US company SpaceX's Dragon cargo ship is set to make a fiery return journey to Earth on Thursday after a landmark mission to the International Space Station.
The release of the unmanned Dragon is set for 5:35 am Eastern time (0935 GMT) on Thursday, with an intact splashdown in the Pacific Ocean off California planned for 1444 GMT, NASA said.
"We have a lot ahead of us on the SpaceX side," said mission director John Couluris in a briefing with reporters on the eve of the spacecraft's return after a seven-day mission to the orbiting outpost.
"We have done it once," he said, referring to the Dragon's test flight in December 2010, when the capsule entered and returned safely from orbit for the first time.
"But it is still a very challenging phase of flight," he added. "We are not taking this lightly at all."
The spacecraft will come streaking back to earth "like a burning comet," protected from the extreme reentry temperatures with a sophisticated heat shield and directed to the landing spot with powerful thrusters, NASA said.
The gumdrop-shaped capsule is supposed to make an ocean landing 490 nautical miles (907 kilometers) southwest of Los Angeles, where three vessels are standing by as recovery boats.
It will then be transported to Texas so that the cargo it is bringing back can be returned to NASA, though the US space agency cautioned that if anything goes wrong, there is nothing irreplaceable on board.
"There is not anything coming home that we couldn't afford to not get back," said Holly Ridings, NASA flight director.
The cargo ship was launched on May 22 with 521 kilograms (1,148 pounds) of gear for the space lab, including food, supplies, computers, utilities and science experiments. It plans to return a 660-kilogram load to Earth.
On May 25, the Dragon became the first privately owned spacecraft to berth with the ISS, an event that US officials hailed as the start of a new era in spaceflight in which commercial enterprise will take on a larger role.
If Thursday's reentry succeeds, Dragon will be the only cargo capsule capable of return shipments to Earth, as the cargo craft operated by other countries are destroyed after making deliveries to the station, NASA said.
The United States retired its space shuttle fleet last year, leaving cargo missions up to the space agencies of Russia, Japan and Europe.
Until private US ventures come up with a replacement vehicle that can carry humans to the $100 billion orbiting lab, the world's astronauts must rely on Russia's Soyuz capsules, at $63 million a ticket.
US astronaut Don Pettit, who is part of the six-member crew at the ISS and helped unload and restock the capsule, described it as "roomier than a Soyuz" and said it boasts about as much space for cargo as his pickup truck.
The white Dragon capsule stands 4.4 meters (14.4 feet) high and is 3.66 meters in diameter, and could carry as much as 3,310 kilograms split between pressurized cargo in the capsule and unpressurized cargo in the trunk.
It was also built to carry up to seven humans to the ISS. The Soyuz carries three at a time.
SpaceX, owned by billionaire Internet entrepreneur Elon Musk, says it aims to begin taking people to the space station by 2015.
SpaceX and its competitor Orbital Sciences Corporation, both of which have received funding from NASA, will likely become the chief cargo servicers of the space station, which is set to remain operational until 2020, NASA has said.
SpaceX has a $1.6 billion contract with NASA to supply the station over the coming years, and Orbital Sciences has a $1.9 billion contract to do the same. Orbital's first test flight is scheduled for later this year.
