International space station planners are debating the possibility of launching a two- or three-man caretaker crew in late April or early May aboard a Russian Soyuz spacecraft to replace the lab's current crew and to keep the outpost occupied until space shuttle flights resume.

The current station crew - Expedition 6 commander Kenneth Bowersox, flight engineer Nikolai Budarin and science officer Donald Pettit - originally planned to come home in March aboard the shuttle Atlantis after a 114-day stay in space.

But that was before the shuttle Columbia disintegrated in the sky above Texas last Saturday, indefinitely grounding the shuttle fleet.

The Russians already planned to launch a Soyuz "taxi" mission on April 26 to deliver a fresh lifeboat to the station. Soyuz capsules are delivered every six months by so-called taxi crews who then return to Earth in the lab's older Soyuz. The spacecraft are certified for six months in orbit and must be regularly replaced.

Russian commander Gennady Padalka and European Space Agency astronaut Pedro Duque - one of John Glenn's crew members on shuttle mission STS-95 - make up the currently assigned taxi crew. The third seat in the Soyuz is unoccupied.

The current Expedition 6 crew has enough on-board supplies to remain aloft until late June in a worst-case scenario.

But U.S. and Russian planners are studying a variety of options to keep the station manned with so-called "caretaker" crews until space shuttles can resume flights.

One leading option is to launch a replacement crew aboard the April taxi flight. Under that scenario, the Expedition 6 crew would return to Earth in the lab's current Soyuz.

But several questions remain to be resolved.

The Expedition 6 astronauts trained on Soyuz systems before flight, but they never planned on returning to Earth aboard one. Additional training may be required.

Because of concern about how much drinking water will be available on the station - the shuttle is a primary source of fresh water - the size of the proposed caretaker crew could be limited to two. The commander must be a Russian cosmonaut familiar with Soyuz systems and an American astronaut presumably would be needed to operate and maintain systems in NASA's modules.

But if the station can support a three-member caretaker crew, the European Space Agency could see its first long-duration representative in orbit for an extended flight.

NASA originally planned to replace Bowersox and company with Expedition 7 commander Yuri Malenchenko, flight engineer Alexander Kaleri and science officer Edward Lu, a NASA astronaut.

But the Expedition 7 crew is specially trained to carry out research operations and station assembly work. NASA does not want to waste that involved training on an expedition that is unlikely to see any additional construction.

Launch of Padalka and whoever might accompany him is targeted for April 26, but the flight could slip to as late as May 10 depending on crew training requirements. The next taxi mission after that is scheduled for October.