NASA is pressing ahead with plans to use part of the International Space Station as a national laboratory, a move that would reserve about half of the outpost's U.S. science facilities for outside use by 2011.

The plan hinges on the completion of the half-built $100 billion space station by September 2010, when NASA's shuttle fleet is due to retire, the space agency said Monday.

"What we're trying to do is open up the U.S. segment of the space station to be utilized by a variety of folks, both governmental and maybe commercial, in the future so that we can take the maximum advantage of the space station that we're in the process of assembling right now," said Bill Gerstenmaier, NASA's associate administrator for space operations, during a teleconference with reporters.

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In late May, NASA presented Congress with a 14-page report detailing the agency's plan to use the U.S. segment of the ISS as a national laboratory as directed under the NASA Authorization Act of 2005.

NASA and its international partners plan at least 12 more space shuttle flights to complete ISS construction by 2010.

Still awaiting launch to the station are the European Space Agency's Columbus module, Japan's three-part Kibo laboratory, additional Russian-built modules and supporting solar arrays, connecting nodes and other hardware.

"Previously, we had anticipated that all the research conducted on the station would be research within NASA's mission portfolio," said Mark Uhran, the space agency's associate administrator for the ISS. "Now what we're looking to do is make the facility available to other government agencies or private firms to pursue their own research interests."

NASA's restructuring in recent years, which shifted away from earlier ISS science efforts to a more focused exploration program to support the planned return of astronauts to the Moon, led to the surplus of science time available aboard the station, space agency officials said.

The level of interest among non-NASA agencies to use the ISS could ultimately decide how long the space station remains in operation beyond its current 2016 design lifetime, they added.

"Technically, the space station could fly to 2020 or 2022," Gerstenmaier said, adding that a decision on whether to extend the station's lifetime would have to be made around 2014. "What really drives the practical lifetime of the space station is how useful it is and does it fit a niche."

When complete, the space station's pressurized U.S. science facilities will include the 12 experiment racks inside NASA's Destiny laboratory already in orbit, as well as five others inside both the Columbus and Kibo laboratories.

NASA has also reserved about five payload slots on the Kibo laboratory's porch-like exposed platform and has room for science experiments on other U.S.-built external cargo carriers.

About half of all those U.S. assets are expected to be available for use by private commercial enterprises or non-NASA government agencies, Uhran said.

Science payloads will be skewed to include more automation, since the amount of crew time available for non-NASA work remains to be determined, as well as focused on experiments that return data only, rather than require a return trip to Earth, he added.

While NASA would continue to assume the current operational and maintenance costs of the ISS, as well as those to launch astronaut crews and cargo, national laboratory users would be responsible for the cost of their payloads and research under the current plan.

The anticipated cost of NASA's ISS operations once the station is complete is estimated at about $1.5 billion per year, Gerstenmaier said.

The space agency is also relying on the anticipated availability of private launch services to the ISS, such as those sponsored by NASA's Commercial Orbital Transportation Services program, as well as the station's planned expansion to a six-person crew in 2009 to maintain the orbital laboratory.

"The success of this plan does clearly rely on being able to reach a six-person crew," Uhran said.

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