Heidemarie Stefanyshyn-Piper on the end of the robot arm is aided by fellow spacewalker Steve Bowen (L) as the pair move a nitrogen tank assembly to the Space Shuttle Endeavour's cargo bay for storage in this image from NASA TV Nov. 18, 2008. (Xinhua/Reuters Photo)Photo Gallery>>>

    WASHINGTON, Nov. 18 (Xinhua) -- Two U.S. astronauts from the shuttle Endeavor partially cleaned and lubed a solar panel on the space station Tuesday during the first of four planned space walks.

    Mission Specialists Heidemarie Stefanyshyn-Piper and Steve Bowen stepped outside the orbital outpost at 1:09 p.m. EST (1809 GMT) and wrapped up their work at 8:01 p.m. EST.

    The astronauts first replaced a depleted nitrogen tank and a device used to help coolants flow along the station truss during the excursion that lasted nearly seven hours. They also removed covers from the front of the Japanese Kibo module to prepare for the installation of the module's exposed facility during the next Endeavor mission in 2009.

Heidemarie Stefanyshyn-Piper on the end of the robot arm with the earth in the background as she moves a nitrogen tank assembly to the Space Shuttle Endeavour's cargo bay in this image from NASA TV Nov. 18, 2008.  (Xinhua/Reuters Photo)Photo Gallery>>>

    The astronauts then began their main task -- inspecting, cleaning and lubricating the station's starboard solar array rotary joint (SARJ) -- or the right-side joint. The mechanism is a wagon-wheel-shaped joint on the station's truss that allows electricity-generating solar panels to rotate so that they can always collect as much sunlight as possible.

    NASA ground controllers noticed last year that more power than normal was being required to rotate the starboard joint, and that the device was vibrating excessively.

    During several space walks to inspect the joint, engineers narrowed the cause to insufficient lubrication.

    The two astronauts began work on the problem by removing the joint's old bearings. They still have to clean metal shavings off the joint's surface, lubricate the device and install new bearings.

Heidemarie Stefanyshyn-Piper's tool bag moves away from the International Space Station in this view from her helmet camera as the tools were lost accidentally during her work cleaning and replacing the station's solar array trundle bearing Nov. 18, 2008.  (Xinhua/Reuters Photo)Photo Gallery>>>

    Altogether, the job will take four space walks. Stefanyshyn-Piper and Bowen replaced just a few of the 11 bearings on the gear during their first space walk.

    Stefanyshyn-Piper reported that her tool bag unexpectedly floated away during the job. But NASA ground controllers believe the loss will not affect the next three space walks. "There is enough equipment for the space walkers to complete the planned SARJ assembly and cleaning," NASA said on its website.

    The Endeavour arrived at the space station late Sunday. "This mission is all about home improvement," Commander Chris Ferguson said. "Home improvement inside and outside the station."

    In addition to the outside work, the seven shuttle crew members and the three station residents will install a series of new facilities that will allow the station to house six residents by spring 2009. Outside, spacewalkers will service the starboard joint and also lubricate the left-side joint to prevent future problems.

Space Shuttle Endeavour Commander Chris Ferguson (C) greets International Space Station Commander Mike Fincke (R) as ISS flight engineer Yury Lonchakov looks on after the opening of the hatches between the two spacecraft in this view from NASA TV November 16, 2008. (Xinhua/Reuters Photo)Photo Gallery>>>

U.S. shuttle Endeavour docks with space station

The Space Shuttle Endeavour is seen docked to the International Space Station with the Earth in the background in this image from NASA TV November 16, 2008. (Xinhua/Reuters Photo) Photo Gallery>>>

    WASHINGTON, Nov. 16 (Xinhua) -- After a nearly-two-day pursuit, the U.S. space shuttle Endeavour arrived at the International Space Station and docked with it at 5:01 p.m. EDT (2201 GMT) on Sunday, according to NASA TV.

    The space linkup occurred under the shroud of darkness as both spacecraft passed over the Earth's night side. Commander Christopher Ferguson was at the controls of space shuttle Endeavour docking to the station's Harmony Node. Full story