Space experts predict the Tiangong-1 space station, or "Heavenly Palace", will turn into a spectacular fireball as it enters Earth's atmosphere sometime during the Easter long weekend.
This TV grab taken on 2 November 2011 shows an animated video clip of the docking of the Tiangong-1 space lab module and the Shenzhou VIII (Shenzhou-8) spacecraft in space. Photo: AFP
According to the latest predictions, it will begin its fiery descent into Earth's atmosphere somewhere in a window between 30 March and 3 April - possibly around 1 April.
But there are concerns the bus-sized spacecraft is out of control.
That means some debris from the fireball could hit the Earth anywhere in a band between 43 degrees north and south of the equator.
But you would have to be extremely unlucky to be taken out by a chunk of debris from Tiangong-1, according to space engineer Warwick Holmes, executive director of space engineering at the University of Sydney's School of Aerospace.
For a start, he said, 70 percent of the world was covered by ocean, and large parts of Australia were sparsely populated.
"Everyone thinks they're going to get hit by the Chinese space station. I promise you it's just not going to happen," Mr Holmes said.
"You've got a greater probability of getting hit by a car crossing a Sydney road today than you're going to get hit by the Chinese space station.
"But you may get to wave it goodbye as it streaks across the sky."
What is it?
The Tiangong-1 is the Chinese Manned Space Agency's (CMSA) version of the International Space Station.
But China's first space station is a lightweight compared to the 420-tonne ISS.
The 8.5-tonne craft is made of two cylindrical modules: the experiment module which housed crew, and the service module, which contains thruster engines, attached to two solar panels.
China's first space station was launched in 2011 and completed three missions, one uncrewed and two crewed, said Xiaopeng Wu, a space engineer at the University of Sydney.
"The original design was for two years but they extended the lab time to 2016," said Dr Wu.
In 2016, the CMSA advised the United Nations that the spacecraft had completed its mission.
How do we know if it's out of control?
Not long after it was decommissioned, rumours began to circulate that the Chinese agency had lost control of the craft.
"It seems they lost communication link to the space station so there is no data link between Tiangong-1," Dr Wu said.
Dr Wu said it had been planned to deorbit in September 2017, but this did not happen.
If there is no data link, ground engineers cannot fire up the engines to help control where the spacecraft lands.
"You can't activate or turn [the thruster engine] on so in this case you can't control the orbit," Dr Wu explained.
But, he added, the status of the craft was unknown.
William Ailor, a US space debris expert, said it was still unclear whether or not there was a possibility the Chinese agency still had the capacity to fire up the engines at the last minute.
"We know that they are not doing motor burns as it's coming down," Dr Ailor, principle engineer for the Aerospace Corporation, a not-for-profit think tank that advises US space and defence agencies said.
"Then again, if you're going to dispose of something, you might save your propellant where it might get into an orbit where you could use that and drive it into an ocean area. We don't know.
"But if they can't communicate with it, they're sort of done. It will come in where it wants to."
Even though we do not know the status of the craft, however, we know its trajectory so it can be tracked.
Are some places more likely to get hit than others?
Tiangong-1 orbits Earth once every one-and-a-half hours in a rollercoaster orbit pinging between the latitudes of 43 degrees north and 43 degrees south.
That is as far north as Boston and as south as Hobart.
Hurtling at a speed of around seven kilometres a second (the equivalent of travelling between Sydney and Perth in about eight minutes), it is never anywhere long.
Originally it was intended the craft would burn into smithereens somewhere over a remote part of the south Pacific Ocean, between New Zealand and South America at the bottom of this band.
This location, known as the "spacecraft cemetery", is the final resting place of space junk from a multitude of large spacecraft sent up into the sky by the US, Russia, Europe and Japan.
But, if communication has been lost with the craft, the re-entry point is much less certain.
According to Aerospace and the European Space Agency, the areas at the edges of this band had a slightly higher risk.
Dr Ailor said the reason for this was that the spacecraft spent slightly longer in these areas as it turned in its orbit than it did as it whizzed straight over the equator.
But Dr Ailor said the probability of increased risk in this area was not significant.
"It's equally likely it will happen any place within those two latitude bands," he said.
"It could crash anywhere it likes."
Why is it so hard to predict where it will come down?
Every day the spacecraft completes 16 orbits, flying over a slightly different part of the Earth's surface each orbit.
At the same time, it is slowly losing altitude. According to the CMSA, it is currently around 227km above Earth.
As it gets closer to Earth, drag will increase and it will drop faster as it loses energy.
The descending craft is being tracked by radar stations around the world, and space engineers are trying to predict which orbit path it will be on as it enters the atmosphere.
By plotting the spacecraft's descent and modelling atmospheric conditions, they will start to narrow down which trajectories will be its last.
"But it will probably never be less than a couple of orbit revolutions," Dr Ailor said.
So the re-entry will happen somewhere on a path that is two times around the Earth, but it will never be pinpointed to a specific location.
"We just don't have data good enough to do that," Dr Ailor said.
"Even if we had really good data, the atmosphere does things that are unpredictable that affects where it's going to come down."
For example, the craft can experience more drag if the atmosphere expands due to solar activity or variations in day/night temperatures, said Dr Ailor.
Even once you can narrow the re-entry point down to hours there will still be an error margin of 20 minutes.
"That makes predicting where it's going to enter the atmosphere and start breaking up uncertain."
Can I see where Tiangong-1 is?
You can track the spacecraft for your specific location on websites such as Heavens Above (just select Tiangong-1) or N2YO, which enables you to see the spacecraft's progress in real time (just select Tiangong-1 from the "most tracked" tab).
If you are really lucky, you could see it as it turns into a fireball, Dr Ailor said.
"These things are really beautiful when they come in and break apart.
"It's like a meteor but it's much smaller so it's really quite a beautiful sight.
"But, most likely it will come down where no-one will see it and that will be sad, but good in a sense too."
What happens when spacecraft re-enter the atmosphere?
The spacecraft re-enters the atmosphere at around 120km above Earth. From that point, it is on its way down.
By around 70km, the intense heat of re-entry starts to melt the craft and it begins to break apart, Dr Ailor said.
"The whole re-entry event, entry interface to debris impact, will last around 15 to 20 minutes," he said.
Based on his experience studying space debris that has fallen to Earth, about 10 to 40 percent of the dry mass of a spacecraft will survive the fireball and make it to the ground.
How does Tiangong-1 compare to other space debris?
Tiangong-1 is not the only potentially uncontrolled spacecraft to fall back to Earth. The largest was SkyLab, a 77-tonne US space station which disintegrated over Western Australia in 1979.
There have also been several uncontrolled landings of both Russian and US craft in the early days of space exploration.
"Most of these heavy things that have come in came in years ago, back when we weren't quite so careful with them," Dr Ailor said.
Since then, many large spacecraft have made controlled entries back to Earth.
On 23 March 2001, bits of the 130-tonne Russian space station Mir splashed down in the south Pacific's spacecraft cemetery.
More recently, cargo ships which supply the International Space Station with food and fuel and return to Earth with waste material were ditched in the cemetery, Mr Holmes said.
"Six times this has happened. A 20-tonne vehicle has re-entered and no-one even knew about it because it was perfectly vectored over the south Pacific Ocean," he said.
And sometime in the future the ISS will return to Earth. NASA started planning the ISS's re-entry three years ago, said Dr Wu.
"The ISS is ... about 50 times the size of Tiangong-1," he said.
- ABC