International Space Station commander Peggy Whitson and flight engineer Dan Tani completed a seven-hour spacewalk Saturday — the third in the past 15 days.

The two finished the external outfitting of the U.S. Harmony lab module, paving the way for the Dec. 6 launch of shuttle Atlantis and the European Columbus laboratory.

It was the third spacewalk at the outpost in the past 15 days and the fourth spacewalk for Whitson, the first female commander of the station. She now has chalked up 25 hours and 40 minutes working in the vacuum environment in low Earth orbit. It was also Tani's fourth spacewalk.

The Harmony will serve as a pressurized passageway to the Columbus lab and the Japanese Kibo science research facility. Ground controllers began checking out Harmony's systems Saturday so the station crew can enter it and finish internal outfitting of the module.

Station program managers will meet Tuesday to determine whether the outpost will be ready for the planned Dec. 8 delivery of the Columbus lab. A flight-readiness review for the Dec. 6 launch of Atlantis will be held Friday, and if all goes well, a three-day launch countdown would begin Dec. 3.

Saturday's spacewalk was the 99th to be performed at the space station since the first two building blocks of the outpost were linked in orbit in late 1998. Total spacewalking time tallied so far in the assembly and maintenance of the complex: 617 hours and 29 minutes.