Astronauts go spacewalking as NASA watches storm

HOUSTON, Aug 18 (Reuters) Two astronauts floated out of the International Space Station today for the final planned spacewalk of a mission that could be shortened by the threat of powerful Hurricane Dean back on Earth.

Dave Williams and Clay Anderson were to perform several tasks to prepares the 100 billion dollar station for upcoming additions in an outing that will be cut to 4 1/2 hours, down from the originally scheduled six hours.

At the same time, NASA is preparing for possibly landing space shuttle Endeavour on Tuesday -- one day earlier than scheduled. The U.S. space agency was concerned that Dean, racing through the Caribbean Sea with 240 kph winds, could force evacuation of the Johnson Space Center in Houston, home of Mission Control, if it enters the Gulf of Mexico as forecast next week.

The shuttle is set to land at Kennedy Space Center in Florida, which is not expected to be affected by Dean. NASA said Mission Control's functions could be moved to Kennedy if necessary.

Mission management team chairman Leroy Cain said NASA was looking for ways to speed up Endeavour's schedule in case an early return is ordered.

NASA dropped some of the planned workload for Williams and Anderson to truncate their spacewalk and hasten preparations to undock the shuttle from the station. After they are back inside, the hatch to the station will be closed, which was originally set for tomorrow.

Cain said the decision to land early or not would be made on Saturday afternoon at a mission management team meeting.

Endeavour launched from Florida on August. 8 and arrived at the station August. 10, carrying a metal beam that astronauts installed on the space complex in the first of four spacewalks.

NASA spent much of the mission pondering whether a gouge in Endeavour's heat shield, caused by loose fuel tank insulation foam that struck the ship's belly at launch, needed to be repaired before the shuttle's fiery return to Earth.

It finally decided on Thursday, after six days of deliberations, the 3-inch gash posed no threat and no patch job was needed.

Shuttle Columbia was destroyed and seven astronauts killed in 2003 because of a heat shield break caused when a piece of flying tank foam hit the ship's wing at liftoff. The damage was not detected and Columbia disintegrated shortly before landing.

Space shuttle Discovery is scheduled to launch in October to install a small compartment called Harmony on the station. It will be the connection hub for laboratories built by Europe and Japan that are scheduled to be launched in December and in early 2008, respectively.

NASA plans to fly 11 more shuttle missions to the space station to finish its construction before the aging shuttle fleet is retired in 2010.

REUTERS SKB PM2002