Updated Tiangong-1 reentry forecasts (updated April 2)

[post last updated April 2, 3:00 UT, 3:45 UT, 16:50 UT and 21:30 UT]

Final orbit and reentry position of Tiangong-1 (click map to enlarge)

TIANGONG-1 has reentered the atmosphere at 00:16 UT on April 2, over the central Pacific Ocean, JSpOC and the 18th Space Control Squadron have announced.

The decay message is, as expected, listing an uncertainty window of only +- 1 minute, indicating this determination was likely based on Space-Based observations by US Early Warning satellites (SBIRS).

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So, how did the final pre-reentry forecasts from various sources fare, compared to reality? Here is a map summarizing nominal last pre-reentry forecasts:

click to enlarge map

Note how well the "amateurs" did compared to the professionals!

Note that the map only shows the nominal positions, ignoring the (hefty!) error bars. When the error bars are taken into account, all predictions overlap with the real position.

It gives you an idea about how much weight to attach to these nominal positions.

Sources of these forecasts: ESA, JSpOC, CMSA, Aerospace Corporation, Elecnor Deimos, Jon Mikkel (@Itzalpean, priv .com, last prediction not issued publicly but privately in a message), Josep Remis and myself.

*****

I am currently issuing a daily estimate of the reentry date for the Chinese Space Station Tiangong-1 on Twitter. This current blog post consolidates these estimates and is daily updated. My current and previous predictions:

SatAna/SatEvo:

Date issued       Date predicted (UT)
April 1 III       2 April 00:56 ± 130 min (re-issue)

April 1 III       2 April 02:02 ± 150 min
April 1 II        2 April 00:52 ± 130 min
April 1 I         1 April 22:30 ± 5.6h
March 31 III      1 April 20:30 UT ± 7h
March 31 II       1 April 22:55 UT ± 9h
March 31 I        1 April 21:15 UT ± 11h 
March 30 II       1 April 20:30 UT ± 14h
March 30 I        1.9 April ± 17h
March 29 II       1.5 April ± 0.7 day
March 29 I        1.4 April ± 0.8 day
March 28          1.1 April ± 1.0 day
March 27 II       1.3 April ± 1.2 days
March 27 I        1.1 April ± 1.3 days
March 26          1.1 April ± 1.6 days
March 25          1.2 April ± 1.9 days
March 24          2.6 April ± 2.4 days

March 23          3.5 April ± 3 days

March 22            2 April ± 3 days

March 21           31 March ± 3 days

March 20           31 March ± 3 days

March 19            3 April ± 4 days

March 18            1 April ± 4 days

March 17            1 April ± 4 days

March 16            4 April ± 4 days

March 15            7 April ± 5 days

March 14            6 April ± 5 days

March 13           13 April ± 6 days

GMAT:

Date issued       Date predicted (UT)
April 1 III       2 April 00:36 ± 130 min (final)
April 1 II        2 April 00:21 ± 125 min
April 1 I         1 April 23:20 ± 5.8h
March 31 III      1 April 23:08 UT ± 8h

March 31 II       1 April 22:46 UT ± 9h
March 31 I        1 April 22:05 UT ± 11h
March 30 II       1 April 18:00 UT ± 13h
March 30 I        1.7 April ± 15h
March 29 II       1.6 April ± 0.7 day
March 29 I        1.6 April ± 0.9 day
March 28          1.6 April ± 1.1 day
March 27 II       1.6 April ± 1.3 days
March 27 I        1.7 April ± 1.5 days
March 26          2.2 April ± 1.8 days
March 25          2.3 April ± 2.2 days
March 24          3.6 April ± 2.6 days

March 23          3.8 April ± 3 days

March 22            3 April ± 3 days

(all times are in UT = GMT: while earlier predictions were  expressed in decimal days, I am issuing the latest predictions with a nominal time. Note the large error margin on this time, however!)

Currently indicated is a reentry late April 1 or early April 2 (in GMT ), depending on how the periodic atmospheric density variation develops.

