Tiangong-1, China’s first prototype space station, is spiralling down towards Earth’s surface and is expected to crash to the ground soon.
The Chinese space agency launched Tiangong-1, whose name translates to ‘Heavenly Palace’, in 2011 as a precursor to a larger, more permanent space station set for launch in 2020. Though it was supposed to be decommissioned in 2013 and replaced by larger modules, Tiangong-1 remains in orbit around Earth.
The station is currently in orbit around 240 km from the ground and travelling at about 29,000 km/hr. It is the largest object in recent times set to reenter Earth’s atmosphere without guidance from the ground.
Scientists usually guide spacecraft that they predict to have more than a 1-in-10,000 chance of causing injury due to debris to perform a controlled re-entry, and steer it to a patch on the southern Pacific Ocean, according to The Guardian. The odds that Tiangong-1 will cause an injury are currently 1-in-5,000. But since ground control lost contact with the station, controlled reentry is not an option.
The Aerospace Corporation has estimated that the station will reenter between March 27 and April 10, 2018. The European Space Agency expects reentry will occur between March 29 and April 9, although it did also warn that the projection wasn’t final. Based on Tiangong-1’s path in orbit, Aerospace also predicted the station will reenter between 42.7° N and 42.7° S latitudes.
Uncontrolled reentries have been common since have been sending things into space. The upper stages of SpaceX’s Falcon 9 and Russia’s Soyuz rockets often perform uncontrolled de-orbits manoeuvres, according to The Verge. In fact, the Chinese station is not even the largest object to have performed an uncontrolled reentry. That record belongs to Skylab, which reentered in 1979 and weighed 69 tonnes. Tiangong-1 only weighs 8.5 tonnes.
According to Jonathan McDowell, an astrophysicist at Harvard University, “Every couple of years something like this happens, but Tiangong-1 is big and dense so we need to keep an eye on it.”
Analysts expect that the bigger parts of the station’s modules will burn up on reentry but also that 10-40% could survive almost intact. Since a significant swath of the projected reentry areas is covered by the ocean, the chances that Tiangong-1 will cause damage to humans is fairly low. (So far, that’s happened only once in history, and even then the human escaped unhurt.)
“When considering the worst-case location, 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,” according to Aerospace.
McDowell was a bit less conservative. He told The Guardian, “Yes there’s a chance it will do damage, it might take out someone’s car, there will be a rain of a few pieces of metal, it might go through someone’s roof, like if a flap fell off a plane, but it is not widespread damage.”
However, everyone agrees that it is very hard to pinpoint the exact location of Tiangong-1’s reentry because the smallest atmospheric disturbance can shift the location from “one continent to the next”, to use McDowell’s words.
Experts believe that the remains of the spacecraft will contain hydrazine, a corrosive substance that is a component in rocket fuel and also a suspected carcinogen. Aerospace has recommended that people “not touch any debris [they] may find on the ground nor inhale vapours it may emit”.