Endeavour astronauts Heidemarie Stefanyshyn-Piper and Steve Bowen have switched their spacesuits to battery power, officially starting the mission's third spacewalk at 1:01 p.m., 44 minutes ahead of schedule.
"You're go to open the hatch," said mission commander Chris Ferguson.
The spacewalk is expected to last at least seven hours, all spent on repairs to a faulty joint that rotates the station's starboard solar wings.
To watch the spacewalk live, click on the NASA TV viewer above.
Stefanyshyn-Piper is wearing a spacesuit with solid red stripes on the legs, and her helmet camera shows No. 16 in the bottom-right corner. Bowen's spacesuit has solid white legs and his helmet camera shows No. 18
They'll be working about 180 degrees apart from each other on the circular joint, at the 5 o'clock and 8 o'clock positions if you were facing the joint. Stefanyshyn-Piper will attempt to remove and replace four sets of bearing assemblies circling the damaged joint, while Bowen will replace two.
Both will continue the laborious task of cleaning around each of those bearings sets. To do that, they will lubricate the joint's gear ring with grease, then wipe and scrape off debris on its surface.
If they are able to complete all the work today, NASA plans to test the rotary joint's performance early Sunday to see if the repairs have helped.
Engineers will let the joint rotate automatically for two orbits, or about three hours. They'll monitor the joint's vibration levels and the amount of current needed to turn it.
Those levels were discovered to be too high last September, so NASA stopped turning the joint continuously. If working properly, the joint should allow the station's solar wings to track the sun all the time so they can generate the most power.
The joint turning the station's port solar wings has worked fine. But the mission's fourth spacewalk plans to grease that joint as a preventative measure.