Some news in various outlets following some twitterings including this tweet by Jonathan McDowell about Tiangong-2 performing a sizeable orbit-lowering, followed by a re-raising: ‏

OK now that's weird. New orbit data for Tiangong 2 shows it back in the 390 km orbit after spending 10 days in the lower 295 km orbit. Wonder what that was about??

Sure enough, I propagated TLEs from 2018 (using standard SGP4 within Skyfield) and found a substantial drop and then raising shown below. Interestingly, there are a couple of wiggles at the bottom, and the raising is differently-shaped (slower towards the end) than the lowering.

While other's have seen what looks like eccentricity noise in Tiangong-1's propagated orbit from TLEs, these wiggles near 320 km look quite real since it's the semimajor axis and therefore period that is wiggling, not just eccentricity.

Having a spacecraft make such a large, apparently unannounced orbital maneuver in LEO is kind-of a big thing.

  1. Is it common, or at least within international norms to do this unannounced?
  2. Does "giving the engines a thorough test" make sense as a potential explanation? As one of the other tweets in the thread mentions, this is quite a lot of fuel!
  3. Are there other plausible explanations out there?

(script used for propagation in pastebin)

Tiangong-2 peri apo June 2018

Tiangong-2 peri apo 2018 twittering

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    $\begingroup$ Let's don't forget that Tiangong-1 and -2 are testbeds for future chinese space station en.wikipedia.org/wiki/Chinese_large_modular_space_station. So if Tiangong-2 has enough propellant a testing would be good - for engines technical performance evaluation as well as for orbit-rising maneuvers training. $\endgroup$ – Heopps Jun 27 '18 at 8:55
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    $\begingroup$ @Cristiano sorry, yes I used the standard SGP4 propagator within Skyfield and I've added the link in the question as well. Here is a quick link to it directly: github.com/brandon-rhodes/python-sgp4 and to Skyfield github.com/skyfielders/python-skyfield $\endgroup$ – uhoh Jun 27 '18 at 12:45
  • $\begingroup$ @uhoh I'm just trying to understand what you plotted, because your graph looks strange to me. Assuming that 1 dot is 1 TLE, it's simply impossible to see those dots during the reboost on day 175. We should see something like this: cristianopi.altervista.org/TIA2_DeRe.png $\endgroup$ – Cristiano Jun 27 '18 at 15:48
  • $\begingroup$ I have nothing to ask. You asked: "What is Tiangong-2 doing?". I answered that if you plot the correct graph, you'll see that they fired the engines during the low orbit phase, but with your wrong graph you cannot see anything. That's all. $\endgroup$ – Cristiano Jun 27 '18 at 16:15

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