A large segment of a Chinese rocket is expected to make an uncontrolled re-entry into the Earth's atmosphere early Sunday, but Beijing has downplayed fears of damage on the ground and said the risk is very low.
A Long March-5B rocket launched the first module of China's new space station into Earth's orbit on April 29.
Its 18-tonne main segment is now in freefall and experts have said it is difficult to say precisely where and when it will re-enter the atmosphere.
Space-Track, using US military data, tweeted that the projected re-entry was at 0211 GMT over the Mediterranean basin, with an uncertainty window of an hour either side of that estimate.
The US Space Force's 18th Space Control Squadron at Vandenberg Air Force Base in California "will not know the precise location until AFTER" the rocket has landed, Space-Track said in an earlier tweet.
Chinese authorities have said most of the rocket components will likely be destroyed as it descends.
"The probability of causing harm... on the ground is extremely low," Chinese foreign ministry spokesman Wang Wenbin told reporters on Friday.
Although there has been fevered speculation over exactly where the rocket -- or parts of it -- will land, there is a good chance any debris that does not burn up will just splash down into the ocean, given that the planet is 70 percent water.
"We're hopeful that it will land in a place where it won't harm anyone," said Pentagon spokesman Mike Howard.
Howard said the United States was tracking the rocket segment but "its exact entry point into the Earth's atmosphere cannot be pinpointed until within hours of its re-entry".
Harvard-based astronomer Jonathan McDowell tweeted on Saturday: "New 18SPCS Space Force prediction narrows things down to one orbit: Costa Rica, Haiti, Iberia, Sardinia, Italy, Greece and Crete, Israel, Jordan, Saudi Arabia, Australia, New Zealand."
- No need for 'too much' worry -
Defence Secretary Lloyd Austin has said the US military has no plans to shoot the rocket segment down, but suggested that China had been negligent in letting it fall out of orbit.
"Given the size of the object, there will necessarily be big pieces left over," said Florent Delefie, an astronomer at the Paris-PSL Observatory.
"The chances of debris landing on an inhabited zone are tiny, probably one in a million."
Last year, debris from another Long March rocket fell on villages in the Ivory Coast, causing structural damage but no injuries or deaths.
McDowell said that although there was no need to worry "too much", the rocket's design needed a re-think to stop such a scenario happening again.
"There is a real chance of damage to whatever it hits and the outside chance of a casualty," he said.
"Having a ton of metal shards flying into the Earth at hundreds of kilometres per hour is not good practice, and China should redesign the Long-March 5B missions to avoid this."