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.
- Is it common, or at least within international norms to do this unannounced?
- 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!
- Are there other plausible explanations out there?
(script used for propagation in pastebin)