Shaking off jitters, SpaceX counted down Friday to liftoff of its Dragon capsule to the International Space Station in what may be a historic mission for private spaceflight.

The blastoff of SpaceX's Falcon 9 rocket, carrying the unmanned Dragon and over half a ton of cargo toward the orbiting lab, is scheduled for 4:55 am (0855 GMT) Saturday from Cape Canaveral Air Force Station in south Florida.

Air Force weather officials said the forecast was 70 percent favorable for the first-of-its kind attempt to send a privately built spacecraft to the research outpost, where it plans to do a fly-under followed by a berthing in the coming days.

Until now, only the space agencies of Russia, Japan and Europe have been able to send supply ships to the ISS.

The United States had that capacity too, with its iconic space shuttle that long served as part astronaut bus, part delivery truck for the lab.

But the 30-year shuttle program ended for good in 2011, leaving Russia as the sole astro-taxi to the ISS until private industry could come up with a replacement.

SpaceX is the first of several US competitors to try sending its own cargo-bearing spacecraft to the ISS with the goal of restoring US access to space for human travelers by 2015.

The company made history with its Dragon launch in December 2010, becoming the first commercial outfit to send a spacecraft into orbit and back.

"If successful, there is no doubt this is a historic flight," said SpaceX president Gwynne Shotwell. "We really stand in awe of the opportunity to attempt this."

However, she acknowledged that even if liftoff goes as planned, many complicated and risky maneuvers lie ahead as the Dragon attempts to berth with the space outpost, which has six astronauts on board.

"I think we are going to be biting off our fingers between now and hour 75," she said, referring to the time span between Saturday's launch and berthing, scheduled for Tuesday.

"I don't think there is going to be a lot of sleep in the next 70 or 75 hours for folks at SpaceX."

Another key hurdle is the near-instantaneous launch window, which, if not met within seconds, would force the mission to be postponed until May 22, 25 or 29, Shotwell said. Slightly less favorable windows also open on May 23 and 26.

SpaceX founder Elon Musk, an Internet entrepreneur, billionaire and cofounder of PayPal, also admitted to having sleepless nights in an interview with AFP earlier this month.

Musk has also publicly fretted over the the complicated matter of latching on to the space station, which he described as moving faster than a speeding bullet.

"The space station is zooming around the Earth every 90 minutes, and it is going 17,000 miles (27,000 kilometers) an hour," he said.

"So you have got to launch up there and you've got to rendezvous and be backing into the space station within inches really, and this is something that is going 12 times faster than the bullet from an assault rifle. So it's hard."

Speaking to reporters on Friday, NASA's acting director of Commercial Spaceflight Development Phil McAlister played down expectations for the flight.

"This is a test flight. NASA views test flights primarily as learning opportunities," said McAlister.

"If it gets us in a better posture to fly next time, that is a good thing."

SpaceX, a California-based company, has benefited from NASA dollars in its quest but has also poured its own money into the endeavor.

Shotwell said SpaceX is firmly in the black and has been for years, even after spending about $1.2 billion on its space projects so far.

SpaceX and Orbital Sciences Corporation both have billion-dollar contracts with NASA to supply cargo to the ISS in the coming years, and they get NASA funds in exchange for meeting key milestones in their projects.

NASA has given SpaceX about $390 million so far of the total $680 million SpaceX has spent on cargo development, she said.

SpaceX also gets funding from NASA on a separate effort to develop a commercial crew vehicle for carrying astronauts to space, along with competitors Blue Origin, Boeing and Sierra Nevada.

In a few years' time, Shotwell said she hopes SpaceX will be able to undercut the hefty price NASA pays Russia for US astronauts to get a seat aboard the Soyuz space capsule -- around $63 million a ticket.

With seven seats aboard the Dragon capsule, she said SpaceX could someday offer that to NASA for $140 million per mission -- about $20 million per seat.