JSpOC, the US Military tracking organization,  is issuing periodic TIP messages for Tiangong-1 on their Space-Track webportal. Their lastforecast (issued late April 1st) was 2 April 00:49 UT ± 2 h.
Their final post-reentry, post-mortem Decay Message gives reentry at 2 April, 00:16 UT +- 1 min.

click diagram to enlarge

click diagram to enlarge

The first set of forecasts is made using Alan Pickup's SatAna and SatEvo software, with current and predicted Solar F10.7 cm flux. The error margins are a standard 25% of the number of days between the last elset used for the estimate, and the estimated moment of reentry. This might be a bit conservative, certainly well before the actual reentry. Note that from March 23 onwards, I am using slightly different settings for SatEvo than before that date, in an attempt to correct for SatAna/SatEvo results being noted to be a bit on the early side using standard settings with recent reentries.

The second set of forecasts (the most reliable, it turns out) is made by modelling the orbital evolution in GMAT, using the MSISE90 model atmosphere, historic and predicted solar flux, and a Prince-Dormand78 integrator. Drag surface is taken from an ongoing analysis of the variation in apparent drag surface as indicated by the NDOT/2 value (see below). The error margins are a standard 25% of the number of days between the last elset used for the estimate, and the estimated moment of reentry. In addition, nominal values for modelling at minimum and maximum drag surface are shown as grey crosses.

Here is the GMAT prediction diagram in a bit more detail, with the actual moment of the reentry indicated by a red x:

click diagram to enlarge

The rest of this post below was written pre-reentry and not updated post-reentry:

Uncertainties

The diagrams above shows you how the GMAT and SatAna/SatEvo predictions develop. When the reentry is still several days away, there will remains quite an uncertainty and prediction-to-prediction shift in the estimated moment of reentry, mostly due to periodic variations in the atmospheric density not well represented in the F10.7 cm solar flux variation that is used by most atmospheric models to account for solar activity.

Solar activity has a strong influence on the density of the upper atmosphere - and from that on the drag that the space station experiences. For a forecast, solar activity over the coming days has to be estimated - and those estimates might be off. One -unpredictable- solar flare can completely change the situation.

In addition, the drag surface of Tiangong-1 is unknown and might vary over time (see below, where I discuss an attempt to get some grip on this. And we do know it is spinning). And there is also some leeway in the current mass of Tiangong-1. These all combine to create uncertainty, even with the best reentry models.

As the predicted reentry moment comes nearer, the uncertainties become less. Still even 1-2 hours before a reentry, uncertainties in the moment of reentry (and from that in the position) can still be many tens of minutes. AS these objects move at almost 8 km/s, a 10 minute uncertainty in time amounts to thousands of kilometers uncertainty in the position.

Within the uncertainty of the current JSpOC TIP message, this is the resulting track, i.e. the line where Tiangong 1 could currently come down. Cities with populations of more than 1 million people between 42.8 North and 42.8 South latitude are marked on the map as well, with those under or very near the projected trajectory indicated by white dots:

click map to enlarge

A note about "Live" tracking websites

There are several websites where you can (seemingly) "Live" track objects like Tiangong-1. They are often causing confusion after reentries: people still see the object orbiting on such websites even when it already has come down, and as a result mistakenly think it must still be on-orbit!

How is that possible? Well, contrary to what many people assume, these sites do NOT live track the object. The positions on their maps are not based on a live feed of data.

Instead, the positions on their map are predictions based on orbital elements gathered earlier in the day by the US tracking network and released through JSpOC's webportal. These elements are hence always "old", at least a few hours and sometimes half a day or more.

So even after it has come down, these websites sometimes still depict a spacecraft as on-orbit for a while (untill they update their orbit database). But they show you a ghost!

So never rely on on-line tracking websites to judge whether Tiangong-1 is still on-orbit or not.

