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Apparently ISS is so close to Earth's atmosphere that it requires periodical boosts to stay in orbit. If the station were completely abandoned and it wouldn't get the boosts anymore, how long would it take until it crashed on Earth? Would it burn completely in the atmosphere or would falling debris pose any danger if it fell on populated areas?

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  • $\begingroup$ possible duplicate of How often does ISS require re-boosting to higher orbit? $\endgroup$ – Rikki-Tikki-Tavi Jun 11 '15 at 13:07
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    $\begingroup$ Sorry, but I really don't think this is a duplicate. The other question answers how often a reboost is required. I'm asking how long it would take for the station to de-orbit without reboosts. $\endgroup$ – Moyli Jun 12 '15 at 11:41
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Looking at this recent data, which conveniently has a long period with no substantial reboosts, we see about a 3 km/month drop at around 410 km.

ISS decay

Using a standard atmosphere model, I backed out a ballistic coefficient of $165\,\mathrm{kg/m^2}$. Running the current altitude to decay, I get that we would have a deep sea space station in about 15 months. I think they recently raised the average altitude. It used to more like 350 km. From there, the lifetime would be more like six months.

Here is a plot of the decay:

altitude vs. time

However your mileage may vary. This assumes that ISS maintains its roughly edge-on attitude which gives that high ballistic coefficient. If ISS is in a different attitude, or if it's tumbling, its ballistic coefficient could be much lower, and so it would come down much faster. Also, the atmosphere is never "standard". We could have solar activity that heats and blooms the upper atmosphere, increasing the density and again bringing down the station much faster. Furthermore, the atmosphere in December and January was probably not "standard", so my backed-out ballistic coefficient is only approximate.

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    $\begingroup$ How long depends on altitude, attitude, and atmospheric density. That rate of 3 km/month pertains to late 2014, the peak of solar cycle 24. The upper atmosphere will presumably be considerably less dense in 2019-2021, when the Sun should be at solar min (and maybe longer according to some solar scientists). $\endgroup$ – David Hammen Jun 12 '15 at 20:31
  • $\begingroup$ I'm sure someone there in Houston is calculating this all the time. Do you have a current number for the predicted lifetime with no maneuvers? $\endgroup$ – Mark Adler Jun 12 '15 at 20:36
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According to this article, the ISS has an orbital decay of 100 metres every 24 hours, which means around a kilometre every ten days. In it's stable orbit, there is a difference of about ten kilometres between the highest and lowest points in it's orbit.

You can see, the decay is not hugely significant, and this is why boosts are usually performed months apart. However if boosts were to stop altogether, the ISS would fall lower faster as time went on, because the atmosphere would thicken as it fell.

As for the second part of your question, we already have information about station de-orbiting from the Mir station. Indeed, debris did survive the reentry, and splashed down in the Pacific Ocean. This is why the eventual de-orbiting of the ISS will be planned and controlled so that the debris field can be far away from populated areas.

Further Reading:

Does the orbit of the ISS decay?

How often does ISS require re-boosting to higher orbit?

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