2017 Apr 30
Sunday, Day 120
Shijian 10 was a retrievable microgravity experiments satellite carring payloads from several Chinese academic institutions and the European Space Agency. It spent nearly twelve and a half days in orbit following launch from the Jiuquan Space Centre 2016 April 5.
Many months before launch the Chinese Academy of Sciences described the vehicle as being aimed at a 220 x 482 km orbit with 63 degrees inclination. However, at some point (mid/late 2015 or early 2016), a decision was made to change the inclination to 43 degrees. At the same time, the landing zone was switched from Sichuan Province to the Siziwang Banner of Inner Mongolia.
It took a long time for that information to find its way into the public arena. Even on the day of launch, the European Space Agency (a science partner in the mission) was giving out the 'old' orbit but there were references to the new inclination coming out as rumours through China-based Internet discussion groups.
Launch and Questions
Shijian 10 used a CZ-2D rocket and departed Jiuquan Space Centre 2016 Apr 5 at 17:38 UTC (Apr 6 at 01:38 Beijing Time). Acording to data issued by the US Air Force Joint Space Operations Command, it entered an orbit of 234 X 269 km at 42°.89 inclination. The Argument of Perigee was 320°.
The orbit parameters immediately led to questions, two in particular. The first concerns the 200+ kilometres difference between the expected apogee and the orbit achieved. Was there a deliberate decision to fly the mission at a lower altitude or did the CZ-2D underperform and deliver Shijian 10 to a lower-than-planned orbit?
A second question comes from the Argument of Perigee - why was it 320 degrees rather than about 120 degrees that might be expected?
Most launches result in an initial Argument of Perigee a few degrees further round the orbit arc than the launch site. Normal practice is for a satellite to enter orbit at an altitude that will become orbital perigee. It is usually imparted sufficient velocity to produce an apogee on the other side of the globe. The reason is that with the best of endeavours neither the altitude nor the velocity can be guaranteed to meet the desired parameters. A small shortfall in the injection velocity will reduce apogee noticably. If the new value is within the denser regions of the Earth's atmosphere, a satellite might re-enter unintentionally before completing one circuit of the Earth.
By way of example, Soyuz TMA-20M - launched a few weeks before Shijian 10 - was aimed at a 242 kilometre apogee but trajectory planners allowed for the Soyuz-FG launch vehicle to display an error of up to 42 km.
The 320° Argument of Perigee results from Shijian 10 being injected into orbit at an altitude that became its orbital apogee. Taken together with the actual, lower-than-expected, apogee the 320 degree value for Argument of Perigee reinforces the question of whether the CZ-2D may have delivered Shijian 10 to space with a lower velocity than intended.
While the planned orbit was being publicised as having a 63 degree inclination, the landing zone was reported to be in China's Sichuan Province where all of Shijian 10's predecessors (the FSW series of spacecraft) set down. When the planned inclination was changed to 43 degrees, the landing zone was also changed - to the Siziwang Banner area of Inner Mongolia. The revised location is where the Chang'e 5 Test re-entry module was recovered and where all Shenzhou spacecraft landed.
Earlier FSW missions carried film cameras for Earth imaging. In order to see their targets, orbits needed inclinations in the region of 55-65 degrees. As a microgravity mission, Shijian 10 did not need a view of the Earth. A switch to 43 degrees had no implications for the experiments payload.
The Shenzhou Connection
Forty three degrees is the standard inclination used by Shenzhou for its solo missions and the rendezvous missions with Tiangong 1. To support the return to Earth, China's space engineers developed some standard segments of the mission profile that can be repeated from mission to mission. One of them is the re-entry and landing procedure.
Previous missions involving FSW craft in orbits near 60 degrees inclination overflew the Sichuan landing zone twice each day although only one of the passes, involving an approach from the SSW, was suitable for a landing. If a return to Earth had to be 'waved off' for any reason then it would have to wait until the next day when the next suitable overflight of the zone could potentially be anything up to 800 kilometres east or west of the originally-intended landing point, a long distance for the recovery team to re-located itself.
Switching to the Shenzhou inclination offered a lot of benefits, not least of which was being able to make full use of the infrastructure China has put in place to support a re-entry and landing from that specific inclination. Shijian 10's re-entry ground track was planned to be similar to the one in the diagram which is the Shenzhou 10 return to Earth.
Important points to note are the labelled locations along the re-entry path. China has put a big effort into establishing ground stations on foreign soil, aimed mainly at supporting its piloted missions and the future space station. At the end of its final circuit of the Earth, Shijian 10 passed over tracking stations in Namibia, Kenya and Pakistan immediately before entering the atmosphere to land in Mongolia.
