North Korea recently launched Kwangmyongsong-4 but it wasn't entirely successful and is tumbling out of control. I read on a Wired article that “This thing is probably a failed attempt to get higher up, and is going to re-enter soon.”

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How soon is soon? Does anyone know when this satellite is likely to re-enter?


No one can say for sure, but the appropriate ISO standard can be used to make an estimate:

Space systems — Estimation of orbit lifetime (ISO 27852) (see page 20)

Figure B5 says that an object in polar orbit with about 500 km perigee (taken from your picture) will reenter in about 25 years. Depending on how much drag it produces, it could be longer.

If you want to put in some effort to get a more precise answer, try UTIAS Review 43. It tells you that at 500 km altitude, the free path length of a molecule is 77km, so it's not so much a gas, but rather some few stray molecules. From this alone, we can see that it won't reenter "soon". To make a calculation, you would need to estimate the area of the surfaces exposed to the orbital direction and the mass. The review specifically talks about spinning satellites. If you can't make a good estimate, you could try to get the orbital parameters as precisely as possible now, and then again in a year and modify the drag coefficient to fit the change. With this you can make a better guess at the orbital lifetime.

If you do this, please tell us what you found! If you post it as a comment, I will include it in the answer.


The answer Rikki-Tikki-Tavi provided is right, with one small exception. The periapsis, not the apoasis, is the key to using such charts for reentry purposes. The 466 km periapsis should be used. Using the same source, I would estimate the lifetime to be no more than 5 years, 2-3 being more likely. I suspect the satellite was a bit low, it certain was compared to the previous satellite North Korea launched, and would also be consistent with it's tumbling.

  • $\begingroup$ At no point did I say anything about the apogee, or would I have. I simply did not mean to make an unwarranted impression of precision, so I did not include more than one decimal place. The ISO norm still predicts "can be 25 years" in any case. $\endgroup$ – Rikki-Tikki-Tavi Feb 15 '16 at 13:06

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