Here is a dynamic forecast of when Tiangong 1 is likely to be back to the Shenzhou operating height. For any particular date, the forecast is based on the decay rate remaining at the current level. The windows shown are for launch into a 14-day mission. The ones drawn in blue and marked "AM" lead to a landing in the morning (around 10:15) UTC. The ones in green ("PM") lead to landing in the evening (around 21:30) UTC.

In order to understand the plot, it is necessary to look at it in terms of the average position of the points plotted over the last 3-4 weeks and take into account the trend in the space weather index.

The right hand line shows the level of solar radiation at 10.7 centimetres wavelength as provided by NOAA. Although there is not a continuous connection between the shapes of the two curves, noticeable peaks and troughs in the weather plot are echoed 3-4 days later by the Tiangong forecast date. In order to see the relationship between Solar Flux and drag, the two date axes have been shifted relatively to take account of the lag between a change in Flux and the resulting effect in air density at orbital altitude.

Drag and the ensuing decay rate are also influenced by the cross-sectional area that Tiangong 1 presents to its direction of travel. On at least two occasions, Tiangong's orientation was changed to counteract short-term variations in upper air density.

2011 Nov 26 - Tiangong 1 had settled down to a steady rate of decay in its new, high, orbit established after Shenzhou 8 departed. It looked as though it would be back to a suitable height for further Shenzhou operations (340 km, 91.25 min period) during 2012 March. Assuming that controllers planned to minimise propellant use, it was reasonable to take it as an indication of when Shenzhou 9 was planned to be ready for launch.

2011 Dec 15 - Chinese news agencies announced that a change had been made in Tiangong 1's stabilised attitude. It resulted in a reduced rate of decay meaning that it would be an extra 4-6 weeks before it got down to the Shenzhou operating height. The change reduced the cross-section of Tiangong 1 as presented to its direction of motion, using it as a means of controlling the rate of orbital decay. It meant that it would be the mid-May at the earliest, before Tiangong 1 got back down to the Shenzhou operating height. December may be when China set a planning date for the Shenzhou 9 mission.

2012 Feb 15 - China announced that the launch is planned for June and will carry a biological payload including test animals. Tiangong's orientation was changed again in order to speed up orbital decay.

2012 Feb 16 - Xinhua, China's state-run news agency announced that Shenzhou 9 will be launched 2012 between June and August. It will carry a crew of three that will undertake a manually-controlled docking with Tiangong 1, open the hatches and go inside it. The announcement coincided with a change of orientation by Tiangong 1 that seemed to be aimed at bringing it down to operating height for the late May window.

2012 Mar 23 - A decision seems to have been made to move the Shenzhou 9 launch date from late May to mid-July. The orbit was raised, meaning that Tiangong 1 would have to make a significant propellant-wasting downward orbital adjustment in order to meet a Shenzhou launch in the earlier window.

The chart below of Tiangong 1's orbital period covers from 2011 Oct 1 until today. Once the period is back below 91.15 minutes period (the horizontal blue line), Tiangong 1 will be available for its orbit ground tracks to be set up for a new rendezvous mission.

The effect on the rate of decay of the December 15 orientation change is very obvious when the trace is compared with the dotted line that continues the original slope. There is more on the effects of the two orientation changes on zarya.info's Tiangong 1 web page