After extensive checkout, it undertook a rendezvous and docking test mission with the unpiloted Shenzhou 8 which was not carrying a crew. Later, both missions aimed for 2012, Shenzhou 9 and Shenzhou 10 will conduct further tests, and crew members will enter Tiangong.

As of 2011 mid-December, China was still talking of Shenzhou 9 being orbited without a crew. Mid-February, Xinhua let it be known that Shenzhou 9 will be launched during 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.

If Shenzhou 9 is launched as late as August, ten months after its predecessor, it raises the possibility of Shenzhou 10 not being launched until 2013. For China it should not present a problem as it will still be within the two year design life of Tiangong 1.

The chart below is an extract of Tiangong's orbital period from the Twoline Orbital Elements sets issued by Spacetrack. It plots the anomalistic period, or the time it takes to complete one circuit of the orbital ellipse - from perigee to perigee, for example. Some satellite and launch listings give the nodal period which takes account of the fact the ellipse rotates in the plane of the orbit measuring, for example, the time between successive northbound equator crossings. In Tiangong's case, perigee moves forward round the orbit (see the chart below showing argument of perigee) so the nodal period is about 0.1 minute less than the anomalistic period.

Tiangong was injected initially into a slighlty elliptical orbit then it was changed to near-circular.

Apogee and perigee relate to the Twoline Orbital Elements model where the orbit is expressed as an ellipse with the Earth's geographical centre sitting at one focus. The heights are then calculated with respect to the Earth's mean equatorial radius of 6378.145 kilometres. It is an idealised representation and ignores variations in the Earth's shape and the gravitational effect of the Earth's centre of mass being south of the equator. Differing figures published elsewhere may be the same orbit expressed through a different geographical and geometric model.

Perigee is traced in blue and apogee in red.

Argument of perigee can quite sensitive to the effects of orbital manoeuvres but it depends on the magnitude of the change and whether is occurs at one of the orbital apses (apogee or perigee). When nothing is happening, the argument of perigee increases slowly. An orbit change can to produce a sudden change in argument of perigee. It immediately settles down again to the slow drift.

Eccentricity is very sensitive to small changes in orbit parameters. Its tendency is to reduce because air drag causes apogee to decrease at a greater rate than that of perigee unless they are very close to each other in value. Use of thrusters makes eccentricity change suddenly and noticeably, but does not effect apogee and perigee measurements - see the small 'jump' early Oct 10 followed by erratic behaviour. When Tiangong arrived in orbit, the eccentricity was about 0.01. It reduced considerably after the circularisation manoeuvre raised perigee.

The cyclic nature of the changes from the end of 2012 November is due to rotation of the Line of Apsides. The rotation cycle has a period of about 50 days and is driven by variation in the real heights of apogee and perigee above the Geoid. They can differ significantly fom the idealised figures produced by the Twoline Orbital Elements model, and are dependent on the argument of perigee.

Within a Twoline Orbital Elements set is a parameter called 'ndot2'. It is a measure of the rate at which an orbit is shrinking. For a low orbit, it is almost exclusively controlled by air drag.

After departure of Shenzhou 8, the orbit of Tiangong 1 was raised to a height where air drag would have brought it down to the rendezvous height for Shenzhou 9 by 2012 mid-March. The value of ndot2 was averaging at about .00041.

China then announced that, on December 15, Tiangong 1 had changed its stabilisation attitude. There was no explanation as to why. Tiangong had actually reduced the cross-sectional area that it was presenting towards its direction of motion. It resulted in ndot2 reducing to an average of about .00034, and pushed back the potential date for reaching the Shenzhou operating height into May.

2012 Jan 12 there appeared to be another drag-reducing manoeuvre that seemed to reverse after about 12 days. Further analysis shows that this particular event was due to the Sun entering a slighly less energetic period. The solar wind reduced and it led to the upper atmosphere becoming less dense for a while. Affectively, the top of the atmosphere fell back to a lower height and drag at Tiangong 1's altitude was thereby reduced.

There is a chart showing how the changes in Tiangong 1's orientation are regulating the rate at which the orbit is reducing in order to get it down to Shenzhou's operating height. It also illustrates the influence of the Sun.

Charts on this page are produced using JpGraph.