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.
This blog analyses Missile tests too.
In my previous post I debated at length a claim by the Financial Times on October 16 that China recently did a FOBS test. This claim seems to be currently disintegrating, as suspicion is rising (following rebuttal comments by a Chinese Government offcial) that it all seems to refer to a Chinese reusable Space Plane test flight on July 16 instead. A test flight which was already reported earlier and hence known.
At the time when that Chinese Space Plane test flight was reported in July, it was reported as having been suborbital. This was (I think) mostly based on the proximity of the reported launch site (Jiuquan) and landing site (Badanjilin Airport), which are only some 220 km apart (indicating a short suborbital "hop").
But I now think that judgement was in error.
If the whole FOBS-story indeed actually refers to the July 16 test flight, then it seems that it was orbital, completing one revolution.
Indeed, upon looking into it and trying some orbital scenario's, I found that a launch from Jiuquan into a 41.07 degree inclined orbit would actually very well match with a landing at Badajilin Airport at the end of one full revolution. I have depicted the resulting trajectory in the map in top of this post.
So: FOBS or space plane? Jeffrey Lewis has a point when he tweets:
"China just used a rocket to put a space plane in orbit and the space plane glided back to earth. Orbital bombardment is the same concept, except you put a nuclear weapon on the glider and don’t bother with a landing gear." - Jeffrey Lewis
But then, Mark Gubrud is likely also right about design differences when he tweets that:
"A FOBS/hypersonic missile would be optimized differently from a space plane. The plane would be designed to slow as much as possible on reentry (hence the blunt design of the Shuttle). It would have landing gear. It would have a payload bay, instead of an integral warhead."
The trajectory of the July 16 space plane flight, if my interpretation is correct, is not very FOBS-like. But this was only a test flight. Jeffrey is right that a space plane or glider in principle is suited for a deorbit with something in it's cargo bay that can go BOOM. This is why some other nations look with suspicion at the US X-37B space plane, currently on its sixth mission.
About the airport at Badanjilin: it seems to have been constructed rather recently. It is not present in Google Earth imagery from as recent as 2016. The landing strip is some 2.4 km long, oriented northwest-southeast, azimuth 313-133 degrees, and located at 39.2264 N, 101.5477 E. Below is a Copernicus Sentinel 2A image of the airport taken on 31 July 2021 (note the Camel!):
click image to enlarge
UPDATE 18 Oct 2021 19:45 UT:
Something very confusing is that, while the Chinese Government now seems to suggest that the orbital
flight reported on by the Financial Times in fact was this July 16 space plane
flight, Chinese news items on this space plane flight from that time seem to state that the flight was "suborbital", as Jonathan McDowell has pointed out.
Still, I am not convinced as the word is used several times in a context where it is odd (e.g. when describing the space plane as a technology). The same in an English language bulletin by Xinhua, which also talks about a "reusable suborbital carrier". That sounds more like an aircraft to me (perhaps one launched with a booster stage and then flying through the upper atmosphere), than a space plane. I have no knowledge of the Chinese language at all (the only thing I can say in Chinese is "thank you") so do not know what is possibly lost in translation here.
Even more confusingly, the first Chinese item linked above seems to name yet another airport (so another one than that pictured above) as the landing site: Youqi airport. I found a Youqi airport which is near 48.5764 N, 116.9377 E. The runway of that airport however seems a bit short for a space plane landing. Jonathan McDowell thinks I got the wrong Youqi and points out that the Badanjilin Airport imaged above is also called Youqi....oh well. He probably is right (he usually is). We can use some more confusion in this already confusing case...
Both with regard to "FOBS or not", and the July 16 "space plane", many things remain very ambiguous.
It could be that the Chinese Government is now seizing on the July 16 test to explain away a later FOBS test.
[this post was updated on October 18 to reflect new information and a refutal of the claim by the Chinese Government. It was again updated on October 21 to reflect new information, including claims that it concerned *two* tests, on July 27 and August 13]
I bet some people connected to US Missile Defense hear this whisper in their sleep currently, given news that broke yesterday about an alledged Chinese FOBS test in August.
FOBS(Fractional Orbital Bombardment System) has menacingly been lurking in the background for a while. In the earlier mentioned podcast it was brought up in the context of discussing new pictures from North Korea showing various missile systems: including a new one which looks like a hypersonic glide vehicle on top of an ICBM (which is not FOBS, but FOBS was brought up later in the podcasts as another potential future exotic goal for the North Korean missile test program).
image: Rodong Sinmun/KCNA
But the days of FOBS being something exclusively lurking as a menace in the overstressed minds of Arms Control Wonks like Jeffrey are over: the whole of Missile Twitter is currently abuzz about it.
The reason? Yesterday (16 Oct 2021) the Financial Times dropped a bombshell in an article, based on undisclosed intelligence sources, that claims China did a test in August with a system that, given the description, seems to combine FOBS with a hypersonic glide vehicle. [edit: but see update at end of this post]
That last element is still odd to me, and to be honest I wonder whether things have gotten mixed up here: e.g., a mix-up with a reported Chinese suborbital test flight of an experimental space plane from Jiuquan on 16 July this year. [edit: and this might be right, see update at the end of this post]
"tested a nuclear-capable hypersonic missile in August that circled the globe before speeding towards its target".
