Why it's so hard to predict where Tiangong-1 will fall

An out-of-control space station is rapidly losing altitude and could come crashing back to Earth as early as Saturday, astronomers say.

Chinese authorities lost control of Tiangong-1 in 2016, and without regular adjustments to its orbit, the bus-sized station has been slowly getting closer to the planet.

Two years ago it was about 350km up. In January it was at 280km, but the drag of the atmosphere has pulled it down to 200km since then. It's now losing more than 2.5km every day, and the closer it gets to Earth, the thicker the atmosphere - slowing its sideways movement down and accelerating its fall.

Tiangong-1's descent.
Tiangong-1's descent. Photo credit: heavensabove.com

At some point the atmosphere will become so thick the friction will cause the 8.5-tonne station to burn up, with a small possibility some parts may make it to Earth. That could happen anywhere between Christchurch in the south and London in the north.

The European Space Agency (ESA) has narrowed the timeframe for Tiangong-1's descent to between March 31 and April 2, while the Chinese space programme has kept a wider window - March 31 to April 4.

You might be wondering how scientists are able to land probes on comets and take HD photographs of faraway dwarf planets like Pluto, but can't predict when a space station less than the distance between Auckland and Rotorua away will fall.

In an article for The Conversation, University of Glasgow engineering lecturer said it's because of Tiangong's odd shape, its chaotic and tumbling motion and the inconsistent density of the atmosphere at that height.

"The main problem here is that Tiangong-1's shape is complex, and the object is uncontrolled and tumbling chaotically."

Photographers take pictures of a model of the Shenzhou-9 spacecraft docking with Tiangong-1.
Photographers take pictures of a model of the Shenzhou-9 spacecraft docking with Tiangong-1. Photo credit: Reuters

Unpredictable solar activity also has an effect on the density of the ionosphere, the part of the atmosphere Tiangong currently inhabits.

And when the station starts to burn up, it will break apart - further complicating the calculations.

The ESA says a likely impact zone won't be known until about 24 hours before it falls, and even then, it could be thousands of kilometres out.

In 1979, the Skylab space station burned up on re-entry, raining down debris on Western Australia. Engineers had tried to land it in the Indian Ocean, but missed.

Skylab was 77 tonnes - nine times heavier than Tiangong, three times wider and more than twice as long - and no one was hurt. In January, three metal spheres landed in Peru - believed to be fuel tanks from a satellite.

"In the history of spaceflight, no casualties due to falling space debris have ever been confirmed," the ESA said in a blog post on its site.

So even if Tiangong makes it back to Earth, it's highly unlikely anyone will be hurt - about a one-in-292 trillion chance.

The risk is slightly increased for those living 43 degrees north and 43 degrees south of the equator - the ESA predicts there's about a 3 percent chance Tiangong will fall on those parallels respectively.

Christchurch is one point along the 43rd parallel south - but the vast majority of it is ocean, with small slivers in Chile, Argentina and Tasmania.

If a piece does land on your property, space lawyer Maria Pozza recommends not touching it, and calling the police.


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