Monday, 29 May 2017

Analysis: The re-entry of the CZ-4B r/b 2014-049C observed by a Dutch pilot on May 27

click to enlarge. Image (c) Christiaan van Heijst, used with permission
click to enlarge. Image (c) Christiaan van Heijst, used with permission

The beautiful, spectacular images of a rocket stage re-entry above were made by the Dutch aviation photographer and pilot Christiaan van Heijst,  the co-pilot of a Cargolux freight aircraft (flight CV760, a Boeing 747-8 with registration LX-VCC) en route to Brazil on May 27, 2017.

While cruising at FL 340, 34 000 feet (10.360 km) over the mid-Atlantic, Christiaan noted a group of 7 to 10 bright yellow, very slow fireballs appearing in the corner of his eye. Here is the story as told by Christiaan on his facebook page:

Suddenly I noticed something in the corner of my eye. I looked to my right and to my own surprise I saw a huge group 7-10 of bright yellow lights move parallel to our track with a much faster speed and very high altitude. This was not an airplane, nor was it a meteorite. Where shooting stars / meteorites often leave a bright trail, they move with very high speed and burn up quickly. This cluster of lights moved far too slow to be a meteorite and its light was far too constant to be an ordinary meteorite. 

Immediately, a lot of excited chatter in Portuguese and other (African) languages I could not identify. was opening up on the frequency we had tuned in. Apparently lots of pilots were seeing the same lights, which is not surprising with such a high and bright appearance. All in all, the lights appeared abeam our aircraft and disappeared on the horizon in roughly two minutes time, keeping their intensity and appearance along the way.

Evidently, what Christiaan and his colleagues were witnessing was a spectacular re-entry of space debris, with the re-entering object breaking up in multiple pieces while it was plunging through the atmosphere. The time of this re-entry event was around 23:18 UT on May 27, 2017, while the aircraft was over the mid-Atlantic near 11o.93 N, 33o.28 E (see also later in this post).

In this blog post, I identify the object responsible and provide some model results for this re-entry.

click map to enlarge
Christiaan van Heijst initially thought that this re-entry event was related to a NOTAM issued mid-May, a warning for the splash-down of a Soyuz 2nd stage during the SES-15 launch from Kourou. This launch however had already happened 10 days earlier, on May 18, so evidently was no explanation for this event. Christiaan next posted his story on Facebook, hoping that someone could identify the object responsible.

I was allerted to Christiaan's Facebook post by one of my Twitter followers, Theo Dekkers and could quickly identify the event as the re-entry of 2014-049C, a Chinese Chang Zheng (Long March) 4B (CZ-4B) upper stage from the launch of the Chinese Gaofeng 2 and Polish Heweliusz satellites in August 2014. Time, location, and movement of the witnessed event agree extremely well.

Two days before the sighting, JSpOC had started issuing TIP (Tracking and Impact Prediction) messages for this object via their Space-Track portal. The final TIP message, issued after the actual re-entry, lists the re-entry time as 27 May 23:17 +- 1 min UT, near 15o.7 N, 34o W (by the way: we actually believe that such times accurate to 1 minute originate from infrared observations of the re-entry fireball by US SBIRS early warning satellites).

click to enlarge

This time and position closely agrees with the observations of the aircraft crew and the aircraft position. Christiaan van Heijst provided me with a photo of the aircraft flight instruments taken about one minute after the event. It shows the time of that moment, 23:20:43 UT, and the aircraft's GPS coordinates and altitude: 11o 56.1' N (11.935 N), 33o 17.3 W (33.288 W) at a flight level of 34 000 ft (10.360 km). [edit: the altimeter in the image above says 33 960 feet but Christiaan informed me that it has a small error and they were flying at FL 340]. The aircraft was heading towards a magnetic bearing of 219 deg, which corresponds to a true bearing of 204 degrees (towards the S-SW).

The time and position are very close to that of the TIP: a difference of about 425 km between the TIP re-entry location and the location of the aircraft, and 1-2 minutes in time.

