China's defunct space lab, Tiangong-1, should fall to Earth over the weekend.
At over 10m in length and weighing more than 8 tonnes, it is larger than most of the man-made objects that routinely re-enter Earth's atmosphere.
China has lost all communication with the module and so the descent will be uncontrolled.
However, experts say there is very low risk that any parts of Tiangong that do not burn up will hit a populated area.
"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.
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% 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
Launched in 2011 and visited by six Chinese astronauts, Tiangong 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 all command links were abruptly lost in 2016, and now nothing can be done to direct the fall.
Tiangong is now brushing through the top of the atmosphere, which is dragging on the spacecraft and causing it to lose altitude rapidly.
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 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.
Tiangong means 'Heavenly Palace'
The module was launched in 2011 to practise rendezvous and docking
Two astronaut crews visited in Shenzhou capsules - in 2012 and 2013
They included China's first female astronauts Liu Yang and Wang Yaping
China plans a more permanent space station in the next decade
It has developed a heavy-lift rocket, Long March 5, for the purpose
Although about 5.2 billion people live within the re-entry zone, most of it is ocean, which explains the high probability that any debris that survives to the surface will hit water.
Dr Krag said: "We know from similar events that on average between 20% and 40% of the initial mass has the chance to survive re-entry heating.
"We could apply this rule of thumb also to Tiangong, I believe, because typically the same amount of heat-resistant material in relative terms is onboard all spacecraft.
"So that would mean between 1.5 tonnes and 3.5 tonnes might be able to survive," he told BBC News.
The components that most often seem to avoid burning up in the atmosphere are tanks. These objects are interior to the spacecraft and so are protected for much of the descent.
But they are also made from steel, titanium or carbon-reinforced plastics and these materials are generally more resistant to high temperatures should they become exposed.
Tiangong is certainly on the large size for uncontrolled re-entry objects but it is far from being the biggest, historically.
The US space agency's Skylab was almost 80 tonnes in mass when it came back partially uncontrolled in 1979. Parts struck Western Australia but no-one on the ground was injured.
Nasa's Columbia shuttle would also have to be classed as an uncontrolled re-entry. Its mass was over 100 tonnes when it made its tragic return from orbit in 2003.
Again, no-one on the ground was hit as debris scattered through the US states of Texas and Louisiana.
The redoubtable cataloguer of space activity, Jonathan McDowell from the Harvard-Smithsonian Center for Astrophysics, reckons Tiangong is only the 50th most massive object to come back uncontrolled.
China is participating in the IADC campaign and is sharing some of its data.
The nation has since launched a second lab, Tiangong-2, which continues to be operational. It was visited by a re-fuelling freighter, Tianzhou-1, just last year.
The Tiangongs were put up to demonstrate orbital rendezvous and docking capabilities - to be testbeds to rehearse activities ahead of China's more permanent space station.
This facility, which is expected to comprise a large core module and two smaller ancillary modules, will be in service early next decade, the Asian nation says.
A new rocket, the Long March 5, was recently introduced to perform the heavy lifting that will be required to get the core module in orbit.
A Chinese space station is hurtling toward Earth. Where will it land?
The Chinese lost control of the Tiangong-1 space station in 2016. And it’s expected to crash around April 1.
In 2016, China lost control of its space station, a school-bus-size outpost called Tiangong-1. Up until that point, the plan was to bring the out-of-service station back to Earth through a controlled final descent. A few well-timed thruster blasts would have made sure the station landed safely in one of the world’s oceans.
But for reasons China hasn’t fully explained, its first-ever space station malfunctioned. And due to a phenomenon called orbital decay, the empty, rogue space station has been steadily cruising back toward Earth on its own.
Tiangong-1, which means “heavenly palace,” is now about 124 miles above the Earth and is expected to crash into the atmosphere around April 1, give or take a day, the European Space Agency (ESA) reports. It should mostly burn up in the atmosphere as it crashes, but a few heavy, dense pieces may hit the ground.
