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In March 2016 China's Manned Space Engineering Office announced that all Tiangong-1 telemetry had failed leaving no ability to safely control the space station's decent.

With what we know about the station, and the last known telemetry data, is it possible to calculate the station's orbital decay? Do we have any idea when Tiangong-1 will deorbit?

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This issue was discussed recently in SpaceDaily where it explains why this is a known unknown:

Right now, it's impossible to make an educated guess on even a rough "window" for Tiangong's return. The orbit is still high. Unpredictable factors such as solar activity will influence its orbital decay. We won't have an estimate until it starts to fall much lower.

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    $\begingroup$ To explain the problem further: we have pretty decent models of descent "in ideal conditions", but solar flares do cause massive loss of altitude (see the end of Skylab, when the final reboost was all for naught, the station deorbiting way earlier than expected due to unexpected strong solar activity); with the timescale expected there will be solar flares, but we can't foresee how many or how strong and that renders all theoretical calculations moot. We can put an upper limit on the time, but the Sun can shorten it arbitrarily. $\endgroup$
    – SF.
    Commented May 10, 2016 at 21:59
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    $\begingroup$ Skylab deorbited "way earlier" as in two to four years earlier. $\endgroup$
    – Mark
    Commented May 11, 2016 at 1:20
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Tiangong-1 will deorbit sometime in late 2017

As recently reported in Popular Mechanics The China National Space Agency recently announced that Tiangong-1 will deorbit sometime late in 2017, although they can't say exactly when or where the former space station will fall.

Source

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  • $\begingroup$ "Harvard astrophysicist Jonathan McDowell, among others, has noted that parts of the craft such as the rocket engines are probably too dense to be burnt up upon re-entry and may result in chunks of debris up 100 kg in weight falling to the Earth's surface, with little possibility of predicting where they may crash" Lovely... $\endgroup$
    – Ezra Bailey
    Commented Jul 19, 2017 at 18:43

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