"Circled the globe" before reaching it's target: that is FOBS.
FOBS
So, for those still in the dark: what is FOBS?
FOBS stands for Fractional Orbital Bombardment System. Unlike an ICBM, which is launched on a ballistic trajectory on the principle of "what goes up must come down", FOBS actually brings a nuclear payload in orbit around the earth, like a satellite.
For example, a very Low Earth Orbit at an orbital altitude of 150 km, which is enough to last a few revolutions around the earth. At some point in this orbit, a retrorocket is fired that causes the warhead to deorbit, and hit a target on earth.
FOBS was developed as an alternative to ICBM's by the Soviet Union in the late 1960-ies, as a ways to evade the growing US Early Warning radar system over the Arctic. Soviet ICBM's would be fired over the Arctic and picked up by these radar systems (triggering countermeasures even before the warheads hit target). But FOBS allowed the Soviet Union to evade this by attacking from unforseen directions: for example by a trajectory over the Antarctic, which would mean approaching the US from the south, totally evading the Early Warning radars deployed in the Arctic region.
In addition, because FOBS flies a low orbital trajectory (say at 150 km altitude), whereas ICBM's fly a ballistic trajectory with a much higher apogee (typically 1200 km), even when a conventional trajectory over the North Pole would be used, the US radars would pick up the FOBS relatively late, drastically lowering warning times (the actual flight times of an ICBM and a FOBS over a northern Arctic trajectory are not much different: ~30 minutes. Over the Antarctic takes FOBS over an hour. But of relevance here is when the missiles would be picked up by US warning radars).
The Soviet Union fielded operational FOBS during the 1970-ies, but eventually abandoned them because new western Early Warning systems made them obsolete. This notably concerned the construction of an Early Warning system in space, consisting of satellites that continuously scan the globe for the heat signatures of missile launches. DSP (Defense Support Program) was the first of such systems: the current incarnation is a follow-up system called SBIRS (Space-Based Infra-Red System). This eliminated the surprise attack angle of FOBS, because their launches would instantly be detected..
Reenter FOBS
But now China has revived the idea, moreover with an alledged test of an actual new FOBS system (while Russia also has indicated they are looking into FOBS again). From the description in the Financial Times, which is based on undisclosed intelligence sources, the Chinese FOBS system moreover includes a hypersonic gliding phase. [edit: but see update at the end of this post]
Initially this surprised me: I was of the opinion (and quarrelled with Jeffrey Lewis about this, but am man enough to now admit I was wrong and he was right. Sorry Jeffrey, I bow in deep reverance...) that FOBS in 2021 had very little over regular ICBM technology and was therefore a very unlikely strategy, feasible only as a desperate last defensive act of revenge before total annihilation in case of an attack by others. Because using FOBS in an offensive tactical role would guarantee you to lead to Mutually Assured Destruction.
I still stand behind that last part, but clearly, China thinks they nevertheless need FOBS. Why?
FOBS still has one advantage over regular ICBM's. That is, that a southern trajectory over Antarctica approaching the US mainland from the south, while not going undetected by SBIRS, still avoids warhead intercepts by the US anti-Ballistic Missile Defense (BMD) systems, that are currently geared to intercept a regular ICBM-attack over the Arctic or from the west (North Korea).
I should ad here: "for the time being".... The logical answer by the US (unless they chose to continue to ignore China with regard to BMD, as they did untill now) will now be to extend their BMD coverage to the south. For countering FOBS, they could use the same AEGIS SM-3 technology that they used to down the USA 193 satellite in 2008 (Operation Burnt Frost).
Here are two maps I made, one for a FOBS attack on Washington DC from China and one for a FOBS attack on Washington DC from North Korea. The red lines are ballistic ICBM trajectories (over the Arctic), and current BMD sites are meant to intercept these kind of trajectories. The yellow lines are FOBS trajectories over the Antarctic, showing how these attack the USA "in the back" of their missile defenses by coming from the south instead.
hypothetical FOBS attach from China. Click maps to enlarge
hypothetical FOBS attack from North Korea. Click map to enlarge
As the USA is currently putting much effort in Ballistic Missile
Defense, developing a new FOBS capacity could be a way by which China is warning
the USA that even with BMD, they are still vulnerable: i.e. that they
shouldn't attempt a nuclear attack on China from a notion that their
BMD systems make them invulnerable to a Chinese answer to such an
attack.
FOBS is hence a way of creating and utilizing weaknesses in the current BMD capacity of the USA, as a counter capacity.
It should be remarked here that the US BMD capacity is geared towards
missiles fired by Russia or by 'Rogue Nations' like North Korea and Iran.
The USA seems to have largely ignored China so far
with regard to BMD. Meanwhile, China is concerned with the US
BMD development, particularly deployment of BMD elements in their
immediate region.
So this FOBS experiment could also be a way
in which China tries to force the US to finally take the Chinese
concerns about US BMD deployments and the inclusion of their region into
such deployments, serious.