But again, the descent will not be controlled. The thrusters cannot be turned on. We don’t even know where those pieces are going to land.
Where will Tiangong-1 crash?
On March 26, the ESA released this map marking in green where the space station couldconceivably crash. It’s ... pretty unspecific.
The area covers much of China, India, the United States, and other densely populated areas. Scandinavia seems safe. The polar bears in the Arctic face many threats, but thankfully, Tiangong-1 is not one of them. Ditto for the penguins in Antarctica.
For what it’s worth, the ESA reports Tiangong-1 is slightly more likely to impact the northern or southern bound of the green-shaded area. The space station spends less time traversing the Earth near the equator than it does these more northern and southern regions. “Only from one day before the actual reentry will it become possible to roughly predict which ground tracks, and hence which regions on Earth, might witness the reentry,” the ESA reports.
The space agency suspects the station will crash on April 1, though there’s some uncertainty in their prediction.
Will Tiangong-1 hit me?
It’s hard to predict exactly when and where the space station will crash. The rate at which Tiangong-1 descends is determined in part by the friction it encounters as it collides with the very top layers of the atmosphere. And the conditions there are hard to measure and hard to model in computer simulations.
That said, the likelihood of being hit by a falling piece of space debris is infinitesimally small — just one in a trillion, according to the nonprofit Aerospace Corporation. “By comparison, the risk of being hit by lightning is one in 1.4 million and the risk that someone in the US will be killed in a hurricane is about one in six million,” it explains. And remember, the space station could very well crash into the ocean or over an uninhabited area, and the pieces could spread out over hundreds of miles.
Space lab re-entry expected by Monday
China's Tiangong I space lab is expected to re-enter the atmosphere between Saturday and Monday, according to the China Manned Space Agency.
The latest announcement, posted on Thursday afternoon on the agency's website, said Tiangong I was still in orbit at an average altitude of 196.4 kilometers on Thursday, which means it had descended 5.9 km since Wednesday. It also narrowed the earlier predicted window from the Beijing Aerospace Control Center, which had estimated that the re-entry would take place between Saturday and Wednesday.
Sources close to the matter told China Daily that it is extremely difficult to predict the exact time and location of the spacecraft's re-entry due to a host of factors such as the unpredictability of upper atmospheric drag and the ultrafast speed of the spacecraft.
In the latest note sent on Monday by China's permanent mission to the United Nations in Vienna to the UN Committee on the Peaceful Uses of Outer Space, Chinese space authorities said Tiangong I's re-entry area will be located between 43 degrees north latitude and 43 degrees south latitude.
The note said most structural parts of the spacecraft will burn up during re-entry and "the probability of damage to aviation activities and human life and facilities on Earth is extremely low".
It added that China will continue to closely monitor the status of Tiangong I and provide information on its orbit and estimated time of re-entry through the China Manned Space Agency's website.
The 8.5-metric-ton Tiangong I, the country's first space lab, was launched by a Long March 2F carrier rocket at Jiuquan Satellite Launch Center in northwestern China in September 2011. The spacecraft had a designated life span of two years but was in service for four and a half years before its retirement was announced by the Chinese space authorities in March 2016.
During its operation, the space lab conducted six automatic and astronaut-controlled dockings with the nation's Shenzhou VIII, Shenzhou IX and Shenzhou X spacecraft.
China launched its second space lab, Tiangong II, in September 2016. It was boarded by two Chinese astronauts during the Shenzhou XI mission in October and November of that year.
Every week, on average, a substantial, inert satellite drops into our atmosphere and burns up. Monitoring these reentries and warning European civil authorities has become routine work for ESA’s space debris experts.
Each year, about 100 tonnes of defunct satellites, uncontrolled spacecraft, spent upper stages and discarded items like instrument covers are dragged down by Earth’s upper atmosphere, ending their lives in flaming arcs across the sky.