The Mongolian landing zone is near the northern apex of Shijian 10's orbit. It meant that a spread of final ground track locations all passed over the area while still being within range of the African and Asian tracking stations. A missed prime landing opportunity could be replaced by one on the next circuit of the Earth. A one day delay could still have got the re-entry vehicle to the ground in the same area without needing a significant re-location of ground forces.
A further similarity with Shenzhou was the time of day at which Shijian 10 set down. It was perfectly in line with a 'Type 1' Shenzhou re-entry - see zarya.info's web page explaining the Shenzhou windows.
The diagram shows the Shijian 10 landing in the context of Shenzhou events.
One of the possible factors creating the best conditions for a Shenzhou return to Earth is Solar illumination of Shenzhou's solar panels. Given that Shijian 10 was designed to run on storage batteries with no solar panels, it seems to knock that on the head. The reason for the relationship between the Sun and the Shenzhou/Shijian 10 orbit may be more to do with navigation, with the Sun as reference point during the mission phase immediately before re-entry.
It is also worth noting that Shijian 10, an automatic vehicle, sits with the group of Shenzhou missions that flew in automatic mode without a crew.
Manouevre of Apr 9, Return to Earth - Initial Estimate
Unusually for this type of mission, Shijian 10 used its thrusters to make a minor orbital adjustment on April 9. Such manoeuvres are normally avoided because they disturb the microgravity environment. The most likely reason for the would have been to set up the ground track for re-entry and landing.
Using Shenzhou as a model, an 'ideal' landing in the Siziwang Banner results from a final Equator Crossing at 322° West longitude but a value a few degrees either side will still reach the zone while still passing in view of the uprange tracking stations. There is more scope for ground tracks to lie west of 322° than for tracks to lie to the east.
Following this orbit adjustment Shijian 10 was predicted to cross the Equator at 322° West on April 19. It matched the Shenzhou model and fitted the 'two week' duration mentioned by China. One day earlier the track was 5° of longitude to the east and a day later it was 5° to the west of 322°.
Manoeuvres of April 13 and April 16
JSpOC/SpaceTrack issued an orbital element set indicating that Shenzhou 10 raised its orbit again on April 13, by about three kilometres, possibly as a trimming manoeuvre to refine the re-entry ground track. It seems to have occurred around 07:00 UTC on a pass across eastern China.
The effect of raising the orbit was to move daily ground tracks to the west of their pre-manouvre locations and hinted at the possibility of landing on either April 18 or April 19. A further orbit boost on April 16 set up the landing for April 18.
There is a summary of the manoeuvres in the zarya.info launch list for 2016.
On April 15, a 'Notice to Airmen' issued by the Beijing Area Control Centre flagged the landing zone closure in the Siziwang Banner for April 18 between 08:05 and 08:31 UTC. Following the April 16 thruster firing, orbital predictions for Shijian 10 showed an anticipated Equator Crossing at 08:02 UTC with longitude 320° West. The subsequent ground track terminated in the Siziwang landing zone but was about two degrees of longitude to the east of the one shown in the diagram above - still well within view of the tracking stations.
Shenzhou requires 35 minutes to travel from the Equator to its touchdown point. Shijian 10, in reality, would be likely to arrive a little more quickly because the permissible G-loading during re-entry is higher, and it was probably designed to spend less time than Shenzhou under its parachute. The trip seemed likely to take about 30 minutes so it appeared that Shijian 10 would reach the ground at approximately 08:32 UTC (16:32 Beijing Time).
On April 18 re-entry occurred as expected and the Xinhua news agancy reported that the Shijian 10 re-entry module touched down in the Siziwang Banner of Inner Mongolia at about 08:30 UTC in the area used for Shenzhou landings. Shijian 10 left a service module in orbit that was expected to continue operating some of the microgravity experiments for a few days until its chemical batteries were depleted.
To Sum Up
There is evidence to suggest that Shijian 10's CZ-2D launch vehicle may have delivered it to a lower than intended orbit but only China's space engineers can confirm if that is the case.
A relatively-late change in the mission plan resulted in the target inclination being changed from 63 to 43 degrees in order to take advantage of the infrastructure and plans that are in place to support China's piloted space programme. It also gave the Shenzhou team an opportunity to rehearse the re-entry/landing plan for Shenzhou 11 - currently set to launch later in 2016.
Shijian 10's launch and landing times were selected to match the on-orbit lighting conditions experienced by Shenzhou. Landing, in particular, appeared to be controlled by exactly the same factors as an unpiloted mission of a Shenzhou spacecraft.
Shijian 10 returned to Earth April 18 with a landing at about 08:30 UTC in the Shenzhou landing zone located in the Siziwang Banner area of inner Mongolia.
Original Page Date: 2016 Apr 7
Updated as the mission progressed
Finalised: 2016 Apr 20
Copyright © Robert Christy, all rights reserved
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