The cargo capsule blasted off May on 22 from Cape Canaveral Air Force Base in Florida and three days later became the first privately owned spaceship to reach the International Space Station, a $100 billion project of 15 nations that flies some 240 miles above Earth.

Dragon splashed down in the Pacific Ocean on May 31 and was returned by barge to the Port of Los Angeles before dawn on Tuesday.

The successful test flight not only means Space Exploration Technologies, or SpaceX, can start working off a $1.6 billion contract with NASA to fly cargo to the space station. It also clears a key hurdle for SpaceX to compete for Department of Defense business as well, which would mean launching military satellites.

Dragon's launch was the third successive flight of SpaceX's Falcon 9 rocket, which debuted in June 2010.

Flying three times successfully was among the criteria the company needed to meet to become eligible to compete for military business under a new program designed to draw competition into a field now monopolized by United Launch Alliance, a partnership of Boeing and Lockheed Martin.

“"The new entrant criteria did say three launches are required (for Falcon 9) before certification can happen for national security payloads," said SpaceX Communications Director Kirstin Brost Grantham.

There are several paths toward certification, and the requirements can vary, Air Force spokeswoman Tracy Bunko said.

"“If the new entrant has a launch vehicle with a more robust, demonstrated successful flight history, then we may require less technical evaluation for certification. But, it also depends on the risk assessment of the mission," Bunko wrote in an email to Reuters.

Near term, United Launch Alliance, or ULA, will remain the sole provider of heavy- and medium-lift commercial launch services to the U.S. military with its Delta 4 and Atlas 5 rockets.

But the wall is cracking. The Air Force is expected to award a non-ULA launch services contract this year for the Deep Space Climate Observatory, a NASA Earth-monitoring satellite that is being repurposed by the National Oceanic and Atmospheric Administration (NOAA) into a solar observatory. The Air Force issued a request for bids on May 11.

A second satellite, the Air Force's Space Test Payload-2, also has been set aside for a new launch services provider.

ELUSIVE MARKET

In addition to 12 cargo-delivery flights for NASA, SpaceX has booked Falcon rocket flights for more than 28 other launches for a variety of companies, foreign governments and other customers.

"The one market that we have not yet been successful with is launching Defense Department satellites, although we're hopeful that we'll win one or two demonstration launches this year," Musk said after Dragon's return from orbit.

"Hopefully the successive flights of Falcon 9 in a row will give them the confidence they need to open up the defense contract for competition," he said.

Robert Bigelow, the president of privately owned Bigelow Aerospace, said the Falcon 9 would create a paradigm shift within the global launch industry.

"The Falcon 9 has clearly arrived and proven itself as a reliable and affordable launch system for NASA, the Air Force and commercial payloads," said Bigelow.

His company plans to build, fly and operate commercial space stations and habitats in orbit, and has a marketing agreement with SpaceX for flight services.

Last week, SpaceX added Intelsat as the first customer for its planned Falcon Heavy rocket, which is expected to have twice the lift capacity of ULA's Delta 4 Heavy, currently the biggest booster in the U.S. fleet.

A Falcon Heavy mission costs between $83 million and $128 million, according to SpaceX's website, a fraction of a Delta 4 Heavy rocket launch.

For now, ULA isn't worried.

"In order for a fair competition, a new entrant would need to support the full set of mission and technical requirements. In addition, entrants also will be faced with stringent government oversight, accounting and reporting requirements - none of which is part of a commercial business plan," ULA spokeswoman Jessica Rye wrote in an email to Reuters.

"ULA also understands that the issue is not about competition, but how can our customers enable the reliable delivery of important space capabilities that protect our nation and promote science at the most cost-efficient method," she added.

(Editing by Jane Sutton)

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