( dpa ) - Two US astronauts from the space shuttle Endeavour began the fourth space walk of their mission to the International Space Station to repair a circuit breaker on the station's truss and test a new technique for repairing shuttle heat shields.
Astronauts Robert Behnken and Mike Foreman began the spacewalk at 6:04 pm (2204 GMT) Thursday; it was to last six hours.
Behnken and Foreman will test a material and caulk gun-type dispenser to repair holes in demonstration heat shield tiles that will then be examined back on Earth, NASA said. Damage caused to the space shuttle Columbia by falling pieces of its heat shield has been blamed for its explosion on reentry in 2003, killing all seven astronauts on board. Since then, NASA has focused on inspecting the tiles on each shuttle mission for damages.
The fifth and final spacewalk is set for Saturday.