 

Drag variability

There is a periodic variability in the drag parameter B*, which is due to a periodic atmospheric density variation under the influence of periodic solar wind speed variations that are not well represented by the F10.7 cm solar flux variation (see below), as can be seen in the diagram below. It is a complex variation of periodicities dominated by ~5.5 and ~6.8 day periods. I expect the reentry prediction to rock back-and-forth a bit with a similar periodicity.

click diagram to enlarge

If fact, the daily shift in SatAna/SatEvo reentry estimates indeed clearly mimics this periodicity:

click diagram to enlarge

Drag surface reconstruction

For the orbital data of the past weeks I have calculated area-to-mass ratio's, in an attempt to get some grip on the drag surface to be used in my reentry modelling.

I initially used a  mass for Tiangong-1 of 8500 kg, but in an e-mail discussion with Jon Mikkel, he convinced me that that mass might be too high as that value likely refers to a fully fueled Tiangong-1. If we assume ~1000 kg of fuel initially at launch but now spent, i.e. a mass of 7500 kg, the resulting drag surface is lower, varying between 16 m² and 31 m² for a 7500 kg mass.

In the diagram below, apparent drag surface values for a 7500 kg mass are shown:

click diagram to enlarge

The calculation was done using the MSISE90 model atmosphere as incorporated in GMAT. For each elset, one full revolution was modelled in GMAT, and atmospheric model densities sampled over that revolution. These values were then averaged to get an average atmospheric density. This density was used in this area-to-mass equation:

A/m = 5.0237*10^-9 * ndot/2 / ( Cd * rho * n^(4/3) ) 

(where n is the Mean Motion taken from the orbital elements; rho is the atmospheric density as modelled in GMAT; Cd a drag coefficient (2.2); and NDOT/2 is taken from the orbital elements)

The drag surface thus modelled from the data between March 4 and March 28 appears to vary between 16 m² and 31 m² (for a mass of 7500 kg). These seem reasonable values: the body of Tiangong-1 measures 10.4 x 3.35 meter (this is excluding the solar panels however), which gives an approximate maximum cross section of 35 m².

My initial (wrong!) interpretation was that over the two week analytical timespan, the drag surface varied between ~90% and ~50% of the maximum surface, suggesting that the attitude of Tiangong-1 appeared to be slowly varying. As will be discussed below, this was a misinterpretation.

The case was solved and my error of interpretation revealed after Eelco Doornbos of TU Delft suggested an alternative explanation:

It turns out he is right! The diagram below plots the drag of Tiangong-1, and that of the Humanity Star (2018-010F, which reentered 22 March near 13:15 UT). The Humanity Star is a nice test object, because it was orbiting low in the atmosphere too and more importantly, it was semi-globular, i.e. we know it had no variation in drag surface. Any variation in drag visible in the data for Humanity Star therefore must be atmospheric in origin.

click diagram to enlarge

As can be seen, the periodic variation in drag of the Humanity Star and Tiangong-1 closely mimics each other. So the cause is NOT attitude variation of Tiangong-1 (a variable drag surface due to a slow spin, as I initially interpreted it), but periodic variations in atmospheric density that are not well represented in the MSISE90 model atmosphere.

After all, to quote Monty Python: "It is only a model...!".

This periodic density variation of the atmosphere is the result of periodic variations in the solar wind speed, which in turn are the result of the distribution of coronal holes over the solar surface. The 5.5-6.8 day periodicities I find are actually quite typical values for this variation. More can be read in this paper.

Note that the same variation is not present in the F10.7 cm solar flux, which most models use to calculate atmospheric density variations under the influence of solar activity. This is why it appears as an apparent drag surface variation in the area-to-mass ratio analysis.

For me, this case has thus produced an interesting lesson regarding area-to-mass ratio analysis: variations in apparent drag surface can in reality reflect atmospheric variations not well represented in the model atmosphere, rather than real drag surface variations. In other words: one should be very careful in interpretating the results of an area-to-mass ratio analysis. Lesson learned!

