Space station commander Gennady Padalka and flight engineer Michael Fincke staged a five-hour 21-minute spacewalk today, successfully installing a new coolant system component and three antennas that unmanned European cargo craft will use for future dockings.

The spacewalk, the 56th devoted to station assembly and maintenance, began at 12:43 p.m. and ended at 6:04 p.m. when the hatch to the Pirs airlock module was closed. It was the fourth and final spacewalk for Fincke and Expedition 9 commander Padalka, who will be replaced next month by a fresh crew.

Total space station EVA time to date: 337 hours and 17 minutes. Total EVA time for Expedition 9: 15 hours and 45 minutes.

There were no problems of any significance today. While the station's control moment gyroscopes once again had to contend with an unexplained torque that acted to pull the complex out of its proper orientation, Russian rocket thrusters were re-enabled in plenty of time to prevent problems. See the report below for additional details.

Spacewalk preview story:

Space station commander Gennady Padalka and flight engineer Michael Fincke are taking a planned six-hour spacewalk today aboard the international space station, the crew's fourth and final excursion before returning to Earth next month.

The goal of today's spacewalk is to replace a coolant system pump control panel on the Russian Zarya module and to install three S-band antennas on the aft end of the Zvezda command module for use next year by a new unmanned European cargo vehicle. The astronauts also will install new safety tether guides and photograph a space-exposure experiment panel.

This will be the 56th spacewalk devoted to space station assembly and maintenance, the 31st staged from the station itself. It will be the sixth spacewalk for Padalka and the fourth for Fincke. Going into today's excursion, 39 NASA astronauts, nine Russian cosmonauts, one Canadian and one Frenchman had logged 332 hours and 56 minutes of station spacewalk time.

Running slightly ahead of schedule, Padalka and opened a hatch in the Pirs airlock module at 12:43 p.m., seven minutes early, to officially begin a planned six-hour spacewalk.Carrying a pressurized, cylindrical case housing a new coolant control panel, the spacewalkers will make their way forward to the top, or zenith, side of the Zarya module. The container will be temporarily mounted near the work site and depressurized.

After peeling back flexible insulation, the crew will disconnect electrical cables, remove the old unit and mount it atop the cargo container. The new assembly then will be removed and mounted in place. Three quick-disconnect fittings must be engaged, along with electrical connections.

The old coolant system panel then will be placed inside the cargo carrier. After installing four new safety tether guides, the crew will move back to the Pirs airlock, stow the cargo carrier and pick up three S-band antennas needed by the European Space Agency's Automated Transfer Vehicle, or ATV, during unmanned dockings starting late next year.

Up until this point, the station's orientation in space will be maintained by U.S. gyroscopes assisted, as necessary, by Russian thrusters. Those thrusters will be disabled when Padalka and Fincke move to the aft end of the command module to install the new antennas.

Flight controllers will be monitoring the station's orientation very carefully throughout the spacewalk in an ongoing effort to track down the source of an unexplained torque that acts to move the station out of its normal orientation during spacewalks at the far end of the complex. During the most recent EVA, the station's control moment gyroscopes because "saturated" trying to maintain orientation. The lab went into free drift, it's long axis ultimately tilted 80 degrees away from the desired position.

Engineers believe the mystery torque could be the result of air venting from the Russian airlock, from sublimators on the Russian spacesuits or the normal activity of astronauts working at the aft end of Zvezda, far from the CMGs and the station's center of mass.

The concern is that if the station drifts too far out of "attitude," its solar arrays will not be able to generate enough electricity. During the most recent spacewalk, flight controllers powered down non-critical systems and, due to an oversight, the S-band radio system used by the spacewalkers when not in contact with Russian ground stations.

This time around, a less extensive powerdown procedure will be implemented, one that does not include the communications gear. The station can safely be in free drift for a full orbit, officials say, and if necessary, the astronauts can simply move back to the airlock while Russian flight controllers re-establish the proper orientation using Zvezda's aft thrusters.

In any case, after dropping off the old flow control valve module and picking up the new ATV antennas, Padalka and Fincke will move to Zvezda's aft and install the antennas at the 8 p.m., 10 p.m. and 2 p.m. positions. Two such antennas were installed previously and three more will be put in place during a February EVA by the next station crew.

With the antenna work complete, Padalka and Fincke will make their way back to the Pirs airlock. Before re-entering the station, they plan to install safety tether guidance brackets around the hatch and to photograph a space exposure experiment panel.