SpaceX's Dragon cargo ship heads back to Earth

SpaceX's unmanned Dragon cargo ship on Saturday left the International Space Station carrying 1,800 kilograms of gear and prepared to splash down in the Pacific Ocean by mid-afternoon, NASA said.

The white supply vessel detached from the orbiting outpost at 9:23 AM (local time), fired its engines three times and slowly began its journey to Earth.

"Release confirmed," said commentator Rob Navias on NASA TV, noting that separation occurred as the ISS was 411 kilometers above the Earth, passing over just south of Australia.

"Dragon is safely on its way."

A parachute-assisted splashdown is expected off the coast of California around 3 PM (local time), but will not be broadcast on NASA TV.

The spacecraft is bringing back a host of science experiments, including lab mice that were studied in orbit to see how their bones changed in microgravity.

"Other critical biological samples preserved in science freezers, such as  plants, insects and human tissue, have also been transferred into Dragon for retrieval and analysis," said a NASA statement.

SpaceX's Dragon is currently the only cargo ship designed to return to Earth intact. The other US commercial supply ship, Orbital ATK's Cygnus cargo carrier, burns up on re-entry to Earth's atmosphere.

The cargo ship arrived April 4 after launching from Cape Canaveral, Florida, with 2,600 kilograms of food, supplies and science experiments to enable the study of thunderstorms, anti-cancer drugs, and technology to remove debris in orbit.

The mission was the 14th for SpaceX under a USD 1.6 billion contract with NASA to resupply the space station over multiple years.