Picking up where the Discovery astronauts left off, two space station astronauts began a spacewalk today to prepare the lab's shuttle docking port for a robotic transplant operation next week. Using the station's robot arm, the crew plans to remove the docking port from the front of the station Monday, attach it to the just-delivered Harmony module and then move both segments back to the front end of the Destiny laboratory module next Wednesday. Two more spacewalks are planned Nov. 20 and 24 to connect Harmony to the station's power and cooling systems, clearing the way for long-awaited shuttle flights in December, February and April to install European and Japanese research modules.

Expedition 16 commander Peggy Whitson and flight engineer Yuri Malenchenko planned to begin today's spacewalk at 6 a.m. But they completed their preparations an hour ahead of schedule, switching their spacesuits to battery power at 4:54 a.m. to officially kick off a planned six-and-a-half-hour excursion.

This is the 97th spacewalk devoted to station assembly and maintenance since construction began in 1998 and the 20th so far this year. For identification, Whitson, call sign EV-1, is wearing a suit with red stripes around the legs while Malenchenko, EV-2, is wearing an unmarked suit.

Whitson and Malenchenko, assisted by Dan Tani inside the lab complex, had hoped to carry out today's spacewalk during Discovery's just concluded mission, but the excursion was delayed because of emergency work to fix a torn solar blanket.

The delay has compressed the station crew's timeline, prompting extensive replanning to get the Harmony preparations completed in time for the launch of Europe's Columbus module aboard the shuttle Atlantis Dec. 6. The shuttle launch window closes Dec. 13 and if the Harmony work cannot be completed in time, the Atlantis flight will be delayed to early January.

"Launching in December is a priority for all of us, not just the U.S. side, but all of the partners," said Expedition 16 increment manager Pete Hasbrook. "You might remember that after Columbia, we did a lot of replanning and our partners were very, very patient in watching us and helping us through that whole mission replanning process. You might remember we changed the order of some of the flights.

"Again, the partners have been working on these modules and looking forward to their payback, to their countries, the European Space Agency, the Japanese exploration agency. They've been very patient about getting their laboratories into space, getting their science return. So it's important to us to recognize the support they've given us and the patience they've given us. It's not just an American mission that we're going into. It's really expanding the station to be more international than it is."

That said, "the managers on both sides, the shuttle program and station program, understand the amount of work we have to do," Hasbrook said.

The station's international laboratory modules will be attached to the Harmony module, which was brought up aboard Discovery. The station's design calls for Harmony to be inserted between the shuttle's forward docking port, known as pressurized mating adapter No. 2 (PMA-2), and the U.S. Destiny laboratory module.

Harmony was delivered by the Discovery astronauts and temporarily mounted on the left side of the central Unity module. Before it can be moved to its permanent position on the front end of Destiny, the station astronauts must remove PMA-2 and bolt it to Harmony. Then both attached segments can be moved to the front of the station to complete the operation.

Today's spacewalk is devoted to completing preparations for PMA-2's detachment and Harmony's relocation next week.

After floating out of the Quest airlock chamber on the right side of the Unity module, Whitson and Malenchenko will make their way to the front end of PMA-2 and detach electrical cables that feed solar array power to docked shuttles. They also will disconnect eight umbilicals connecting PMA-2 to Destiny, make preparations for installation of cooling lines between Destiny and Harmony and remove a floodlight that's in the way.

With that work complete, Whitson and Malenchenko will move back to Harmony and remove a protective shower cap-like cover on the port where PMA-2 will be attached Monday. Whitson then will complete electrical connections to a robot arm mounting fixture while Malenchenko reconfigures an electrical jumper. He also plans to replace a faulty circuit breaker.

If all goes well, Tani, operating the station's robot arm, will disconnect PMA-2 Monday and mount it on the far end of Harmony. The next day, the astronauts will work inside Harmony, making internal connections between the module and the docking port, before moving both segments to the front of Destiny on Wednesday.

Whitson and Tani then plan to stage a pair of spacewalks Nov. 20 and 24 to connect electrical cables that will route solar array power to Harmony and the lab modules that eventually will be attached. They also will unstow and attach long trays to Destiny's hull that will carry ammonia coolant lines from the solar array truss to Harmony.

"The big technical part of my stay on station is the EVAs that will follow, where we take fluid trays that have been stored on the station for years and we install them on the lab to provide cooling and power to the node (Harmony) so it can offer it to the Columbus module and the Japanese Experiment Module," Tani said before launch.

"We talk about this as a 45-day shuttle mission in terms of pace," he added. "Shuttle missions are scheduled down to 10-minute increments and generally, usually station timelines are a bit more relaxed. But we are not, we are all 'go' from the moment of launch to probably until (Atlantis) comes to get me to bring me home, we are go, go, go."

Station flight director Holly Ridings said the lab crew will have to work through the Thanksgiving holiday to get the work done in time for.

"They are not going to have Thanksgiving off," she said. "Even pre-flight, the crew knew that this month of November was going to be very, very challenging. And so they said right up front, if it works out that we need to work on Thanksgiving, we are more than happy to do that. And it did work out that way. So they will be working on Thanksgiving with a lot of us here working as well to support them."