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.
“A confidence of one hour is only reached about four hours beforehand. And one hour still means almost one revolution around the Earth,” said Holger Krag, the head of Esa’s space debris office. “But that’s still good enough to exclude many countries and even some continents.”
What can be said with certainty is that nothing will fall outside of 43 degrees from the equator, north or south.
This encompasses a region up to the Mediterranean and down to Tasmania, for example. It is governed by the inclination on which Tiangong was launched.
China has limited national tracking facilities around the globe and so had no choice but to keep the vessel on a reasonably tight equatorial path.
The International Space Station by contrast reaches 52 degrees north and south.
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. (Courtesy BBC)