Astronauts begin spacewalk at space station

HOUSTON, Aug 11 (Reuters) A pair of rookie spacewalkers floated outside the International Space Station today to bolt an extension onto the orbital outpost's frame while NASA made plans to inspect potentially troublesome damage to the shuttle Endeavour's heat shield.

Shuttle crewmembers Rick Mastracchio and Dave Williams left the station's US airlock at about 2200 IST to begin a planned 6-1/2 hour spacewalk.

Their main job was to bolt a two-tonne, 11-foot-long 3.3-metre-long aluminum extension onto a solar array support beam installed during NASA's last space station construction mission in June.

The final piece of the half-finished 100 billion dollar station's backbone is due to arrive late next year.

NASA engineers on the ground meanwhile tweaked plans for an additional inspection tomorrow of the shuttle's heat shield.

Photographs taken by the station crew before the shuttle docked yesterday revealed a small but possibly deep gash in the center of one of the ship's belly tiles.

Although shuttles have returned from space several times in the program's 26-year history with far worse damage, NASA did not have the tools and procedures to look for heat shield damage until after it lost the shuttle Columbia and seven astronauts in 2003 due to an undetected hole in the wing.

ICE SUSPECTED Teacher-turned-astronaut Barbara Morgan and crewmate Tracy Caldwell are scheduled to pull out Endeavour's 15 metre robot arm tomorrow and mount a sensor-studded extension boom on it to takes images of the damaged tile.

The tile is on the underside of the shuttle near the right, rear landing gear compartment and may have been damaged by ice falling off the shuttle during launch.

The shuttle is covered with ceramic heat-resistant tiles and carbon panels to protect the ship's aluminum skin from melting during the plunge back through the atmosphere for landing.

Temperatures around the damage site can reach up to about 2,000 degrees Fahrenheit (1,093 degrees Celsius).

NASA needs more information about the tile damage before determining if the shuttle crew needs to make repairs before Endeavour returns to Earth.

The shuttle is using a new power converter that allows it to tap into the station's electrical grid and remain docked for at least an additional three days.

Even before the prospect of repairing the heat shield was raised, NASA had planned to extend Endeavour's stay at the outpost from seven to 10 days and add a fourth spacewalk if the converter worked.

The extra time would allow the Endeavour crew to finish more tasks needed to prepare the station for the arrival of labs built by Europe and Japan in December and 2008.

NASA managers also could decide to use the time to make repairs to the shuttle's heat shield if needed.

REUTERS RSA RAI2335