Two astronauts, Mission Specialist Rick Mastracchio and Dave Williams, completd their first space walk Saturday of a scheduled 11-day mission to expand the International Space Station (ISS).

During the 6-hour, 17-minute excursion, they successfully installed the Starboard 5 (S5) truss segment onto the International Space Station and continued preparations to relocate the Port 6 (P6) truss.

Pilot Charles Hobaugh used the station’s robotic arm to attach the S5 and Mastracchio and Williams permanently bolt the S5 in place on the end of the Starboard 4 truss segment. They also replaced a defective gyroscope on the ISS and installed an external stowage platform.

During the spacewalk, a computer onboard the ISS failed, and a backup system immediately kicked in. There was no immediate explanation for the problem, which did not hinder the installation work outside the orbiting space station.

During the spacewalk the Station-to-Shuttle Power Transfer System, which was activated after Friday’s docking, was shut down and it will be activated later in the day.

Also on Sunday, NASA is planning to a detailed inspection of the detected damage to Endeavour’s heat shield.

Barbara Morgan and crewmate Tracy Caldwell will use the space shuttle's robotic arm and laser-tipped boom to sweep over the area. The process was expected to take about three hours. After this task will be completed NASA engineers will study how deep the gouge is and if the repairs are needed.

In this case, Endeavour’s mission may be extended with three more days and an additional spacewalk.

NASA engineers have studied the 296 pictures photos sent in by Endeavour’s team during the docking maneuvers and are optimistic about solving the problem.

"The consensus in the mission management team is -- considering the flight history we had, the location of it -- all are very good signs that it will not be something that we will have to be worried about," said John Shannon, NASA Programme Manager, during a press conference.

Damage to the heat shield - much larger than that discovered on Endeavour - was what led to the Columbia shuttle disaster in 2003, in which seven astronauts were killed.

There won’t be a problem even if the damage is important and the shuttle crew would have to abandon Endeavour. According to NASA officials there are enough supplies to last to the end of October, whereas another shuttle could be launched as early as Oct. 5.

"I think we will have a final resolution on Monday when we do the thermo analysis," Shannon added.

The Endeavour last flew nearly five years ago and was the last shuttle to make the trip to the International Space Station before the Columbia exploded on reentry in February 2003. The launch date was postponed for one day after NASA discovered and replaced a leaky valve in Endeavour's cabin.