Outer Space Treaty
China (like the US and Russia) is a signatory of the Outer Space Treaty (or, in full: the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies).
FOBS seems to be a violation of this treaty, as Article IV of the treaty clearly states that:
"States Parties to the Treaty undertake not to place in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction"
This is exactly what FOBS does: it (temporarily) places a nuclear weapon in orbit around earth, so that they can later bring it down over a target.
The Soviet Union, when testing FOBS in the late '60-ies, tried to get out from under this by claiming that, as their FOBS did not complete a full orbit around the Earth, article IV of the treaty didn't apply. The US Government, surprisingly -and for opportunistic reasons- went along with this interpretation (see this article in The Space Review). Which is, pardon me the word, of course bullshit: in the sense of orbital mechanics (that is to say; physics), FOBS clearly does place an object in orbit, and it is very clear too by the fact that after launch it needs an actual, separate deorbit burn to get it down on the target.
North Korea and FOBS
How about North Korea? As I mentioned, FOBS has been repeatedly mentioned as a potential route North Korea might take with its nuclear missile program. Some fear that NK could be developing a FOBS capacity in order to have a means of final-revenge-from-over-the-grave from the Kim Jong Un regime in case of a 'decapitation' attempt (an attempt to end the Kim Jong Un regime by a targetted military strike on KJU and his family members).
One reason behind this fear is that North Korean Kwangmyŏngsŏng (KMS) satellite launches were on a trajectory over Antarctica, bringing the payload over the US only half a revolution after the launch.
Compare this launch trajectory of KMS 3-2 in December 2012 for example (which comes from this 2012 blog post), to the hypothetical FOBS trajectory in the map below it: the similarities are obvious (if perhaps superficial).
KMS satellite launch trajectory (above) and hypothetical FOBS attack from North Korea (below). Click map to enlarge
So, can we think of something even more sinister than FOBS? Yes, yes we can, even though so far it is completely fictional and a bit out there (pun intended).
Let us call this very hypothetical menace DSBS. It is truely something out of your nightmares.
DSBS is a name I coined myself for a so far nameless concept: it stands for Deep Space Bombardment System. DSBS at this point is purely fictional, with no evidence that any nation is actively working on it: but the concept nevertheless popped up, as a distant worry, in a recent small international meeting of which I was part (as the meeting was under Chatham House rules, I am not allowed to name participants). So I am not entirely making this up myself (I only made up the name to go with this so far unnamed concept).
The idea of DSBS is that you park and hide a nuclear payload in Deep Space, well beyond the Earth-Moon system: for example in one of the Earth's Lagrange points. There you let it lurk, unseen (because it is too far away for detection). When Geopolitical shit hits the fan, all is lost and the moment is there, you let your DSBS payload return to earth, and impact on its target.
With the current lack of any military Xspace (Deep Space) survey capacity, such an attack could go largely undetected untill very shortly before impact. Your best hope would be that some Near Earth Asteroid survey picks it up, but even then, warning times will be short. Moreover, with the kind of impact velocities involved (12+ km/s), no existing Ballistic Missile Defense system likely is a match for these objects.
Far-fetched? Yes. But that is also something once said about FOBS...
(Note: I hereby claim all movie rights incorporating DSBS scenario's)
(added note: I only now realized, when answering a comment to this blogpost below, that, unlike FOBS or placing something in GEO, a DSBS parked in one of the Lagrange points would NOT violate Article IV of the Outer Space Treaty, because the device would NOT be in orbit around Earth (but co-orbital with Earth).
Of course, as Jeffrey Lewis rightfully remarks, spaceplane technology shares a lot with FOBS technology. In Jeffrey's words: "China just used a rocket to put a space plane in orbit and the space plane glided back to earth. Orbital bombardment is the same concept, except you put a nuclear weapon on the glider and don’t bother with a landing gear."
At the time, this space plane test was interpreted to have been suborbital, as the space plane reportedly landed in Alxa League, 800 kmBadanjilin Airport, 220 km from the launch site, Jiuquan. I today however realised that this might have been a misinterpretation: it might actually have been an orbital, not suborbital, test fligth landing at the end of the first revolution.
Indeed, I managed to create a hypothetical 41.2 degree inclined proxy orbit for a launch from Jiuquan that brings it over Alxa League Badajilin Airport at the end of the first revolution.
Slightly more on this in this follow-up blogpost. which also points out that a Chinese source confusingly points to yet another airport as the landing site of the July 16 space plane test (if it was a space plane at all and not some upper atmospheric aircraft vehicle).
It could be that the Chinese Government is now seizing on the July 16 test to explain away a later FOBS test.
click map to enlarge
UPDATE 21 October 2021 10:25 UT:
New information circulated by Demetri Sevastopulo, the FT journalist that broke the story, indicates that there were *two* tests, on July 27 and August 13. The first date tallies with rumours that reached me on July 29 about an 'unusual' Chinese test apparently having taken place (that I at the time erroneously though might refer to the July 16 'space plane' test).
