Oxygen and food will be worrisomely low on the international space station (search) by Christmas, and it's crucial that a Russian supply ship get there by then, a top NASA (search) official said Friday.

Complicating matters is the breakdown of the station's primary oxygen generator. The unit has barely worked this month despite intensive repair efforts by the two astronauts on board, and the men have had to tap into backup oxygen supplies.

The next Russian cargo ship is scheduled to launch on Dec. 23. That's right around the timod — will be getting tight, said space station program manager Bill Gerstenmaier (search).

The situation won't be nearly as dire if the astronauts manage to get their main oxygen generator working again. But if they don't and a cargo ship cannot be launched by year's end or blows up during liftoff, the space station would have to be abandoned, with the crew returning to Earth in their docked Russian capsule.

"We're a long way away from demanning," Gerstenmaier stressed.

Gerstenmaier said the current crewmen — Russian Gennady Padalka and American Mike Fincke — are in no immediate danger because of the multiple oxygen reserves on board. He said the same will be true for their replacements, who will rocket away from Kazakhstan on Oct. 11 on a six-month mission. Their capsule will carry up more spare parts for the balky oxygen generator, but have room for little else.

"When you couple all of those things together, you take a look at that and it's worth it to go ahead and put the crew on orbit," Gerstenmaier said. "They're not at any undue risk. There's plenty of capability. We have plenty of time to react and we've got a lot of time to really work this issue to see what happens and what comes."

The space agency would not want to keep a crew on board if oxygen and food dwindled to less than an approximately 45-day supply, Gerstenmaier said. That so-called red line would be reached right around the time the next supply ship is due to arrive, on Christmas Day.

NASA has had to rely on the Russian Space Agency for all station crew and cargo deliveries ever since last year's Columbia accident. The shuttle fleet is not expected to resume flying until next spring at the earliest.

The Russian-built oxygen generator — a source of repeated trouble — shut down more than two weeks ago. Padalka has replaced suspect parts and flushed out clogged lines, but to little avail.

Engineers' latest theory is that the valve used for venting hydrogen overboard may be contaminated. The machine produces oxygen by breaking down waste water into its two basic chemical elements.

Another oxygen generator is on board and might be usable, but it has its own history of malfunctions, Gerstenmaier said.

The Russians have no spare generators available for launching, and it will be next year before a new one can be built and flown, Gerstenmaier said. A backup method for producing oxygen in orbit — using small, portable canisters — is in the midst of a Russian redesign and also won't be ready to fly before next year.

A U.S. oxygen-generating system won't be ready before 2006.