WASHINGTON — The unmanned Dragon space capsule set off from the International Space Station Sunday for the cargo-laden return trip to Earth after successfully delivering its first commercial payload, NASA said.

Using a robotic arm, an astronaut aboard the floating laboratory detached and released the capsule at 1329 GMT after an 18-day mission to resupply the space station, the first ever by a privately-owned company, SpaceX.

The next step will be to bring the capsule out of orbit by intermittently firing its onboard engines to slow its speed.

It is then supposed to parachute into the Pacific Ocean off the California coast at 1920 GMT.

The Dragon's descent will be controlled by SpaceX from a center in Hawthorne, California, although NASA, which was in charge of the decoupling operation, will continue to provide communications.

The mission -- the first of 12 planned trips in SpaceX's $1.6 billion contract with NASA -- is a milestone for American efforts to privatize the space industry, aimed at reducing costs and spreading them among a wider group than governments alone.

The capsule delivered about 1,000 pounds (450 kilograms) of cargo to the space station and is taking home 1,670 pounds (758 kilograms) of supplies, hardware and scientific tests and results.

Owned by billionaire Paypal co-founder Elon Musk, SpaceX is one of several private firms working with the US space agency to send flights to and from the ISS, but SpaceX is the first to become operational.

The next SpaceX flight is scheduled for early January 2013.

NASA has been relying on Russian spacecraft for the last year, after retiring its fleet of shuttles -- but the Soyuz craft does not have room for cargo on the return flight.