The Chinese space station Tiangong-1 is slowly but surely getting closer and closer to the time when it re-enters the atmosphere. The Chinese government lost control of the station back in 2016, and its re-entry is now unpredictable rather than controlled.
The station will likely burn in the atmosphere, but there are bits of it that may survive the fiery journey through the air. People are worried that it may fall on their head, so we decided it would be quite informative to work out just how likely we are to be hit by this large bit of space junk. The estimations are approximations since the factors that come into play in such an event are numerous.
First some data. Tiangong-1 weighs about 8,500 kilograms (18,800 pounds) and is about 10.4 by 3.4 meters (34.1 by 11 feet). The space station could fall anywhere between latitude 42.5 degrees north and 42.5 degrees south, equivalent to an area of 345 million square kilometers (133 million square miles). It is mostly made of materials that should burn up in the atmosphere, but some people estimate that up to 100 kilograms (220 pounds) of metallic materials could survive the trip through the air.
To estimate the effect such objects would have if they fell to the ground, we used the Impact: Earth! Calculator developed by geoscientists from Imperial College London. There are a few constraints, including the fact that the metal fragments that survive may be too small to be modeled and that Tiangong has such a low density (2 percent of ice) that it can’t be modeled. So we assumed the full spaceship melted (which won't happen) in a single object about 1 meter (3.3 feet) across, which has the density of iron. This space ball would be landing with a speed of 3,300 kilometers per hour (2,050 miles per hour), creating a crater around 23 meters (75.5 feet) across and 4.95 meters (16 feet) deep.
Now, if we are realistic, it is twice more likely that it hits a body of water than it hits land. But we want to talk about the worst-case scenarios, so let’s forget realism. Clearly, the impact won’t be powerful enough to create a cataclysm, but it could be enough to knock down a building. What’s the chance that it hits a city? Still considering our back-of-the-envelope calculations, the probability remains extremely low. It’s about four chances in a million. If you are curious about the likelihood of it hitting a specific building, well we are talking about one in a trillion. But let’s make it more extreme.
What would happen if, like in disaster movie tradition, Tiangong-1 was going to land on a nuclear power station? There are 450 nuclear reactors in the world and most of them are outside the danger zone. Let’s assume that there are 150 potential targets just to give us a round number. The average size of a nuclear facility is roughly 20,000 square meters (215,00 square feet). To give us a bit more leeway on the danger, we can assume that any damage to the facility would cause the reactor to melt. Our scenario is extremely unlikely at about one chance in 200 million.
An object like Tiangong certainly poses a risk, but we need to be pragmatic when it comes to the danger. It is unlikely to be anything major, but it's not impossible. We cannot predict when it will come down or where, and we hope that the most extreme scenarios don’t come to pass.