THE SECRET SPIES IN THE SKY - Imagery, Data Analysis, and Discussions relating to Military Space
SatTrackCam Leiden (Cospar 4353) is a satellite tracking station located at Leiden, the Netherlands. The tracking focus is on classified objects - i.e. "spy satellites". With a camera, accurate positional measurements on satellites of interest are obtained in order to determine their orbits. Orbital behaviour is analysed.
In early evening twilight of 16 June 2018, around 21:11 UT (23:11 local time), a brilliant fireball at least as bright as the full moon and fragmenting into multiple pieces, appeared over NW Europe. It was widely seen and reported by the public in the Netherlands, Belgium, France and Germany. It garnered a lot of press attention, especially in the Netherlands.
The fireball notably rose to fame because it appeared over the stage of a concert by the Foo Fighters at Pinkpop, the large annual music festival at Landgraaf in the Netherlands. Here is footage of the event over the stage:
From this video, we can determine that the fireball duration was at least 1.65 seconds, and probably longer as the video clearly did not record the start of the fireball but only part of the apparition.
At first it seemed there were no records of the fireball by our dedicated meteor camera network, as it was still very early in twilight. But as it turned out the All-Sky meteor camera of Jean-Marie Biets in Wilderen in Belgium, where it is slightly darker than more north like in the Netherlands at this time of the year, had captured it in a still bright blue sky with only a few stars (and bright planet Jupiter) visible. Here is the image:
The fireball as seen from Wilderen, Belgium. Image (c) Jean-Marie Biets.
click image to enlarge
Another image, that popped up through Twitter, was made by a German amateur astronomer, Uwe Reichert from Schwetzingen, who was photographing the conjunction between the moon and Venus low in the west when the fireball shot through the field of view of his camera. That yielded this very nice picture, which also clearly shows the fragmentation into at least two fragments:
image (c) Uwe Reichert
Detail of previous image showing fragmentation. Image (c) Uwe Reichert.
(Note: while it appears as if the fireball pierces a cloud, it in reality appeared behind the cloud, being bright enough to shine through the thinner edges of the cloud. It ended well above cloud levels.)
The Landgraaf video shows at least 5 separate fragments near the end of the fireball apparition:
Fragmentation into 5 pieces on the Landgraaf video. Click to enlarge.
Based on the Wilderen and Schwetzingen images and some quick azimuth determinations for the fireball endpoint using Jupiter, Venus, the moon and the few bright stars visible on the Wilderen image as reference, I made this cross-bearing as a quick initial assessment, suggesting the fireball appeared over the Belgian Ardennes in the southeast Belgian province of Wallonia, close to the border with Luxemburg:
click to enlarge
Next, it turned out that there was a second meteor camera image, from the All Sky camera located at Bussloo Public Observatory (Mark-Jaap ten Hove):
The fireball as seen from Bussloo, the Netherlands. Image (c) Mark-Jaap ten Hove/Bussloo Public Observatory.
click image to enlarge
Like the Wilderen image, only a few stars are visible, not enough to do serious astrometry. I therefore used a trick to get decent astrometry on the images: I asked both photographers for images from somewhat later that night. By measuring star positions on those, I could create an astrometric grid over the camera field, yielding the positions of the start and end of the fireball on both images. This means that, with triangulation, a proper atmospheric trajectory could be reconstructed.
The result is this trajectory, with the endpoint of the fireball only a few km from where my initial crude cross-bearing analysis had placed it:
click to enlarge
The fireball started over the Luxemburg-Belgian border, at 70 km altitude. It came in from the southeast under a steep angle (48 degrees with the horizontal), and ended over the Belgian Ardennes at 30 km altitude. The endpoint is located some 30 km south of Liege.
The apparent radiant of the fireball is on the Ophiuchus-Hercules border. As alas no speed information is available (the Wilderen image has no discernable sektor breaks; the Bussloo camera is unsectored), a precise geocentric radiant cannot be given, and a precise heliocentric orbit cannot be computed either.
