The two-man crew aboard the International Space Station could be sequestered inside the Russian Zvezda module for several days beginning Wednesday as other portions of the orbiting complex are closed off to find a still-mysterious pressure leak.

Expedition 8 commander Mike Foale and flight engineer Alexander Kaleri have helped ground controllers in the hunt. But without success, officials are moving forward with a plan to shut the station's various modules to narrow the search.

Foale and Kaleri are seen in the Zvezda module. Credit: NASA

The station cabin's atmospheric pressure began decaying around December 22. Over the past few weeks, the rate of decline has been a few hundredths of a pound per square inch (psi) of pressure per day.

"I would encourage you to keep in mind this is very, very small," said Mike Suffredini, space station operations manager, told reporters Friday.

As of Friday, the station pressure was 14.0 psi instead of the normal level of 14.7 psi.

"The way we go after this is we create this very methodical, meticulous approach to figuring out a fault tree and working down the list of all of the potential components," Suffredini said.

This week, Foale and Kaleri examined valves and seals throughout the station using an ultrasonic leak detection system and found no problems. On Friday, the Russian Vozdukh system that removes carbon dioxide from the air and several other Russian systems were checked without uncovering the leak.

"We are wrapping up our fault tree to understand all of our possible sources of a leak and beginning the process of determining most-likely vs. down to least-likely so we can determine which items to go check and in which order," Suffredini said.

"As part of that effort, systems that, as part of their operation, vent overboard are the ones we want to take a look at first. In keeping with that, we did check the Vozdukh today. That pressure indication of the Vozdukh when the valves were not moving indicated it is not leaking. In addition, our Russian counterparts today checked...a scrubbing system much like our trace-contaminate system we have on the U.S. system. They did a leak check of that system and it also indicated that it was not leaking."

The International Space Station. Credit: NASA

The next phase of the search involves cordoning off sections of the station to determine which module has the problem.

"We continue to put together the plan to isolate the individual elements to try to take things off the fault tree in large chunks. So the first thing we will do, on Saturday, is isolate the Progress vehicle and monitor it for pressure drop.

"On Sunday we intend to isolate the Docking Compartment and Soyuz combination as well as the U.S. Airlock and monitor those for an approximate 24-hour period to see if they, perhaps, are a source.

"Right after we finish that, we will repress the vehicle back up to about 14.4 psi using nitrogen from our tanks in the airlock.

"All of that is in preparation for a Wednesday effort to isolate the Service Module, FGB, Node and Lab from one another and then monitor the pressure of the environments in those four modules to see if we can figure out which module potentially has a leak."

The resident crew will live inside the Russian Zvezda service module for the four-to-five-day period because that is where the life support systems, toilet, kitchen and other items are located. It is also close to the Soyuz escape capsule that would ferry the crew back to Earth in an emergency.

"When we isolate the big modules like that, particularly on the Service Module side where we are producing oxygen and scrubbing CO2 and the crew is breathing, it is hard to see this needle in the haystack when all of that is going on...We are looking at probably four or five days before we can see (the leak) through all of those pressure variations," Suffredini said.

"So starting Wednesday and probably through the weekend, we'll be in an isolated state where we isolate the modules. While this is no impact to the crew themselves, it will impact some of the operations we planned to do. So we will reschedule accordingly those operations.

"In the week after that, if we determine we do have a slight leak in a particular element, we will begin the process of trying to isolate what the source of the leak is."

Officials are hoping the leak can be found quickly so any spare parts or repair hardware can be launched to the station aboard a Russian Progress resupply ship scheduled for liftoff January 29.

Engineers report that given the current rate of pressure decay, the station has enough consumables onboard to pump up the cabin to counteract the leak for six months before needing replenishment.

Meanwhile, Russian flight controllers also are continuing to evaluate the possible replacement of parts in the oxygen-generating Elektron system that has broken down. The Russian system produces oxygen by recycling wastewater aboard the complex. Spare parts are aboard the station that engineers believe can fix the system. Plans are being formed to perform that work possibly next week, NASA said.

While the Elektron trouble is sorted out, the crew has been burning Solid Fuel Oxygen Generators.