CSpOC/18th Space Control Squadron report that the CZ-5B rocket booster 2021-035B met a fiery end at 2:14 UT last night (May 9) over the Arabian peninsula and Indian Ocean. China reports reentry at 2:24 UT in the Indian Ocean near the Maldives
click map to enlarge
UPDATES 9 May 2021, 9:00 UT and 14:30 UT:
In the map above, I have plotted the approximate final trajectory (based on a SatEvo-evolved orbit propagated to reentry) and both the reentry locationsreported by China (2:24 UT, 2.65 N, 72.64 E, in the Indian Ocean near the Maldives) and by CSpOC/18th Space Control Squadron (2:14 UT, 22.2 N 50 E, over the Arabian Peninsula).
Note how within minutes of the reentry, the rocket passed over the city of Riyahd in Saoudi-Arabia!
In looking at the plot, one should realize that a reentry is not an instantanious moment, but a process stretching over many minutes, where the object starts to break up and fragments burn up in the upper atmosphere. During this process, the object continues to move over a swat of trajectory that can be hundreds to a few thousands of kilometers long. It is very likely that this proces started over the Middle East or even over the Mediterranean already [edit: imagery from Jordan suggest that the object was still intact when passing over that location, but see a potential video from Oman below]. If fragments survived, they are scattered somewhere along the yellow line in the map, within the current uncertainties of the reentry location.
The video footage in the tweet below appears to show the (start of the) reentry, imaged from Oman, but is unverified for now:
فيديو من سلطنة عمان يبين بداية دخوله للغلاف الجوي.
بحسب مصادر صينية، سقط الحطام في المحيط الهندي شمال المالديف.
بحسب مصادر أمريكية دخل الغلاف الجوي الساعة 02:14 أثناء مروره فوق سلطنة عمان وسقط الحطام أخيرا شمال المالديف. pic.twitter.com/aR6FEUHMd0
— مركز الفلك الدولي (@AstronomyCenter) May 9, 2021
I have tried, by fiddling with the area-to-mass ratio for the rocket in GMAT, to create a reentry trajectory that would match a splashdown in the Maldives around 2:24 UT, as reported by China. The closest I can get to the location reported by China is this result:
click map to enlarge
Of course, this is a simplified model (the proverbal "spherical cow in a vacuum") that does not take into account both mass loss from ablation and fragmentation over the reentry trajectory.
It however suggest that, if the time and location reported by China is right, the rocket stage would have been at an altitude of ~100 km around the position and time reported by CSpOC (the US military tracking network). It could be that the CSpOC position, with the quoted very small uncertainty in time of +- 1 minute, is based on a SBIRS satellite detection of the fireball (we have long suspected that TIP's with such 1-minute uncertainty quotes are based on satellite detections of the reentry fireball).
About 100 km is a fair altitude for the ablative phase to start, similar to the altitude at which meteoric fireballs start. If the video from Oman above is the real deal, it is indeed suggested that ablation (and break-up) was starting over southern Arabia.
As a further update: interesting information about infrasound detections of the reentry from Djibouti, with source over the Arabian peninsula:
(below is the pre-reentry version of this frequently updated post):
(this part below last updated 8 May 23:00 UT)
On 29 May April, China used a CZ-5B rocket to launch the core module of it's new Space StationTianhe-1 ("Harmony of Heavens-1"). The module was initially placed in a 41.5 degree inclined, 382 x 171 km orbit and subsequently raised to a 385 x 352 km orbit.
The massive CZ-5B core stage from the CZ-5B rocket used for this launch was left in a 375 x 170 km orbit. Like it's predecessor that flew in May 2020, it was not deorbited after payload release. This likely means that it will come down in an uncontrolled reentry somewhere on May 8 or 9.
Compared to 'normal' rocket upper stages (which are typically 3 to 10 meters long and say 1000-2500 kg in mass), the CZ-5B core stage is huge. It is 31 meters long, 5 meters in diameter, and very heavy: sources differ on the dry mass, but it is somewhere between 17 and 22 tons.
The informal Space industry standard for objects that are heavier than 10 tons, and launched into Low Earth Orbit, is to have a deliberate deorbit over an empty stretch of Ocean. This did not happen with the CZ-5B core stage from the 5 May 2020 launch a year ago, and does not seem to happen now following the Tianhe-1 launch either.
An uncontrolled reentry of an object this large and heavy means that sizable fragments can survive reentry and reach the surface of the earth, with the risk that this happens over inhabited parts of the world. This is not something that you want, from safety concerns. Several analysts have gone on the record calling the lack of a controlled reentry for the CZ-5B core stage 'irresponsible' for that reason.
I’ll just reiterate that uncontrolled re-entries of large rocket stages in 2021 is absolutely irresponsible behavior https://t.co/Q6RZAyFPpZ
With the first lauch and uncontrolled reentry of a CZ-5B core stage a year ago, the question was whether a deliberate deorbit was planned but failed, or whether the CZ-5B core stage simply does not have a deorbit capability. As history now seems to repeat with this second CZ-5B launch, it starts to look like the CZ-5B indeed has no deorbit capability. This is highly surprising, and, indeed, irresponsible imho.
At the same time, while there is a risk (see what happened a year ago in Ivory Coast), the risk should not be overstressed. The risk that a random passenger aircraft ends up falling on your house is still orders of a magnitude larger, and we all have come to accept that risk.
