Update: Spacewalk Ahead of Schedule

Halfway into a planned seven-hour excursion, two Endeavour spacewalkers are working through their tasks smoothly and about 30 minutes ahead of schedule.

At their current pace, it appears Heidemarie Stefanyshyn-Piper and Steve Bowen may complete repairs to a rotary joint on the station's starboard side, which was the pre-flight plan by the end of three spacewalks.

That could set the stage for a test early Sunday to see if the repairs have allowed the damaged joint to turn more easily.

Bearings have been grinding against the joint's gear ring since last year, preventing it from turning so solar wings on the station's starboard side can continuously track the sun as the station orbits Earth.

Stefanyshyn-Piper has replaced two of the four bearing assemblies she plans to tackle today. Bowen has replaced one, moved on to a second and will clean around a third that doesn't need to be replaced.

If the repairs are completed, NASA commentators say a two-orbit test of the joint's ability to automatically track the sun could be run Sunday, starting as early as 6 a.m.

Before the mission, commander Chris Ferguson requested that the "auto-track" test be done at a time when the crew could watch it, particularly the astronauts who have worked on the joint during three spacewalks so far: Stefanyhsyn-Piper, Bowen and Shane Kimbrough.

"Heide, Steve and Shane are just going gangbusters outside, and I think it would be nice just to see a little bit of the fruits of our labor, or their labor," Ferguson said Friday during a crew news conference.