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.
Today the first 57 orbital element sets for Microsat-r debris, debris from the Indian ASAT test on March 27, appeared on CSpOC's data-portal Space-Track (I have posted on aspects of this Indian ASAT test earlier: here, here and here). They have catalogue numbers 44117 - 44173. The analysis below is based on these orbital element sets.The elements confirm what we already knew: that Microsat-r (2019-006A) was the target of the ASAT test.
The image above plots the orbit of the 57 debris fragments, in red. The white orbit is the orbit of the International Space Station ISS, as a reference. Below is a Gabbard diagram of the debris pieces, plotting their perigee and apogee values against their obital period. The grey dashed line gives the orbital altitude of the ISS, as a reference:
click diagram to enlarge
Again, it is well visible that a large number of the 57 fragments (80% actually) have apogee altitudes above the orbit of the ISS, well into the altitude range of operational satellites. This again shows (see an earlier post) that even low-altitude ASAT tests on orbiting objects, creates debris that reaches (much) higher altitudes. The highest apogee amongst the 57 debris pieces is that of 2019-006AR at 2248 km.
Below is the apogee altitude distribution as a bargraph (including a kernel density curve), again showing how pieces do reach the altitudes of operational satellites:
click diagram to enlarge
Most of the created debris in the current sample of tracked larger debris has apogee altitudes between 400 and 700 km. It is interesting to compare this to a similar diagram for debris from the 2008 US ASAT demonstration on USA 193, "Operation Burnt Frost":
click diagram to enlarge
The Operation Burnt Frost debris distribution peaked at a somewhat lower apogee altitude, ~250 km (the same orbital altitude as the target, USA 193) while the peak of the Indian ASAT debris apogee distribution is higher, ~400-500 km (there could however be detector bias involved here).
It is interesting to note that both distributions appear to be double-peaked, both having a secondary peak near 700-800 km. I remain cautious however, as that could be due to detector bias.
The question now is, how long this debris will survive. To gain some insight into the expected lifetimes, I used Alan Pickup's SatEvo software to make a reentry forecast for the debris fragments. It suggests that most of the debris will stay on orbit for several weeks to months: by half a year from now, most of it should be gone however, except for a few lingering pieces. Note that this forecast should be taken with some caution: it assumes a constant solar activity at the current level, and takes the NDOT values of the element sets face value.
The following bar diagram charts the forecast number of debris objects reentering per week (the x-axis being the number of weeks after the ASAT test) resulting from the SatEvo analysis:
click diagram to enlarge
Again, the result is quite similar to the actual lifetimes displayed by the USA 193 debris fragments after Operation Burnt Frost in 2008 (see an earlier post, with the same diagram), as expected:
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.
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.
On March 21 at 17:44 GMT, a Soyuz rocket (Soyuz MS-08) was launched from Baikonur in Kazakhstan, bringing three new astronauts to the International Space Station.
The upper stage from this rocket (2018-026B) reentered the atmosphere last night, producing a nice spectacle in the sky. The reentry was seen from southern Europe, and filmed from Italy. The still below is from video footage that you can find here on the Italian Ondanews website.
The US Military tracking network JSpOC gives a final TIP bulletin placing reentry at 25 March 1:25 UT ± 1 minute near 41.9 N, 8.1 E, depicted as a star symbol in the map in top of this page. The ± 1 minute indicates that this time and position come from an Infrared observation by one of the US Early warning satellites and hence should be very accurate.
I had been issuing forecast on twitter prior to this reentry, based on modelling in SatAna/SateEvo and GMAT. In addition to the JSpOC TIP position and time, the map above also gives some of my own modelling results for this reentry. The open circles were my two last proper forecasts, made before the actual reentry happened. The red dots are two "post-casts", i.e. forecasts made after-the -fact using orbital elements that were not yet available when I made my last forecast the evening before. The nominal position of the SatAna/SatEvo post-cast is only 10 minutes from the JSpOC TIP.
(This post was updated April 4, 2018, with the results of lifetime-modelling. The update is at the end of the post)
The Humanity Star. Image: Rocket Labs
The Humanity Star reenteredinto the atmosphereyesterday, 22 March 2018, near 13:15 UT.
Humanity Star (2018-010F) was a surprise payload launched on 21 January 2018 as part of the first successful orbital flight of fledgeling New Zealand space company Rocket Lab's Electron rocket. In addition to three cubesats, the launch featured an unannounced surprise in that it brought a 3-feet, 10.4 kg geodesic sphere into a 530 x 295 km, 82.9 degree inclined Polar orbit.
The idea was that the reflective surfaces would produce a conspicuous flashing object that would attract people's attention so that they would look up at the sky and ponder their place in the Universe. As a non-functional "art-for-arts-sake" satellite, it scooped (and was perhaps inspired by) a similar but much better thought through project by Trevor Paglen that is to be launched in August 2018.
Rocket Lab claimed that the Humanity Star would be visible as a very bright object in the sky. In reality, very few people have seen it. It mostly stayed faint, producing occasional very brief bright flashes (I saw one of these myself, at magnitude -1). Moreover, during the first 1.5 months of being on orbit, it stayed in Earth shadow, only becoming visible in twilight in March, when it already was close to reentry. The visibility window hence was short. As a project to attract public attention to the night sky, it largely failed. And the fuzz made by some astronomers about Humanity Star being "sky vandalism", clearly was over the top (and was in fact somewhat ridiculous from the start. Some people appear to take issue with everything nowadays).
Rocket Lab claimed the object would stay on orbit and be visible for nine months. Apparently, they had not realized that the area-to-mass ratio of this object was much different from a usual payload (it was a carbon sphere very lightweight for its size) and apparently they did not seriously model the lifetime. Because in reality, it lasted not nine months but only 60 days, a mere two months, on orbit. The orbital decay was very fast:
Apogee and perigee of Humanity Star over time. Click diagram to enlarge
I have modelled the last few days of Humanity Star's existence, producing reentry estimates in the two days leading to the reentry. I used two methods: one was the combination of Alan Pickup's SatAna and SatEvo software; the other was a simulation in GMAT.
click map to enlarge
The reentry occured in the early afternoon (UT) of March 22, somewhere along the white line in the map above, and most likely near the two locations marked halfway that line, i.e. over southwest Asia.
JSpOC issued a final TIP bulletin estimating reentry at 13:15 UT ± 29 min, nominally near 14 N, 61.8 E. My final GMAT simulation gives a result very close to that time and location, at 13:12 UT ± 45 min, nominally near 10.8 N, 61.9 E.
The final SatAna/Satevo result appears to be a bit early (indicating that I have to adjust some settings), placing reentry near 12:07 UT ± 28 min, nominally near 72 N, 126.5 W. For the upcoming Tiangong-1 reentry (see my daily updated post with reentry estimates) I am going to work with revised SatAna/SatEvo settings from now on.
UPDATE added 4 April 2018
I wrote: "apparently they [Rocket Lab] did not seriously model the lifetime".
To emphasize this, I ran a GMAT model for Humanity Star today, to see what modelled orbital lifetime would result.
I used the MSISE90 model atmosphere, a low solar activity regime, and modelled for a mass of 8.16 kg and diameter of 0.91 meter. Starting point was a TLE from 4 days after the launch.
The resulting lifetime was 51 days. My model has it reenter on March 13.
The real lifetime was 60 days. The real reentry was on March 22.
So my modelling resulted in a lifetime that was 85% of the real lifetime, which is not bad for modelling over a 2-month period.
[later added section]
There are also other values for Humanity Star floating around: a mass of 10.34 kg and diameter of about 1 meter.
Running the model with those figures ads 2 days to the orbital lifetime, i.e. brings it at 53 days, i.e. 89% of the real lifetime. [end of added section]
It also shows that applying a model (like GMAT) would have yielded Rocket Lab a much more realistic orbital lifetime than the 9 months which they claimed.
