Tiangong-1 demise, 1 April, no 1 April joke

We’ve been following the demise of the Tiangong-1 for quite sometime and now the prediction of its re-entry is from about midnight tomorrow through to midnight Sunday (pretty much all of 1 April 2018, New Zealand time). It’s not easy to predict the exact time because now the space station is in an unstable orbit. There are many variables at play to determine how it will be affected by drag caused by the atmosphere as it gets lower and lower. For the latest information keep an eye on the European Space Agency’s Space Debris Office website. It’s currently at about 185km in altitude and dropping quickly.

Re-entry prediction in UTC (Credit: ESA)

This re-entry process has also been a bit worrying for Milky-Way.kiwi because, being in New Zealand, we could potentially be in the debris field. The nearly 8500kg weight of the space station will probably mostly burn up and not cause anyone any problems so there’s not too much need for alarm. Notwithstanding that comforting thought, there is still the possibility that some bits will survive re-entry and not be completely burnt up. ESA’s current prediction is that the probability of anything surviving re-entry hitting the same latitude as New Zealand is about 3%.

ESA’s probability of impact (Credit: ESA)

The map above shows there is a higher probability of impact in the vicinity of 42.8N and 42.8S latitudes. Fortuntaly for Wellington’s latitude there is plenty of ocean in the Southern Hemisphere where it could impact, rather than New Zealand. More worrying is the graph on the left which shows the population densities for various latitudes and the potential risk of a northern hemisphere re-entry.

The shape of the spacecraft is basically two cylinders with a combined length of 10.4m and two solar panels about 3m x 7m each. It’s likely the solar panels will break off and burn up first, due to their light weight construction and shape. The heavier and more aerodynamic central structure is a little harder to predict how it might break up. The weight of Tiangong-1 means it’s a long way off the largest uncontrolled re-entries of space craft. That prize goes to NASA’s Skylab which was about 74,000kg back in 1979, though, unlike the Tiangong-1, it wasn’t completely uncontrolled.

Generally spacecraft conduct a re-entry procedure at the end of their life in a controlled way so that the impact point can be chosen, generally in the ocean and far from any populated areas. The Chinese lost control of the Tiangong-1 in about March 2016, meaning they could no longer command the space station to fire its engines to maintain a stable orbit. Since that time there has been no ability to counteract the upper atmospheric drag that is slowly lowering the orbit of Tiangong-1.

Radar image of Tiangong-1 (Credit: Fraunhofer Institute)

Hopefully by this time tomorrow the Tiangong-1 has safely burnt up in the atmosphere and any bits left over have impacted a remote part of the ocean rather than landing on anyone’s head.