The first of two companies hired by NASA to fly cargo to the International Space Station is preparing for a test flight on Saturday, a harbinger of a new type of public-private partnership.

"It is,  by all accounts, an important step, bordering on a giant leap, for commercial space," said Michael Lopez-Alegria, a former astronaut who now heads the Commercial Spaceflight Federation trade organization.

"We are at a brink of a milestone moment in our space history," added NASA deputy administrator Lori Garver.

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The upcoming mission by Space Exploration Technologies, a 10-year-old firm owned and operated by internet entrepreneur Elon Musk, is billed as a practice run to the space station, a $100 billion research laboratory owned by the United States, Russia, Europe, Japan and Canada that circles about 240 miles above the planet.

Liftoff is set for 4:55 a.m. EDT Saturday from Cape Canaveral Air Force Station in Florida.

Since the space shuttles were retired last summer, NASA is dependent on its  partner countries to  fly cargo and crew to the outpost while it focuses on developing new spacecraft that can travel beyond the station's orbit.

Rather than continuing to pay other governments for flights, NASA has hired Space Exploration Technologies, also known as SpaceX, and a second company, Orbital Sciences Corp., to deliver cargo to the station beginning this year under contracts worth a combined $3.5 billion. It wants to use commercial space taxis to fly its astronauts as well.

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The difference between these work agreements and traditional NASA contracts is that the companies contribute financially to their rockets' development, they retain ownership of their spacecraft and they are responsible for launch, in-flight operations and, in the case of SpaceX's capsule which returns to Earth, landing.

The arrangement also leaves the companies free to sell their spaceflight services to other customers.

SpaceX built two versions of its Falcon rocket, the Dragon capsule, its Cape Canaveral launch site, a  mission operations center and other support and testing facilities for about $1 billion, about one-third of which came from NASA.

An  analysis by the U.S. Government Accountability Office shows that a  similar program under a traditional NASA procurement would have cost four to 10 times as much, said NASA's Alan Lindenmoyer, a manager overseeing the agency’s new commercial initiatives.

SpaceX launched its first Dragon capsule in December 2010 for a trial run around the planet. This time it plans to go all the way to the space station.

"It's a very ambitious mission," said XCOR Aerospace's Jeff Greason, a long-time commercial space advocate. "If they get even half of their mission objectives done successfully, that would still be a historic first."

(Falcon 9 rocket and Dragon capsule are being prepared for launch on Saturday. Credit: SpaceX)