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.

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 manual docking with Tiangong 1, open the hatches and go inside it.

Below is a dynamic forecast of when Tiangong 1 is likely to be back to the Shenzhou operating height ready for a potential rendezvous mission in June. The chart covers the current eight week period and updates automatically as new orbit information becomes available. February 18, as an experiment, the plot was amended to use 91.15 minutes as the target rather than 91.25 minutes that was used previously.

The reason the date oscillates is variation in the rate of decay, which is controlled by two things. First is air drag and second is the cross-sectional area that Tiangong 1 presents to its direction of travel. One 'jump' at least is due to a change in the latter.

The smaller, slow, periodic variations are down to air density changes at orbital altitude. They are driven mainly by the solar wind and solar radiation. The right hand plot 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 dips in the 10.7 cm index (eg Dec 16, Jan 11, Jan 28 and Feb 1) are followed 3-4 days later by the Tiangong forecast date moving a few days later. In order to see the relationship between Solar Flux and drag, the two date axes have been shifted relatively by four days. It takes account of the lag between a change in Flux and the resulting increase in air density at orbital altitude.

There is also a long term element of variation due to the rotation of the Line of Apsides periodically dipping perigee deeper into the upper air layers where the atmosphere is more dense.

The chart below of Tiangong 1's orbital period covers from 2011 Oct 1 until today. Once the period is back below 91.25 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 now 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'sTiangong 1 web page