Today's spacewalk plan includes hooking up more electrical and fluid connections between the Harmony Node 2 and the Destiny laboratory.

As was the case in the Nov. 20 spacewalk, Whitson, the lead spacewalker, will wear the suit with the red stripes while Tani will be in the suit with the barber-pole stripes.

Earlier this week Whitson and Tani already moved another 300-pound, 18.5 foot fluid tray, the Loop B fluid tray, from a temporary location on the station’s main truss to the Destiny lab. The Loop B fluid tray was placed on the opposite side of where the Loop A fluid tray was placed on Tuesday.

International Space Station (ISS) commander Peggy Whitson and flight engineer Dan Tani began their six-and-a-half hour spacewalk at 4:50 a.m. EST, NASA said.

After leaving the airlock and setting up tools and equipment, Whitson will remove, vent and stow an ammonia jumper, part of a temporary cooling loop. Removing it allows connection of the hookup of the permanent Loop B ammonia cooling loop on a second fluid tray on the station's exterior.

Tani meanwhile will configure tools, then remove two fluid caps to prepare for connection of that permanent cooling Loop B.

As they did with the Loop A tray Nov. 20, they'll use a kind of relay technique, one moving ahead and attaching tethers to be ready to receive the tray, then the other moving farther forward to take the next handoff.

Once they reach the installation point they'll bolt down the tray, and then hook up its six fluid line connections, two at S0, two at the tray and two in between.

The astronauts also planned to inspect a rotary joint of one of the station's solar arrays, which has experienced vibration and increased electrical current draw. Fixing the joint is important for the station's continued assembly in the future.

Tani will take digital pictures of the joint and collect samples of any debris there. Tani will return the cover to the airlock, leaving the joint available for a video survey by a camera on the station's robotic Canadarm2. That survey will be done after the STS-122 mission will end.

Today’s spacewalk is the final phase of Harmony preparations for the arrival the next module, Columbus.

The Columbus module is set to arrive on the next shuttle, Atlantis, due to launch December 6 from the Kennedy Space Centre on Florida's Atlantic Coast at Cape Canaveral.

Yesterday, NASA announced that the seven-member crew of STS-122 completed a full dress rehearsal for their upcoming mission. The crew participated in the terminal countdown demonstration test, familiarizing themselves with equipment and launch-related procedures.