Food is running so low aboard the international space station that the two crewmen have been instructed to cut back on calories, at least until a Russian supply ship arrives in a little over two weeks, NASA (search) said Thursday.

If anything goes wrong with the Christmas Day delivery, the space agency will have no choice, given the grounding of its shuttle fleet, but to abandon the station and bring the men home in early January.

This cargo ship "is very critical, there's no question about that," said NASA's space station program manager, Bill Gerstenmaier. Supply runs to the space station have been conducted exclusively by the Russians ever since last year's Columbia disaster.

Gerstenmaier estimated there is enough food to last seven to 14 days beyond Christmas Day, after which there will be nothing left if the supply ship does not arrive.

The food supply has never gotten this low before, and no mandatory dieting has ever been in effect before in the four years that the station has been permanently occupied.

American astronaut Leroy Chiao (search) and Russian cosmonaut Salizhan Sharipov (search) are barely two months into their six-month stay aboard the space station. Last week, after a pantry audit found supplies running surprisingly low, they were put on restricted diets in hopes of trimming 5 percent to 10 percent of their daily intake of 3,000 calories.

So far, there have been no complaints from the husky, healthy men, said Dr. Sean Roden, their NASA flight surgeon. Just last month, Chiao told The Associated Press said he and Sharipov were eating a lot to keep up their weight and stay strong, and had warned Mission Control about it.

Roden said cutting out 300 or so calories a day is "really quite minimum," and has not affected the crew's rigorous daily exercise.

"These are consummate professionals and they will do whatever is required and asked of them," Roden said. He added: "They're in good spirits, they're doing well. I am in no way, shape or form worried about their mental mood with this menu change."

NASA and the Russian Space Agency (search) were stunned to learn last week that the astronauts had begun digging into the 45-day food reserve — which exists to protect against a delayed supply shipment — in mid-November. Flight controllers knew food and water were tight when the crew was launched from Kazakhstan on Oct. 13, but had not expected to dip into the reserves for another week.

Gerstenmaier said an independent team is looking into how the food inventory ended up being tracked so poorly and how it can be improved in the future.

Meals and drinks are contained in pouches and scattered throughout the space station, so the crew had no idea the situation was getting bad until flight controllers requested three audits, Gerstenmaier said. He said it was not until the third audit that everyone realized: "This is very, very close."

Some food had to be removed from a previous delivery because of the need to fly spare parts for a broken oxygen generator, and that aggravated the situation, Gerstenmaier said. The Russian cargo ships can carry about only a third of what a space shuttle can.

The station's water supply, while low, is not nearly as dire and the two men have been encouraged to drink as much as they want.

NASA is drawing up plans to evacuate the orbiting outpost, in case the Russian rocket carrying the cargo ship explodes during liftoff from Kazakhstan on Dec. 23, or the ship cannot dock two days later.

Extra food and water have been packed into the supply ship, including some Asian delicacies — dim sum dumplings for Chiao, a first-generation Chinese-American, and fried rice for Sharipov, who was born in what is now Kyrgyzstan in central Asia and has an Uzbekistan heritage.

Chiao and Sharipov put in the takeout orders some time ago. Their normal fare is half American and half Russian.