Confined to their orbiting capsule at least until April in the wake of the Columbia accident, the three men aboard the international space station are putting a brave face on their predicament.

"We enjoy the environment on the space station," astronaut Kenneth Bowersox said the first time the crew spoke publicly after the Feb. 1 accident. "And we're going to enjoy the next two-and-a-half, three months."

Their bosses on the ground don't have that luxury. With the remaining three space shuttles out of service indefinitely, NASA and its international partners face some difficult choices about how to operate the station while its main link with Earth is severed.

Space station boosters believe losing the space shuttle for up to a year will have little net impact on the scientific output of the orbiting laboratory. But more critical voices say the station itself could be threatened by a long period of reduced activity.

NASA has grounded the three remaining space shuttles until the cause of Columbia's destruction Feb. 1 is found and any flaws are corrected. After the Challenger exploded over Cape Canaveral in 1986, no shuttle flew for almost three years.

For the moment, neither the space station nor its crew is in danger due to the shuttle stand-down. But in some sense, as long as the three remaining shuttles remain out of service, the space station will languish.

Most significantly, the loss of the shuttles halts construction of the space station, a process that began in 1998 and is scheduled to continue through at least 2006.

"We cannot continue assembly without the shuttle fleet," said Michael Kostelnik, NASA's deputy associate administrator for spaceflight.

A 16-nation project, the construction of the station got under way when the United States and Russia each launched capsules that were joined by shuttle astronauts. After 40 deliveries of building materials and supplies spanning almost a decade, the $60 billion complex is intended to have six laboratories and a total interior volume comparable to a 747.

Six crews of three have inhabited the station continuously since 2000. It now consists of living quarters for three, an attached Soyuz lifeboat and a 28-by-14-foot laboratory where crewmembers conduct experiments — about 15,000 cubic feet of space in all.

Five shuttle flights in 2003 were to add an array of solar panels. Missions in 2004 and 2005 were scheduled to attach European and Japanese laboratory modules.

Now all of that is on hold. Only the shuttle has the capacity to carry large pieces of equipment into orbit — Russia's Soyuz and Progress capsules are too small. A French-built transport system that is expected to be available in September 2004 could help with supplies and some construction materials, but would not be big enough to carry the major pieces.

"What would make sense," said John Logsdon, director of the Space Policy Institute at George Washington University, "is to send up a crew and experiments for the crew to do until assembly resumes."

Alex Roland, a Duke University history professor who specializes in NASA, has a different idea.

"I don't think we should have it up there anyhow," Roland said. "My recommendation is surely to get those astronauts out of it and just close it up."

Right now water appears to be the limiting factor aboard the space station. An astronaut or cosmonaut in orbit needs roughly 1,000 gallons a year.

Because it weighs more than eight pounds a gallon, most of the station's water is delivered on the space shuttle rather than the much smaller Russian vehicles; NASA officials have said that the current station crew has enough water to last through June. After that, more Russian resupply flights will have to be added to the existing schedule to support more than two astronauts.

That means funding the construction of more Progress spacecraft by the Russian space agency. Prior to the Columbia accident, Russia had committed to fly two Soyuz and three Progress ships to the space station this year at a cost of $130 million.

Each additional Progress, an unpiloted ship that can boost two tons of supplies to the space station, would cost $22 million to build.

The Soyuz are used primarily as lifeboats for the station crew. One is docked to the station at all times, and the Soyuz ships are rotated every six months to ensure their reliability.

"What was previously done by the United States and Russia will have to be done by Russia alone," Russian Aerospace Agency director Yuri Koptev said Feb. 13.

Criticized in the past for including the poorly funded Russians in the space station, NASA now finds itself dependent on a beleaguered space program that has muddled along for a decade with almost no government support.

"It's a wicked situation, but it does provide the opportunity for some creative diplomacy," Logsdon said. "It's an opportunity to demonstrate the benefit of having this program be international."

Some experts question the Russians' ability to provide sufficient transportation to support the space station. They point to financial problems that delayed the occupation of the station by two years, and conflicts between the Russians and the other partners over allowing paying tourists to visit the outpost.

"Having the Russians along from the beginning is what bankrupted the station budget," said James Oberg, a space policy analyst and former NASA flight controller. "Now they're in the driver's seat."

Right now all attention is on the next Soyuz flight. Scheduled for April, the flight was to have been a minor operation. A crew of three was to fly a Soyuz to the station and return in another one that had been serving as the station's lifeboat for six months.

The current station crew was to have been replaced by a new one arriving on a March shuttle flight. But with the shuttle grounded, NASA now has to decide whether to send the replacement crew on the Soyuz or leave the three men now on the station for another six months.

NASA must also decide whether to keep three people aboard the station or reduce its crew to two. If only two people fly on the April Soyuz mission, the small spacecraft's third seat can be loaded with water and other critical supplies.

"I think everything will be done to avoid leaving the space station without a crew aboard," Logsdon said.

It generally takes two people just to keep the space station running. So if NASA does decide to reduce the crew, much if not all of the science will fall by the wayside.

"We would see a limitation and a diminution of the science focus," NASA administrator Sean O'Keefe told Congress Feb. 12, if the shuttle was grounded for more than a few months.

No great loss, critics of the space station say.

"It's not as if they have a lot of science to get done on it," said Oberg.

But Don Pettit, the science officer on the current space station crew, believes even two busy astronauts can conduct plenty of valuable research, much of it just by being in space.

"An orbiting environment is rich in discovery, and I cannot fathom a moment when there wouldn't be some new investigation or observation to make," Pettit said. "By virtue of having people here, you are always doing research on your body itself, looking at the effects of long-duration weightlessness on human physiology."

More than anything else, experts' opinions of the space station's prospects during a shuttle-free period depend on their feelings about human space flight generally.

"Flagpole-sitting with the lights turned down is just sillier than what they're normally doing," Roland said.

He contends that NASA sends humans to space just for the sake of it. In Roland's eyes, the talk of exploration, adventure and discovery is just a weak excuse for repeatedly flying an overpriced spacecraft to a pointless destination.

"There's nothing up there for people to do and so they give them busy work and call it science," he said.

But Logsdon, who sees the station's science program as a necessary prologue to Mars exploration and even more ambitious space travel, considers that criticism old news.

"The scientists that oppose the station had basically lost their argument until the Columbia accident and now they're back out," Logsdon said. "The ideal outcome of all this is to have the shuttle flying within a year."