Space shuttle Endeavour crewmembers Heidemarie Stefanyshyn-Piper and Shane Kimbrough completed the second of the mission's four spacewalks at 00:43 GMT, having moved service carts on the exterior of the orbiter and lubricated moving parts of the solar array, which provides power to the station.
During the first spacewalk on Tuesday, Stefanyshyn-Piper failed to properly secure her tool bag, and $100,000-worth of equipment floated into space. This time, she returned to the station with a full complement of kit.
The third spacewalk is scheduled for Saturday, when cleaning and lubrication of the solar array joint will continue.
Thursday marked the 10th anniversary of the launch of the first segment of the 16-nation project. The U.S.-funded and Russian-built module Zarya - which means "dawn" - was fired into orbit from Kazakhstan on November 20, 1998.