Spacewalking astronauts from the U.S. space shuttle Discovery have returned to the orbiter - having succeded in their attempt to fold and store a set of solar panels outside the International Space Station.

The two astronauts began a previously unplanned spacewalk Monday to correct problems with the panels which got stuck and failed to retract by remote control.

The old solar panels were replaced by a new set of panels.

The extra spacewalk delays the shuttle landing, now set for Friday at the Kennedy Space Center in Florida.

Monday's walk was the fourth by crew members during the mission. Other tasks have included re-wiring the space station to use energy from the new solar panel system.