With a weight of 9.4 tons (8.5 metric tons) at launch, which occurred in September 2011, the craft will be one of the heftiest chunks of space debris to re-enter our planet’s atmosphere, according to the European Space Agency (ESA). Does that mean you’re in danger of being struck?
In two words? Not really.
"The odds of being hit are very small," Marco Langbroek, a consultant with the Space Security Center of the Royal Dutch Air Force and Leiden Observatory, told Space.com. [China's Tiangong-1 Space Lab in Pictures]
Langbroek, who tracks spy satellites and writes the blog SatTrackCam, has been posting re-entry predictions for Tiangong-1 since March 13, 2017. "We should not overdramatize the dangers," he said.
The risk is low for several reasons. First, although Tiangong-1 is about the size of a school bus, most of it will break apart and disintegrate as the friction of Earth’s atmosphere burns up the space lab. The surviving bits will likely scatter along a path projected to be about 1,240 miles long by 43 miles wide (2,000 by 70 kilometers), according to researchers with the Aerospace Corporation, a California-based company.
On a planet with a total surface area of about 197 million square miles (510 million square km), that’s a pretty small strip.
And that strip will probably fall on the ocean, which covers 70 percent of Earth's surface. Combine that with the fact that most people live clumped together in cities, and the chances of getting whacked on the noggin from a falling piece of space debris are less than 1 in 1 trillion, according to an Aerospace Corp. Tiangong-1 re-entry fact sheet.
ESA researchers deem the odds to be even slimmer — 1 in 300 trillion, or "about 10 million times smaller than the annual odds of being struck by lightning," they wrote in an FAQ on Tiangong-1’s re-entry.
There are no guarantees, of course. In January 1997, Lottie Williams of Tulsa, Oklahoma, was struck in the shoulder by a hand-size piece of space junk, later determined to have come from a Delta II rocket. She was not hurt. And at least 166 pieces of space junk have been recovered over the last 55 years, according to ESA.
Tiangong-1's orbit dictates that it will come down somewhere between 43 degrees north latitude and 43 degrees south latitude — a huge portion of the globe that harbors most of the world's population. But that's about as specific as researchers can get right now, and perhaps that lack of details is why people are feeling skittish.
Attributes such as the craft’s current mass and the exact atmospheric drag it's experiencing are unknown, Langbroek said. That’s because some of Tiangong-1's fuel was used before China lost contact with it in March 2016, and it might shed parts, such as its solar panels, as it falls, he added. The space lab is also tumbling in an unknown way, which varies the strength of the drag. In addition, solar outbursts that rile up Earth’s atmosphere can increase this drag, throwing off calculations, Langbroek said.
Finally, Tiangong-1 is moving incredibly fast, at about 15,660 mph (25,200 km/h).
"Even 10 minutes of uncertainty on either side of the nominal prediction corresponds to almost 8,500 kilometers [5,280 miles] of uncertainty in where it will come down," he said.
The real danger may not lie in people being struck by the debris, but rather in curious people inspecting it, said Langbroek. According to the Aerospace Corp. fact sheet, chunks of Tiangong-1 that survive re-entry may harbor hydrazine, a toxic and corrosive rocket fuel.
Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, said that any hydrazine would most likely leak out and burn up high in the atmosphere. "But just to be cautious, don’t handle the debris, keep people perhaps 100 yards away from it, and report it to local emergency services," he told Space.com.
If that piece fell in the United States, local police or firefighters should pass the information along to NASA or the U.S. Air Force, who, under the United Nations' Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space, must return the debris to China, McDowell said.