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I cited objects 41332 and 41333 launched in early 2016 as counterexamples to this answer on my highly down voted and closed but otherwise "perfect" question What regulations, agreements, or other forces can help mitigate “PrankSats”?

Answers to When (roughly) will North Korea's Kwangmyongsong-4 satellite re-enter? range from

no more than 5 years, 2-3 being more likely.

to

in about 25 years. Depending on how much drag it produces, it could be longer.

Question: After almost 4 years, how has their altitude decreased so far? Are they loosing it at a rate expected for dead spacecraft of their estimated sizes and shapes?

Will they reenter in time to be the "Christmas present" mentioned in recent news?


  • 41332, 2016-009A: Wikipedia, GSFC
  • 41333, 2016-009B: n2yo says "Rocket Body". Strangely I couldn't confirm n2yo's TLE in Celestrak
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  • $\begingroup$ The title was almost North Korean objects 41332 and 41333; Where are they now? but that would have been too "clever" to be an effective SE question titile. ;-) $\endgroup$ – uhoh Dec 24 '19 at 23:33
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    $\begingroup$ I post a comment because it's not a real answer. If you take a look here: cristianopi.altervista.org/as/ele_graphs-regre.html and select KMS 4 (ID 41332), you'll see a purely natural decaying satellite (no human intervention). It's currently loosing about 7/8 m/day in the semi-major axis. If you are interested in a reentry prediction, let me know. $\endgroup$ – Cristiano Dec 25 '19 at 11:02
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Here’s what I get from all the TLEs currently available at www.space-track.org processed with the SGP4 library freely downloadable from the same site.


Definitions

T: orbital period.

1-orbit mean radius vector: numerically integrated radius vector against the eccentric anomaly (it’s the semi-major axis). The integration starts from –T/2 and ends to T/2 before and after the TLE epoch (1 orbit). Not to be confused with the osculating semi-major axis.

1-orbit minimum radius vector: find the smallest radius vector from –T/2 to T/2 before and after the TLE epoch (1 orbit). Not to be confused with the osculating perigee.

1-orbit mean air density: numerically integrated air density against the time divided by T. The integration starts from –T/2 and ends to T/2 before and after the TLE epoch. The air density is calculated at the satellite position with the NRLMSISE-00 atmosphere model updated with the solar and geomagnetic indices in the file www.celestrak.com/spacedata/SW-All.txt.

For the graphs that show the radius vector, the vertical axis is scaled to a sphere with a radius of 6371 km, just to show an approximate altitude.


The following graph shows the mean radius vector of both the objects:

enter image description here

We see two strange things: 1) the blue plot shows an higher decay rate when the satellite is flying higher; 2) the orange plot shows something similar to a small reboost (it seems strange to me because a decaying rocket body should be a non-maneuvering object).

With the help of the following graph, we see that the blue plot is not strange:

enter image description here

The air density is clearly very high on the left side. That seems sufficient to me to explain the higher decay rate.

Now consider the UNHA 3 R/B plot:

enter image description here

and its detailed view:

enter image description here

The radius vector starts to increase at the air density peaks, but I guess that a 250-meter increase in mean radius vector cannot be explained by just a lift effect. Moreover, if we look at the minimum radius vector, we see a much bigger increase: about 2.1 km:

enter image description here

After almost 4 years, how has their altitude decreased so far?

Do you really mean “altitude”? If the radius vector is good for you, the values are readily available from the graphs (if you are interested in my excel file, I can upload it somewhere).

Are they loosing it at a rate expected for dead spacecraft of their estimated sizes and shapes?

KMS 4: yes; UNHA 3 R/B: no.

Will they reenter in time to be the "Christmas present" mentioned in recent news?

KMS 4: probably Christmas 2026 (from my simulations).

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    $\begingroup$ Very thorough answer, thank you very much! The rocket body's behavior around January 2018 is really curious. $\endgroup$ – uhoh Jan 3 at 1:56
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    $\begingroup$ @uhoh is there a possibility of some medium being slowly vented (from March 2018 till April 2018) and therefore providing thrust which resulted in this anomalous behaviour? $\endgroup$ – Sergiy Lenzion Jan 3 at 2:26
  • $\begingroup$ @LeoS that would require the venting to be preferentially in the retrograde direction in order to produce a net prograde thrust, which I suppose is possible. These are slightly related; they are about intentional vents and who knows what's going on with the NK rocket body! What is “propulsive passivation” and why will the SpaceX STP-2 mission do it? and What is a non-propulsive vent? $\endgroup$ – uhoh Jan 3 at 2:54
  • $\begingroup$ I found this page: sattrackcam.blogspot.com/2016/02/… that show a flash pattern of the UNHA 3 third stage (it is tumbling), but it's 2 years before the strange "reboost". $\endgroup$ – Cristiano Jan 4 at 11:56
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41332 is staying pretty steady, although it is currently listed as "operational" in the databases I'm seeing. It is getting to be a bit lower, although I wouldn't expect it to reenter anytime soon, maybe 3-4 years left (I guess my original prediction was wrong...). Thus far the altitude has gone down about 20 km in 4 years, so 5 km/ year, although the rate seems to be increasing.

enter image description here

41333, however, is a spent booster, and obviously not operational at all. The average perigee right now seems to be around 420 km. It seems likely it has maybe another few years tops. From the plot the altitude hasn't gone down a huge amount, maybe 15ish km, so say 4 km/year.

enter image description here

Neither object is predicted by C-SPOC (Space Track) to reenter within the next 2 months.

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  • $\begingroup$ Snazzy plots, thanks! How curious that '32 is maintaining its low but non-zero average eccentricity as it slowly drops, while '33 is sort-of circularizing. $\endgroup$ – uhoh Dec 25 '19 at 13:07
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    $\begingroup$ If you think about it, an object should circularize. The most drag happens at periapsis, and thus the largest decrease in altitude will happen at the opposite part of orbit. $\endgroup$ – PearsonArtPhoto Dec 25 '19 at 13:12
  • $\begingroup$ I did think about it a while ago, but that was for an extremely fast case. Thinking further, yes you're right, at any reasonable altitude, even 400+ km, the density is still varying exponentially, so yeah, in cases where there's a big BSTAR it can tend to circularize even that high up. $\endgroup$ – uhoh Dec 25 '19 at 13:18
  • $\begingroup$ What's the reason for those secondary order oscillations, and why are they symmetrical to the max/min points? Is it measurement/processing noise or is it real orbital elements behaviour? $\endgroup$ – Sergiy Lenzion Dec 26 '19 at 2:12
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    $\begingroup$ @LeoS: Great question. Short is the magic of orbital dynamics. For a better answer, you might ask a question directly, I don't fully understand myself the change in the apogee/ perigee as shown on these plots. $\endgroup$ – PearsonArtPhoto Dec 26 '19 at 12:40

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