttle's exterior during liftoff.

Foam damage has been a worry for NASA since a chunk pierced the space shuttle Columbia's left wing during its launch, causingthevehicleto break up upon re-entry on Feb. 1, 2003.

At a later news conference in Cape Canaveral, NASA official John Shannon said the particles they were tracking "looked more like ice than foam."

The focused inspection planned for Sunday is an extra precaution.

"We perked up our ears a bit because ice is much denser than foam and could do much more damage on tile …so that was interesting," Shannon said.

A severe penetration could let in searing gases when the shuttle returns to Earth in a possible replay of the Columbia accident, in whichall seven astronauts on board were killed.

Accident investigators concluded a suitcase-sized chunk of foam tore a hole in Columbia's left wing 82 seconds after liftoff on Jan. 16, 2003.

In an emergency, Shannon said, Endeavour could remain at the space station for at least two months and a rescue shuttle could be launched as early as October.

With files from the Associated Press