We should also realize that much of the 17 to 22 tons mass of the stage will burn up before it reaches earth surface. Moreover, as a large part of the Earth consists of Ocean, it is likely that it will come down harmlessly over some Ocean.
Yet, it cannot be excluded that it will come down over a populated area, so some worry is justified. With an orbital inclination of 41.5 degrees, locations between 41.5 N and 41.5 S are in the danger zone for this reentry. The danger is slightly elevated at the extremes of this (41.5 N and 41.5 S). The latitude range where the CZ-5B booster can come down includes the whole of the United States, Australia, Africa, the southern parts of Asia and much of southern America. Europe is mostly safe, except for Spain, Italy and the Balkans:
When the reentry happens, the rocket stage will break up in the atmosphere and any surviving parts that have not burned up completely will rain down along the ground path along a very long stretch of earth: the area where fragments fall down can be hundreds of kilometers long.
PREDICTIONS
The diagram in the very top of this post gives reentry predictions. I will update it every time I run a new prediction. Below are the same predictions in table form. They are based on modelling of the orbital evolution in the General Mission Analysis Tool (GMAT).
Modelling is done for a mass of 17 tons (note that the true dry mass of the rocket stage is a bit uncertain: quoted values range between 17 and 22 tons, depending on the source) and with a drag surface at 60% of the maximum surface, as I have found that this generally fits well with tumbling rocket stages (which as a result of the tumbling have a variable drag surface).
Each new prediction is based on a new orbital update from CSpOC. The MSISE90 model atmosphere is used, with past, current and estimated future Space Weather.
Note that these predicted reentry times are nominal values only. PLEASE NOTE THE VERY LARGE UNCERTAINTY IN THESE TIMES!
The uncertainty margins shown are calculated as 20% of the time between the orbital epoch on which the prediction is based, and the predicted reentry time.
Note that the nominal position shown is only nominal: it is for the center of the uncertainty interval, but as long as the uncertainties in the reentry time measure in the hours, it is basically meaningless.
DATE *NOMINAL* TIME (issued) (*nominal* position)
8 May 19:23 UT +- 22 hr (May 4.22)
8 May 21:23 UT +- 21 hr (May 4.46)
8 May 20:40 UT +- 21 hr (May 4.53)
8 May 19:41 UT +- 20 hr (May 4.59)
8 May 20:46 UT +- 18 hr (May 5.08)
8 May 21:36 UT +- 18 hr (May 5.20)
8 May 21:18 UT +- 16 hr (May 5.51)
9 May 00:10 UT +- 16 hr (May 5.58)
9 May 04:40 UT +- 13 hr (May 6.50)
9 May 03:52 UT +- 10 hr (May 7.18)
9 May 04:01 UT +- 9 hr (May 7.36)
9 May 03:36 UT +- 5 hr (May 8.17)
9 May 03:10 UT +- 3.5 hr (May 8.40) (07 N 108 W)
9 May 03:11 UT +- 1.8 hr (May 8.77) (11 N 103 W)
9 May 02:54 UT +- 1.3 hr (May 8.86) (31 S 155 W)
Uncertainties in the predicted reentry times will remain very large untill very shortly before the actual reentry.
click map to enlarge
Within the 2 hour wide uncertainty window of the current CSpOC TIP (9 May 2:04 UT +- 1 hr), which no doubt is more reliable than my amateurish efforts, the rocket stage can come down anywhere on the lines drawn in the map above (the lines show the rocket's trajectory over the uncertainty window). Within the risk window, the trajectory runs over central America, southern Europe, Arabia and the southern tips of Australia. China itself now appears to be out of the risk zone, ironically enough. The nominal point (but: with such a wide uncertainty that it is still basically meaningless!!!) is over Spain
The yellow dots in the map are those cities with populations of a million or more between 41.5 N and 41.5 S.
Other sources of predictions:
Most notably: Space-Track (the CSpOC portal: requires an account), which should be regarded as the most authoritive one.
Earlier this month I wrote a post about China's brand new, recently launched and landed 'Reusable Test Spacecraft' (2020-063A), probably a 'Spaceplane' similar to the US X-37B. It was launched on September 4 from Jiuquan, and landed on September 6 at Lop Nor, after two days on orbit (see a previous post).
As I noted near the end of that post, it left something in orbit: an object of unknown character, which the US Military tracking network now calls 'Object A' (a bit confusing I think, as the COSPAR code is 2020-063G - so I'd called it 'Object G'). It is in a 347 x 331 km orbit.
click diagram to enlarge
This does not appear to be just a piece of debris - e.g. some discarded cover. Radio observers discovered that it sends a signal in the L-band near 2280 MHz, something debris doesn't do. So, this appears to be an interesting object that had or has some function, including a radio data signal downlink. It does not appear to have manoeuvered so far, and if it is tumbling (see below) it isn't likely to do so..
I initially thought that it might be a cubesat, but it appears to be rather large for that. At maximum brightness it reaches magnitude +4, i.e. it is visible to the naked eye. Speculation is that it is either an inspector satellite used to inspect the outside of the Chinese spaceplane before landing: or maybe some jettisoned support module. The ejection from the 'Reusable Test Spacecraft' appears to have taken place some two revolutions before landing, or perhaps even earlier (see brief analysis at the bottom of a previous post).
