Last week, the Long March 5B rocket launched the first stage of the Chinese space station Tianhe into low-Earth orbit.
Under international guidelines, rockets are supposed to return to Earth in a controlled way, but the 30-metre-long rocket is spiralling out of control.
Weighing around 22.5 tonnes, it's one of the most massive pieces of space junk to make an uncontrolled re-entry to Earth.
"This is probably among the top 10 largest objects to come down in one piece," said space junk expert Ted Muelhaupt of the Aerospace Corporation, a not-for-profit organisation that advises US government space and defence agencies.
The rocket is about twice the mass of the Chinese space station Tiangong-1, which plunged back to Earth in 2018.
But at this stage, no-one has got a good idea about when and where it might land.
Aerospace Corporation has been using publicly available radar data supplied by the US Space Surveillance network to plot and predict the rocket's return.
At the time of writing, they predicted the rocket would fall to Earth at lunchtime this Sunday (AEST) give or take a day or so.
Others have given a time range of between 11:00am on Saturday and 5:00am on Monday.
But expect that to change as new data comes in.
Here's what we do – and don't – know about this piece of space junk.
What is it?
The Long March 5B is one of the newest and heaviest rockets in the Chinese fleet.
"This was specifically designed to launch their new space station," Mr Muelhaupt said.
This is not the first time this type of rocket, known as a CZ-5B, has plummeted back to Earth in an uncontrolled way.
Guo Cheng/Xinhua via AP
)On its first flight exactly a year ago, the prototype of this rocket launched an uncrewed module into low-Earth orbit.
While that proved it could lift modules into space, the rocket was sent spiralling back to Earth in an uncontrolled orbit.
It landed in the Atlantic Ocean, but debris from the rocket landed near a village on the Côte d'Ivoire in West Africa. Luckily, there were no casualties.
Since then, the space community had been speculating about whether same thing would happen again, said space journalist Andrew Jones, who follows the Chinese program.
“We were wondering this time whether they would do the same thing again where they have an uncontrolled re-entry or they had upgraded the booster [so it was capable of a controlled descent]," Mr Jones said.
"It seems [the controlled descent] didn't happen. Whether that was simply that they didn't make any changes or they did make some changes and they didn't work, we don't know, because we haven't heard anything."
What are the chances of it hitting me?
The chance of getting taken out by the rocket is very slim.
About three-quarters of the Earth is covered by water and a large chunk of the part that isn't is sparsely inhabited.
"The odds of it landing in an inhabited area are not great," Mr Muelhaupt said.
While there have been some tales of near misses (a truck destroyed in Minneapolis), only one person in the entire history of spaceflight has been hit by debris, Mr Muelhaupt said.
Lottie Williams of Oklahoma lived to tell the tale of being struck on the shoulder by a small piece of rocket in 1997.
In saying that, the rocket's orbit covers an area between 42 degrees north and south; that's as far north as Boston and as far south as Hobart.
"A great big chunk of the world's population is under it," he said.
It orbits around Earth about 15 times a day, covering a slightly different patch of the planet each orbit.
Can you track what it's doing?
A number of websites such as orbit.ing-now use radar data to show you in real time where the rocket is over Earth.
On maps, this looks like a wave (the technical term is "ground track"), but in reality the rocket is moving like a corkscrew oscillating in altitude.
As time goes on, it moves closer and closer to Earth.
At the moment, the Aerospace map shows all of the possible ground tracks in the timeframe the rocket is predicted to land.
But as the expected entry date draws closer, the number of tracks on the map will decrease.
"We may not be able to tell you where it's going to land, but we can start to rule out huge swathes of the world where it's not going to land," Mr Muelhaupt said.
Predicting when and where the rocket will come down is tricky, thanks to a range of factors.
Supplied: Aerospace Corporation
)The position of the spacecraft changes along the wave as it gets closer to Earth.
As the atmosphere gets thicker, there's more drag and the rocket will start to drop faster.
