The American SpaceX company's Dragon cargo capsule has fired its thrusters to bring itself back to Earth.
The burn will put the unmanned vehicle on a path to an ocean splashdown off the coast of California.Dragon is projected to hit the water about 08:44 Pacific Daylight Time (15:44 GMT; 16:44 BST).
The craft made history last Friday by becoming the first privately operated ship to visit the International Space Station (ISS).
It delivered half a tonne of food and other supplies to the outpost's astronauts.
The encounter was intended as a demonstration of the freight service that SpaceX plans to run to the platform.
The firm has a $1.6bn (£1bn; 1.3bn-euro) contract with US space agency Nasa, pending the successful recovery of the capsule from the ocean.
Dragon's trip home began early on Thursday when it was unberthed from the ISS by the station's 17.5m (58ft) robotic arm.
Astronaut Don Petit, at the controls of the Canadarm2, then released the cargo ship to fly free at 09:49 GMT, just as the station was moving over the Southern Ocean.
Dragon fired its thrusters three times to take itself down and away from the platform, and spent the next five hours preparing for the fiery re-entry.
A final 10-minute de-orbit burn by Dragon's thrusters was initiated at 14:51 GMT.
The capsule has a heatshield that should protect it from the extreme temperatures it will experience during the fall through the atmosphere.
A range of ships, planes and ground stations has been organised to track the return, which will be slowed in the final minutes by three parachutes.
The planned splashdown zone is about 760km (470 miles) west of the Baja California Peninsula.
Once recovered, Dragon will be returned to port, and then transferred to Texas for inspection and for its cargo to be unloaded. Astronauts put 660kg (1,400lb) of experiments and redundant equipment inside Dragon before it left the station.
SpaceX - Space Exploration Technologies Corporation - has been engaged by Nasa to fulfil a logistics role at the ISS.
The current mission was designed to see the company complete a last set of performance milestones.
Nasa has another such arrangement with Orbital Sciences Corporation of Virginia, although its freighter, known as Cygnus, is still several months from making its maiden flight.
The agency hopes that by contracting out the carriage of freight it will save money which can then be re-invested in more daring activities beyond the station, at destinations such as asteroids and Mars.
The commercial cargo approach will be followed later this decade by crew transport services.
SpaceX wants this business as well, and is developing the safety and life-support equipment that would allow Dragon to double up as an astronaut taxi.
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