NASA hinted that it might rely on Russian rockets to deliver crew and cargo to the International Space Station when the shuttle fleet is retired in 2010 if private space companies fail to fill the gap.

The space agency in 2006 earmarked nearly $500 million US in seed money over five years to two budding commercial space companies — SpaceX and Rocketplane Kistler — to build and test new spacecraft to possibly fly to the space station.

The program ran into trouble last year when Oklahoma-based Rocketplane, which received about $32 million US from NASA, failed to raise enough money from private sources. NASA cancelled the contract and reopened the bidding process for the remaining $173 million.

A new winner is expected to be selected this month.

The space agency would prefer to give American aerospace firms the chance to deliver cargo to the space station but may be forced to buy such services from the Russians if commercial rockets are "not yet demonstrated and available," said NASA deputy administrator Shana Dale.

"While I do not like the idea that the United States may have no option other than to purchase crew transport services from Russia … I am glad that the Russians are our partners and have such capabilities," Dale said during NASA's budget briefing Monday.

NASA is under orders to finish the space station and retire the shuttle fleet by 2010. It needs to find a launch vehicle to ferry cargo to the space station while it works on the Orion program, the shuttle replacement with a first launch planned around 2014.

SpaceX, or Space Exploration Technologies, dismissed the idea that its rocket would not be ready in time.

The Hawthorne-based company founded by entrepreneur Elon Musk is building a 55-metre-long reusable launch vehicle called Falcon 9, named after Star Wars' Millennium Falcon. It plans to ship its Falcon 9 to Cape Canaveral, Fla., by the end of the year and start test flights at an unspecified date.

"There will be no gap in American access to the space station," Musk said in a statement.