round and round she goes and where she stops nobody knows*

The unmanned Chinese space station Tiangong I (launched in 2011) is due to reenter Earth’s atmosphere any day now. As of Wednesday March 28, 2018 EADT reentry is due anytime from late March 30 to April 02 (See ESA for updates). The chance of being hit by parts is very close to zero, but if you are lucky enough to see the reentry it will look quite spectacular!

The station is a large object, roughly 11×4 metres (plus a couple of 3x7m solar panels), and weighs about 6 tonnes – that’s big enough for at least some parts to potentially survive reentry and reach the ground. Or more likely reach the ocean given most of Earth’s surface is liquid. However, to put it into perspective Tiangong I is far smaller than the 74 tonne Skylab spacecraft that came down over western Australia in 1979.

Unfortunately, no-one has a clue where the pieces will come down. Reentry predictions are difficult. Each orbit lasts just under 90-minutes and the effects of atmospheric drag are extremely difficult to determine so its likely reentry point can be predicted only a few hours prior to it happening. Its orbit takes it between latitudes from Tasmania to northern Japan and most of that region is ocean-covered. So most likely no-one will even see the reentry.

However, if you are very lucky enough to witness its reentry it might appear something like this ATV-1 reentry of 2008 – a slow moving fireball breaking into pieces. The chance of being hit is close to zero (10 million times less likely than being hit by lightning),  but not quite zero. Typically it is heavy, strong parts like the fuel tanks which reach the ground. In 1979 Skylab came down over Western Australia and MAAS collected this piece of an oxygen tank. There are warnings that if the fuel tanks survive reentry they may contain residual, and toxic, hydrazine fuel – keep clear and call the emergency services!

You might expect a spacecraft of this size to be reentered in a controlled manner and brought down over unpopulated areas, the southern Pacific ocean for instance. However, communication with Tiangong I was lost in 2016 and it appears to have been uncontrolled since.

If you want to see Tiangong I passing overhead before it reenters (it looks like a fast moving star) go to Heavens-Above, define your location at the top-right of the screen and select Tiangong I from the list of ‘Satellites’, follow the instructions to get a map of the sky showing the stations path across the sky.

For more detailed information: The Aerospace Corporation has an excellent FAQ and the European Space Agency (ESA) are hosting a campaign to monitor and predict the reentry. Follow the ESA twitter feed @esa or the China Manned Space pages for the very latest updates.

Postscript: April 5, 2018. Tiangong I fell harmlessly, and apparently unobserved, into the mid Pacific ocean at about 10:16 AEST (00:16 UTC).

• I’m not sure of the origin of this quote but this may help.