In that case, NASAs next flight to the station would be the planned Aug. 5 launch of shuttle Discovery on a mission to ferry a new crew to the outpost and return with its current tenants: Yuri Usachev, Susan Helms and Jim Voss.

"Thats a possibility," Kennedy Space Center spokesman Bruce Buckingham said.

At issue is a problem that cropped up in May with the shoulder joint of the robot arm, which is required to mount the airlock to the station. The shuttles shorter robot arm, which has been used to complete all station construction to date, is not long enough to do the job.

Over the course of several weeks in May and early June, the arms shoulder joint failed to work properly when back-up systems were being used to operate the crane-like device.

Then the shoulder joint inexplicably began operating as advertised last week.

NASA and Canadian Space Agency engineers so far have been unable to pinpoint the cause of the original problem or duplicate it during a series of tests over the past week.

Consequently, there still are questions about whether the problem might recur during the Atlantis mission, leaving the airlock stranded at the end of the crane while astronauts are trying to mount it to the station.

And unless those questions can be resolved in short order, the Atlantis flight likely will be delayed until late September.

"There still are a number of scenarios floating around out there that could give (mission managers) some pause," Buckingham said. "But if the managers can gain enough confidence in the arm, they might feel comfortable enough to proceed with the missions in the current order."

With another round of robot arm testing scheduled for Thursday, Atlantis was joined in the KSC Vehicle Assembly Building Wednesday by Discovery, which now will be outfitted with an external tank and twin solid rocket boosters.

Atlantis had been scheduled to move out to a KSC launch pad last week but the move was delayed to give engineers more time to sort out the situation with the robot arm.

Mission managers will meet Monday to decide whether to send Atlantis out to the launch pad the next day. Doing so would provide enough time to prepare for a July 12 launch of the 11-day airlock installation mission.

NASA engineers, however, require about a week or so to analyze shuttle performance data between flights. So managers likely would opt to launch the Aug. 5 Discovery mission first if the ongoing robot arm analysis delayed the Atlantis flight beyond July 12.

NASA also is facing another deadline of sorts for the Atlantis flight.

The mission cannot be launched between July 17 and August 4. The station during that time will be flying in an orbit that would expose a docked shuttle to high temperatures that could foul its station docking system as well as cargo in its payload bay.

The Discovery mission also faces time constraints. That flight must be launched by Aug. 17 because a supporting network of Air Force ground tracking stations is to be temporarily shut down for systems upgrades between Aug. 18 and Sept. 21.

Meanwhile, NASAs next station crew which includes U.S. astronaut Frank Culbertson and Russian cosmonauts Vladimir Dezhurov and Mikhail Turin already are planning for a lengthy delay in the Atlantis mission.

The three recently started training for the airlock installation, a job that is supposed to be carried out by the current outpost crew.