On average, the rocket is moving at around 7 kilometres per second.
"That's one of the reasons it's so hard to predict," Mr Muelhaupt said.
"If you get the prediction wrong by one minute, it's half a continent away."
Pinning down the precise position of the spacecraft is also hindered by gaps in radar or telescope tracking and the fact the rocket is tumbling, so it's harder to get an accurate read.
The other thing that really drives this is the Sun, which causes the atmosphere to expand and contract, changing the amount of drag on the rocket.
As the space junk closes in, it does get easier to predict, but it's still not precise.
"If I'm predicting a day out, I'm usually good at saying within a couple of [orbits around the Earth]," Mr Muelhaupt said.
"But it's still a whole hemisphere."
What happens when a rocket comes back to Earth?
Once a piece of space junk hits around 120 kilometres from the Earth's surface, it starts to burn up, creating a spectacular fireball.
As it re-enters, it peels apart like an onion, scattering debris across a distance of up to 500 kilometres.First to go are lightweight things like insulation and empty tanks.
Lightweight things tend to slow down quicker so fall first, while heavier and denser things such as engines tend to fall later, Mr Muelhaupt said.
These items often tend to survive the re-entry because they are too big to melt.
"In a rocket body, maybe 40 per cent of the structure will survive," he said.
Using that rule, he estimates around 8 tonnes of material from the Chinese rocket will fall back to Earth intact.
Mr Muelhaupt said a number of objects were photographed near the African village when the prototype CZ-5B rocket crashed back to Earth last year.
It is unknown whether or not the rocket is carrying any chemicals that could cause environmental damage, but Mr Muelhaupt said it was unlikely to be carrying the types of more toxic propellants used to keep other types of spacecraft in orbit.
Does this happen to every rocket we send into space?
It shouldn't.
The very first piece of space junk was the rocket that launched Sputnik, the first Russian satellite in 1957 (Sputnik itself became the second piece of space junk).
Back then we used to leave rockets in space, but Mr Muelhaupt said the chance of them exploding and creating a shower of space junk was too high.
Today, countries are supposed to follow guidelines set by the Inter-Agency Space Debris Coordination Committee (IADC) about managing risk.
"That's why when a large object is going to come in, you want to put it some place where people aren't," Mr Muelhaupt said.
Usually that's in a place in the South Pacific Ocean quaintly known as "the spacecraft cemetery".
"Most of the world has been launching rockets where they deliberately re-enter [the rocket back to Earth]."
Today's rockets are designed so they can be controlled by firing the motor to turn the rocket around, or installing extra motors and gyroscopes to guide them back to Earth.
Some companies such as SpaceX have moved to reuse rockets, rather than ditching them.
"[Designing rockets that can be controlled is] expensive and difficult, and there are things that can go wrong with that," Mr Muelhaupt said, citing the recent break-up of a SpaceX Falcon 9 rocket over Portland in the north-west Pacific.
Will there be any repercussions?
Astrophysicist Jonathan McDowell said it was unacceptable to let the rocket, which is seven times bigger than the Falcon 9 rocket, to re-enter in an uncontrolled way.
“Since 1990, nothing over 10 tonnes has been deliberately left in orbit to re-enter uncontrolled,” he told SpaceNews.
The Chinese Space Agency plans to launch two more stages of the space station next year, using the same type of rocket.
Mr Jones said it would be good to have some clarification of what happened this time.
“Also, it would be interesting to know what the design is. Are they taking steps to deal with this, or is this just how they are going to do it?”
The guidelines set by the IADC are not law and no-one enforces them, Mr Muelhaupt said.
"The European Space Agency recently put out a study that says the world's doing a lousy job following these rules," he said.
Mr Muelhaupt would like to see the world adopt what he calls "the backpacker's rule".
"You pack it in, you pack it out," he said.
"If you put it up there, bring it back down with assurance that you're going to bring it back down [safely]."
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