The Landgraaf video however puts some constraints on the maximum speed: that cannot have been above 29 km/s, and was probably much less as the Landgraaf video did not pick up the fireball from the start. This is an interesting constraint. For a range of likely speeds up to 24 km/s, the resulting orbits are all asteroidal in character with inclinations smaller than 23 degrees and aphelion within the orbit of Jupiter.
The map below shows the observed apparent radiant (blue) and geocentric radiant positions for a range of assumed speeds (red):
click map to enlarge
The fireball penetrated deeply into the atmosphere and showed fragmentation, but the lack of speed data precludes a definite statement on the end velocity and on whether something could have survived. An end altitude of 30 km is a borderline case: most meteorite droppers end lower, at 25-15 km altitude.
Acknowledgement: I thank Mark-Jaap ten Hove and Jean-Marie Biets for making available their all-sky images for analysis.
Note: the radiant map initially had a labelling error in the declination. This has been corrected
The two images above show Orbital ATK's Cygnus AO-9 cargoshi chasing the International Space Station (ISS), a few hours prior to berthing. The Cygnus OA-9 cargoship, launched on May 21 from Wallops Island, brings supplies (food, equipment etc.) to the Space Station.
I could observe three passes of the two objects during the night of May 23-24: in all three cases the two objects could be seenr at the same time in the sky, with the Cygnus (the fainter trail in the images above) somewhat behind ISS.
The images above are from the first pass (21:48 UT, 23:48 local time), a high pass, and the third pass (01:00 UT, 03:00m local time), low over the southwest horizon. The Cygnus spacecraft was about 22 seconds behind the ISS on the third pass. The sky over Leiden was somewhat hazy.
The very short third trail near the ISS on the first image is Kosmos 2392.
As usual, the Cygnus spacecraft was quite faint (mag +4.5), so not an easy naked eye target. The brightness of these Cygnus spacecraft is strongly phase-angle dependent. The Dragon spacecraft of their competitor SpaceX are much brighter and easier to see.
As related in a previous post, the X-37B robottic space plane OTV 5 made an orbital manoeuvre on the 19th, lowering its orbital altitude from ~355 km to ~315 km.
It has been observed in its new orbit enough by now (pass predictions for yesterday evening were spot on), to allow an analysis to reconstruct the time and location of the manoeuvre. This can be done by looking for a moment where the positions in the old orbit and the new orbit were close.
Using Mike's pre-manoeuvre OTV 5 orbit of epoch 18104.41928168 and my own post-manoeuvre orbit solution of epoch 18112.84880111, and feeding these into the COLA program written a long time ago by Rob Matson, the resulting time of coincidence is 19 April 2018 at 5:20 UT.
OTV 5 was near perigee and in its descending node at the time, over west Africa, as can be seen on the map above. Manoeuvres typically happen near the nodes and near either perigee or apogee, so that fits well with this reconstructed moment of manoeuvre.
Since the manoeuvre entailed both a lowering of the perigee and a lowering of the apogee, the time and location listed above is likely the second of two manoeuvre moments.
The first manoeuvre burn probably happened near 4:35 UT, near apogee and the ascending node of the original orbit, south of Hawaii. This burn lowered the perigee altitude of the orbit to 310 km. Next, a second burn lowering the apogee altitude to 323 km was conducted half an orbital revolution later at 5:20 UT, near perigee and the ascending node of the orbit over west Africa. The two points are depicted by red circles in the map above.
Past OTV missions frequently made such manoeuvres between different orbital altitudes. They probably are meant to be able to test experimental technology in the payload bay under various thermospheric density and irradiation regimes.
Meanwhile, we continue to track OTV 5 in its new orbit. My observations yesterday were hampered a bit by an untimely field of clouds, but I did get some astrometry. Here is some imagery from yesterday, showing OTV 5 ascending amidst a thin cloud cover (bright star in clouds at right is Capella):
The image above shows the US Air Force's "secret" X-37B space plane OTV 5 ascending in the western sky (the two bright stars above the roof are Castor and Pollux), in the evening of 21 April 2018.
I was asked the question: "how do we know this is OTV 5? Why can't it be Zuma?". I will explain here why it is definitely OTV 5 and definitely not Zuma.