The sky track of the re-entering space debris that can be seen on the photographs also agrees well with the predicted sky track of 2014-049C for the aircraft's location. Below is the predicted track for 2014-049C for the location of the aircraft based on a propagated version of the last available orbital element set for the object. The blue line is the predicted track in the sky, the yellow arrow the approximate trajectory for the brightest fragment visible on Christiaan's photographs:


click to enlarge

There is a discrepancy, in that the observed trajectory is some 11 degrees lower in the sky than the predicted trajectory, with a time lag as well. However, this is what you expect. The track shown is for the pre- re-entry orbital altitude (about 134 km). During the re-entry phase, the altitude of the object however quickly drops, and as a result the observed track will be located significantly lower in the sky. As the object is slowed down by increasing drag of the atmosphere, it starts to lag behind predictions in time as well. At the time of the re-entry, the object was already below 80 km altitude,  40% or more below its orbital altitude.

To gain insight into the positions and altitude of  the re-entering debris over time relative to the aircraft, I have modelled the re-entry event. I propagated the last five known orbital element sets (TLE) for 2014-049C to its last ascending node passage before re-entry, using SatAna and SatEvo. The resulting, final ,pre- re-entry TLE was next used as the starting point for a ballistic simulation in GMAT, using the MSISE90 model atmosphere and current Space Weather data. With this input, I had GMAT calculate positions and altitudes of the re-entering object over time.

Such modelling always is an approximation only. There are a number of unknowns, one of which is the spatial orientation of the major axis of the re-entering rocket stage with regard to its flight direction. This adds uncertainty to modelling the atmospheric drag experienced by the re-entering rocket stage, which introduces uncertainties in the position and altitude of the stage for a certain time. A CZ-4B 3rd stage is a tube measuring 6.24 x 2.90 meter with a dry mass of about 1 metric ton. The drag experienced depends on whether its longest dimension is facing the flight direction, its narrow end, or whether it tumbles. For the modelling, I choose to use a drag surface that is 50% of the maximum drag surface possible. Breakup of the rocket body, which is evidently happening (see the copious fragmentation in the photographs) adds more uncertainty, as fragmentation drastically alters the drag surface and surface-to-mass ratio. As the images show, the trajectories of individual fragments clearly start to diverge as a result of this. The model, however, treats the re-entering body as one single body with no mass loss.

So, Caveat Lector. But let us look at the results. Mapping the GMAT results along with the position and bearing of the aircraft a minute after the event, yields this positional map and this altitude versus time profile:

click map to enlarge
click diagram to enlarge

For the reasons mentioned above, the altitudes versus time in the diagram are approximations only, with a possible uncertainty of perhaps 25% for a given time instance.

Compared to the JSpOC TIP data, the resulting trajectory I modelled seems to be slightly on the 'early' side, in that it passes the JSpOC location about a minute too early. On the other hand, the time in the TIP is given with an accuracy of no better than 1 minute, and an unspecified inaccuracy in the coordinates of the geographic location as well. What we can conclude from the modelled positions relative to the aircraft, is that the sighting definitely matches the 2014-049C re-entry data closely.

If my modelling is somewhat correct, the re-entering debris was moving from altitudes of ~95 km at the start of the sighting to below 50 km near the end. It is uncertain whether anything survived to sea level c.q. aircraft flight level. Usually, most materials have burned up before they could reach the surface: it is however not impossible that some pieces nevertheless survived and splashed down in the Atlantic. Notably the pressure spheres of rocket engines tend to survive. If anything, the modelling shows that any surviving debris was well ahead of the aircraft once it reached the flight level of the aircraft.

Ted Molczan has done a similar modelling with similar results. The differences that do exist between Ted's analysis and my results, are due to the choice of slightly different starting parameters for the model.

The final spectacular demise of 2014-049C was the result of a long drop that started short after launch. Below I have mapped the evolution of the orbital altitude of the rocket booster over the past years, starting just after launch:

click diagram to enlarge

The quick decay of (notably) the apogee altitude, but also perigee, can be clearly seen. Early 2017, the drop in altitude starts to increase exponentially. At 23:17 UT on 27 May 2017, after 15772 revolutions around the planet since launch, it was the final end for 2014-049C.

Christiaan asked me why there was no NOTAM issued for this re-entry. NOTAMS or Area Warnings are however generally only issued for controlled de-orbits, and first and second stage splashdowns during launches. Reasonably accurate locations can be predicted in advance for these. For uncontrolled re-entries, such as this event, this is not the case. There are so many uncertainties that anything approaching an accurate prediction can only be issued during the last hour or so before re-entry.

(note: for some Frequently Asked Questions about re-entries, see an earlier post here)

Acknowledgement: I thank Christiaan van Heijst (www.jpcvanheijst.com) for providing extra information and for his permission to use his photographs. I thank Theo Dekkers for pointing me to Heijst's observations.

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