U.S. astronauts on the space shuttle Endeavour have begun a spacewalk to continue construction on the International Space Station and examine possible damage to the shuttle.

The U.S. space agency NASA says during a six-and-a-half hour spacewalk Saturday the astronauts will install a truss on the space station, moving it into place with the help of a robotic arm.

They will also examine a gouge in the space shuttle's heat-resistant tiles that was detected Friday as the orbiter approached the space station for docking.

Video images beamed down from space indicate the damage to the heat shield measures about 7.5 square centimeters. NASA officials suspect a piece of ice or insulating foam from the shuttle's external fuel tank hit the heat shield during launch Wednesday.

NASA says more information on the condition of the heat shield will be gathered Sunday using sensors attached to a robotic arm.

Based on the crew's findings, engineers will determine whether the heat shield damage is significant enough to warrant repairs in orbit.

In February of 2003, a damaged heat shield caused the fiery breakup of the shuttle Columbia. All seven astronauts aboard the craft died as it disintegrated in the upper reaches of Earth's atmosphere.

Endeavour will be docked at the space station for seven to 10 days. The crew of two women and five men includes 55-year-old school-teacher-turned-astronaut Barbara Morgan. NASA says she will create a "classroom in space" during the mission.

Some information for this report was provided by AP.