SpaceX, as the privately owned, California, company is known, is one of two firms hired by NASA to fly science experiments and supplies to the $100 billion station, a project of 15 countries, after the shuttles' retirement.

Following a successful test flight in May, SpaceX launched its first operational Dragon cargo ship on October 7 and reached the station three days later.

On Sunday, as the station soared 255 miles over Burma, Dragon was released by the station's robot arm to begin the return trip back to Earth. It splashed down in the Pacific Ocean west of Mexico's Baja California about 3:20 p.m. EDT (1920 GMT).

Unlike the Russian, Japanese and European freighters that also ferry cargo to the station, Dragon was designed for round-trip flights. It returned with 1,673 pounds (759 kg) of equipment and science samples, including hundreds of frozen urine and blood samples from the crew.

"It was nice while she was on board," station commander Sunita Williams radioed to Mission Control in Houston. "Literally and figuratively, there is a piece of us on that spacecraft going home to Earth."

She leads a six-member crew that includes Russian cosmonauts Oleg Novitskiy, Evgeny Tarelkin and Yuri Malenchenko; U.S. astronaut Kevin Ford and Japanese astronaut Akihiko Hoshide.

SpaceX's next supply run is expected in January. Orbital Sciences Corp, NASA's second cargo hauler, plans to debut its Cygnus capsule in February or March. Combined, the companies' contracts with NASA are worth $3.5 billion.

SpaceX also is working under a separate $440 million NASA contract to upgrade the Dragon capsule and its Falcon 9 launcher to carry astronauts. The company plans a test flight with its own employees in 2015.

Boeing and privately owned Sierra Nevada Corp., also have investment funds from NASA to develop alternative space taxi designs. NASA hopes to be able to buy rides for its astronauts by 2017, breaking Russia's monopoly on crew ferry flights, a service that costs the United States more than $60 million a ride.

(Editing by Jane Sutton and Sandra Maler)

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