It will all be over in a flash. At some point this weekend, a dazzling fireball will tear across the sky as China’s out-of-control space station tumbles back to Earth at 16,500mph and burns up in the atmosphere.

The Tiangong-1, or “Heavenly Palace”, has been hopelessly adrift since the Chinese space agency lost control of the prototype space lab in 2016, five years after it launched as a bold symbol of the nation’s ambitions in orbit.



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From the moment it was lost, scientists around the world began plugging information on the stricken craft into computer models to predict how its final act would play out. On Friday, the European Space Agency said that the unoccupied wreckage would crash back to Earth between Saturday night and Sunday evening UK time.



“If you’re in the right place at the right time, and the sky is clear, it will be quite spectacular,” said Holger Krag, head of ESA’s space debris office in Darmstadt. “It will be visible to the naked eye, even in daylight, and look like a slow-moving shooting star that splits into a few more shooting stars. You might even see a smoke trail.”



Compared with the International Space Station (ISS), Tiangong-1 is a minnow. The 400-tonne ISS would barely fit inside a football field, while the 8.5 tonne Chinese station is no bigger than a bus. Visitors to Tiangong-1, including China’s first two female taikonauts, Liu Yang and Wang Yaping, had two sleeping berths at their disposal, but the toilet and cooking facilities were on the Shenzhou module that ferried them to the orbiting outpost.

Though Tiangong-1 will be the largest lump of space junk to fall back to Earth so far this year, it is nowhere near a record-breaker. In 2001, the Russian space agency steered their 120 tonne Mir space station safely into the Pacific Ocean. The far less controlled re-entry of Nasa’s 74 tonne Skylab in 1979 scattered pieces of space hardware over hundreds of miles of Western Australia. The re-entry of Tiangong-1 will be uncontrolled too.

About 100 tonnes of spent rocket stages, defunct satellites, and other space debris come down each year. Most of the material burns up in the atmosphere as air ahead of the object is compressed to generate ferocious heat. As a rule of thumb, only about 20 to 40% of a spacecraft survives the inferno of re-entry. The components that hit the ground tend to be heat-resistant fuel tanks, thrusters and other parts, such as metal docking rings, which can be the size of a rear tractor wheel.

On Friday, the Chinese space station was still hurtling around the planet more than 180km above the surface. The atmosphere is tenuous at that altitude, but thick enough to drag on the solar panels and body of Tiangong-1. Gradually this slows the spacecraft down until gravity can pull it from orbit. When the spaceship falls below 100km, it will begin its re-entry in earnest.

“Within minutes it will be fully decelerated and the energy it carries will be translated into heat and aerodynamic forces that rip it apart,” said Krag. Between 1.5 to 3.5 tonnes of the spaceship may survive the ordeal and reach the Earth’s surface in dozens of pieces cast over hundreds of miles. The most dangerous could be fuel tanks that typically hold toxic hydrazine propellant.



Space agencies around the world have sophisticated computer models to predict where and when space debris may land, but even the best cannot give anything close to a precise answer. The speed at which spacecraft travel, and the unpredictability of their breakup high in the sky, means the calculation is beyond the means of modern science.



“We will never be able to say upfront where debris will fall because these objects are moving so fast,” said Krag. “Even if you can narrow the re-entry window to two hours, that still means you have tens of thousands of kilometres where debris may come down.” The situation would not be much better if the exact point of re-entry, 100km above the Earth, could be calculated, either. “From that altitude, you still have a fallout zone of 1,000km which might stretch over several countries,” Krag said.

The Chinese space station flies over land and sea from 43 degrees north and 43 degrees south, which rules out re-entry over the UK and much of Europe. But more than five billion people do live beneath its flightpath, in vast stretches of North and South America, China, the Middle East, Africa, and Australia.

With most of the Earth covered in water, and huge areas home to few if any people, the chances of being struck by space debris are remote. To date, the only person thought to have won this unenviable lottery is Lottie Williams from Tulsa, Oklahoma, who had a lump of Delta II rocket strike her shoulder in 1997 while on a stroll through a local park. “The risk is not zero, but history tells us that the risk is not that high,” Krag said. “Inside the zone, the risk of being hit is about the same as being hit by lightning twice in the same year.”



• This article was amended on 18 April 2018 to more accurately describe the process by which heat is generated as a spacecraft re-enters the atmosphere.

