China’s defunct space lab, Tiangong-1, should fall to Earth over the weekend.
At over 10m in length and weighing more than 8 tonnes, it is larger than most of the man-made objects that routinely re-enter Earth’s atmosphere.
China has lost all communication with the module and so the descent will be uncontrolled.
However, experts say there is very low risk that any parts of Tiangong that do not burn up will hit a populated area.
“Given Tiangong-1 has a larger mass and is more robust, as it is pressurised, than many other space objects that return uncontrolled to Earth from space, it is the subject of a number of radar tracking campaigns,” explained Richard Crowther, the UK Space Agency’s chief engineer.
“The majority of the module can be expected to burn up during re-entry heating, with the greatest probability being that any surviving fragments will fall into the sea,” he told BBC News.
Launched in 2011 and visited by six Chinese astronauts, Tiangong was supposed to have been de-orbited in a planned manner.
The intention was to use its thrusters to drive the vehicle towards a remote zone over the Southern Ocean. But all command links were abruptly lost in 2016, and now nothing can be done to direct the fall.
Thirteen space agencies, under the leadership of the European Space Agency, are now following Tiangong’s path around the globe, modelling its behaviour as it descends deeper into atmosphere.
The collective, known as the Inter-Agency Space Debris Coordination Committee (IADC), are trying to forecast the most likely time and place for the laboratory’s re-entry.
The many uncertainties involved mean definitive statements can only be made close to the end of Tiangong’s flight.
Although about 5.2 billion people live within the re-entry zone, most of it is ocean, which explains the high probability that any debris that survives to the surface will hit water.
Dr Krag said: “We know from similar events that on average between 20% and 40% of the initial mass has the chance to survive re-entry heating.
“We could apply this rule of thumb also to Tiangong, I believe, because typically the same amount of heat-resistant material in relative terms is onboard all spacecraft.
“So that would mean between 1.5 tonnes and 3.5 tonnes might be able to survive,” he told BBC News.
The components that most often seem to avoid burning up in the atmosphere are tanks.
These objects are interior to the spacecraft and so are protected for much of the descent.
But they are also made from steel, titanium or carbon-reinforced plastics and these materials are generally more resistant to high temperatures should they become exposed.
Tiangong is certainly on the large size for uncontrolled re-entry objects but it is far from being the biggest, historically.
The US space agency’s Skylab was almost 80 tonnes in mass when it came back partially uncontrolled in 1979. Parts struck Western Australia but no-one on the ground was injured.
Nasa’s Columbia shuttle would also have to be classed as an uncontrolled re-entry. Its mass was over 100 tonnes when it made its tragic return from orbit in 2003.
Again, no-one on the ground was hit as debris scattered through the US states of Texas and Louisiana.
The redoubtable cataloguer of space activity, Jonathan McDowell from the Harvard-Smithsonian Center for Astrophysics, reckons Tiangong is only the 50th most massive object to come back uncontrolled.
China is participating in the IADC campaign and is sharing some of its data.
The nation has since launched a second lab, Tiangong-2, which continues to be operational. It was visited by a re-fuelling freighter, Tianzhou-1, just last year.
The Tiangongs were put up to demonstrate orbital rendezvous and docking capabilities – to be testbeds to rehearse activities ahead of China’s more permanent space station.
This facility, which is expected to comprise a large core module and two smaller ancillary modules, will be in service early next decade, the Asian nation says.
A new rocket, the Long March 5, was recently introduced to perform the heavy lifting that will be required to get the core module in orbit.