International Space Station planners developing options for operating ISS without support from NASA's space shuttle fleet could decide as early as this w eek to begin training a special replacement crew, perhaps with only two members, to relieve the three men on board.

This would give the station partnership a little more time to decide how to proceed, driven as it is in the near term by tight onboard water supplies and the dwindling service life of the station lifeboat. At issue is whether and how to use a scheduled Apr. 26 "taxi flight" with a fresh Soyuz rescue vehicle to replace Expedition 6, which had been slated to return to Earth in March on the shuttle Atlantis.

Because Atlantis and the two other surviving orbiters are grounded, managers may decide to substitute Expedition 7-Russian cosmonauts Alexander Kaleri and Yuri Malenchenko and U.S. astronaut Ed Lu-with a less-specialized team that would maintain the station until normal operations resume or another crew replaces them. The planned taxi crew-cosmonaut Gennady Paldaka and Pedro Duque, a European Space Agency astronaut from Spain-is likely to be replaced, since NASA officials want at least one U.S. astronaut on the station who is trained to operate and fix U.S. systems.

Russia believes it can provide enough dry goods, propellant and orbit-raising reboosts with its Progress cargo vehicles to keep the station operating indefinitely. But water produced as a byproduct of the shuttle's fuel cells has been the main source of supply on ISS. Without it, providing the 4-6 liters a three-person crew uses daily has become the "pacing item" for future station operations.

Conservation can only go so far, according to Donald R. Pettit, NASA science officer on Expedition 6. "I probably use 3-4 oz. of water every time I take a shower," Pettit said during an air-to-ground press conference last week. "It doesn't buy you that much."

As a result, managers in Houston and Moscow are considering a shift to a two-person crew to conserve resources while the shuttle fleet is grounded in the wake of the Columbia accident. Even with a go-ahead to start contingency training, the decision must be made quickly because the Soyuz vehicle attached to ISS needs replacing by early May at the latest.

That vehicle is the first Soyuz-TMA, a modification built specially for ISS use. Its Russian designers consider this mission a qualification flight, and they do not want it to remain in orbit much longer than the 188-day period that will end with its scheduled landing Apr. 26, according to William F. Readdy, associate NASA administrator for space flight.

Beyond the near-term decision on a crew for the April taxi flight, space station program engineers at Johnson Space Center have been preparing a "decision tree" to guide future choices about station operations, according to Mark L. Uhran, senior systems integration manager for ISS at NASA headquarters. The training issue aside, NASA has some flexibility because the station is stable and its crew is safe, he said.

Uhran said engineers were "moving through all the distributed systems, moving through all the consumable provisions, assembling all the technical data, building the options matrices and determining in each of these option sets when a decision has to be taken."

The Progress vehicle that docked with the station on Feb. 4 boosted it about 6 mi. on Feb. 11 using its onboard engines, according to Mission Control Center-Moscow. Russian specialists estimate one additional Progress vehicle this year and six next year would be sufficient to sustain ISS in orbit.

RSC Energia, which manufactures the Progress, has 12 vehicles in different stages of completion and could provide the necessary number of spacecraft in time if funding is available. However, the Columbia accident left the chronically under-funded Russian space program even harder-strapped than usual.

The Russian Aerospace Agency had hoped to take at least two paying space tourists to the station this year on taxi flights, earning some $40 million, but it tabled those plans after the accident. That amount was intended to supplement the modest $130-million Russian budget for the ISS this year, which would be just enough to cover the cost of the three Progress ships and two Soyuz vehicles originally planned, according to Russian space officials.

NASA's international partners may be able to help Russia speed its manufacturing, an option complicated for NASA by anti-proliferation legislation linked to Russia's sale of military technology to Iran. The ISS partnership organization is addressing the issue, but U.S. and partner officials said it isn't likely to act until there is a clearer idea of just how long the shuttles will be grounded.

"A lot of our decisions depend upon what is, right now, an independent variable, and that's the shuttle return to flight," Uhran said. "Until we get a little better understanding of what that might be, we're trying to [defer] our decisions until we absolutely have to make them."

Another area under study is how much science can be conducted on the station without the shuttle. Uhran said experiments requiring power in transit from the ground to the station can't be delivered without the middeck locker space on the shuttles, but otherwise samples and hardware can be shipped in Progress vehicles. In addition to experime nts already active on the station, others are in storage there for future use, according to Pettit.

Crewmembers' role as science subjects is one reason to keep the station manned, Uhran said, but having a crew on board also reduces the risk to the facility. While it is possible to operate the station from the mission control centers in Houston and Moscow, he said, Russian experience on earlier space stations showed it is much better to have humans available to fix what breaks and deal with other emergencies.

Kenneth D. Bowersox, the Expedition 6 commander, said the desire to maintain a permanent human presence in space is also a factor.

"If we were to have to deman the station, it would not be a huge setback," he said. "The station would keep flying. We would be able to send people back, but on an emotional level I really want us to see people stay."

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