I filmed the object this morning with the WATEC 902H equipped with a 1.8/50 mm lens - see the movie above. The mysterious object showed slow but marked brightness variations, between magnitude +4 and invisible (= fainter than +7). This confirms reports by radio observers of periodic fading in the signal.
Below is the brightness curve that I extracted from my video, using LiMovie. I was handtracking the object, and halfway lost it for over half a minute when it became too faint for the WATEC 902H (equipped with a 1.8/50 mm lens): hence the half-minute gap in the curve. The other, smaller gaps in the curve are moments that I repositioned the camera. One of these days, I really have to start using a motorized mount tracking on the satellite for this kind of endeavours.
The curve shows two brightness peaks, and two major fading episodes. Peak-to-peak period is about 80 seconds, so if this is due to a tumble, it is a slow tumble.
click diagram to enlarge
When I first picked it up (it had just come out of earth shadow), it initially was very bright and steady (see the movie in top of this post). But then it started to get fainter, untill I momentarily lost it. When I picked it up again, it was becoming brighter again, and after a slow peak, it faded again to invisibility. The fades are faster than the brightening phase and brightest phase.
Early September 2020, the space tracking community was in nervous anticipation of a rather mysterious Chinese launch. Amidst tight security measures, a Changzeng-2F (CZ-2F) rocket was readied at SLS-1 of Jiuquan's Launch Area 4. Chinese tracking ships were taking up positions near South America and in the Arabian Sea. Two NOTAM's appeared suggesting a launch between 5:20 and 6:00 UT on September 4. Something was afoot! Speculation was, that this was the long anticipated inaugural launch of a robottic Space Plane, a version of the Shenlong, China's answer to the American Air Force's X-37B robottic Space Plane.
Then, on September 4th, the Chinese news agency Xinhua published a very brief news item announcing that a CZ-2F from Jiuquan had launched a 'Reusable Experimental Spacecraft' earlier that day.
The bulletin was scarce in information but stated that "after a period of in-orbit operation, the spacecraft will return to the scheduled landing site in China. It will test reusable technologies during its flight, providing technological support for the peaceful use of space".
No further details were given on launch time, orbit or character of the spacecraft. The description of the spacecraft is a bit ambiguous. Instead of a space plane, a 'reusable spacecraft' could in theory also be some sort of capsule (e.g. like the SpaceX Dragon): but most analysts think this indeed refers to the long rumoured space plane, China's answer to the US X-37B.
Pre-launch, and based on the positions of the hazard zones from the two NOTAM's, I calculated a launch into an orbital inclination of ~45 degrees, incidentally similar to the orbital inclination of the X-37B OTV 6 mission currently on-orbit. What's more, the launch window given (the NOTAM windows were from 5:23 to 6:05 UT) indicated the possibility of a launch into the orbital plane of OTV 6! The orbital plane of OTV 6 passed over Jiuquan at 6:00 UT - near the end of the launch window.
I published the following expected track for a launch into a 45 degree inclined orbit (which we now know is wrong):
Initial pre-launch trajectory guess. Click map to enlarge
When later that day the first orbital elements by the US military tracking network appeared on the CSpOC portal, it turned out that the orbital inclination was not ~45 degrees, but 50.2 degrees, 5 degrees higher than I anticipated. The reason for the mismatch, is that the rocket apparently did a dog-leg manoeuvre during ascend. This is very clear when we plot the orbital ground track in relation to the launch site and hazard zones from the two NOTAM's: it passes obliquely between them rather than lining up.
Actual orbital track. Click map to enlarge
A 'dog-leg' manoeuvre is usually done for safety reasons, to avoid overflying a particular area downrange (e.g. a city or a foreign nation), but can also be done to insert the spacecraft into an orbital inclination that otherwise cannot be reached from the launch site. The latter is however not the case here - [editted] the orbital inclination is higher than the launch site latitude (you cannot reach an orbital inclination that is lower than your launch site latitude without a dog-leg, but higher you can.). So the reason must be range safety.
It is clear that the launch occurred well outside the NOTAM time window (why, is not clear). My analysis, based on a proximity analysis using the orbits of the spacecraft, the upper stage of the CZ-2F rocket, and that of four engine covers ejected upon spacecraft separation, indicate spacecraft separation and insertion into orbit around 7:41 UT on September 4th, over the Chinese coast with the orbital plane lining up with Jiuquan (see image below which depicts the orbital position at orbit insertion). The launch itself then should have occured some 8-10 minutes earlier i.e. around 7:30 UT, give or take a few minutes.