The key is in the orientation of the orbital plane. Both OTV 5 and Zuma were launched from Cape Canaveral into a northwest direction, towards azimuth 40-50 degrees (see map with launch hazard zones below). That direction establishes the orbital plane the objects were launched into.
click to enlarge
From our tracking of the OTV 5 candidate the past 10 days, we have the orbital plane this object is moving in. We can project that orbital plane back to the launch dates of both OTV 5 and Zuma.
For the launch date and launch time, it should pass over the launch site, with a correct orientation in terms of direction. That means, in this case: it should pass over Cape Canaveral, into a northeastern direction.
Now let us first do that for OTV 5, which was launched by SpaceX from Cape Canaveral pad 39A on 7 Sept 2017 at 14:00 UT. The 3D plot below shows the orbital plane of the object we track projected backwards, for the moment of OTV 5 orbit insertion (7 Sep 2017, ~14:09 UT):
click to enlarge
As can be clearly seen, the orbital plane we established for the object we have been tracking the past few days, for this date and time lines up with the launch site, and it is oriented into the correct direction (southwest to northeast). This strongly indicates that the object we track is from the OTV 5 launch.
If we do the same for the Zuma launch, we do not get a good match. Zuma was launched by SpaceX on 8 Jan 2018 at 01:00 UT from Cape Canaveral pad 40. The 3D plot below shows the orbital plane of the object we track
projected backwards, for the moment of Zuma's orbit insertion (8 Jan 2018, ~01:09 UT):
click to enlarge
As we can see, the orbital plane we established for the object we have been tracking the past few days, for this date and time does not line up with the launch site, and it is moreover oriented into the wrong direction too (northwest to southeast instead of southwest to northeast: a 90-degree angle!). This strongly indicates that the object we track is not from the Zuma launch.
(As avid readers of this blog know, Zuma presumably failed to detach from the Falcon 9 upper stage due to a faulty adapter provided by the satellite's builder Northrop Grumman, and reentered with the upper stage a few hours after its launch).
So the object's orbital plane lines up with a launch from Cape Canaveral on 7 Sept 2017 and orbit insertion at 14:09 UT, the launch date of OTV 5. Ad to this the very low orbit which was also typical for past OTV missions, and it is very clear that the object we are currently tracking is the X-37B mission OTV 5.
Below is a video of OTV 5 which I shot yesterday evening, 21 April 2018:
The image above shows the secretive X-37B Space Plane OTV 5, a robottic mini space shuttle flown by the US Air Force, over my house in Leiden, cruising through Leo (the bright star above the chimney is Regulus). It was a bright, easy naked eye object with a brightness of magnitude +1.
In a previous post I detailed how (and why), following the launch in September 2017, we had a hard time tracking down the whereabouts of this fifth OTV mission. Untill Cees Bassa located it on April 11th, in a 54.4 degree inclined orbit. It is the first OTV mission bringing it to the latitudes of the Netherlands.
Clouded weather in the Dutch coastal region after Cees' recovery prevented me from seeing it untill yesterday. During the past week, OTV 5 moved from morning passes to evening passes. Weather improved too medio last week, but still OTV 5 initially escaped me. Because it manoeuvered!
On April 18th, a week after it was first located in orbit, OTV 5 made a manoeuvre. It was a no-show for several observers, including me, on the 19th, but two observers, Tristan Cools in Belgium and Marian Sabo in Slovakia, reported an "unidentified" object some 8 minutes earlier (which means it passed while I was setting up my camera on the 19th). Based on Tristan's photograph of that object, a post-manoeuvre orbit was guessed by Mike McCants as well as by me. Yesterday evening on the 20th, we were ready to look for it, and we did recover OTV 5, a few minutes in front of the estimated new orbit.
The new orbit is still preliminary, but it seems as if the orbit has been lowered from a ~355 km circular orbit to a 307 x 320 km orbit. In a few days, when we have more observations, we'll know more about the new orbit, and when the manoeuvre exactly happened.