Spinning

We do know that Tiangong-1 is spinning, as a matter of fact: high resolution RADAR data gathered by Fraunhofer FHR with their TIRA radar  shows that the space station is in a flat spin with a period that was about 4 minutes a week ago, and is about 2.5 minutes currently. TIRA by the way also captured amazingly detailed RADAR images of Tiangong-1, which can be seen here.

click diagram to enlarge

Perigee of the Tiangong-1 orbit is currently below 145 km altitude and rapidly decreasing.

click diagram to enlarge

This diagram shows the frequent orbital raising manoeuvres, ending late 2015, after which the station goes steadily down:

click diagram to enlarge

The rate of decay, clearly going up:

click diagram to enlarge

Where can Tiangong-1 come down?

The map below shows the area where Tiangong-1 potentially can come down: included land areas at risk are southern Eurasia, Australia and New Zealand, Africa, South America, Meso-America and the United States. Northwest Europe including my country (the Netherlands) is not at risk.

In theory, the extreme margins of this zone (i.e. near 42.8 S and 42.8 N) have an elevated risk. In reality, it is notably the position of the perigee which matters, as reentries tend to happen just after perigee passage.

Note that at this moment, the uncertainty in the reentry estimates is that large, that it is not meaningful to provide nominal estimated reentry positions. Any newspaper claims that it will reenter over a particular region, are simply false.

click map to enlarge

Within the uncertainty window of the current JSpOC TIP, the lines on the map below are where Tiangong-1 could come down (cities with populations lager than 1 million people between latitude 42.8 N and 42.8 S are also shown: those under or very near the trajectory of Tiangong-1 are indicated by white dots):

click map to enlarge

Only during the very last few hours before the actual moment of reentry, we can start to point to a particular part of the orbit where it might reenter. But even then, uncertainties in location still will remain large. Satellites near atmospheric reentry move at speeds of almost 8 km/s, so a mere 10 minutes uncertainty in time on either side of the nominally predicted time already means an uncertainty in position of almost 8500 km! And  usually, short before reentry the uncertainty is still much larger than 10 minutes...

An article in the International Business Times has recently appeared which suggests that Chinese officials claim to still have control of Tiangong-1, and that they will do a deliberate deorbit over a designated Ocean area.

In that case, I would expect to see a NOTAM and Maritime Broadcast Warning being issued in advance by Chinese authorities for a specified location and time window. No such NOTAM or Maritime Broadcast Warning has been issued so far, so for the moment I am skeptic of the claim.

What if...?

Tiangong-1 is big enough to almost certainly see pieces survive reentry and hit the ground or the Ocean surface.

Surviving elements of reentries are often parts of the rocket engines and fuel- and inert gas tanks.
The tank below is an example: this was part of the second stage of a Falcon 9 rocket (2014-052B) that reentered over Brazil on 28 December 2014. This tank impacted on Brasilian farmland and was subsequently recovered:

photograph (c) Cris Ribeiro, Brasil

The chances of being hit by falling space debris are however very slim: you have a much higher chance of being struck by lightning.

In fact, the biggest risk of freshly reentered space debris is not being hit, but curious people checking out the fallen objects. If the part in question contains a fuel tank with remnants of fuel still in it, this can be very dangerous. Most rocket fuels are highly toxic, and fumes from a ruptured tank still containing some remnant fuel could easily kill you. It can also do nasty things when your skin or eyes come into contact with it.

The video below shows a spent rocket stage that came down downrange from a launch in China in January (this is not "space debris" persé: but rather "launch debris" as it concerns a primary stage that was jettisoned early in the launch, so the stage itself stayed suborbital).

In the second part of the video, you can see people filming the burning wreckage from close by.
DON'T DO THIS! This is extremely dangerous...!




So if by change the reentry does occur over your region and you come upon debris lying in the field, hold your distance and call the emergency services. Let them deal with it.

At the same time, do not worry too much about the risks. It is still most likely that Tiangong-1 will come down over the Ocean, as most of our planet is Ocean.

And finally...

To get into the mood, here is the Hollywood version of a Tiangong reentry for you... ;-)
(Tiangong-1 in reality is much smaller by the way)



Note: this post has been updated, and parts added or rewritten, repeatedly. Text and figures are updated daily


Note 2: a very nice background piece on my reentry estimate efforts was written for Atlas Obscura by Jessica Leigh Hester.