The "technology of our time now permits companies to build with their own capabilities" systems for travelling into space, Griffin said.

"Exploring and getting a presence on the frontier will always be the province of government because operating on the frontier is a good way for the companies to go broke. But lower orbit with the station is no longer the frontier," he said.

The growing role includes private companies transporting people and cargo to the International Space Station (ISS).

NASA's first choice for supplying the ISS would be to buy the service from a private venture if it becomes available, said Griffin.

At present only the US space shuttle program and the Russian Soyuz spacecraft can transport astronauts to the ISS. Shuttle flights will be discontinued in 2010.

The transport of cargo to the ISS is provided by the shuttle, and by automated European ATV craft and Russia's Progress vehicle.

Griffin stressed the technical difficulties and cost of developing a launcher that can reach the ISS, but is confident the journey will be viable for the private sector soon after the shuttle is retired.

"Sometime in the next five to 10 years I expect the industry will be able to provide without government help the capabilities to transport cargo and people to the ISS," he said, giving a nod to SpaceX, a private space company whose previous launch attempt in August failed.

SpaceX is privately funded, but the technical aspects of the project are supported by NASA and the US Air Force.

Griffin expressed "deep sympathy" with the SpaceX's August failure, but said he "absolutely believes that they will succeed."

Founded by 37-year-old South African entrepreneur Elon Musk -- who also founded the Internet payment company PayPal -- SpaceX hopes to win lucrative US government contracts to launch satellites of less than 570 kilograms (1,256 pounds).

The company began test launches in 2006, followed by tests in 2007 and 2008. All three tests failed and the next is scheduled on September 23, 2008.

But the past failures have not dampened Musk's confidence in the SpaceX project.

"There should be absolutely zero question that SpaceX will prevail in reaching orbit and demonstrating reliable space transport. For my part, I will never give up and I mean never," Musk said in a statement after the August failed launch.

A number of other commericial space enterprises have ended badly.

The first such effort, Space Services Inc., did not survive successive launch failures in the 1980s.

The Texan entrepreneur Andrew Beal also conceded defeat in the late 1990s after constructing a number of space rockets.

Potential for growth in "space tourism" is a more promising direction for private companies because the technical and financials costs are more manageable.

The rewards for such a venture are also lucrative, with companies seeking to capture a share of a market which estimates say could carry nearly 15,000 tourists a year into space by 2025.

British billionaire Richard Branson unveiled the final version of his WhiteKnight Two (WK2) rocket in July 2008. The craft is projected to start sub-orbital flights for tourists by 2010.

Since 2001, five multimillionaire space tourists have visited the ISS, courtesy of the Russian Soyuz craft.

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