Some of these objects are big and chunky, and pieces of them survive the fiery reentry to reach the surface. Our planet, however, is a big place, mostly covered by water, and much of what falls down is never seen by anyone, sinking to the bottom of some ocean, or landing far from human habitation.
While still in orbit, these and many other objects are tracked by a US military radar network, which shares the data with ESA, since Europe has no such capability of its own.
Informing European and international partners
It’s the task of ESA’s Space Debris team to look at these data and issue updates to ESA Member States and partner civil authorities around the globe.
They mix in additional tracking information gleaned from European sources, such as Germany’s Fraunhofer research radar near Bonn or telescopes and other detectors run by a mix of institutional and private researchers, to generate reentry forecasts – a challenging and imprecise art.
“With our current knowledge and state-of-the-art technology, we are not able to make very precise predictions,” says Holger Krag, head of ESA’s Space Debris Office.
“There will always be an uncertainty of a few hours in all predictions – even just days before the reentry, the uncertainty window can be very large.
“The high speeds of returning satellites mean they can travel thousands of kilometres during that time window, and that makes it very hard to predict a precise location of reentry.”
Most larger descents, about 50 per year, happen unseen by anyone and never make the news. In the history of spaceflight, no casualties from falling space debris have ever been confirmed.
Around once a year, ESA takes part in a joint tracking campaign run by the Inter Agency Space Debris Coordination Committee, which consists of experts from 13 space agencies and organisations such as NASA.
Radar image of China’s Tiangong-1 space station
“Members use these campaigns to pool their predictions of the time window, as well as their respective tracking datasets from radar and other sources. The aim is to cross-verify, cross-analyse and improve the prediction accuracy for all members,” notes Holger.
“Today, everyone in Europe relies on the US military for space debris orbit data – we lack the radar network and other detectors needed to perform independent tracking and monitoring of objects in space,” says Holger.
“This is needed to allow meaningful European participation in the global efforts for space safety.”
Chinas Raumlabor-Reste über Pazifik abgestürzt
Chinas Raumstation „Tiangong-1” ist am frühen Montagmorgen um 2.15 Uhr über dem Südpazifik in die Erdatmosphäre eingetreten, war ab diesem Moment im Sturzflug Richtung Ozean!
Das acht Tonnen schwere Weltraumlabor sei dabei „größtenteils“ verglüht, teilte die chinesische Raumfahrtbehörde CMSEO in Peking mit. Die übrigen Teile werden demnach ins Meer stürzen – sechseinhalb Jahre nach dem Start ins All! Auch das US-Militär bestätigte den Eintrittsort.
Als erwartete Eintrittsstelle in die Atmosphäre wurde von den Chinesen ein Punkt im Südatlantik genannt: 40,24 Grad westlicher Länge und 27,4 Grad südlicher Breite, also vor der Küste der brasilianischen Stadt São Paulo.
„Es wäre natürlich besser gewesen, wenn sie gar nicht abgestürzt wäre, aber sie ist im Südpazifik gelandet und das ein Ort, wo man es sich wünscht“. sagte Brad Tucker, Astro-Physiker von der Australian National University in Canberra.
Von der ursprünglich 8,5 Tonnen schweren Raumstation würden etwa etwa 1,5 bis 3,5 Tonnen nach dem Eintritt in die Atmosphäre als Einzelteile auf die Erde stürzen, verteilt über einen Ausdehnung von rund 1000 Kilometern, so Krag.
Sicher waren sich die Experten nur darin, dass die Trümmerteile in einem Band von 43 Grad nördlicher Breite (dort liegt etwa Marseille) und 43 Grad südlicher Breite (dort liegen etwa Argentinien oder Australien) auf die Erde treffe werde. Deutschland, das nördlich davon liegt, war somit also nicht gefährdet.
