The largest section of the rocket that launched the main module of China's first permanent space station into orbit is expected to plunge back to Earth as early as Sunday (NZ time) at an unknown location.
Usually, discarded core, or first-stage, rockets re-enter soon after lift off, normally over water, and don't go into orbit like this one did.
An astrophysicist earlier this week revealed New Zealand is one of a number of worldwide locations in the path of the rocket's trajectory - though it is more likely the debris will fall in the ocean.
China's space agency has yet to say whether the core stage of the huge Long March 5B rocket is being controlled or will make an out-of-control descent. Last May, another Chinese rocket fell uncontrolled into the Atlantic Ocean off West Africa.
Basic details about the rocket stage and its trajectory are unknown because the Chinese government has yet to comment publicly on the re-entry. Phone calls to the China National Space Administration weren't answered on Wednesday, a holiday.
However, the newspaper Global Times, published by the Chinese Communist Party, said the stage's "thin-skinned" aluminum-alloy exterior will easily burn up in the atmosphere, posing an extremely remote risk to people.
The situation is "not worth panicking about," it said, citing industry insiders.
"Most of the debris will burn up during re-entry ... leaving only a very small portion that may fall to the ground, which will potentially land on areas away from human activities or in the ocean," Wang Yanan, chief editor of Aerospace Knowledge magazine, was quoted as saying by the newspaper.
The US Defence Department expects the rocket stage to fall to Earth on Saturday (Sunday NZ time).
Where it will hit "cannot be pinpointed until within hours of its re-entry," the Pentagon said in a statement on Tuesday.
White House press secretary Jen Psaki at a Wednesday briefing said the US Space Command was "aware of and tracking the location" of the Chinese rocket.
The nonprofit Aerospace Corp. expects the debris to hit the Pacific near the Equator after passing over eastern US cities. Its orbit covers a swath of the planet from New Zealand to Newfoundland.
Harvard-based astrophysicist Jonathan McDowell said potentially dangerous debris will likely escape incineration but in all likelihood would fall into the sea, given that 70 per cent of the world is covered by ocean.
McDowell, a member of the Harvard-Smithsonian Centre for Astrophysics, said the rocket's main stage core would likely break into a shower of debris equivalent to that of a small plane crash and come down in a narrow trail stretching about 160km.
Based on its current orbit, the debris trail is likely to fall somewhere as far north as New York, Madrid or Beijing and as far south as southern Chile and Wellington, New Zealand, or anywhere in between, McDowell said.
The Long March 5B rocket carried the main module of Tianhe, or Heavenly Harmony, into orbit on April 29. China plans 10 more launches to carry additional parts of the space station into orbit.
The roughly 30m-long stage would be among the biggest space debris to fall to Earth.
The 18-tonne rocket that fell last May was the heaviest debris to fall uncontrolled since the former Soviet space station Salyut 7 in 1991.
China's first space station, Tiangong-1, crashed into the Pacific Ocean in 2016 after Beijing confirmed it had lost control. In 2019, the space agency controlled the demolition of its second station, Tiangong-2, in the atmosphere.
In March, debris from a Falcon 9 rocket launched by US aeronautics company SpaceX fell to Earth in Washington and on the Oregon coast.
- additional reporting Reuters