enter image description here

The graph shows the ISS semi-major axis.

EDIT: the blue plot represents the ISS mean radius vector minus 6371 km (just to show an approximate altitude), which is equivalent to the sami-major axis (I use "mean radius vector" to avoid confusion with the osculating semi-major axis). The mean radius vector is obtained from the TLEs downloaded from https://www.space-track.org/ and all the calculations are done with the CSpOC's SGP4 library downloaded from the same site.

There are no reboosts for an unusually long period of time and according to https://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html there will be no reboosts at least for the next 10 days.
Does anyone know the reason?

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    $\begingroup$ possibly related to rendezvous activity in the next few days; Cygnus with ~3.4 tons of goodies? $\endgroup$ – uhoh Nov 17 '18 at 12:49
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    $\begingroup$ I've just asked What are the causes of these episodes of faster than average altitude loss by the ISS? By the way, are you able to mention the original source for the plot? $\endgroup$ – uhoh Nov 18 '18 at 5:55
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    $\begingroup$ Sure, it's me! :-) I use TLEs + CSpOC's SGP4 library. $\endgroup$ – Cristiano Nov 18 '18 at 10:21
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    $\begingroup$ or related to Progress 71 w/ another 2.6 tons $\endgroup$ – uhoh Nov 19 '18 at 3:41

This question is asking for an official answer, and that would take someone who is official to answer. I can at best hazard some guesses.

One reason is that the Sun is approaching the end of what was a low activity solar cycle. We can look forward to several months, minimum, of very low solar activity. Several months of inactivity is the norm for the lull between solar cycles. The prognostications are that solar cycle 25 will be even weaker than the now ending solar cycle 24. If that's the case, the very low activity currently being seen may be the norm for the next few years, perhaps longer. The quiescent period between solar cycles 23 and 24 was abnormally long, October 2005 to November 2009. A four year lull in solar activity or longer might well be what we're about to see with the upcoming transition.

Given the low solar activity, there's no particular reason to maintain the ISS at a high altitude (high for the ISS, that is). ISS altitude was kept considerably lower than it is now back when the Shuttle was operational. This enabled the Shuttle to lift huge amounts of mass to the ISS.

Another possible reason is the upcoming SpaceX Dragon launch, which has slipped from November 16 to November 27 to November 29 to December 4.

  • $\begingroup$ I'm more inclined for the latter, including @uhoh's comments about Cygnus and Progress 71. Thank you. $\endgroup$ – Cristiano Nov 19 '18 at 16:10

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