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DRAGON SPACE
Tiangong 1 Parked And Waiting As Shenzhou 10 Mission Prep Continues
by Robert Christy FBIS,
Scarborough, UK (SPX) Nov 05, 2012


illustration only

Following the July-August period when controllers used Tiangong 1's thrusters for strict control of the orbit decay rate, things went quiet and Tiangong 1 settled down to a regime of orbit decay dictated by solar activity. As the solar wind increased and heated up the Earth's atmosphere, decay increased and it then slowed down again as the solar wind reduced.

Early October, small inconsistencies in orbit data coming from SpaceTrack indicated that thruster firings were occurring but this time the effects were consistent with attitude control changes. Thrusters were being used to change Tiangong's orientation but the firings were also making subtle changes to the orbit.

After Shenzhou 9, Tiangong was 'parked' in a 42 degrees .8 inclined, 355 x 366 kilometre orbit. By October 15 it was down to 346 x 353 kilometres. A major two-impulse thuster firing on that day raised it to 367 x 374 kilometres. It was only two kilometres below the highest orbit it had ever achieved back in 2011 November following the post Shenzhou 8 boost.

For a few days thrusters were again used to control the decay but on October 20, it was left to its own devices.

Waiting for Shenzhou 10
Once the orbit had settled, it once more became possible to work out when orbit decay will bring Tiangong 1 down to Shenzhou's operating height around 330 kilometres. It will now reach that point mid-late March in 2013. Any future Tiangong 1 orbit adjustments will change this.

Orbital manoeuvres after Shenzhou 9 departed at the end of June had indicated various, but changing, dates through the winter of 2012-2013 when Tiangong 1 might be available for a Shenzhou rendezvous.

In hindsight, a piloted launch in that period is unlikely. Dates used so far for its piloted missions indicate that China probably has a preference for them to run between late-Spring and early-Autumn.

It avoids returning to Earth during a period of harsh winter weather. It is reminiscent of the pre-Salyut-6 years of the Soviet piloted programme when there was a very obvious preferred 'shooting season' running from April to October.

As yet, the earliest in the year that a piloted Tiangong mission has been flown is Shenzhou 9 during June. Shenzhou 5, 6 and 7 all flew in September or October. There may yet be another Tiangong 1 orbit raising to push the potential Shenzhou date nearer to the summer.

Shenzhou 10 Mission
Shenzhou 9 seems to have accomplished every task set for it with the initial Tiangong 1 docking, occupation of the space lab and a second, manually-controlled docking.

The only public moment that caused anxiety among its watchers was the end-over-end 'nose flip' of the Descent Module when it hit the ground at mission end.

Apart from from providing more astronauts with docking and space station experience, there seems little that can be added.

Shenzhou does not have the capability to bring significant new equipment to Tiangong and China has hinted that the mission duration will not be significantly longer than Shenzhou 9.

One thing that has surfaced is a report through the UK's Flight Global web site that Shenzhou 10 might try a different approach trajectory and may perform a fly-round of Tiangong.

Both the automatic Shenzhou 8 and piloted Shenzhou 9 joined Tiangong 1 by getting in front of it, matching its orbit and then approaching along the flight path.

The vehicles were aligned with their long axes parallel to the ground and pointing exactly along the flight trajectory.

Flight Global's report indicates that Shenzhou 10 will try an approach where it is placed in an elliptical orbit with apogee at the Tiangong 1 height.

This will have the effect of making it approach Tiangong 1 from below and behind as the two orbit paths converge.

It is the approach previously used by the Space Shuttle when visiting Mir and the ISS. It is one of many alternatives, requiring different computations and new thruster firing patterns for Shenzhou.

Peoples Daily - Nov 1
Towards the end of October, several Chinese news items appeared around the same time looking forward to Shenzhou 10 sometime in 2013.

The crew will be mainly based around flight experienced astronauts and may well include a female astronaut. One intriguing statement in a Peoples Daily English language report said "After the Shenzhou 10 mission, a second module for the Tiangong space station will be sent into orbit...".

This may be ambiguous translation by the Peoples Daily but it sets the hare running over whether Tiangong 2 will actually be configured with two docking ports so it can act as an extension to Tiangong 1.

Previous mentions of Tiangong 2 have it being a similar design to Tiangong 1 and being a new, independent, vehicle in the series.

As to Tiangong 1 operating beyond its two year expected life, China has said that it expects bacterial contamination from human habitation to make it unusable eventually.

Unlike the ISS, it does not have the complex life support system required to keep contamination permanently down to acceptable levels. Joining it to a 'clean' Tiangong 2 poses an immediate risk to the newer vehicle.

Briz-M Explosion
October 16, the Briz-M upper stage from a failed Proton launch earlier in the year exploded and put a cloud of debris into a 50 degrees inclined, 260 x 1500 kilometre orbit.

At the time, it was noted that the cloud is a threat to the International Space Station. It is also a threat to Tiangong 1.

Both stations are in orbits that intersect the cloud's orbit and orbital dynamics dictate that both will spend periods of time when large segments of their orbits are in the path of the fragments' orbits.

Tiangong 1 Diary

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Related Links
Tiangong 1 Orbit History
Tiangong 1 and Shenzhou 10
China National Space Administration
The Chinese Space Program - News, Policy and Technology
China News from SinoDaily.com






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