Russia proposed in late March doubling the length of space station visits so that it could set aside one Soyuz launch a year for two paying customers -- either so-called space tourists like Dennis Tito and Mark Shuttleworth or European astronauts sponsored by their home countries.

NASA said Monday that it was not opposed to longer stays in principle, but that October was too soon to make such a significant change. NASA spokesman Robert "Doc" Mirelson said the U.S. space agency had recently informed its Russian partners that it could not go along with extended space station stays.

"We believe the timing is not appropriate, in part, because the station is currently in a reduced operational state," Mirelson said. "We don't have sufficient countermeasures in place yet to offset the effects that a longer duration space flight would have on the crew. Also, the impacts on planned station biomedical research still needs to be addressed."

With the U.S. space shuttle grounded, the three-seat Soyuz capsule is the only means of transporting crew members to and from the station. Russia has been launching the vehicles twice a year, with two of the three seats occupied by a Russian cosmonaut and U.S. astronaut heading to the station for six month stays. Only the third seat is available for sale. Extending crew stays by six months would allow Russia to fill one Soyuz a year with two paying passengers instead of a fresh expedition crew.

On Monday's Soyuz launch, Dutch astronaut Andre Kuipers accompanied Russian cosmonaut Gennady Padalka and U.S. astronaut Michael Fincke. Padalka and Fincke are scheduled to remain onboard the space station until October. Kuipers, meanwhile, will come back to Earth in about a week with Expedition 8 crew members Michael Foale and Alexander Kaleri. The three will travel home in the Soyuz vehicle docked at the station since last fall.