An out-of-control Chinese space station with "highly toxic" chemicals on board could hit a number of major world cities, including Christchurch, research suggests.
China's first prototype station, Tiangong-1, will come crashing back to the planet between March 30 and April 6, experts say.
It has the highest chance of crashing into cities along a narrow strip around latitudes of 43 degrees north and south.
As well as Christchurch, this includes a number of highly populated cities including New York, Barcelona, Beijing, Chicago, Istanbul, Rome and Toronto.
It is most likely to hit these places because it is travelling parallel to the equator at the most northern and southern points of its orbit, the Daily Mail reports.
From our perspective on Earth it appears to be travelling more slowly above these regions, thanks to its geometry relative to the Earth, although its speed actually remains constant.
Because it takes longer to cross the surface of the Earth at these latitudes it has a higher risk of coming down here.
The doomed 8.5-tonne craft, which has been hurtling towards Earth since control was lost in 2016, is believed to contain dangerous hydrazine.
Scientists will only know the precise date it will impact and exactly where debris will fall during the finals weeks of its decline.
Explaining why this is so Dr Hugh Lewis, senior lecturer in Aerospace Engineering at the University of Southampton, compared the geometrical processes at work to crossing the road.
Speaking to MailOnline, he said: "The spacecraft is travelling around a more or less circular orbit, which is tipped with respect to the equator at 43°.
"If you plot this path on a map of the Earth, it produces a sine wave pattern, with the slower curve of the wave in northern and southern latitudes and the faster straighter sections running from east to west.
"If you imagine the green low risk area on the map is the part of the road we're trying to walk across, the quickest way is to go at 90 degrees – straight across.
"When the spacecraft crosses the equator, it's crossing the road at this point, and it does so really fast.
"When it goes across the red bands further north and south, it's crossing at a steeper angle - almost parallel to the road.
"It takes longer to cross at these latitudes, which is why it has a higher risk of coming down here."
Predictions of Tiangong-1's most likely point of impact come from Aerospace, a US research organisation based in El Segundo, California, that advises government and private enterprise on space flight.
It says the space station will enter the Earth's atmosphere on April 4, give or take a week, and debris will fall no further north than 42.7° N latitude or south of 42.7° S latitude.
These zero probability areas, marked safe as Tiangong-1 does not fly over them, constitute about a third of the Earth's total surface area.
At particular risk in the northern hemisphere are northern parts of the US, including Boston, Des Moines, Detroit, Milwaukee, Philadelphia and Salt Lake City.
Florence, Italy, Monaco City and Sochi, Russia, are also in the higher risk region.
In the southern hemisphere, other cities which might be affected include Trelew, Argentina, Christchurch, New Zealand, and Sapporo, Japan.
Experts from the European Space Agency (ESA), based in Paris, are among those tracking Tiangong-1, which means "heavenly palace".
Their Space Debris Office in Darmstadt, Germany, made a revised re-entry prediction date of March 30 and April 6 in recent days.
This narrows down from their previous estimate of March 29 and April 9.
Exactly where it will hit is slightly harder to predict.
Speaking to MailOnline, Dr Lewis added: "We can't say precisely where as we don't know which orbit it will come in on. At this point in time it's very difficult to say.
"If you take how far in advance you make your prediction, the rule of thumb for error is around 10 per cent.
"At the moment, that's roughly 10 days, or 160 possible orbits.
"If we were to predict again with a week to go, this would narrow to less than one day, or 16 possible orbits.
"My expectation is that what little of the craft survives the atmosphere will impact the ocean."
While most of the satellite will burn up during re-entry, around 10 to 40 per cent of it is expected to survive as debris and some parts may contain dangerous hydrazine.
In recent months, the spacecraft has been speeding up and it is now falling by more than 6km (3.7 miles) a week. In October it was falling at 1.5km (0.9 miles) a week.
"Every couple of years something like this happens, but Tiangong-1 is big and dense so we need to keep an eye on it", Jonathan McDowell, an astrophysicist from Harvard University told the Guardian.
