The latest assessment on the status of the Chinese space lab Tiangong-1 confirms the likelihood that it will fall to Earth later than expected.
Students ask Wang Yaping questions as she delivers a lesson to them from Tiangong-1 space module in the morning of June 20, 2013. Photo: AFP
Experts are tracking the module as it orbits at an altitude of 160-180km and are now forecasting a re-entry window centred on late Sunday at 23:25 GMT.
The window runs from the afternoon to the early morning of Monday.
China has lost all communication with the module and so there is no way to control the end stages of the descent.
However, the risk of any parts of Tiangong hitting a populated area is considered very low.
"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, the 10m-long, 8.5-tonne Tiangong module 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 the loss of command links in 2016 took away that option.
Tiangong is now brushing through the top of the atmosphere, which is dragging on the spacecraft and causing it to lose altitude rapidly.
Thirteen space agencies, under the leadership of the European Space Agency, are now following its path around the globe, modelling its behaviour.
This collective, known as the Inter-Agency Space Debris Coordination Committee (IADC), is trying to forecast the most likely time and place for the laboratory's final, destructive plunge.
The many uncertainties involved mean definitive statements can only be made very 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.
* Precise knowledge of the re-entry time and location will come late
* Typically, only in the last hour or so are experts very confident
* Most of the module's components will burn up in the high atmosphere
* Its orbital path means any debris is restricted in where it can fall
* Perhaps 20-40 percent could survive to the surface - that's 1.5-3.5 tonnes
* The highest probability is that this material would hit the ocean
* Any debris path at the surface would be hundreds of km long
* Tiangong is the 50th most massive object to come back uncontrolled
- BBC
