"I'm still quite nervous about it because it's just our second mission to the station," said Elon Musk, SpaceX's 41-year-old billionaire founder and CEO.

"We're hoping that this mission goes as smoothly as the last one."

With last year's retirement of the space shuttle fleet, Nasa is eager to give private industry the job of carrying cargo and crews, in the hope of cutting costs.

Meanwhile, the space agency will focus on deep-space missions to land probes on asteroids and Mars.

Another aerospace firm, Orbital Sciences, is nipping at SpaceX's heels with a test flight of its commercial rocket set for later this year. Orbital has a US$1.9 billion cargo-hauling contract with Nasa. The company is running tests on its Antares rocket at a launch pad at Nasa's Wallops Flight Facility in Virginia for a separate resupply mission.

Critics, including some former astronauts and members of Congress, have voiced concerns about Nasa's move toward private space missions. They contend that private space companies are risky ventures with unproven technology and say that the missions should be handled by Nasa flight-proven hardware.

But SpaceX, formally known as Space Exploration Technologies, has silenced many opponents after its successful demonstration mission, though it still faces opposition.

"A SpaceX failure back then, or indeed a slip-up on the next launch, would give ammunition to congressional critics, who in many cases are trying to bring home the bacon for their own constituents," said Tim Farrar, president of the consulting and research firm Telecom, Media & Finance Associates. "Continued success on SpaceX's part makes it much harder to argue for continuing to invest in traditional contracts."