Astronaut Michael Foale has apparently located the small air leak that has bedeviled the international space station for the past three weeks.
NASA’s Mission Control Center in Houston on Sunday suspended impending plans to lock down the entire station, close the hatches between its major modules and monitor the air pressure in each one separately.
“The news couldn’t have come at a better time,” Mission Control advised the crew in their pre-sleep briefing on Sunday afternoon. “It looks like we found our culprit.”
When the crew jokingly asked for the rest of the day off, Houston responded, “Not so fast!” But after completing a short discussion of the next day's activities, Foale and his Russian shipmate, Alexander Kaleri, were told to begin their pre-sleep activities and not to expect further communications until the morning.
A leaky window?
The leak appears to be at the main window in the U.S.-built Destiny laboratory module. A flexible cable called a vacuum jumper, used to help equalize air pressure across the multipaned window, showed telltale signs of leakage where the hose entered a steel harness at the edge of the window.
The structure can be capped off with equipment already aboard the station. It can be entirely replaced later in the year when the needed spare part is sent up on a robotic Russian cargo ship.
First detected on Jan. 2, the leak had been slowly building up since ten days earlier, Mission Control discovered. The air pressure drop has not reached the point where it poses a risk to Foale and Kaleri: At its highest rate, the station was leaking about 5 pounds of air per day overboard. The actual pressure drop was only from 14.7 pounds per square inch to about 14.0 psi, the equivalent of moving from sea level on Earth to an altitude of 2,500 feet.
Foale spent what was supposed to be a day off rechecking suspected areas in the U.S. modules with a device that listens for the ultrasound shriek of escaping air. While wearing headphones, he placed a probe near potential leak areas and listened for noise.
“I got a pretty strong signal when I flex the vacuum jumper,” he called down excitedly.
“That’s awesome news,” Mission Control radioed back. “We’re going to ponder it here a minute.”
Houston then requested a series of more tests at precise locations, and Foale also set up the video camera to downlink images.
Foale expressed disappointment he hadn’t found the leak earlier.
“Sasha [Kaleri] and I wonder why we didn’t come to this conclusion a day or two earlier,” he told Houston. He explained that they had checked the window a few days earlier, but that background noise from an operating science instrument had overwhelmed the signal then. Kaleri had still been suspicious about the window and had tried various techniques to isolate the noise, but in the end both men had concluded at that time that what they were hearing was from the science instrument.
Mission Control congratulated them for Sunday’s discovery. “Great work, it’s going to save us a lot of time,” Houston told them.
Tracking down the culprit
Earlier analysis had led ground controllers in Houston and Moscow to suspect a Russian air-purifying apparatus as being the source of the leak. As it was switched between different operating modes, the leak rate appeared to react by increasing or decreasing.
The space station normally leaks anyway, NASA official Michael Suffredini told reporters on Friday. At its worst, the pressure drop corresponded to a leak five times the normal rate, but by late last week it was only twice the normal rate.
The possible leak was so gradual that accurately determining its size was challenging.
The pressure inside the station is affected by temperature changes caused by repositioning the station relative to the sun, and pressure also is increased periodically by routinely dumping more oxygen into the cabin. As a result, the "signature" of the pressure drop was very garbled, and sources told MSNBC.com that some space engineers argued there was no increased leak at all, but merely pressure changes caused by these other factors.
In order to determine the location of the leak, the crew had been sealing off small peripheral modules of the space station while Mission Control looked for accelerated pressure drops inside the sealed-off sections. Detailed plans had been drawn up to close the main hatches between the large modules on Wednesday, and have Foale and Kaleri retreat to the Russian-made service module for four or five days while pressure readings were watched in all the different modules.
That plan, which would have affected station operations, has now been put on hold pending confirmation that the leak has been found.
During the Russian side of the pre-sleep debriefing on Sunday, Mission Control in Moscow congratulated Kaleri for fixing the leak and stabilizing air pressure. “It’s not confirmed it’s stabilized,” Kaleri answered cautiously, “but that may turn out to be the case.”