CAPE CANAVERAL, Fla. -- Spacewalking astronauts drifted outside shuttle Atlantis and the International Space Station late Saturday, aiming to help mount a $164 million airlock at the 17-story frontier outpost.

Outfitted in spacesuits designed to protect them from the vacuum of space, Michael Gernhardt and James Reilly floated out into the cargo bay of Atlantis about 11:20 p.m. EDT (0320 GMT) as the linked shuttle-station complex flew high above the Pacific Ocean west of South America.

"Welcome to space," Atlantis pilot Charles Hobaugh told Reilly, a rookie spacewalker.

"Thank you, man. This is cool."

Running about an hour behind schedule, the astronauts set out to attach the two-room airlock to a berthing port on the U.S. Unity Module, where it will serve as a dual locker room and doorway for future spacewalking construction and maintenance workers.

The first order of business: Removing a large "shower cap" and eight metal seal covers from common berthing mechanisms on either end of the airlock, which was cradled for launch in the rear of the shuttle's cargo bay.

Measuring about 10 feet (3 meters) in diameter, the round cap provides a thermal cover for the airlock berthing mechanisms, one of which will be connected to an identical device on the starboard side of the Unity module.