In September 2016, Chinese officials confirmed that they had lost control of the space lab and that it would crash into Earth sometime in the latter half of 2017. In May, China told the United Nations that the lab would reenter Earth between October and April 2018.
This week, the Aerospace Corporation — a California-based research center — said Tiangong-1 “is predicted to reenter in mid-March ... ± 2 weeks.”
Much of the space lab, which measures 34 feet in length and weighs more than 19,000 pounds (9½ tons, or about 8½ metric tons), is expected to burn up during its reentry.
But Jonathan McDowell, an astrophysicist from Harvard University, told the Guardian that pieces weighing up to 220 pounds could make it to the Earth's surface.
Where exactly the craft will fall is anyone's guess.
Even slight changes in atmospheric conditions can alter the landing site “from one continent to the next,” McDowell told the Guardian.
“You really can’t steer these things,” he said. “Even a couple of days before it reenters, we probably won’t know better than six or seven hours, plus or minus, when it’s going to come down. Not knowing when it’s going to come down translates as not knowing where it’s going to come down.”
The Aerospace Corporation noted that “there is a chance that a small amount of Tiangong-1 debris may survive reentry and impact the ground. Should this happen, any surviving debris would fall within a region that is a few hundred kilometers in size and centered along a point on the Earth that the station passes over.”
The nonprofit research center provided a map showing “the relative probabilities of debris landing within a given region.”
From the center: “Yellow indicates locations that have a higher probability while green indicates areas of lower probability. Blue areas have zero probability of debris reentry since Tiangong-1 does not fly over these areas (north of 42.7° N latitude or south of 42.7° S latitude). These zero probability areas constitute about a third of the total Earth’s surface area.”
Aerospace noted that “when considering the worst-case location (yellow regions of the map) the probability that a specific person (i.e., you) will be struck by Tiangong-1 debris is about one million times smaller than the odds of winning the Powerball jackpot.”
Importantly, the center added: “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.”
In other words, no matter where Tiangong-1 lands, it almost certainly won't land on you. Or, as Aerospace noted in an FAQ:
It is highly unlikely that debris from this reentry will strike any person or significantly damage any property.
Uncontrolled crashes of larger spacecraft, while rare, have happened before. The Soviet Salyut 7 space station crashed to Earth in 1991, while NASA's Skylab space station fell over Western Australia in 1979.
China launched Tiangong-2, its second experimental station, in September 2016. China is aiming to have a permanently manned space station in orbit by 2020.
The 2011 launch of Tiangong-1 was seen by some as a “potent political symbol” that marked an important step forward in China's expanding space program. It was considered a geopolitically significant event, part of China's broader space program through which it wants to assert its emergence as a new superpower.
Tiangong-1 ended its service in March after it had “comprehensively fulfilled its historical mission,” Wu Ping, deputy director of the manned space engineering office, was quoted as saying at a news conference by Xinhua, China's state news agency. The lab had served as a base for space experiments for 4½ years, two years longer than originally planned. It hosted two three-person crews, including China's first female astronaut, Liu Yang, in 2012.
The station has been descending gradually since its service ended. More recently, it has started to fall faster, reaching the denser layers of Earth's atmosphere, the Guardian reported.
The odds that the crashing craft will damage aviation or ground activities is “very low,” China told the United Nations, adding that it will closely monitor Tiangong-1's descent.
For spacecraft that remain under control, scientists carefully guide their reentry to a place on Earth called the Oceanic Pole of Inaccessibility, a 2½-mile-deep spot in the ocean known as the “spacecraft cemetery” about 3,000 miles off the eastern coast of New Zealand and 2,000 miles north of Antarctica.
As of June 2016, more than 263 spacecraft had crashed at the cemetery since 1971, according to Popular Science.
This post has been updated.