Astronauts begin third spacewalk for station work

HOUSTON, Aug 15 (Reuters) Two US astronauts left the International Space Station today for a spacewalk that will prepare the complex for its first new module in six years.

NASA hopes the arrival of the new node, scheduled for October, will not be delayed due to renewed concerns about the space shuttles' fuel tanks.

Shuttle Columbia was destroyed and seven astronauts killed in 2003 because of a heat shield breach caused when a piece of foam insulation fell off its tank and hit the ship during liftoff.

The tanks were remodeled, but a piece of falling insulation hit shuttle Endeavour during its launch on Aug. 8.

Managers today were still evaluating whether a small but deep cut in two of Endeavour's heat-resistant belly tiles would need to be repaired during another spacewalk. A decision was expected later today.

On Friday, NASA planned to begin addressing the larger issue of whether any changes would be needed before the shuttle Discovery is cleared for flight on the next station assembly mission in October.

Discovery is to carry a small compartment called Harmony that will serve as the connection hub for laboratories built by Europe and Japan that are scheduled to be sent aloft in December and in 2008.

NASA has just three years to finish at least 11 space station assembly missions before it retires the shuttles, which are the only vehicles suited to the job as space freighters.

Today's spacewalk was the third since Endeavour's arrival at the station on Friday. Shuttle astronaut Rick Mastracchio and space station flight engineer Clay Anderson stepped outside the station to prepare a solar array truss segment for relocation during Discovery's mission.

The spacewalkers planned to install a new communications system and relocate two of the station's mobile work carts.

A final spacewalk of the mission is scheduled for Friday, but NASA is considering delaying the outing to Saturday if the shuttle heat shield needs to be repaired.

Another option is to add two more days and a fifth spacewalk to Endeavour's mission, which already has been extended from 11 days to 14.

Today's outing, which began around 1975 IST and is expected to last 6-1/2 hours, paired a shuttle astronaut with a space station crew member for the first time since spacewalks to build the station began eight years ago.

REUTERS RSA RN2236