The answer is known only to the planners and leaders of China's space programme. However, with eight missions under the belt of China's piloted spacecraft, some clues have come to light from those earlier flights that point to what China takes into consideration when planning its launch dates and times.

The controlling event is the landing at the end of the flight. In the run up to retro fire there are constraints relating to an acceptable direction and angle of solar illumination. They dictate what time of day Shenzhou will actually touch down. With the exception of Shenzhou 8, times of day for lift off for all Shenzhou missions were determined simply by the number of days they were to fly.

Once Shenzhou is in orbit there is a constant westwards drift of its orbit plane relative to the Sun. It takes about 51 days to complete one cycle and get back to where it started. This moves the orbit into and out of the required alignment twice per cycle.

Once planners decided how long a mission will last, then the time of lift off on launch day is set so that the correct solar alignment occurs on the last day of the mission.

With Shenzhou 8 there was an additional constraint as the mission plan required it to make rendezvous with Tiangong 1 which was already in orbit. On any particular day the launch time was going to be dictated by when the plane of Tiangong 1's orbit was carried past the Jiuquan launch site by the Earth's rotation. Like Shenzhou, the Tiangong orbit experiences the constant westward drift.

It creates regularly repeating, and distinct, periods of time when a rendezvous mission can be run and permit re-entry under the right lighting conditions. It has the added advantage for space watchers of making it possible to calculate when those conditions will occur on the Tiangong orbit and hence be able to predict the dates of rendezvous launch windows.

Shenzhou 9 is also going to be aimed at Tiangong 1 so it has been possible to work out when launch widows will occur. There are other features of mission preparations that now allow the actual launch to be narrowed down to two specific sets of dates.

At the beginning of the year China said Shenzhou 9 would be launched in the period June-August so already, the range of opportunities was limited. Two further things have happened that give pointers to possible dates. First is the slow decay of Tiangong 1's orbit since it was boosted following the Shenzhou 8 mission.

Initially, it looked as though it would be back down to the right height for a Shenzhou rendezvous launch late-May with the mission running into June. Then on March 23, the orbit was boosted slightly and it had the effect of pushing back the critical date to July. It may be that planners made a decision to delay launch.

China has been fine-tuning Tiangong 1's orbital decay by changing it's orientation periodically to vary the cross section that is experiencing air drag. Currently, Tiangong 1's orbit will now be down to rendezvous height about July 1 which lies between two launch windows: June 17 - June 25 and July 9 - July 17. A small orbit manoeuvre can bring either set of dates into play and set up a rendezvous.

China watchers have noted transport of the launch vehicle and Shenzhou 9 to Jiuquan, and have made comparisons of the timescales with previous launches. They point to the possibility of launch during the June 17 window.

The dates in this article are calculated on the basis of thirteen days in orbit, a figure mentioned in passing by China. The dates for alternative mission lengths can be found here at the zarya.info.

Robert Christy has been analysing and documenting space events since the early 1960s and currently provides informtion via his web site