The two crewmembers -- one American and one Russian -- would dock to the station and the Expedition Six crew now in orbit would return to Earth in the Soyuz capsule presently docked to the outpost.

The station would then remain occupied by rotating two-person crews on Soyuz spacecraft every six months until shuttle flights resume.

At the same time, unmanned Progress supply ships would be launched to the station as previously scheduled in June, September and January.

But current Expedition Six skipper Ken Bowersox wasn't so sure even a two-person crew could be sustained that long without an increase in Progress supply missions.

"If we were to keep the same number of Progresses we have per year in the current plan, it would be difficult to maintain the station as we currently are running it," Bowersox told CBS News in a space to ground interview Thursday.

"We'd probably have to back off a lot from what we're doing, maybe even go to less than two people on board. But if we increase the number of Progresses, we should be able to maintain operations, at least from all the data we've seen from the ground. And of course, that's still being studied."

Financial issues in Russia and U.S. laws about buying hardware from the Russians are complicating the picture and adding stress to the decision making process on how to operate the ISS during the next few months.

Russia could accelerate its assembly and launch of Progress and Soyuz spacecraft if needed, but would need the cash to do it -- cash that would either be given directly to Russia or come from the sale of seats on the Soyuz spacecraft.

Russian news media have reported the current asking price for a full Progress supply mission is roughly $22 million.

But a U.S. law enacted in response to Russia selling technology to Iran forbids NASA from buying any new Russian space hardware beyond that which is already contracted and procured.

And NASA officials have said this week they would not seek loopholes to thwart the intent of that law.

Moreover, the two-person caretaker crews would have to be made up of a Russian Soyuz commander and American astronaut fully trained in ISS systems, so it's not likely any space tourists or professional astronauts from the international partners are going to fly.

O'Keefe did not directly characterize the discussions among Russia and the partners as being tense or difficult, offering a diplomatic summary instead.

"Every partner is acting like a partner, a very strong participant in what the solutions on this should be," he said. "How we treat each other and work through this in times of challenge like we see today is the reason this kind of partnership was put together."

O'Keefe's visit to the site where space shuttle main engines are test fired was preceded by a stop at NASA's Michoud Assembly Facility near New Orleans, where the shuttle's external tanks are made.

His message to workers at both places was to keep safety the first priority, cooperate with investigators but most importantly to know that there is a future with manned spaceflight and what they're doing is important.

O'Keefe also used the visits -- like the Columbia Accident Investigation Board did last week -- to learn more about the hardware and discuss some of the theories as to why Columbia was lost that involved the foam insulation on the external tank.

A piece of foam was seen falling from Columbia's external tank some 80 seconds after its Jan. 16 launch from the Kennedy Space Center. The material appeared to strike Columbia's left wing and shatter into a shower of debris.

It's possible that incident was a factor in whatever happened to allow superhot air to get inside Columbia's left wing and eventually lead to the break up of the vehicle and the loss of the crew over Texas.

O'Keefe once again stressed that every possibility is being considered.

"The NASA position since Day One, and to this date, has been there is no favorite theory. There is no one cause that we believe is more likely than another," he said.