"Once we get one oxygen and one nitrogen tank (on the chamber)," he added, "then we essentially have a fully functional airlock."

Look for the action to pick up just before 10 p.m. EDT Tuesday (0200 GMT Wednesday) as station flight engineer Susan Helms plucks an oxygen tank out of the shuttle's cargo bay with the outpost's $600 million Canadian robot arm.

Atlantis mission specialists Michael Gernhardt and James Reilly then will venture outside the joined shuttle-station complex to help install the 1,200-pound (540-kilogram) tank on the outer shell of the airlock, which is mounted to the station's U.S. Unity module.

That same process then will be repeated with the nitrogen tank. Hoses connecting the tanks to the airlock also will be hooked up during the spacewalk, which is expected to wrap up about 3:44 a.m. EDT (0744 GMT) Wednesday.

The five-and-a-half hour excursion will follow a fruitless effort to fix a leaky Unity module valve designed to regulate the flow of air into the station's new airlock, dubbed Quest.

The regulator sprang a leak during a pressure test early Monday, but the joined crews of the shuttle and the station failed to fix it during extensive troubleshooting efforts late Monday and early Tuesday.

Instead, the astronauts put a temporary cap on the valve so they could carry out a dress rehearsal for an inaugural spacewalk to be staged out of the station's new airlock late Thursday.

The faulty valve is one of two located near a submarine-like hatch that separates the Unity module and the new station airlock. Designed to be opened and closed by remote control, the electrically actuated regulators enable atmospheric pressure within the airlock to be reduced -- a standard operating procedure before astronauts venture into the deadly vacuum of space.

The valves also serve another important purpose: They would enable ground controllers or the station crew to re-pressurize the airlock within six seconds in an emergency.

That capability could be a lifesaver if a fire, a serious cabin leak or any other crisis cropped up on the station while astronauts were sealed up in the airlock prior to a spacewalk.

NASA mission managers, meanwhile, say only one of the two valves must be operating properly to stage a spacewalk from the new station airlock.

While the capped valve cannot be opened and shut by remote control, the airlock still could be re-pressurized within 12 seconds in a crisis situation. What's more, astronauts could use a socket wrench to pop off the cap off the faulty valve, speeding up that response time in an emergency.

Consequently, the valve problem is considered more of a headache than a safety hazard.

Said NASA flight director Paul Hill: "I really do see this as a nuisance."

Hill said the valve ultimately will be replaced with a spare, but mission managers are not in a rush to carry out that job, which would take about two hours.

Coupled with an airlock water line leak late Sunday, the failed effort to fix the valve has put the joint shuttle-station crew about a half-day behind schedule, so the valve replacement work likely will not take place until after the Atlantis crew departs the outpost.

NASA mission managers, meanwhile, are considering adding an extra day to the shuttle's planned weeklong stay at the station, and that effectively would delay the planned spacewalk Thursday by 24 hours.

In that case, Atlantis and its four-man, one-woman crew would depart the station Sunday and then head for a landing here at Kennedy Space Center on July 24.

As it stands, though, the shuttle's departure from the station -- and the subsequent landing -- remain scheduled for 12:15 a.m. EDT (0415 GMT) Saturday and 1 a.m. EDT (0500 GMT) July 23, respectively.