China's Tiangong-1 space lab was billed as a potent symbol of the country's rise when it launched in 2011.
Now, the lab is out of control and expected to crash-land on Earth by the end of March -- posing a minuscule risk to humans but inflicting a blot on the nation's bold push to become a space superpower.
The risk of debris hitting anyone is minimal, experts say
If wreckage falls on land, it could provide insight into China's space program
"They have a PR embarrassment on their hands," said Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. "The actual danger is small, but it is accepted international best practice nowadays that objects that big shouldn't be able to fall out of the sky in this manner."
Space experts stress the potential danger to humans is minimal -- the odds of debris from the vessel hitting a human are astronomically tiny, estimated to be less than one in 1 trillion. That compares with a one-in-1.4 million chance of a person in the US being struck by lightning.
In the best-case and most likely scenario, the space lab will largely burn up as it enters Earth's atmosphere over the ocean and a few parts will sink to the sea floor.
"The worst realistic case is that the Tiangong-1 reenters over a highly populated area, and a few largest chunks hit the ground, with perhaps some minor property damage," McDowell said. "But this has never happened in the 60-year history of reentering space debris. The chances are small."
Tiangong-1 stops functioning
The 8.5-ton, 40-foot Tiangong-1, or "Heavenly Palace," was launched in September 2011. Along with its successor -- the Tiangong-2, which launched in 2016 -- it was a prototype for China's ultimate space goal: a permanent, 20-ton space station that is expected to launch around 2022.
It's part of ambitious space plans that also include putting a man on the moon and sending a rover to Mars.
The Tiangong-1 was last used by astronauts in 2013.
"The original plan was for it to be retired soon after and sent under control into the ocean, but they were worried that its successor, Tiangong-2, might not get into orbit successfully, so they decided to keep the Tiangong-1 around as a backup," McDowell said.
But Tiangong-1 "ceased functioning" on March 16, 2016, China told the United Nations in May, without specifying why.
China may have lost control of the lab because it ran out of fuel, said Roger Handberg, a professor at the University of Central Florida.
Six months after Tiangong-1 went offline, Tiangong-2 went into orbit successfully.
"While the Chinese would have, of course, preferred this event not to happen, this does not present a threat to their long-term human spaceflight plans," said Joan Johnson-Freese, a professor and former chair of national security affairs at the US Naval War College in Rhode Island.
Fuel remnants could pose risk
As of December 24, Tiangong-1 was 286.5 kilometers (178 miles) up, compared with 348.3 kilometers (216 miles) in March, according to a weekly update on the space lab's location published at the website of China's manned space program.
In its UN submission anticipating the craft's fall to Earth, China said "most parts of Tiangong will be burned and destroyed in the process of reentering the atmosphere."
"It's of little probability that it will cause harm to aviation or ground activities," China stated.
One risk is that humans could come into contact with remnants of the toxic hydrazine rocket fuel that might remain in the space vessel, Johnson-Freese said.
"Worst case, it hits a populated area and curious individuals decide to investigate, thereby coming into contact with the hydrazine," she said.
Breathing hydrazine for short periods may cause coughing and irritation of the throat and lungs, convulsions, tremors or seizures, according to the US Agency for Toxic Substances.
Landing's location, timing in flux
Though China has said it expects Tiangong-1 to crash-land by late March, unpredictable space weather in the outer atmosphere in the form of solar flares makes it hard to predict exactly when and where that will happen, McDowell said. He estimates the earliest reentry would be at the end of February.
Experts say they know roughly the latitude at which the craft will land, putting places like Canada and the United Kingdom in the clear.
But despite the international attention, it's very likely the reentry will go unnoticed by most people.
"Secret missile warning satellites will see a big flare in infrared when it burns up, but that information doesn't usually come out to the public for a few hours," McDowell said. "Unless it's over a place where people actually see it happening, we won't know until it's all over."
Debris could offer insight
China's path to the stars has so far avoided fatal incidents like the Challenger and Columbia disasters that killed 14 US astronauts, but it's not always been smooth. In 2007, it shot down one of its weather satellites 853 kilometers (530 miles) above Earth.
The strike, which resulted in tens of thousands of pieces of debris, has forced the International Space Station to maneuver several times to avoid collisions -- a scenario that played out to dramatic effect in the Oscar-winning movie, "Gravity."
If wreckage from the Tiangong-1 falls on land, it could potentially provide some insight into China's space program.
"It would be an opportunity for Western analysts to look at remnants of China space hardware. That's a risk for the Chinese," said Michael Listner, a lawyer and space law expert.
US Congress in 2011 barred bilateral contact with individuals of the Chinese space program because of national security fears, although bilateral trade in the aviation industry has made China's rocket program more open than it used to be.
Meantime, China has ratified two international treaties that would require it recover any wreckage and assume liability for damage, Listner said.
Space debris falls often
It's common for space debris, such as spent satellites and rocket stages, to fall to Earth. In 2017, five objects heavier than 3 tons made uncontrolled reentries, McDowell said. All five melted as they entered the Earth's atmosphere, he said.
The last human space outpost to fall to Earth was the 135-ton Russian space station Mir in 2001. That was a controlled landing, with most parts burning up upon return and the rest landing in the ocean.
The first US space station, the 74-ton Skylab, fell to Earth in an uncontrolled reentry in 1979. Some debris fell in sparsely populated Western Australia, incurring no damage save for a $400 fine for littering.
More than 5,400 metric tons of space materials is estimated to have survived reentry in the past 50 years, with no reported casualties. Most space debris ends up in the remote southern Pacific Ocean, an area some call a space graveyard.