After Shenzhou 8 departed in November 2011, controllers raised the orbit of Tiangong 1 to preserve it from potential re-entry brought about by the air drag that causes all low orbits to decay.

In order to conserve propellant, they chose an altitude (about 375 kilometres) that would use the same air drag to bring the orbit down to the standard 335 kilometre Shenzhou operating height for the spring of 2012.

Originally, China seemed to be aiming for a Shenzhou 9 launch sometime around May. Then, mid-March, a decision appears to have been made to put off the launch to June/July.

At the same time, Shenzhou 10 disappeared from China's 2012 launch forecasts, a sure indication that the change was an operational decision with a knock-on effect, and not simply vagueness in what China had been broadcasting through its news agencies. Consequently Tiangong's orbit was raised slightly to delay arrival at the critical altitude.

There did not seem to be a firm date in mind because Tiangong 1 was heading to meet the target orbit between two 8-10 day long launch windows that open mid-June and mid-July respectively. It left China space-watchers uncertain when to expect the actual launch.

During May, Shenzhou 9's CZ-2F launch vehicle was moved from the manufacturing plant near Beijing to the Jiuquan launch site in time to meet the earlier of the two windows.

Controllers continued using Tiangong 1's orientation for fine control of its rate of descent by varying the cross sectional area that was pushing through the thin traces of atmosphere at orbital height.

It kept the critical date part-way between the two windows. May 26, China's intention became clear when Tiangong 1's apogee was reduced by about 10 kilometres, ensuring that it will get down to Shenzhou's operating height just before the mid-June window.

Currently, the window opens about June 17 so a late-morning launch (UTC) will allow Shenzhou 9 and its crew of three to make rendezvous with Tiangong 1.

For a Shenzhou mission, it is the relationship between the orbit and the Sun on the day of landing that determines when the launch will occur. Once a potential window for the landing is calculated, the launch is set the appropriate number of days earlier.

In this case Shenzhou 9 seems to be aimed at a mission lasting just under two weeks, leading to June 17 as the most likely (but not certain) date for its launch.

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