Even as Shenzhou 9 undocked from Tiangong 1, and before it returned to Earth, the Beijing Aerospace Control Centre was starting preparations for the Shenzhou 10 mission. Within a few hours of departure, Tiangong 1 had fired its thrusters to trigger a move to a higher orbit while awaits the next visitor. The move echoed what had happened after Shenzhou 8's departure late-2011.
Tiangong was being 'parked' in an orbit with a height that would ensure that the small amount of air drag it experiences will bring it down to Shenzhou operating altitude about the time China has in mind for the next docking mission.
This time, Tiangong did not go as high as it did after Shenzhou 8, indicating that the aim is for a shorter time between launches. The wait for Shenzhou 9 started at 375 kilometres altitude, for Shenzhou 10 it is 360 kilometres. Tiangong will be back to rendezvous altitude early in December this year.
When in December?
The time of day at which Shenzhou can land safely is controlled by the direction from which it sees the Sun on the last day in orbit. It varies with the Earth's seasons and moves earlier and later as the year progresses. (see main story image)
There are two short periods on any particular day where conditions are just right.
For the Shenzhou 1 through 7 missions a launch could have been undertaken on any day, with lift-off being set by the time the ground track would be passing through the landing area on the mission's final day.
With Shenzhou 8 and 9, where rendezvous with Tiangong 1 was involved, launch had to be at a time when the chasing spacecraft could get into the space laboratory's orbit plane and still meet the on-orbit solar lighting requirement on the last day in space. For a given mission length. The two requirements lead to a pair of five-day windows recurring every couple of months, and bring about a certain level of predictability.
For a thirteen day mission, like Shenzhou 9, the end of year windows for 2012 run November 27 through December 1 and December 12 though December 16. For a 21 day flight, equivalent dates are November 19 through November 24 and December 5 through December 9. Whatever the mission duration, the dynamics of the standard Shenzhou operational orbit dictate that it will be an odd number of days, barring in-space emergencies.
As Tiangong's orbit decays, a small re-adjustment can steer it towards either window of the pair. At present the date is NET, "no earlier than", in NASA parlance. If China decides to run the mission later then a small upward adjustment in the Tiangong orbit will set it up for the next pair of launch windows that open late-January 2013.
China's Candour
In some respects, China has been very open in dealing with the two Tiangong docking missions. Major events were covered briefly, but reasonably well, by both internal and external news services, and there was extensive real-time broadcasting on TV and via webcasts.
In the UK, the local free-to-air satellite-based TV service carried live programmes in English on the CCTV news channel. Mission events were described in detail, as they happened, by knowledgable experts in the studio. Video from orbit was shown as it came in.
In contrast, China tends to be less open in the run-up to a mission. Some events like transport of the spacecraft and launcher from the Beijing factories to the Jiuquan launch site are reported, but mission details like duration and crew names are officially held back until the last moment. Consequently, they tend to arrive in advance by 'leak' and have to be treated with caution.
Right up to the time the Shenzhou 9 crew was presented to the world, some 'experts' claiming to know the names for certain were reporting a different group of people. They then seemed surprised when crew selection was made on practical grounds rather than in accordance with bureaucratic rules.
With Shenzhou 10, some information may turn up that will give away the mission length, and there will be other clues. A major one will be when the Long March launch vehicle is moved from Beijing. On arrival at Jiuquan, it will be assembled to a project plan that has already been used for earlier missions so there will be a fair degree of certainty as to when it will be ready, and hence the likely launch date.
As December approaches, it will become more obvious whether one of that particular pair of windows will actually be used, and there will be indications of which one. Release of the launch date or mission duration will point to the detailed timetable.
The Tiangong 1 orbit is very predictable provided the vehicle makes no significant manoeuvres, even down to the level of being able to say that a 13 day flight in the later of the two windows is likely to be aimed for launch on December 12 at 22:03 UTC (give or take five minutes).
