The station uses four control moment gyroscopes to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. But one gyro, CMG-1, failed in 2002 and cannot be replaced until next year, during the first post-Columbia shuttle mission.
The ISS can operate with just two, but another failure could mean trouble, and a third, CMG-3, has shown subtle signs of unusual behavior in recent months due to presumed lubrication issues.
"The only way to fix is to do a spacewalk, to go outside and replace that. They have spares onboard, but it's a complicated task and it's just come up at a bad time for these guys," said CBS News Space Consultant Bill Harwood.
Expedition 8 commander Michael Foale and flight engineer Alexander Kaleri are preparing to return to Earth next week and in the process of handing over control to newly arrived Expedition 9 commander Gennady Padalka and flight engineer Michael Fincke.
The problem is not with the actual gyroscope, but a control module.
"If they lose another one, they would then have to use rocket thrusters, to attach Soyuzes, the Russian spaceships that are attached to the station, and of course there's a limited amount of fuel," said Harwood. "There's not a terrible rush to fix this, but obviously it's something they want to get done as soon as they can."
"It's not a matter of whether, it's a matter of when," one NASA official said.
Padalka and Fincke already planned to carry out two spacewalks during their six-month stay and officials said the gyro repair work might be wrapped into one of those excursions. But a variety of issues remain to be resolved.
Swapping out the module isn't the problem, getting to it is.
"It's easy to get to from the American airlock, but the just-arrived American astronaut, Mike Fincke, doesn't have a suit that can work out of that airlock," said Harwood. "To wear Russian suits and come out of the Russian end of the station to do this is pretty complicated. There's a lot of territory they have to cover just to get to it."
Another wild card is the lack of a third crew member to operate the station's robot arm. Replacing a remote power control module normally would require use of the arm and new procedures likely will need to be developed.
"If you guys checked the caution-and-warning panel today, you'll see CMG-2 has failed," mission control radioed the station crew early Thursday. "What's happened is the RPC that's powering CMG-2 has tripped, multiple times in fact."
"We copy all," Foale replied. "And yes, we'll follow the plan with great interest."
"No doubt. We'll be busy down here for the next week, for sure, looking at this and we'll keep you guys informed."
Of more immediate concern to Fincke, Padalka, Foale and Kaleri is the completion of handover operations before the Expedition 8 crew departs next week. Foale and Kaleri are giving their replacements a crash course in station operations, sharing the insights that come with six months of daily life aboard the outpost.
The space station continues to deteriorate, and, with the halt to U.S. space shuttle flights, repairing it and continuing to expand it is impossible. The Russian Soyuz rockets now the sole method of resupplying and restaffing the ISS can't carry construction materials.
The station also has had a serious of maintenance problems.
Earlier this month, the crew heard again heard a mysterious noise, first reported in November. Cosmonaut Alexander Kaleri and astronaut Michael Foale describe it as sounding like a flapping sheet of metal. "It sounded sort of like a drum. It sounds sort of like a sheet of something being bent," the cosmonaut reported.
During a spacewalk in February, Kaleri and Foale were supposed to check the exterior of the space station where the noise originated last November. But Kaleri's spacesuit overheated and became damp, and the spacewalk had to be cut short, so the men did not have time to inspect the area.
In January, Foale found a braided flexible hose with an apparent leak in it that could have caused the slight loss of air pressure station engineers and the station crew had been struggling to find.
CBS News Space Consultant William Harwood has covered America's space program full time for nearly 20 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.