Astronauts on the space shuttle Discovery have completed the first of their three scheduled spacewalks for tests on shuttle components and repair work on the International Space Station.

Japanese astronaut Soichi Noguchi and his American crewmate Steve Robinson returned to the shuttle Saturday after a six-hour and 50-minute excursion.

They tested repair techniques on the shuttle's thermal protection system and performed several repair tasks on the International Space Station, where the shuttle docked on Thursday.  On future spacewalks they are to do more work on the space station before Discovery's planned return to Earth on August 7.

This week, NASA said insulating foam from the shuttle's external fuel tank may have struck the orbiter's wing on lift-off, but caused no serious damage.  NASA has grounded further flights until the foam problem can be resolved.

Loose foam doomed the shuttle Columbia in 2003 when a piece punched a hole in the wing during take-off.  The shuttle broke apart during re-entry, killing all seven astronauts aboard.