In the next few days, a 23-tonne piece of rocket will plummet to Earth at about 15,000 miles an hour. Much of it may burn up on re-entry, but a significant amount will not.

It could land as one piece but more probably as many, scattered over an area up to several hundred miles across. Scientists have narrowed down the likely impact zone to within the latitudes of 41 degrees north and 41 degrees south, a region covering much of the US and South America, Africa, the Middle East, most of Asia, and all of Australia except the island of Tasmania.

Beyond that, predictions are iffy.

“A few hours after it re-enters the atmosphere we’ll know where it was,” said Dr Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “A few hours before we might know when to within three hours … But in that time the rocket goes all the way around the Earth at 17,000 miles an hour. So if you’re an hour out you’re also 17,000 miles out.”

In all likelihood the space junk, discarded from China’s Long March 5B launch last Sunday, will not hit a populated area. Despite 80% of the world’s people living in the risk zone, just 0.1% of it is considered populated.

“Everything else is ocean, forest or agricultural land,” said Dr Shane Walsh, a research fellow at the International Centre for Radio Astronomy Research. “It’s extremely unlikely to cause damage or loss of life.”

Space watchers may not be too concerned, but they aren’t exacty happy about the situation. The impact will be akin to a small plane crash, experts say, and probably far less fatal than missile strikes and accidents that are happening elsewhere every day. But the risk could be mitigated.

Sunday’s launch was the third of the 5B series, delivering a new laboratory module to the Tiangong space station. Most nations’ rockets separate the launcher from the payload before leaving the atmosphere, with an extra engine on the payload giving a final boost and allowing the launcher to fall in a more predictable way.

But China appears not to want to spend weight on the second engine, and its 5B rocket – one of the largest in use – instead pushes fully into orbit before separating. The bus-sized launch section then travels through orbit for days or weeks before re-entering Earth’s atmosphere. Somewhere.

In May 2020, two villages in Ivory Coast were hit by objects – including a 12-metre section of pipe – that appeared to have come from a Chinese Long March 5B expected to land that day.

After the second 5B launcher harmlessly landed in seas near the Maldives last year, a Nasa administrator, Bill Nelson, accused China of “failing to meet responsible standards regarding their space debris”.

China’s authorities reject the accusation. This week its foreign ministry spokesperson Zhao Lijian said Chinese space exploration had always acted “in accordance with international law and … customary practice”, and the probability of the debris causing harm was “extremely low”.

Zhao said the unit had been designed with unspecified “special technology”, and the “overwhelming majority” of its components would burn up on re-entry into the atmosphere.

Walsh said: “They claim to have learned from the last two launches and added some method of control, but the EU tracking network showed this unit is tumbling, which means it’s not controlled.”

Prof Chao Chi-Kuang, the head of the space science department at Taiwan’s National Central University, noted there had been other uncontrolled re-entries with debris hitting Earth, and not just by China. Famously, Nasa was fined $400 for littering when parts of its Skylab space station hit Western Australia in the 1970s. (It still has not paid.)

Chao said China’s launches were more unpredictable, and with larger pieces, and “of course people are scared in this case”, but he also accused the media of alarmism. “People think that there is something very heavy and big above our heads. But I think if China can prevent damage, they will prevent it,” he said.

Should the debris hit something or, worse, someone, people affected will be liable for compensation. But otherwise there are no international rules to prevent or restrict uncontrolled re-entries.

“It’s an interesting quirk of space law that if you do damage you’re liable for it, but if you do something risky and get away with it … then you get away with it,” said McDowell.

The US and the EU have embedded risk assessments and will not launch if there is a greater than one in 10,000 chance of causing injury. China appears to have a much lower bar.

In April, villagers in a remote part of India found what appeared to be large parts of a Chinese Long March 3B rocket launched in February. Launches from the inland Xichang satellite launch site routinely rain debris down on communities, with officials issuing evacuation notices for residents to “adjust your location quickly”.

Walsh said China was rightly proud of its space programme, and the launches should be a PR coup. Instead there are global headlines of varying levels of alarm.

McDowell and Walsh hope the bad publicity will encourage changes to future launches. “I do think they’re a bit embarrassed by the bad publicity,” McDowell said. “I think they know this is considered a problem now. They may never admit it but maybe we’ll see – without them mentioning it – that the next generation [of rockets] will be better behaved and re-enter more safely.”