„Tiangong-1“ (deutsch: Himmelspalast), Chinas erste eigene Raumstation war am 29. September 2011 in den Weltraum gestartet, um verschiedene Experimente durchzuführen.
Seit März 2016 gab es keine Verbindung mehr. Zunächst war ein kontrollierter Absturz geplant worden, doch dann wurde das Nachfolgemodell nicht rechtzeitig fertig und der richtige Zeitpunkt verpasst. So blieb die Station im All.
Tiangong-1 crash: Chinese space station comes down in Pacific Ocean
Tributes on Weibo as officials say craft, which had been out of control since 2016, mostly burnt up on re-entry
As China’s Tiangong-1 space station hurtled toward Earth on Monday, burning up as it entered the atmosphere, Chinese residents wished the spacecraft a final farewell.
“Goodbye Tiangong-1. You are our hero,” one user wrote on the Chinese microblog Weibo, under the hashtag “Goodbye Tiangong”. Chinese news outlets posted photos and tributes to the 10.4-metre-long space station as it re-entered the Earth’s atmosphere over the south Pacific.
“Let’s take this moment to applaud and cheer her. This time it’s really goodbye,” wrote the Beijing-based Toutiao News.
Tiangong-1, or Heavenly Palace 1, was a symbol of China’s rise when it launched in 2011 and key part of the country’s ambitious space programme, which aims to place a permanent station in orbit by 2023.
It was originally planned to be decommissioned in 2013 but its mission was repeatedly extended. It became apparent in 2016 that China had lost control of the craft, which had stopped functioning and was no longer responding to ground control.
In December 2017, China alerted the United Nations that Tiangong-1 would come down by late March 2018 but could not predict exactly when or where. On Monday, the country’s space agency confirmed it had crashed into the Pacific Ocean.
The China Manned Space Engineering Office said: “Through monitoring and analysis by Beijing Aerospace Control Centre and related agencies, Tiangong-1 re-entered the atmosphere at about 8.15am, 2 April, Beijing time (0115 GMT). The re-entry falling area is located in the central region of the south Pacific.
“Most of the devices were ablated during the re-entry process.” Ablated, in spacecraft terms, means burned up through atmospheric friction.
Some in China dismissed the agency’s characterisation of the out-of-control satellite. “Re-entry? Everyone knows it’s a crash,” one Weibo user wrote.
The Chinese tabloid Global Times said on Monday that similar descriptions of the spacecraft in international media reflected overseas “envy” of China’s space industry.
“It’s normal for spacecraft to re-enter the atmosphere, yet Tiangong-1 received so much attention partly because some western countries are trying to hype and sling mud at China’s fast-growing aerospace industry,” the paper said.
The US military’s Joint Force Space Component Command (JFSCC) confirmed the craft’s re-entry “through coordination with counterparts in Australia, Canada, France, Germany, Italy, Japan, South Korea and the United Kingdom”.
The European Space Agency had indicated earlier that Tiangong-1 was likely to break up over water, which covers most of the planet’s surface.
It described the probability of someone being hit by a piece of debris from Tiangong-1 as “10 million times smaller than the yearly chance of being hit by lightning”.
Brad Tucker, an astrophysicist at the Australian National University, said Tiangong-1’s re-entry was “mostly successful” though it would have been better if the space station had not been spinning toward Earth. “It could have been better obviously, if it wasn’t tumbling, but it landed in the southern Pacific Ocean and that’s kind of where you hope it would land,” Tucker said.
“It’s been tumbling and spinning for a while, which means that when it really starts to come down it’s less predictable about what happens to it,” Tucker said.
“The biggest takeaway from this is that as we put more things into space, all countries, we have to be aware that we do have to plan for these sorts of issues that are happening.”
A second Chinese space station, Tiangong-2, remains in orbit. Launched in 2016, after Tiangong-1 went offline, its aim is to test capabilities for long-term human presence in space, in anticipation of China’s permanent space station.