American and Russian space officials stressed Tuesday that the drop in air pressure is slow and there is no immediate danger to the crew or the operation of the outpost. If the pressure were to fall dangerously low, astronaut Michael Foale and cosmonaut Alexander Kaleri could abandon the station in the docked capsule and quickly return to Earth.
"We're going to take a very measured and methodical approach to sort through this problem," said Mike Suffredini, the station's operations and integration manager. "If this was in fact a leak, which we're not certain that it is, we have, oh, about a little over half a year's worth of gas on board to feed it and so we're in no particular hurry to overreact."
On Monday and again Tuesday, Foale and Kaleri used an ultrasound leak detector to check hatches, windows and valves throughout the space station, but found nothing suspicious. Mission Control asked them Wednesday to hold off on additional inspections so all the data could be reviewed.
On Friday, a check will be run on the Russian carbon-dioxide removal unit that engineers believe may be the culprit. If it is venting air overboard, then it will be replaced with an on-board spare in the next week or two. If not, the search will widen, with wall panels being removed and supplies moved and stacked elsewhere in order for the spacemen to gain access to other suspect parts.
"Let's keep our fingers crossed that we're narrowing in on this guy," Foale said.
Foale sounded as cheerful as ever.
"Well, you know, we do have a big problem on board," he radioed to Mission Control. "Any regular sailing ship would have chewing gum on board to patch the leak, but we don't have any chewing gum. Someone left it off our menu. How can you be flying in space without chewing gum?"
Suffredini said the space station has lost 2¼ pounds of air a day over the past two weeks. That is 34 pounds gone out of the roughly 900 pounds of air normally inside the complex.
The space station is now down to 14.2 pounds per square inch of pressure, the point at which flight rules call for replenishment, Suffredini said.
But the threshold for equipment to start malfunctioning is 13.9 pounds per square inch, and the real cutoff point for both systems and humans is about 10.2 pounds per square inch, Suffredini said.
Russia's old Mir space station was often plagued by air leaks. Kaleri was on board for some of them, and Foale was present when Mir was rammed by a supply ship in 1997. The ruptured lab module had to be sealed off.
The international space station is larger than Mir, however, and it could take longer to find a leak.
If the carbon-dioxide removal unit and other equipment check out fine, the next step would be for Foale and Kaleri to close the hatches between the U.S. and Russian sides of the space station in an attempt to isolate any leaks. If that does not work, then individual compartments would be sealed off one at a time. And if that still does not turn up any leaks, then a more specific point-to-point search would be conducted.
Although first detected around Jan. 2, the falling and fluctuating air pressure has been traced back to Dec. 22. It coincides with the malfunction and ultimate breakdown of the primary oxygen generator on board and could be related to the use of backup oxygen-producing canisters. The canisters just exceeded their expiration date, but Russian space officials recertified them for another year.
NASA said the falling pressure also could be related to drastic changes recently in the amount of sunlight exposure to the space station, rather than an actual leak.
Whatever the reason, the problem comes at a bad time. With NASA's space shuttles grounded indefinitely because of the Columbia tragedy, there is no way to send up all the necessary spare parts. Russian spacecraft are too small to carry large equipment.
The crew has been reduced from three to two until shuttle flights resume.
Foale, who is the station commander and turned 47 on Tuesday, moved in with Kaleri in October for a six-month stay.
By Marcia Dunn