The video below which I shot yesterday evening shows OTV 5 cruising through the Coma Berenice cluster:
This was my very first observation of an X-37B! Very cool to see this enigmatic object pass in my own sky. Given that previous OTV missions frequently manoeuvered, it will be an interesting object to follow.
All kinds of nefarious motives and purported specific targets have been ascribed to the X-37B program by the aluminium hat brigade, but the reality probably is that the X-37B is an experimental test-bed for new space technologies, testing these under real space conditions and at various thermospheric regimes, over a prolonged time period, before retrieving them.
I do find it interesting though that this new OTV mission is in a 54.4 degree inclined orbit, rather than the previous 38-43 degree inclined orbits (see comparison in my previous post). Over the past year we have now seen three experimental missions going (or planned to go) into 50-55 degree inclined orbits: USA 276; the failed Zuma; and OTV 5. All three are clearly experimental missions. For Zuma, I suspect it was meant as an experimental radar satellite, and maybe OTV 5 tests radar as well. Or maybe not.
At any rate, I welcome this new attention to ~50-55 degrees inclination, as objects in such orbits are well observable from my 52-degree latitude in the Netherlands.
OTV-5, The fifth mission of the US Air Force' X-37B robottic mini-shuttle, was launched from Cape Canaveral on 7 September 2017 on a SpaceX Falcon 9 rocket. Until last week, OTV-5 had not been located by amateur satellite trackers, and that was somewhat curious, as we did locate and track the previous four missions.
But now OTV-5 has beenfinally found. In the early morning of April 11, 2018, Dutch satellite tracker Cees Bassa imaged a bright unidentified satellite in a ~54 degree inclined orbit. It was seen again by Cees two days later, on April 13. Ted Molczan managed to link it to a lone sighthing of an unidentified object done by Russell Eberst in Scotland back in early October 2017 that was already suspected to perhaps be OTV-5 at that time (several of us, including me, had tried to recover the object Russell observed in the next few nights that October, but failed).
OTV-5 immediately was suspected as the identity for this object. It was in a very low, ~355 km circular orbit, which is lower than usual for satellites, but which fits with the characteristics of previous OTV missions.
The orbital plane the object is moving in passed over Cape Canaveral at the moment OTV-5 was launched (see below, which shows the location of the orbital plane for the moment of OTV-5 orbit insertion on 7 September 2017). So that fits nicely, and as a result we are quite confident that this is OTV-5.
click to enlarge
There is a difference with previous OTV missions: OTV-5 is in a 54.5 degree inclined orbit, which is a substantially higher orbital inclination than that of previous OTV missions which were flown at orbital inclinations between 38.0 degrees and 43.5 degrees, as can be seen in this diagram below where the current OTV-5 mission orbit is white, and previous OTV mission orbits are red:
"The fifth OTV mission will also be launched into, and landed from, a higher inclination orbit than prior missions to further expand the X-37B’s orbital envelop."
I am very happy that OTV-5 was launched, as it now turns out, into a 54 degree inclined orbit, as for the first time this will give me a chance to see an X-37B OTV mission from the Netherlands. OTV-5 will actually pass over my country (and even somewhat north of it), while previous OTV missions passed over southern Europe only. The previous four missions therefore were not visible from my country, due to their lower orbital inclination.
An obvious question is: why did it take so long to find OTV-5? I have some answers to this that might explain.
First, I think many amateurs subconsciously reckoned it would be in a 38-43 degree inclined orbit like its predecessors. Indeed, the initial search elements we used were for a 43-degree orbit.
Second, this was an autumn launch and the very low orbital altitude means it is not well visible in wintertime from the Northern hemisphere, where almost all currently active satellite trackers are located. Almost all wintertime passes are in Earth shadow.
Now spring has arrived, OTV-5 is emerging out of these shadows, into the light. Weather has not been cooperating for me in the coastal area of the Netherlands where I am located so far, but I hope to be able to joing tracking this object soon. It is an interesting object to track, as previous OTV missions frequently manoeuvered between different orbital altitudes. Plus, the shuttle-like character of this object makes it a special one to track as well.
[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).
*****
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.
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 doNOTlive 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:
(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.
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.
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.
