"Right now, it's the equivalent of trying to conduct experiments in a laboratory while you are trying to build it," O'Keefe said.

Trying to do science is "no mean feat," O'Keefe said, given ISS construction still underway, with crews speeding around Earth in microgravity. "That sounds like a circus act more than it does anything else."

"No one has ever done anything like this. This is the equivalent of building a pyramid in space," O'Keefe told the NPR listening audience.

Russian resolve

O'Keefe countered reports that the ISS may be left without a crew in the future. Cash-strapped Russian space officials have raised that scenario in recent times.

"I don't think that's a likely prospect at this point," O'Keefe said. Presently, the sixth expedition aboard ISS has settled in, and the station has been permanently occupied for over two years, he said.

"We don't see any break in that activity in the time ahead," the NASA chief said. Earlier this month, he added, Russia renewed their commitment to hold up their end of the space partnership.

"We believe that they will be strong international partners as they have been, all the way through this important, very historic quest," O'Keefe said.

The Russians have reassured NASA they have a strong commitment, interest, and resolve to make the project work, he said.

Russia views the ISS as a "science and research imperative," endorsing and making the facility part of their national policy, O'Keefe said.

Orbiter odometer: miles to go

Noting the fact that NASA's space shuttle program is more than 20 years old, O'Keefe said shuttle orbiters are not old in the sense of lifetime left to serve.

"All of them have fewer than one-quarter of their operational life expended at this stage," O'Keefe said. "So we've got a lot of miles left."

Last month, the White House gave NASA a go-ahead to start work on an Orbital Space Plane -- an alternative way to hurl crews to the space station on a more rapid and routine basis.

In this regard, the shuttle is like "an 18-wheeler truck type asset," O'Keefe said. The Orbital Space Plane "is more like a roadster."

The hope is for an Orbital Space Plane that is capable of "on-demand" takeoffs.

It would be built for speed and greater maneuverability in space, far more than shuttle orbiters. Using the vehicle, NASA could rotate crews in and out of the station more rapidly. Also, select research experts can be flown to the orbiting research base as needed, the space agency chief said.

O'Keefe said the target date for the Orbital Space Plane to streak skyward is 2010.

Round-trip travel tips

O'Keefe was questioned about any NASA plans to replant humans on the Moon or dispatch expeditions to Mars and elsewhere. Such sojourns are more a matter of harnessing new technology coupled with the driving force of science and research needs, he said.

In particular, new forms of propulsion and power are a technological must. Furthermore, ways to thwart the effects of radiation and microgravity exposure on long voyages are mandatory.

"Our mission right now is to be sure that we can go there [the Moon] or anywhere else, quickly and for people to be able to survive the experience," O'Keefe said. "At NASA we believe in round-trips."