Moment of orbital insertion. click to enlarge
The spacecraft was inserted into a 50.2 degree inclined, initially 332 x 348 km orbit. During the hours after launch, the spacecraft made small orbital manoeuvres (see diagram below). At the time of writing (5 September 20:45 UT) it is in a 331 x 347 km orbit.
click diagram to enlarge
The later than initially expected launch time and, through a dog-leg manoeuvre, insertion into a 50.2 degree inclined orbit moved the orbital plane away from that of the X-37B OTV 6, although the two orbital planes are still near. Igor Lissov has pointed out some resemblance to the orbital plane of another US classified payload, USA 276, which has a similar orbital inclination to the Chinese spacecraft (but 50 km higher orbital altitude). The RAAN difference is 8 degrees:
click to enlarge
Based on the current orbits of all three spacecraft, there will be no close approaches of the Chinese spacecraft to either of these classified US payloads over the coming two weeks.
OTV 6 is currently in a 383 x 391, 45.0 degree inclined orbit. The difference in RAAN with respect to the Chinese spacecraft is 13.4 degrees, with a 5.2 degree difference in inclination and about 40-50 km difference in orbital altitude.
USA 276, the mysterious spacecraft that made a close approach to the ISS in May 2017 (see my July 2017 article in The Space Review), is currently in a 397 x 395, 50.0 degree inclined orbit. The difference in RAAN with respect to the Chinese spacecraft is 7.9
degrees, with a 0.2 degree difference in inclination and about 50-60 km
difference in orbital altitude.
The Chinese 'reusable' spacecraft was launched from SLS-1, one of two launch platforms at
Launch Area 4 of the Jiuquan Space Launch Center. Below is a Copernicus
Sentinel 2B image of the launch complex, taken on September 2nd, two
days before the launch. The two launch platforms are indicated: the southernmost one is the platform used for this launch.
click image to enlarge
It will be interesting to see where the 'reusable spacecraft' will eventually land. One likely candidate is a military airfield, the Dingxin Test and Training Base, that is located some 75 km southwest of the launch site. I have indicated both the launch site (A) and the potential landing site (B) in the Copernicus Sentinel 2B image below. The second image gives a more detailed look on the airbase.
Click image to enlarge
Click image to enlarge
We have no clue how long the spacecraft will stay in orbit. It will be interesting to see when and where it lands.
The 'reusable spacecraft' has the CSpOC catalogue entry #46389 (COSPAR ID 2020-063A). The CZ-2F upper stage is object #46390 (2020-063B). The four ejected engine covers (with apogees in the 458 to 566 km range), have numbers 46391-46394 (2020-063A to 202-063F).
UPDATE 6 Sept 2020 8:45 UT:
Xinhua reports on Sept 6 that the spacecraft has landed after 2 days on-orbit. Depending on the landing site, landing should have been (based on orbital overpass) either around 1:55 UT at Lop Nor (an alternative landing site suggested), or 6:45 UT at Dingxin Airbase.
UPDATE 2, 9:30 UT: As the Chinese version of the Xinhua bulletin dates to an hour after the first option (1:55 UT), it seems that the landing was near 1:55 UT near Lop Nur in the Taklamakan desert (HT to Jonathan McDowell).
UPDATE 3, 10:30 UT: This is the potential landing site, a triangular arrangement of 5 km long landing strips in the Taklamakan Desert. The orbital track of the spacecraft passed some 42.5 km northwest of it around 1:54 UT, more or less parallel to what appears to be the main landing strip:
Click image to enlarge
Click image to enlarge
UPDATE 4, 14:00 UT: This is an updated diagram of the orbital evolution over the test flight. It seems no large manoeuvers were tried during this flight.
Click diagram to enlarge
UPDATE 5, 16:00 UT:
Jonathan McDowell noted that a new object related to the launch has been catalogued, object 2020-063G, #46395. My analysis suggests it was ejected from the experimental spacecraft near 22:25 UT on the 5th, two revolutions before landing. It likely is a cubesat of some sort. It is in a 332 x 348 km, 50.2 degree inclined orbit. (Update 8 Sept:on Twitter, Bob Christy has suggested that it might be a small inspector satellite, used to inspect the outside of the experimental spacecraft prior to deorbit)
Just after local midnight of August 7-8, 2019, the South Korean amateur astronomer Mr Lee Won-Gyu was taking images of the night sky at Mount Jiri in Korea when he observed and photographed a cloud-like illuminating phenomena in Corona Borealis that to the expert eye is clearly the exhaust cloud from a rocket engine burn.
Mr Lee Won-Gyu's images of the cloud are featured in this article in the Korea Times, where they were presented as a 'UFO'. The images were taken between 00:14 and 00:24 Korean time (corresponding to August 7, 15:14-15:24 UT). Mount Jiri, the location of the sighting, is at approx. 35.34 N, 127.73 E. In this blogpost, I will identify this 'UFO' as a Chinese ICBM test.
Initial speculation on the internet was that this was perhaps related to the AEHF 5 geosynchronous satellite launch from Florida on August 8, 10:13 UT. The observation was however done 19 hours before this launch (there was some initial confusion due to the date difference in local time and UT), and the cloud was seen in a wrong part of the sky for a launch to geosynchronous altitude. So I suggested it could be a Russian or Chinese ICBM test launch.
