China's vacant Tiangong-1 space laboratory was due to fall to Earth this weekend.
Over 10 meters long and weighing more than 8 tons, it is larger than most man-made objects that routinely return to Earth's atmosphere.
China has lost all communication with the module and the descent will be uncontrolled.
However, experts say there is a very low risk that parts of Tiangong that do not burn will hit an inhabited area.
"The majority of the module can be expected to burn on reentry heating, with the greatest likelihood that all surviving fragments will fall into the sea," he told BBC News.
- Accurate knowledge of reentry time and location is delayed
- Experts are typically very confident in the last hour
- Most components of the module burn in the high atmosphere
- Its orbit means that all debris is limited are where they can fall
- Maybe 20-40% could survive to the surface – that's 1.5-3.5 tons
- The highest probability is that this material hits the ocean
- Any of the rubble on the surface would be hundreds of kilometers long
- Tiangong is the 50th most massive object that returns unchecked
Launched in 2011 and visited by six Chinese astronauts, Tiangong should be orbited as planned manner.
The intention was to use its engines to drive the vehicle to a remote zone above the Southern Ocean. But all command connections were abruptly lost in 2016, and now nothing can be done to steer the case.
Thirteen space agencies, led by the European Space Agency, are now following Tiangong's path around the globe, modeling his behavior as he delves deeper into the atmosphere.
The collective, known as the Inter-Agency Space Debris Coordination Committee (IADC), attempts to predict the most likely time and place for the laboratory to re-enter.
The many uncertainties mean that final statements can only be made shortly before the end of Tiangong's flight.
"One hour's trust is achieved only across four Hours ago, and an hour is still a revolution around the world, "said Holger Krag, head of Esa's space debris office. "But that's still good enough to rule out many countries and even some continents."
What can be said with certainty is that nothing falls north or south from 43 degrees outside the equator.
This includes, for example, a region as far as the Mediterranean Sea and as far as Tasmania. It is dominated by the inclination on which Tiangong was launched.
China has few tracking facilities worldwide, so the ship had to be kept on a relatively narrow equatorial path.
In contrast, the International Space Station reaches 52 degrees north and south.
Tiangong means & # 39; Heavenly Palace & # 39;
- The module was launched for rendezvous and docking in 2011
- Two astronaut crews in Shenzhou capsules – in 2012 and 2013
- Among them were China's first female astronauts Liu Yang and Wang Yaping
- China is planning more permanents Space Station over the Next Decade
- To this end, it has developed a long-range missile, Long March 5
Although about 5.2 billion people live in the reentry zone, most of the ocean is what explains the high probability that Rubble that reaches the surface meets water.
Dr. Krag said, "We know from similar events that on average between 20% and 40% of the initial masses have the chance to survive reentry heating.
" We could apply this rule of thumb to Tiangong, I believe, because, as a rule the same amount of refractory material is relatively aboard all spacecraft.
"That would mean that between 1.5 tons and 3.5 tons could survive," he told BBC News.
The components that most often do not go up in flames in the atmosphere are tanks. These objects are inside the spacecraft and are therefore protected for much of the descent.
But they are also made of steel, titanium, or carbon-reinforced plastics, and these materials are generally more resistant to high temperatures should they be exposed.
Tiangong is certainly on the big size for uncontrolled reentry objects, but it is by no means the largest, historical.
The Skylab of the US Space Agency was nearly 80 tons heavy, as it returned in 1979 partly uncontrolled. Parts hit Western Australia but nobody on the ground was injured.
NASA's Columbia shuttle should also be classified as an uncontrolled reentry. Its mass was over 100 tons when it tragically returned in 2003 from orbit.
Again, no one was hit on the ground as rubble that was scattered throughout the US states of Texas and Louisiana.
Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics believes that Tiangong is only the 50th most massive object to return uncontrollably.
China participates in the IADC campaign and shares some of its data.
The nation has since started a second laboratory, Tiangong-2, which is still in operation. Just last year it was visited by a tanker called Tianzhou-1.
The Tiangongs were built to demonstrate orbital rendezvous and docking skills – being testbeds to rehearse activities in front of China's more permanent space station.
This facility, which is expected to include a large core module and two minor sub-modules, will be up and running in the early next decade, says the Asian nation.
A new rocket, the Long March 5, was recently introduced to carry out the heavy lifting that will be required to bring the core module into orbit.