"It is only in the final week or so that we are going to be able to start speaking about it with more confidence.
"I would guess that a few pieces will survive re-entry. But we will only know where they are going to land after after the fact."
Website Satflare, which provides online 3D tracking of more than 15,000 satellites, has calculated what it thinks are the chances of the space station entering the atmosphere during the next three months.
According to its analysis of orbital elements gathered during the last months, the re-enter may occur in March (20 per cent), in April (60 per cent) or in May 2018 (20 per cent).
These predictions may also change as new orbital measurements become available.
Aerospace Corp has also issued its own forecast over the likelihood of being hit by falling debris.
In a written statement, a company spokesman said: "When considering the worst-case locations, the probability that a specific person will be struck by Tiangong-1 debris is about one million times smaller than the odds of winning the Powerball jackpot.
"In the history of spaceflight, no known person has ever been harmed by reentering space debris.
"Only one person has ever been recorded as being hit by a piece of space debris and, fortunately, she was not injured."
On September 14, 2016, China made an official statement predicting Tiangong-1 would reenter the atmosphere in the latter half of 2017.
The Tiangong-1 spacecraft launched in 2011, with the aim of using the craft to set up a larger space station.
The craft is now at an altitude of less than 300 kilometres (186 miles) in an orbit that is decaying, forcing it to make an uncontrolled re-entry.
Holger Krag, head of ESA's Space Debris Office, said: "Owing to the geometry of the station's orbit, we can already exclude the possibility that any fragments will fall over any spot further north than 43°N or further south than 43°S.
"This means that re-entry may take place over any spot on Earth between these latitudes, which includes several European countries, for example.
"The date, time and geographic footprint of the re-entry can only be predicted with large uncertainties.
"Even shortly before re-entry, only a very large time and geographical window can be estimated.'
Much of the spacecraft is expected to burn up in the atmosphere upon re-entry.
But owing to the station's mass and construction materials, there is a possibility that some portions of it will survive and reach the surface.
In the history of spaceflight, no casualties due to falling space debris have ever been confirmed.
What is the "highly toxic" chemical onboard China's Tiangong-1 space station?
A "highly-toxic" corrosive chemical could land on Earth when parts of an out-of-control Chinese space station crash into our planet.
The chemical, called hydrazine, is used in rocket fuel and long-term exposure is believed to cause cancer in humans.
It is being carried aboard the Tiangong-1 space station which is hurtling towards Earth.
The warning over exposure to the chemical came from Aerospace Corp, a non-profit corporation based in El Segundo, California, which provides technical guidance and advice on all aspects of space missions.
Hydrazine is a colourless, oily liquid or sometimes white crystalline compound with a very highly reactive base.
It has a number of industrial, agricultural and military uses, including in rocket fuel.
Symptoms of short-term exposure to high levels of hydrazine include irritation of the eyes, nose, and throat, dizziness, headache, nausea, pulmonary edema, seizures, and coma, according to the United States Environmental Protection Agency (EPA).
Long-term exposure can also damage the liver, kidneys, and central nervous system in humans.
The liquid is corrosive and may produce dermatitis from skin contact in humans and animals.
Increased incidences of lung, nasal cavity, and liver tumours have been observed in rodents exposed to hydrazine.
The EPA has classified hydrazine as a Group B2, a probable human carcinogen.
What is the Tiangong-1 space station?
Tiangong-1 is China's first Space Station Module.
The vehicle was the nation's first step towards its ultimate goal of developing, building, and operating a large Space Station as a permanent human presence in Low Earth Orbit.
The module was launched on September 29, 2012.
Tiangong-1 features flight-proven components of Chinese Shenzhou Spacecraft as well as new technology.
The module consists of three sections: the aft service module, a transition section and the habitable orbital module.
The vehicle is 10.4 metres long and has a main diameter of 3.35 metres.
It has a liftoff mass of 8,506 kilograms and provides 15 cubic metres of pressurized volume.