Two former schoolteachers have taken the shuttle Discovery mission's third and final spacewalk. But the astronauts were unable to repair a piece of equipment at the international space station.

Before making their spacewalk, the astronauts had to steer clear of some dangerous orbiting junk. On Sunday, the linked shuttle-station complex had to move out of the way of a 10-centimetre piece of debris that was expected to come perilously close during the spacewalk.

Astronauts Joseph Acaba and Richard Arnold II were counting on brute force to free up a jammed equipment storage shelf at the station. But it refused to budge and must now be fixed on another mission.

It was the second spacewalk for both Acaba and Arnold. Each previously went out with another mission astronaut, Steven Swanson.