In this Feb. 11, 2010 photo provided by NASA, astronaut Nicholas Patrick participates in the first session of extravehicular activity as construction and maintenance continue on the International Space Station. (AP Photo/NASA)

(AP) -- Astronauts ran into trouble Saturday while setting up the International Space Station's newest room, Tranquility: A critical insulating cover does not fit.

The fabric, multilayered cover is supposed to go between Tranquility and its observation deck, but the metal bars are not locking down properly because of interference from a hand rail or some other structure at the hatch.

Flight director Bob Dempsey said engineers are trying to figure out how to proceed. Until this snag, everything had been going smoothly in NASA's final major construction job at the space station. Shuttle Endeavour delivered Tranquility and the attached lookout - an enormous bay window - earlier this week.

The cover is needed to protect Tranquility's seals and docking mechanisms from the cold. This port will be exposed, temporarily, when the domed lookout is moved to its permanent location on the other side of Tranquility. That relocation is supposed to happen Sunday, but may be delayed.

Dempsey said the astronauts may be asked to remove the interfering pieces or, possibly, do away with the cover entirely and proceed with the original relocation plan. Once the lookout is moved, a docking adapter will take its place so the port would not be exposed for too long, provided nothing else went wrong.

Another option, Dempsey said, may be to hold off on moving the observation deck until a properly fitting cover can be flown on another shuttle flight.

Only four more shuttle missions remain. The next visit is scheduled for mid to late March.

The $27 million, Italian-built observation deck sports the biggest window ever flown in space. In all, there are seven windows that will offer 360-degree views.

The 11 astronauts aboard the shuttle-station complex opened the door Friday to the $380 million Tranquility, also made in Italy for the European Space Agency. The door leading from Tranquility into the observation deck was opened soon afterward, and that's when shuttle pilot Terry Virts and Kay Hire encountered the cover problem.

That didn't stop the astronauts from routing cables and water lines into Tranquility, and hauling in an exercise machine and other equipment.

"I will tell you, we're filling it up very quickly," Hire said in an interview with The Associated Press on Saturday morning. "We're loading all kinds of equipment in there and starting to bring it to life."

Virts said Tranquility had that "new spaceship smell" when the hatch swung open. He described the odor as unique, "almost a space smell to it."

Tranquility was installed on the space station early Friday during the first spacewalk of the mission. Two spacewalkers will venture back out Saturday night to set up plumbing needed for Tranquility's cooling system.

Virts said this week's events remind him of the completion of America's transcontinental railroad.

"When it was done, it opened up the West, let our country become the nation that we are today," he told the AP. With the addition of Tranquility and the lookout, this "is the beginning of a future of space exploration that a long time from now, we'll look back on and see in a similar vein."

Explore further Space station gets room, huge window to see Earth

©2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.