NEW YORK • Part of China's largest rocket, the Long March 5B, is tumbling out of control in orbit after launching a section of the country's new space station last week.
The rocket is expected to fall to Earth in what is called "an uncontrolled re-entry" some time today or tomorrow.
Whether it splashes harmlessly into the ocean or impacts land where people live, why China's space programme let this happen - again - remains unclear. And given China's planned schedule of launches, more such uncontrolled rocket re-entries in the years to come are possible.
The country's space programme has executed a series of major achievements in the past six months, including returning rocks from the Moon and putting a spacecraft in orbit around Mars. Yet it continues to create danger, however small, for people all over the planet by failing to control the paths of rockets it launches.
"I think it's negligent of them," said Mr Jonathan McDowell, an astrophysicist at the Centre for Astrophysics in Cambridge, Massachusetts, who tracks the comings and goings of objects in space. "I think it's irresponsible."
The piece that will be dropping out of the sky is the core booster stage of the Long March 5B, which was designed to lift the big, heavy pieces of the space station. For most rockets, the lower stages usually drop back to Earth immediately after launch.
Upper stages that reach orbit usually fire the engine again after releasing their payloads, guiding them towards re-entry in an unoccupied area like the middle of an ocean.
Over the past three decades, only China has lifted rocket stages this big to orbit and left them to fall somewhere at random, Mr McDowell said.
For the Long March 5B booster, that could be anywhere between 41.5 degrees north latitude and 41.5 degrees south latitude. That means Chicago, located a fraction of a degree farther north, is safe, but major US cities like New York could be hit by debris.
On Thursday, the Aerospace Corp, a non-profit largely financed by the US federal government that performs research and analysis, predicts re-entry will occur tomorrow at 11.43am Singapore time. If that is accurate, debris could shower down over north-eastern Africa, over Sudan.
Uncertainty over the time - give or take 16 hours - and location remains large. A day before, Aerospace's prediction put re-entry more than one hour earlier, over the eastern Indian Ocean.
Because the booster is travelling at 29,000kmh, a change of minutes shifts the debris by hundreds or thousands of kilometres. It is only a few hours before re-entry that the predictions become more precise.
China plans many more launches in the coming months as it completes construction of the country's third space station, called Tiangong, or "heavenly palace".
That will require additional flights of the mammoth rocket and the possibility of more uncontrolled re-entries that people on the ground will watch nervously, even if the risk from any single rocket stage is tiny.
China has a long history of letting pieces of its space equipment come down where they may. Rockets from one of China's principal launch sites, the Xichang Satellite Launch Centre in Sichuan province, routinely fell on rural areas downrange, occasionally causing damage.
China has since moved many of its launches, including last week's, to a new site in Wenchang, a city on Hainan, an island off the south-eastern coast. From there, rocket stages can fall harmlessly into the sea.