Trying to find a pattern in mission launch times for Shenzhou 1 to 8 produces a rather scattered plot of data points. However, looking at the time of day of landing results in a somewhat clearer picture.

Landing times divide into two groups, one between 08:50 and 11:40 UTC, and the other between 20:30 and 22:30 UTC. They equate to about 17:00-19:30 and 03:40-06:00 Apparent Solar time across the Shenzhou landing zone in Inner Mongolia. All missions of 7+ days duration have returned into the 08:50 UTC slot with most of the shorter ones hitting the 20:30 UTC window. The exception is Shenzhou 7, a three-day piloted mission that returned in the earlier slot.

There is a list of Shenzhou mission timings and orbits here (opens in a new tab/window).

Indication from China is that the Shenzhou 9 mission is planned to last for 13-14 days including 9-10 days docked with Tiangong 1. The analysis of previous flights pointed to it landing in the earlier window, ie - 10:15 UTC ±85 minutes.

For a 13 day mission, Shenzhou 9 seems to be planned for launch at 21:00 UTC (±85 minutes) on the chosen day in order to bring it back over the landing zone at the correct time on the final day of the flight. The chosen day, in turn, is defined by the date on which Tiangong's orbit plane is in position for a rendezvous launch within the planned time window.

Looking at the Tiangong 1 orbit, the desired alignment occurs on three dates around the June-August period announced by China as when the Shenzhou 9 mission will run - the first two are May 24 and July 14, give or take a day. Factoring in the ±85 minutes, it suggests launch in either of the periods May 20 - May 28, or July 9 - July 17. A third launch window runs August 29 - September 6.

If the planned duration of the Shenzhou 9 mission is longer or shorter than 13 days, it shifts the launch earlier by one day, and time of day later by 28 minutes, for each day of extension - or the converse for a shorter flight.

Shenzhou 9 is actually aimed to return to Earth in the second, later in the day (UTC), Earth return window. This window has hitherto been generally reserved for the shorter-duration (less than seven days) Shenzhou flights. The potential launch periods for a thitrteen day mission are Jun 14 - Jun 23 and August 6 - August 14.

For varying mission lengths, windows run as in the table below.

NOTE - June 10, China let it be known that is was aiming for a launch no earlier than June 16, suggesting the mission is aimed at 10-14 days duration.

From data issued by SpaceTrack, it is possible to maintain an ongoing view of when Tiangong 1's orbit is likely to have decayed to a height suitable for operations with Shenzhou. Once it has reached that point, the a small adjustment will allow the ground track to be set up and stabilised in an appropriate location for a rendezvous launch.

The plot assumes that a period of 91.15 minutes is the point at which a phasing manoeuvre will occur. It may not be absolutely the right choice but the answer it produces in not likely to be more than a week out. Shenzhou's operating orbit is about 330 x 338 km, 91.2 minutes period.

Until March 23, it seemed Tiangong 1 was going to reach Shenzhou operational height around the beginning of June.

At that point, the orbit was raised by about five kilometres, delaying arrival at operating height until early July and giving an opportunity for a small orbit adjustment to push it towards either window.

A further adjustment May 26 lowered the orbit slightly and set it up for a launch during the mid-June window.

Tony Quine has been analysing the potential launch date from the point of view of flight hardware arriving at Jiuquan, and his data pointed to launch in the June window. The orbit adjustment by Tiangong 1 on May 26 confirmed Tony's analysis.

Based on orbital data from Jun 6, the launch time for a mission with lift-off on June 14 works out to be 11:35 UTC ±3 minutes. The orbit adjustment May 26 set up the Tiangong 1 orbit so that it offers launch possibilities on alternate days.

The window for a thirteen day mission now runs June 14 - June 23. The times below cover the whole period but on June 10, China indicated launch no earlier than June 16: