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.
A weird Navigational Warning (NAVAREA XII 384/21) for "Space Debris" has appeared defining nine areas, some of them overlapping, in the Pacific for August 9, 16:27 to 17:29 UT and August 10, 17:16 to 18:17 UT.
I have mapped them in the map above. Below is the text of the Navigational Warning:
060929Z AUG 21
NAVAREA XII 384/21(GEN).
EASTERN NORTH PACIFIC.
1. HAZARDOUS OPERATIONS, SPACE DEBRIS
091627Z TO 091729Z AUG, ALTERNATE
101716Z TO 101817Z AUG
IN AREAS BOUND BY:
A. 22-52-40N 137-34-57W, 20-12-47N 134-02-08W,
04-25-05N 146-28-48W, 06-54-48N 149-55-52W.
B. 51-11-05N 141-36-54W, 49-40-18N 142-13-53W,
50-44-15N 170-19-30W, 52-17-11N 170-39-50W.
C. 12-58-15N 130-00-21W, 10-52-28N 127-06-04W,
05-17-31S 138-47-34W, 03-13-54S 141-40-25W.
D. 48-12-47N 135-38-42W, 46-20-17N 136-55-43W,
50-55-14N 165-28-28W, 52-59-09N 165-19-24W.
E. 13-53-47N 126-52-33W, 11-46-05N 123-56-09W,
04-19-41S 135-37-56W, 02-14-45S 138-32-32W.
F. 49-27-33N 135-51-45W, 47-43-47N 136-53-00W,
50-56-51N 168-09-57W, 52-48-04N 168-20-28W.
G. 14-27-06N 127-19-28W, 12-18-52N 124-23-30W,
03-36-29S 136-03-34W, 01-31-24S 138-57-30W.
H. 49-46-04N 136-40-41W, 48-05-08N 137-37-30W,
50-55-01N 168-54-51W, 52-42-19N 169-08-13W.
I. 31-49-12N 124-20-42W, 30-20-18N 122-34-43W,
22-47-14N 130-25-52W, 24-10-15N 132-10-44W.
2. CANCEL THIS MSG 101917Z AUG 21.
The nine areas A to I cluster in basically three regions (which I have colour-coded in the map above).
The directions of the areas point to a series of deorbits from a 51-53 degree inclined Low Earth orbit. As I have indicated in the map in top of this post, two of the three defined regions with warning boxes line up with the ISS groundtrack during the two time windows given, but I think this is coincidence (and the series of boxes south of Alaska do definitely not line up with the ISS during these two time windows. In fact, this points to deorbits from at least two different orbital planes).
Rather, my suspicion is a mass deorbit of Starlink satellites, who move in ~53 degree inclined orbits [but see update below].
UPDATE:
After some discussion, Jan Hindrik Knot rightfully questioned whether Starlink satellites, with their ion thruster propulsion, are capable of a controlled deorbit in a designated area at all. That is a good point, which I overlooked initially.
So it appears we have no idea what will be deorbitted on August 9-10.
The combination of the areas in the mid-Pacific and those south or Alaska, to me point to deorbits from at least two different orbital planes (both inclined 51-53 degrees).
Note that, from the position of the areas, the fact that their shapes clearly point to deorbits from Low Earth Orbit, and that the NavWarning mentions time windows on two successive dates, it is clearly not related to this deorbit (the Spectr-R rocket booster) from Deep Space either.
UPDATE 2:
The plot thickens: the on-line KML version of the Navigational Warning has appeared and mentions:
"Authority: NASA 300917Z JUL 21"
(the versions sent to subscribers to the service doesn't mention the authorities issuing the warnings).
So it appears to be something NASA-related (HT to @john_moe on Twitter).
One possibility could be that these are emergency landing zones for Starliner (which was to be launched on July 30, the date mentioned in the "Authority:" line: but was scrubbed). Still open questions though: why August 9 and 10? Why where these same zones not published before the July 30 launch date? Questions, questions...
UPDATE 3:
I like the suggestion by Bob Christy that these are warnings for the reentry of the Starliner service module (that is jetissoned from the Starliner capsule before landing of the latter). That makes sense.
(this post is updated when I have run new predictions)
click diagram to enlarge
The Proton-M third stage from the July 21 Nauka launch (see previous post) is coming down fast. The current reentry forecast models place the reentry into the atmosphere in the early hours of August 6 UT.
The diagram above shows CSpOC TIP data in red, and my own GMAT model results in black. My GMAT predictions in tabular form:
DATE UT +- LAT LON orbit epoch 6-8-2021 9:55 1.8 day 28-7-2021 12:12 6-8-2021 16:51 1.7 day 29-7-2021 06:01 6-8-2021 9:18 1.4 day 30-7-2021 04:16 6-8-2021 8:29 1.2 day 31-7-2021 05:28 6-8-2021 9:47 1.0 day 1-8-2021 05:09 6-8-2021 18:23 23 hr 1-8-2021 22:54 6-8-2021 21:00 23 hr 2-8-2021 03:19 6-8-2021 13:52 17 hr 3-8-2021 00:31 6-8-2021 11:44 14 hr 3-8-2021 16:12 6-8-2021 9:29 11 hr 15 S 177 W 4-8-2021 00:05 6-8-2021 7:50 8 hr 19 N 180 W 4-8-2021 15:44 6-8-2021 6:40 6 hr 38 S 108 W 4-8-2021 23:04 6-8-2021 6:26 5 hr 4 N 146 W 5-8-2021 03:27 6-8-2021 2:56 2.9 hr 25 N 112 E 5-8-2021 12:14 6-8-2021 5:07 2.5 hr 22 S 104 W 5-8-2021 16:37 6-8-2021 5:34 1.7 hr 29 S 24 E 5-8-2021 21:00 6-8-2021 4:49 1.6 hr 32 N 148 W 5-8-2021 21:00*
The last orbit was re-issued with an epoch almost similar to the previous orbit. This orbit is indicated by an asterisk and my final forecast.
Within current uncertainty windows, no meaningful prediction can be given about the location of the reentry yet. The values nevertheless listed in the tabe for latitude and longitude are nominal values only for the middle of the quoted uncertainty interval (which spans multiple revolutions around the Earth). Given the current uncertainty intervals, they are basically meaningless. Only an hour or so before the actual reentry, the uncertainty interval becomes less than one revolution.
The map below shows the nominal GMAT and last pre-reentry CSpOC TIP positions, plus the trajectory over the uncertainty window [EDIT: see update with final TIP at end of post!!!!]:
click map to enlarge
The rocket stage has a dry mass of about 4 tons and is about 4 x 4 meter wide. The diagram below shows the evolution of the orbital altitude of the rocket stage so far, based on CSpOC tracking data. Perigee is the lowest point in it's elliptical orbit around earth, apogee the highest point. Altitudes refer to the equatorial radius of the earth.
The last few orbits shows signs of trouble in determining the (quickly
evolving) orbit (look at the perigee values for the last four orbits issued). The last available orbit was issued in two versions
click diagram to enlarge
On July 21, a few hours after launch, I filmed the Proton rocket stage during a pass over Leiden, accompanied by three pieces of debris that were never catalogued by CSpOC:
(EDIT: see update below movie!)
UPDATE:
The final CSpOC TIP is in: 4:46 +- 1m UT (August 6) near 37.8 N 155.7 W, north of Hawaii (this is probably based on a SBIRS detection of the reentry fireball, given the very accurate +- of 1 minute).
This is very close to my last nominal GMAT estimate (4:49 UT near 32 N 148 W)!
In all honesty: given the uncertainty intervals, that very good match is down to pure luck....
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.
On 7 September 2020 near 2:14 UT (6 September 22:14 local time) a bright fireball appeared over Mexico, creating some media attention. As part of that attention, a video surfaced and was widely retweeted, purporting to show this fireball. The image above is a screenshot of this video.
However: the object on this video is not the fireball from 7 September 2020.
It is an 'old' recycled video from July 2020, showing a space debris reentry.
The video shows a very slow fragmenting object that is clearly reentering space debris. There was something familiar to it, which was one thing that raised my suspicion (I thought I had seen it before). The other thing that raised my suspicion was that this video clearly does not show the same object as other videos that showed up, which show the genuine September 7 fireball (like this one) .
Doing a Google Reverse Image Search quickly turned up Reddit posts from July 2020 (e.g. this one), featuring this same video, indicating that the footage was at least 2.5 months old (and hence definitely not the fireball of 7 September, confirming my suspicions).
The video does show a genuine reentry. The reentry in question happend on July 18th, 2020. The Reddit post linked above is from that date. Other video's of clearly the same reentry that was also seen from the USA posted on that date exist too.
And this is why the video looked so familiar to me: back in July I already identified footage of the same reentry as the reentry of a Russian Soyuz rocket stage (2019-079C), the second stage from the Soyuz rocket that launched the military Kosmos 2542 satellite on 25 November 2019.
According to a CSpOC TIP message from July 18th 2020, this rocket stage reentered on 18 July 2020 07:02 UT (+/- 1 minute: this time accuracy indicates a SBIRS or DSP infra-red detection of the reentry) near 26.8 N, 101.2 W, over Northeast Mexico near the border with Texas. The map below depicts the final trajectory of the rocket stage and the CSpOC reentry position:
Click map to enlarge
This case highlights again that footage appearing on Twitter or other social media after an event is not always what it purports to be, and one should always check whether it shows what it purports to show.
On May 7 I imaged a pass of the Kosmos 482 Descent Craft (1972-023E), using the WATEC 902H camera and a SamYang 1.4/85 mm lens. This is a very interesting object on which I have blogged earlier.
It is the ascend module of a 1972 failed Soviet Venera probe,
meant to land on Venus but stuck in Earth orbit after its apogee kick
engine failed to push it into Heliocentric orbit towards Venus in 1972. This is the video from (a part of) the May 7 pass of this object:
The object on the video, at that time at an altitude of about 1640 km and range of 1845 km, is about 1 meter large and weighs 495 kg. It should look like this:
photo: NASA. Click to enlarge
The photo above is not Kosmos 482 itself, but an exhibit replica of a sister ship, the Venera 8 landing module in its protective shell. Venera 8 was launched four days before Kosmos 482, and unlike the latter it was successful and did reach Venus.
The failed Kosmos 482 probe still in Earth orbit was launched from Baikonur on 31 March 1972, and put in a highly elliptical 220 x 9200 km parking orbit around Earth. It's apogee kick engine next failed to push it into a heliocentric orbit towards Venus, and the spacecraft then broke up into four pieces.
Three of these four pieces have already reentered, the fourth, that is believed to be the landing module in its protective shell, is still on-orbit and is the object I imaged. It's apogee altitude has been lowering significantly since 1972. The object will probably reenter somewhere around late 2025 or early 2026: I wrote an extensive blog post about it including a lifetime simulation a year ago.
The diagram below is from that post and shows the observed orbital decay up to March 2019, and the future decay (light blue) that I modelled with GMAT:
click diagram to enlarge
The interesting thing is that the Kosmos 482 Descent Craft might survive reentry largely intact! It is, after all, a lander that was meant to survive ascend through the thick atmosphere of Venus. It's parachute system will probably no longer function (so it will impact rather than land), but we can expect the hardware to reach Earth surface largely intact.
From a Space Heritage point of view, both this and its history makes this 48-year-old piece of Soviet Space hardware a highly interesting object. This is material culture that represents humanities' babysteps in the exploration of other planets.
Which makes this an interesting object to image, from a "Space Archaeology" viewpoint, and an interesting object to keep an eye on the coming years, until it reenters about six years from now.
Added Note, 9 May 2020 13:30 UT:
In response to my statement that the object likely is the lander in its enclosing protective shell, several people have pointed me to telescopic imagery that purportedly would show that a part of the main bus is still attached.
I (and many others in the amateur satellite community - we had a heated discussion on it on the Satobs list a few years ago) distrust imagery of this kind. This is imaging at the edge of resolution, in this case also notably from a non-stable imaging platform (handtracking a moving object at the limit of resolution!). It unfortunately includes cherrypicking frames. It is very difficult to objectively determine what is real detail and what is artefact of the imaging procedure. It is easy to overinterpret.
I also want to note that taking the mass and dimension of the lander only, actually give a very good fit to the observed orbital decay.
— Israel Borja Nuñel Timiraos (@Canaan_1983) April 28, 2020
The movie was shot from A Coruña in NW Spain this morning (28 April
2020) around 6:45 local time, which equates 4:45 UT. It clearly shows space debris reentering and breaking up.
Here are a few screenshots from the video:
A brief look in the CSpOC TIP messages showed a very clear candidate: 2020-026B, the upper stage from the Soyuz rocket that launched Progress MS-14 to the ISS on 25 April.
The CSpOC TIP lists the reentry for this object at 4:45 +- 1 minute UT for 28 April, near 38.4 N, 15.5 W, west of Portugal. This matches both time and location of the Spanish movie well.
Below is a map I created showing the final revolution of this rocket stage. The red circle is the nominal CSpOC position for the reentry (we suspect these "+-1 minute" positions are based on SBIRS detections). A Coruña where the video was shot is also indicated in the map.
Note that an observer in A Coruña looking towards the trajectory would see it move from right to left (towards the east), and this matches the video. Also note that while CSpOC gives an instantanious time in its TIP messages, reentries in reality take some time (several minutes). The object would pass A Coruña about 2 minutes after the nominal CSpOC time, which is well within a typical reentry duration.
click map to enlarge
Addition 17:15 UT (28 April):
For clarity: the trajectory above was created by taking the last available orbital elements for 2020-026B (elset 20119.06935500) and evolving these to a final decay orbit with SatEvo.
On 30 January 2020 near 12:30 UT (10:30 pm PST), a bright, slow, spectacularly fragmenting fireball swooped over southern California. It was seen and reported by many in the San Diego-Los Angeles area. The video above was obtained by a dedicated fireball all-sky camera operated by Bob Lunsford. The fireball duration approached 20 seconds.
In the hours after the fireball, the American Meteor Society (AMS) initially suggested that this was a Space Debris reentry, i.e. the reentry of something artificial from earth orbit.
But it wasn't.
Immediately upon seeing the video, I had my doubts. Upon a further look at the video, those doubt grew. To me, the evidence pointed to a meteoritic fireball, a slow fragmenting fireball caused by a small chunk of asteroid entering our atmosphere.
A discussion ensued on Twitter, until NASA's Bill Cooke settled the issue with multistation camera triangulation data, which showed that this was an object from an Apollo/Jupiter Family comet type heliocentric orbit with a speed of 15.5 km/s. In other words: my doubts were legitimite. This was not a space debris reentry but indeed a chunk of asteroid or comet.
I've already set out my argumentation about my doubts on Twitter yesterday, but will reitterate them again below for the benefit of the readers of this blog.
My doubts started because while watching the video I felt that the fireball, while slow and of exceptionally long duration, was still a tad too fast in angular velocity in the sky, and too short in duration, for this to be space debris. In the video, it can be seen to move over a considerable part of the sky in just seconds time.
The image below shows two stills from the video 6 seconds apart in time. The fireball passes two stars, alpha Ceti and beta Orionis, that are 35 degrees apart in the sky, and it takes the fireball a time span of about 6 seconds to do this, yielding an apparent angular velocity in the sky of about 5-6 degrees per second. That is an angular velocity that is a factor two too fast for reentering space debris at this sky elevation, as I will show below.
stills from the fireball video, 6 seconds apart, with two stars indicated
Orbital speed of a satellite is determined by orbital altitude.
Reentering space debris, at less than 100 km altitude, has a very well
defined entry speed of 7.9 km/s. This gives a maximum angular speed in
the sky of about 5 degrees/second would it pass right above you in the zenith (and only then): but gives a (much) slower speed (2-3 degrees/second) when
the reentry is visible lower in the sky, such as in the fireball video.
To gain some insight in the
angular velocity a reentering piece of space debris would have at the
elevation of the California fireball, I created an artificial
70 x 110 km reentry orbit over southern California that would pass the same two
stars as seen from San Diego.
The map below shows that simulated track, with the object (marked by the green rectangular box) at 70 km altitude and positioned 6 seconds after passing alpha Ceti (marked by the green circle):
Simulated reentry track. click to enlarge
The angular velocity in the sky for a reentering object at this sky elevation suggested by this simulation is barely half that of the fireball. During the 6 seconds it took the fireball to move over 35 degrees of sky passing alpha Ceti and beta Orionis, the simulated reentering object would have moved over only15 degrees, i.e with an angular velocity of 2.5 degrees/second rather than the 5-6 degrees/second of the fireball.
So this suggested that the fireball was moving at a speed a factor two too high for space debris. This therefore pointed to a meteoritic fireball, not a space debris reentry.
There were other reasons to doubt a reentry too. There were no matching TIP messages on Space-Track, the web-portal of CSpOC, the US military satellite tracking network. A reentering object as bright as the fireball in the video would have to be a large piece of space debris: this bright is clearly not the "nuts and bolts" category but suggests a large object like a satellite or rocket stage. It is unlikely that CSpOC would have missed a reentry of this size.
To be certain I ran a decay prediction on the full CSpOC catalogue with SatEvo myself: no object popped up that was expected to reenter near this date either, based on fresh orbital elements.
The fragmentation in itself, one of the arguments in the AMS' initial but mistaken conclusion of a "space debris reentry", is not unique to space debris reentries. It is also a common occurence with slow, meteorite dropping asteroidal fireballs, especially when they enter on a grazing trajectory. Take the Peekskill meteorite fall from October 1992 for example:
Likewise, while a 20-second meteor is not everyday, it is not a duration that is impossible for a meteor. Such durations (and even longer ones) have been observed before. Such long durations are especially the case with meteors that enter in a grazing way, under a shallow angle.
At the same time, a 20 seconds duration would be unusually short for a satellite or rocket stage reentry. Such reentries are usually visible for minutes, not a few seconds or a few tens of seconds.
So, to summarize:
1) the angular velocity in the sky appeared to be too large for space debris; 2) the fireball duration would be unusually brief for space debris; 3) and there were no obvious reentry candidates.
On the other hand:
a) the angular velocity would match those of slow ~15 km/s meteors; b) the 20 second duration, while long, is certainly not impossible for a meteor; c) the fragmentation observed occurs with slow asteroidal origin meteors as well.
Combining all these arguments, my conclusion was that this was not a space debris reentry, but an asteroidal origin, slow meteoritic fireball. This was vindicated shortly later by the multistation camera results of Bill Cooke and his group, which yielded an unambiguous speed of 15.5 km/s and as a result a heliocentric orbit, showing that this was not space debris but a slow chunk of asteroid or Jupiter Family comet.
In defense of the American Meteor Society (who do great work on fireballs): it is not easy to characterize objects this slow, certainly not from single camera images and visual eyewitness reports. Given the slow character and profuse fragmentation, it is not that strange that the AMS initially (but incorrectly) thought it concerned a space debris reentry. It does go to show that you have to be extremely careful in drawing conclusions about slow moving fireballs: not every very long duration fragmenting fireball is space debris.
It concerned an unusual object launched 47 years ago, called the Kosmos 482 Descent Craft (1972-023E, CSpOC nr 6073). Word was that it was about to reenter into the atmosphere, maybe even this year. But will it? Short answer: almost certainly not.
The source of the prediction is attributed to Thomas Dorman in the Space.com article, but how the prediction was done is not clear from the news coverage. On the request of David Dickinson, who was preparing an article on the topic for Universe Today, I made my own assessment of the issue. I looked at the orbital decay of 1972-023E since 1973 and did some GMAT modelling to gain insight into how the orbital decay will develop in the future.
As I will show in this post, my modelling suggests the Kosmos 482 Descent Craft is not to come down yet for several years.
Kosmos 482, a failed Venera mission
During the 1960-ies and '70-ies, the Soviet Union launched a number of Venera space probes destined for the planet Venus. Some of these probes did reach Venus and even briefly took pictures before succumbing to the very hostile atmospheric environment on this planet. But not all of the probes reached Venus. Several attempts went awry.
Kosmos 482, a probe similar to and launched only a few days after the Venera 8 probe, was launched from Baikonur on 31 March 1972. Reaching a highly elliptic parking orbit around Earth, its apogee kick motor failed to put it into an heliocentric orbit. The space probe broke up into at least four pieces that remained in Low Earth Orbit. Two of these, parts of the rocket engine, reentered within weeks of the failure. Another piece, presumably the main space probe bus, reentered in 1981.
A fourth piece, 1972-023E, is left on orbit, and it is interesting, as it most likely concerns the Descent Craft, the lander module in its protective cover that was to land on Venus, similar to the Venera lander module imaged in the photograph in the top of this post. That makes this a highly interesting object, as it will likely survive reentry into the atmosphere (it was designed to survive reentry into Venus' atmosphere after all).
Orbital decay 1973-2019
Initially stuck in a highly elliptic ~9600 x 220 km, 52.25 degree inclined orbit 47 years ago, its orbit has since decayed considerably. Currently (March 2019) it is in a ~2400 x 202 km, 52.05 degree inclined orbit:
click to enlarge
The diagram below shows how the apogee and perigee changed between January 1973 and March 2019. The orbit has become markedly less eccentric. Orbital decay strongly acted on the apogee altitude. The apogee altitude (blue line in the diagram) has come down steadily and by a large amount, from ~9600 km to 2397 km.This lowering of the apogee is to continue over the coming years. By contrast, the perigee altitude (red line) has changed only minimally, from 210 to 202 km over the past 46 years.
click diagram to enlarge
The apogee altitude will continue to come down. Once it is below ~1000 km, in combination with the low perigee at ~200 km. decay will go progressively fast.
Modelling future orbital decay
To gain insight into the validity of the claim that object 1972-023E might reenter this year, I modelled the future decay of the orbit using General Mission Analysis Tool (GMAT) software. Modelling was done for a 495 kg semi-spherical lander module 1 meter in size, using the MSISE90 model atmosphere.
The result suggests that the Kosmos 482 Descent Craft still has at least 5 to 7 years left on orbit. My model has it nominally reenter late 2025. Taking into account the uncertainties, a reentry between late 2024 and late 2026 seems most likely. That is still several years away.
click diagram to enlarge
click diagram to enlarge
The model result fits well with the trend in the actual tracking data, which gives confidence in the results (the thick lines in the diagrams above are actual tracking data, the thinner lines the GMAT modelled future orbital decay. The latter extend the previous trend in the tracking data well, there are no clear pattern breaks).
It should be well noted that modelling the decay of highly elliptic orbits with high apogee and low perigee is notoriously difficult. Yet, both the past and current orbital parameters and my modelling forecast do lead me to think a reentry is not imminent.
I am not the only one casting some doubt on a reentry of 1972-023E this year. Both NBCnews and Newsweek
quote earlier results by Pavel Shubin that predict reentry around
2025-2026, quite similar to my results. They also quote well-known and respected space analyst Jonathan
McDowell who is similarly opting for a reentry several years into the
future, rather than the coming year.
Conclusions
From my look at the current orbital decay rate and my modelling of future orbital decay, supported by assessments from other sources, it appears that the newsreports suggesting that the reentry of the Kosmos 482 descent craft is imminent and might even occur this year, are in error.
As to the why of the discrepancy: in the Space.com article, Dorman is quoted claiming "Our guess is maybe as much as 40 to 50 percent of the upper spacecraft bus may still be there". It is not clear at all what this "guess" is based on. My own modelling shows that the mass and size of the landing module only (i.e. without other parts still attached), fits the current orbital decay rather well. It is not clear how Thomas reached his conclusion, but modelling with a wrong mass and/or size might explain the discrepancy between my result and that claimed in the Space.com article.
I am hesitant with regard to accepting the high resolution imaging attempts by Ralph Vandebergh featuring in the Space.com article as evidence for object 1972-023E being more than the lander module only, as the weak and rather irregular protrusions visible might be image artefacts from atmospheric unrest and camera shake rather than real structure. Even when telescopically imaged at minimal range in perigee, we are talking about apparent object sizes at the arcsecond level and single pixel level here, conditions under which it is very challenging to image detail. Under such challenging conditions, spurious image artefacts are easily introduced.
Acknowledgement:I thank David Dickinson for encouraging me to probe this issue.
UPDATEMay 2020:
On 7 May 2020 I imaged a pass of the Kosmos 482 Descent Craft using the WATEC 902H and a Samyang 1.4/85 mm lens. Here is the video:
The image above is a still image from TV-footage shot during the January 5th 2019 cricket match of Sri Lanka against New Zealand at Mount Maunganui, New Zealand. The camera captured a bright, very slow, copiously fragmenting fireball that occurred during the match. Here is the actual footage:
From the video footage, the event had a duration of at last 1 minute, and likely longer. The event was widely seen and reported from New Zealand: more images and more noteworthy video footage, as well as descriptions, can be found in this news article from the New Zealand Herald.
From the footage it is clear that this is a space debris reentry: the event is too slow and of too long duration to be a meteoric fireball.
From a Sri Lankan tv-broadcast of the cricket match, which features a clock in the imagery, the time of the event can be established as 5 Jan 2018 at 07:58 UT (Sri Lanka has a time difference of 5:30 with GMT):
image from Lotus TV broadcast
From the time and location, the event can be identified as the reentry of Kosmos 2430
(2007-049A), a defunct Russian US-K Early Warning satellite launched in
2007. Time and location match well with a near perigee pass of this object over New Zealand. The
map below shows its predicted position for 08:00 UT on Jan 5 (movement is from top to bottom):
click ap to enlarge
CSpOC at the time of writing (5 Jan 2019 14h UT) has a reentry TIP for 6:41 ± 4 m UT on its webportal Space-Track. This is 1h 47m, or one revolution, earlier than the New Zealand sightings.
Nevertheless, I am fully convinced that the event is Kosmos 2430 reentering - the match is too good, and the footage clearly suggests an artificial object reentering from earth orbit. So why the mismatch with the CSpOC TIP?
Kosmos 2430 was in a highly elliptical orbit with perigee over the southern hemisphere. In the diagram below, we see the apogee altitude (the blue line) quickly diminishing in the days before reentry, due to the drag experienced in perigee (diagram based on orbital tracking data from CSpOC):
click diagram to enlarge
The perigee altitude already is very low, near 90-85 km altitude, for days before the reentry and changes minimally untill the actual moment of reentry. The difference between apogee and perigee altitude remains significant up to the last few revolutions, with apogee still at 1000 km only two revolutions before reentry.
This means that, unlike typical objects reentering, Kosmos 2430 only briefly dipped into the upper atmosphere during each orbital revolution, experiencing drag only during brief moments. This is the kind of situation where an object can survive multiple very low perigee passes, and predicting the actual moment of reentry (i.e. during which perigee pass reentry will happen) is difficult. Looking at the CSpOC TIP bulletins for January 5th, this is clear as well as the CSpOC predictions significantly shifted forward in time with the addition of data from each new orbital revolution.
The sightings from New Zealand strongly suggest Kosmos 2430 survived one orbital revolution longer compared to the current (final?) CSpOC TIP estimate.
Note that with such brief but deep dives (well below 100 km) into the upper atmosphere, it is possible that the satellite already developed a plasma tail one or two perigee passes before actual reentry. The copious fragmentation visible in the footage from New Zealand shows that this event, at 7:58 UT was the actual moment of atmospheric reentry and complete disintegration.
[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.