As it turns out, additional evidence suggests this indeed was an ICBM test, by China. As the result of a private request by me, Twitter user @Cosmic_Penguin managed to dig up NOTAM's for the date and time of the event posted on a Chinese forum by a forum member nicknamed 'kktt'. These NOTAM's with temporary airspace closures from "ground to unlimited" in two parts of China corroborate an ICBM test launch:
A4092/19 NOTAMN Q) ZBPE/QRTCA/IV/BO/W/000/999/3909N10940E019 A) ZBPE B) 1908071449 C) 1908071511 E) A TEMPORARY RESTRICTED AREA ESTABLISHED BOUNDED BY: N392016E1092107-N391413E1100213-N385819E1095815-N390419E1091716 BACK TO START.VERTICAL LIMITS:GND-UNL. ALL ACFT SHALL BE FORBIDDEN TO FLY INTO THE RESTRICTED AREA. F) GND G) UNL
A4094/19 NOTAMN Q) ZWUQ/QRTCA/IV/BO/W/000/999/3712N08311E108 A) ZWUQ B) 1908071451 C) 1908071548 E) A TEMPORARY RESTRICTED AREA ESTABLISHED CENTERED AT N371133E0831033 WITH RADIUS OF 200KM. ALL ACFT ARE FORBIDDEN TO FLY INTO THE TEMPORARY RESTRICTED AREA. VERTICAL LIMITS:GND-UNL. F) GND G) UNL
The NOTAM's have a time window between 14:49 UT and 15:48 UT on 7 August 2019, which fits the phenomena observed from Korea (7 August 15:14-15:24 UT). They also fit the direction of the sky phenomena as seen from Korea: the exhaust cloud was seen at 30 degrees elevation in the sky at azimuth 290-291 degrees (west-northwest). This sightline points directly to the area designated in NOTAM A4092/19.
The map below plots the two areas designated in the NOTAM's. The smaller rectangular area from NOTAM A4092/19 represents the launch area near Hongjian Nur in Shaanxi province. The larger circular area from NOTAM A4094/19 at the southern edge of the Taklamakan desert represents the RV target area. The two areas are some 2300-2400 km distant from each other:
click map to enlarge
I have depicted the sightline from Mr Lee Won-Gyu's photographs from Mt. Jiri in Korea on the map as well (white): it points towards the launch area and it lines up with the direction of that rectangular area. Both time and direction therefore fit the Korean sighting. So does the character of the photographed cloud, which is similar to missile exhaust clouds observed during other ICBM launches.
This was an interesting ICBM launch in that it appears to have been highly lofted, with an apogee at approximately 3000 km altitude. This is based on both the estimated flightime (about 37 minutes) deduced from the NOTAM time window durations; and from an assessment of the exhaust cloud sightings from Korea, the direction and elevation of which point to a burn at 3000 km, close to apogee of the orbit, when combined with a ballistic trajectory between the two areas of the two NOTAM's. The launch happened near 15:00 UT (August 7), the missile engine burn seen from Korea happened some 15 minutes later close to mid-course and was probably meant to change the direction of the missile.
The situation is spatially depicted in the diagram below. The sightline from Korea crosses a 3000 km apogee trajectory twice, at about 2300 km altitude when the missile is ascending, and near apogee at 3000 km altitude. The latter altitude is the most likely location of the engine burn. At these altitudes, exhaust clouds are well above the earth shadow and hence brightly sun-illuminated.
click image to enlarge
When launched on a less lofted trajectory, this missile would have had a ground range of at least 6300 km. The reason to launch it into a lofted trajectory, rather than a more typical trajectory with apogee at 1200 km, is that in this way the test could be done completely within the borders of China. We have seen such lofted trajectories earlier with some early North Korean ICBM tests.
The ICBM appears to have done a dog-leg manoeuvre near apogee, changing the course just before mid-course. One piece of evidence for this is that the orientation of the launch hazard area from NOTAM A4092/19 does not match with a simple ballistic trajectory towards the target area. Neither does the sightline direction from Korea. They would result in a target area more to the north than the area from NOTAM A4094/19.
This can be well seen in the map, where I depicted both a direct ballistic trajectory (solid black line) between the two areas from the NOTAM's, as well as a 'dog-legged' trajectory (dashed black line), with the dogleg at the near-apogee burn imaged from Korea and initial launch direction according to the orientation of the NOTAM A4092/19 area:
click map to enlarge
The direct trajectory clearly does not fit the launch area direction and Korean sighting well, whereas a launch into the direction of the NOTAM A4092/19 area and a dogleg near apogee does, with the latter also clearly fitting the Korean sighting.
A reason for such a dog-leg manoeuvre might be to confuse and evade mid-course anti-Ballistic missile intercepts. So I am wondering if this perhaps was an anti-ballistic missile test as well.
This missile test must in theory (and ignoring cloud cover) have been widely visible over Eastern Asia. The Korean Times article presents one other observation, also from Korea, but I have not seen other observations so far.
UPDATE: Twitter user @LaunchStuff sent me this link to a Weibo page, which includes several photographs of the event from various parts of China and a very cool video shot from Inner Mongolia, showing the spiralling behaviour seen during other ICBM tests as well.
Acknowledgement:I thank Ravi Jagtiani for bringing the Korea Times article to my attention; @Cosmic_Penguin for digging up the NOTAM's; and Jim Oberg and Jonathan McDowell for discussions.
[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.
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.
