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Shenzhou 9


Tyneside, UK
2018 Dec 14
Friday, Day 348

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Unidentified Object From the Shenzhou 9 Launch

When Shenzhou 9 was launched, it was catalogued as a matter of course and orbital elements were issued by SpaceTrack for the spacecraft, the launch vehicle and several items of debris. A few days later, orbital elements for a new object started to appear with no immediately obvious explanation of where it had come from.


Catalogued Items

SpaceTrack lists the following as coming from the Shenzhou 9 launch. Apart from the Mystery Object, most items are self explanatory from their names.

International
Designation
Catalogue
No
SpaceTrack
Name
Item
2012-032A38461SZ-9Shenzhou 9 spacecraft
2012-032B38462CZ-2F R/BUpper stage of launch vehicle
2012-032C38463CZ-2F DEBMotor cover?
2012-032D38464CZ-2F DEBMotor cover?
2012-032E38465CZ-2F DEBMystery object
2012-032F38467CZ-2F DEBMotor cover?
2012-032G38520CZ-2F DEBMotor cover?
2012-032H38550SZ-9 MODULEShenzhou 9 Orbital Module

The reason the Catalogue Numbers do not run sequentially in a block is that SpaceTrack released orbit information on a large quantity of debris items from earlier launches at about the same time as the Shenzhou items were being detected so they became intermingled. Numbers were allocated in the order items were brought into the catalogue. The Orbital Module was not catalogued until it was released from Shenzhou at the end of the mission.


Motor Covers

The identification of motor covers from launches by the various versions of the CZ (Long March) rocket is something that has crept in over time and is a plausible explanation of what the debris items are. They are not always catalogued but there seem to be four associated with each launch. Generally, they finish up in orbits with similar perigee and an apogee many tens of kilometres higher than the launch vehicle as they are pushed away either by a spring or the force of an explosive bolt. They tend to re-enter after a small number of days because they have a low ratio of mass to cross sectional area. They may simply be pieces of lightweight insulating material.

The motors are retro-rockets that are used to lower the spent rocket stage's orbit. In some cases they are used to precipitate immediate re-entry after the payload has separated.

They often seem to prove difficult to find and sometimes do not even get catalogued. None were listed for the Shenzhou 2, 3 and 4 missions. Sometimes the similarity of orbits between members of the set from any particular launch suggests that the occasional one may get catalogued twice. The rapid rates of decay make it difficult to analyse their orbits.

Of the nine Shenzhou launches to date, Shenzhou 7 has the best documented set of debris. Each point on the plot represents a Twoline Orbital Elements set issued by SpaceTrack, and all four items re-entered within five days of launch.

Shenzhou 7 Debris


History of 2012-032E

A set of orbital elements was issued for this object, together with four others, immediately after launch. all five had similar orbits and four of them were listed by SpaceTrack as having decayed on June 18, two days after launch.

Data also ceased to be published for 2012-32E until Jun 21 when new data started to flow showing an object in a 263 x 379 kilometres orbit. It was clearly not the same item as the earlier 2012-032E which had an orbit 204 x 488 kilometres.


What It Is Not

2012-032E's perigee of 263 kilometres is similar to the perigee of the orbit used by Shenzhou 9 as it transferred from its initial orbit to the Tiangong 1 rendezvous orbit. Shenzhou's parameters were 262 x 316 kilometres and it was tempting to assume that the debris item had been ejected near perigee in a forward direction.

Another of the orbit's parameters shows that it did not originate from Shenzhou 9 while in the transfer orbit. Argument of Perigee indicates where on the orbit perigee actually lies. It is an angle, measured at the centre of the Earth and taken from where a satellite crosses the Equator from south to north, then round the orbit to perigee.

When Shenzhou was in the transfer orbit, the Argument of Perigee was 136°. At that time, the value for 2012-032E was 224°. When Shenzhou was going through perigee, 2012-032E's orbit was about 70 kilometres above it.

The similarity of numbers is a coincidence and does not indicate that Shenzhou 9 was the source of the debris.


What Might it Be?

One of the characteristics of an orbit just above the atmosphere is that air drag causes it to decay. If the orbit is elliptical, as with 2012-032E, decay manifests itself as a reduction in apogee over time while perigee reduces more slowly. This is due to most of the drag occurring at perigee.

With the Shenzhou motor covers being 'lightweight', their orbital decay is slightly different because the difference in drag between perigee and apogee is less than with a more dense object. The orbit then shows less of a difference in decay between perigee and apogee. This is the type of behaviour being shown by 2012-032E. By July 16, its 263 x 379 kilometre orbit had decayed to 242 x 340 kilometres. Perigee had reduced by double the amount that would be expected for an object with a typical satellite density.

The object would seem to be living up to its SpaceTrack description as a piece of debris.


When Did It Separate

Running a simulation of 2012-032E and the various orbits occupied by Shenzhou 9 and its launcher shows that they come together on June 16, the day of launch, within 2-3 hours of lift off. On that basis, it could come from either Shenzhou or the launch vehicle but it is unlikely to be from the spacecraft because something departing at that speed would most likely cause damage when whatever ejected it was set off. It was never near Shenzhou 9 while it was in the transfer orbit.


What Is It?

On the balance of probabilities, it is one of the four motor covers from the launch vehicle.

The reason for the strange orbit is that it did not eject at the same time as the others. It may simply have ejected late, but the separation velocity seems to have been a little higher than normal. An alternative is that it failed to separate properly when the command was given and it was then blasted away when the motor fired.

Although that would appear to take the count of motor covers to five, there is not enough in the way of published element sets for debris from this launch to be be sure that everything is properly accounted for. For most of them there is only a single measurement. There is nothing like the volume of data that was available for Shenzhou 7 (above). The element set for 2012-032C and the initial 'rogue' one for 2012-32E, for example, are similar enough for them to be the same object.

Shenzhou 9 Debris



Page date: 2012 Jul 13
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