LEAGUE CITY, TX - Commercial space station cargo ships, crew ferries and other spacecraft will prove a vital cog in NASA's engine for future space exploration, the agency's top official said Tuesday.

"We want to be able to buy these services from American industry," NASA chief Michael Griffin told American Astronautical Society (AAS) during its annual conference here, adding that the first space station resupply proposals are expected this fall. "It will not be government business as usual."

Commercial cargo and crew ships to the International Space Station (ISS) are just the tip of what Griffin dubbed 'the dawn of the true space age,' an era which could include private fueling depots in low-Earth orbit to aid NASA's plan to return astronauts to the Moon.

"It is exactly the type of enterprise which should be left to industry and the [commercial] marketplace," Griffin said of the fuel depot.

Relying on additional commercial space services rather than its own infrastructure may be a boon for NASA.

"NASA cannot succeed alone in pursuing the exploration vision," said Courtney Stadd, a former NASA chief of staff and White House liaison, adding that the agency already has its hands full sustaining the ISS while working to develop new cargo and human-rated spacecraft and rockets for Moon and Mars missions. "It's a breathtakingly full dance card."

The agency should embrace partnerships with commercial industry and the international community because "any misstep in human spaceflight could spell a very long hiatus in human-driven exploration in the U.S.," Stadd said.

NASA has already eaten into its ISS research budget to fund the station's completion and the development of the Crew Exploration Vehicle (CEV) - the shuttle's planned successor.

"We can afford to build the station and finish its assembly, or we can afford to use what's there [for research]," Griffin said. "But we cannot afford to simultaneously do both."

NASA's three remaining space shuttles are slated for retirement by 2010, during which time agency officials hope to launch 19 flights - 18 of them to the ISS for station construction or resupply. The additional shuttle mission is earmarked to service the Hubble Space Telescope.

"The station is a great proving ground for exploration," said Bill Gerstenmaier, NASA's associate administrator of space operations, adding that by the time the ISS is complete it will have a mass of about 800,000 pounds (362,873 kilograms) - roughly the same as a potential Mars mission under the space agency's exploration vision. "We've learned a lot about operating internationally and a lot about continuous operation, all of which applies directly to exploration."

ISS construction

The station is currently about the size of a three-bedroom home and only half-built as NASA works to resume space shuttle flights.

NASA orbiters are the only vehicles capable of launching the hefty ISS components awaiting launch at Kennedy Space Center, though a steady stream of Russian spacecraft have kept the orbital platform manned and supplied.

The shuttle Atlantis's STS-121 mission - NASA's second post-Columbia accident test flight after STS-114 this past summer - is currently expected to fly no earlier than May 2006. But it will be the flight after that, STS-115, which will once again resume ISS construction.

Initial ISS shuttle flights will deliver truss and solar array segments required to provide the power necessary to support larger components such as Japan's Kibo experiment module and the European Space Agency's Columbus module.

NASA's international partners would prefer to launch their modules earlier, but because of the truss and power logistics, it has proven difficult to move them earlier in the ISS launch manifest, explained John Elbon, Boeing vice president and ISS program manager for NASA systems.