Two astronauts of space shuttle Discovery have stepped out into space completing the first of the three spacewalks aimed at extending and rewiring the International Space Station [Images].

Mission Specialists US astronaut Robert Curbeam, a veteran spacewalker, and the European Space Agency's Christer Fuglesang added the P5 integrated truss structure to the station on Tuesday.

The P5 was attached to the P4 segment. Mission Specialist Joan Higginbotham used the station's robotic arm to move the new segment with only inches of clearance into its installation position.

Then, the spacewalkers guided Higginbotham with visual cues as the exacting operation was carried out. After the P5's attachment, Curbeam and Fuglesang finalised the installation with power, data and heater cable connections.

They also replaced a malfunctioning camera on the S1 truss. Since spacewalkers worked ahead for the timeline, Curbeam and Fuglesang were able to tackle some get-ahead tasks. Two more spacewalks are on tap for STS-116 to reconfigure and redistribute power on the station.

The seven-member crew also performed a thorough, over five-hour inspection of the shuttle's heat shield tiles to look for damage from debris shed during liftoff.                                                                        

During the inspection, Higginbotham and Indian-origin astronaut Sunita Williams [Images] used the station's 57-foot camera-equipped robotic arm to scan four reinforced carbon-carbon panels on the shuttle's wing.

Preliminary analyses of the images reveal nothing of concern, NASA [Images] deputy space shuttle manager John Shannon said in a news briefing on Wednesday.

While engineers have said further inspections are not necessary, experts are still reviewing data. It was damage in the craft that lead to the loss of Columbia and its seven astronauts during re-entry in 2003.

The astronauts began their day, waking up to the Beatles' song Here Comes the Sun beamed up by ground controllers in Houston.

"Good morning, Discovery, and a special good morning to you, Mark," NASA astronaut Shannon Lucid, serving as spacecraft communicator, greeted from the ground as she hailed the shuttle launch.

Led by commander Mark Polansky, the STS-116 crew will participate in three spacewalks over the next week to install a new $11 million Port 5 spacer segment to the ISS, switch on a thermal cooling system and rewire the orbital laboratory's electrical grid so it can draw power from a new set of solar panels arrays installed last month.

Discovery is expected to stay at the station until Monday, with another inspection of the heat shield planned after the shuttle undocks.

A return to earth is scheduled on Thursday. NASA engineers are keen to have the ship and crew safely on the ground before New Year's Day, as the shuttle's navigation software is not designed to handle the new year roll over.