Orbital Focus - International Spaceflight Facts and Figures
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Shenzhou 9

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Friday, Day 145

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Shenzhou 9 Orbital Module

During the live broadcast of the Shenzhou 9 Landing, Mission Control announced that the Orbital Module was cast off, as anticipated, about one minute before retro-fire. Until July 6, nearly one week later, nothing appeard in the catalogue at SpaceTrack.

Tiangong 1

When Shenzhou 9 departed in the early hours of June 28 (UTC), it was in an orbit 327 x 336 kilometres at 42°.78 inclination. Within a few hours, SpaceTrack showed it had moved to 335 x 338 km.

Later on July 29, Wang Zhaoyao, Director of the China Manned Space Engineering Office gave a press conference to mark the Shenzhou 9 landing. In summarising the mission, he said that after undocking "....Tiangong 1 started to elevate itself into a higher orbit at a height of approximately 370 kilometers for long-term operation". This had been expected because it made a similar move immediately after the Shenzhou 8 flight in 2011.

Apart from the small manoeuvre on July 28, SpaceTrack showed nothing to indicate this was happening.

July 6, SpaceTrack issued its first element set for Tiangong 1 in an orbit 354 x 365 kilometres. In hindsight, it had been at that height since June 29.

Shenzhou 9

After moving away from Tiangong 1, SpaceTrack had it at 334 x 355 km. It apparently remained there until retro-fire early June 29.

Shenzhou 9 Orbital Module

The cast-off Orbital Module should have been catalogued in an orbit very close to the final one for Shenzhou 9. In the event, nothing appeared in the catalogue for nearly a week, until July 6, when an object that SpaceTrack calls "SZ-9 MODULE" entered the list with the expected International Designation - 2012-032H, and catalogue number 38550.

There were no immediate accompanying orbital element sets but the orbit was given as 331 x 338 kilometres. The figures were nearly identical to the most recent ones issued for Tiangong 1. Subsequently, orbital element sets started to appear for it in the 'new' orbit.

What Has Been Going On?

Element sets issued between June 28 and July 6, and tagged as Tiangong 1 are actually for the Shenzhou 9 Orbital Module! SpaceTrack got Tiangong and Shenzhou the wrong way round on June 28 when they separated.

In an analysis published at the NASAspaceflight web site, Phillip Clark pointed out that all of SpaceTrack's "Shenzhou 9" element sets starting with an apparent, but non-existent, orbit adjustment on June 28 are actually for Tiangong 1. They follow it to the point where it had completed the first of two thruster firings to move it the storage orbit to await Shenzhou 10.

Spacetrack's element sets were ceased by administrative decree June 29 when Shenzhou landed because it was known to be no longer in orbit. SpaceTrack would have continued to monitor the real Tiangong but fed any element sets into a temporary catalogue file for unidentified objects until it dawned that an identity switch had occurred.

In SpaceTrack's defence, it would have been difficult initially to tell the two craft apart using radar because they were similar to each other in size and mass. The excuse went away after Shenzhou landed because the Orbital Module is a distinctly smaller object than Tiangong. At that point, bureaucracy ruled - the identities had been fixed. It was an eventual realisation that something known to be in orbit was missing from the public catalogue that caused SpaceTrack to look again about a week later, possibly prodded by independent analysts raising questions.

Late July 6, SpaceTrack resolved it's error and started to issue element sets for both Tiangong and the OM under their correct identities. Unfortunately, SpaceTrack does not have a mechanism to correct already-published element sets so any future analyst will have to be already aware of the error, or work it out independenty.

Page date: 2012 Jul 6
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