Sunday, 15 April 2018

X-37B OTV-5 mission located on orbit

OTV-5, The fifth mission of the US Air Force' X-37B  robottic mini-shuttle, was launched from Cape Canaveral on 7 September 2017 on a SpaceX Falcon 9 rocket. Until last week, OTV-5 had not been located by amateur satellite trackers, and that was somewhat curious, as we did locate and track the previous four missions.

But now OTV-5 has been finally found. In the early morning of April 11, 2018, Dutch satellite tracker Cees Bassa imaged a bright unidentified satellite in a ~54 degree inclined orbit. It was seen again by Cees two days later, on April 13. Ted Molczan managed to link it to a lone sighthing of an unidentified object done by Russell Eberst in Scotland back in early October 2017 that was already suspected to perhaps be OTV-5 at that time (several of us, including me,  had tried to recover the object Russell observed in the next few nights that October, but failed).

OTV-5 immediately was suspected as the identity for this object. It was in a very low, ~355 km circular orbit, which is lower than usual for satellites, but which fits with the characteristics of previous OTV missions.

The orbital plane the object is moving in passed over Cape Canaveral at the moment OTV-5 was launched (see below, which shows the location of the orbital plane for the moment of OTV-5 orbit insertion on 7 September 2017). So that fits nicely, and as a result we are quite confident that this is OTV-5.

click to enlarge

There is a difference with previous OTV missions: OTV-5 is in a 54.5 degree inclined orbit, which is a substantially higher orbital inclination than that of previous OTV missions which were flown at orbital inclinations between 38.0 degrees and 43.5 degrees, as can be seen in this diagram below where the current OTV-5 mission orbit is white, and previous OTV mission orbits are red:

click to enlarge

But this actually fits with information released on the OTV-5 mission by the US Air Force, which prior to the launch of OTV-5 stated that:

"The fifth OTV mission will also be launched into, and landed from, a higher inclination orbit than prior missions to further expand the X-37B’s orbital envelop." 

I am very happy that OTV-5 was launched, as it now turns out, into a 54 degree inclined orbit, as for the first time this will give me a chance to see an X-37B OTV mission from the Netherlands. OTV-5 will actually pass over my country (and even somewhat north of it), while previous OTV missions passed over southern Europe only. The previous four missions therefore were not visible from my country, due to their lower orbital inclination.

An obvious question is: why did it take so long to find OTV-5? I have some answers to this that might explain.

First, I think many amateurs subconsciously reckoned it would be in a 38-43 degree inclined orbit like its predecessors. Indeed, the initial search elements we used were for a 43-degree orbit.

Second, this was an autumn launch and the very low orbital altitude means it is not well visible in wintertime from the Northern hemisphere, where almost all currently active satellite trackers are located. Almost all wintertime passes are in Earth shadow.

Now spring has arrived, OTV-5 is emerging out of these shadows, into the light. Weather has not been cooperating for me in the coastal area of the Netherlands where I am located so far, but I hope to be able to joing tracking this object soon. It is an interesting object to track, as previous OTV missions frequently manoeuvered between different orbital altitudes. Plus, the shuttle-like character of this object makes it a special one to track as well.

Monday, 2 April 2018

Updated Tiangong-1 reentry forecasts (updated April 2)

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

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

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

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


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

click to enlarge map

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

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

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

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


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

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

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:


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

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

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

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

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

click map to enlarge

A note about "Live" tracking websites

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

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

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

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

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


Drag variability

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

click diagram to enlarge

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

click diagram to enlarge

Drag surface reconstruction

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

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

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

click diagram to enlarge

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

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

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

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

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

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

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

click diagram to enlarge

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

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

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

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

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


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.

Sunday, 25 March 2018

The atmospheric reentry of the Soyuz upper stage 2018-026B on March 25

click map to enlarge

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.

click image to enlarge. Link to newsitem with video

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.

Friday, 23 March 2018

The reentry of Humanity Star (updated)

(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 reentered into the atmosphere yesterday, 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.

click diagram to enlarge

Tuesday, 13 March 2018

One month left for Tiangong-1 [UPDATED]

Note: a daily updated post with reentry estimates for Tiangong-1 is here.

image (c) Alain Figer, used with permission

The beautiful image above (used with kind permission) was made by Alain Figer and shows the Chinese Space Station TIANGONG-1 over the French Alps on 27 November 2017.

Tiangong ("Heavenly Palace") 1 was launched on 29 Sept 2011. It was the first Chinese Space Station and was visited by Taikonauts twice, first by the crew of Shenzou 9 in June 2012 and then by the crew of Shenzou 10 in June 2013: six Taikonauts in total.

All eyes are currently on this Chinese Space Station, as it is about to re-enter. Since the station was shutdown in 2016, it has steadily come down, especially so the past year and months. Its orbital altitude has currently descended below 250 km (it currently is ~240 km, with apogee at 251 km and perigee at 229 km on 2018 March 13):

click diagram to enlarge

click diagram to enlarge

Using SatAna and SatEvo, and under the assumption that the re-entry will be completely uncontrolled, I currently estimate it to re-enter one month from now, somewhere between April 7 and April 21  April 1 and April 12.

EDIT:  daily updated re-entry predictions are in a dedicated post here

The station has an orbital inclination of 42.8 degrees, and hence can come down anywhere between 42.8 N and 42.8 S. The map below shows the area that is at risk:

click map to enlarge

Note that newspaper accounts (e.g. this one) that single out a particular area as being at particular risk, are nonsense: At this stage, a month before re-entry, it is impossible to pinpoint a region. That will only be possible during the hours just before actual re-entry (and even then...).

The station has a mass of about 8500 kg and measures 3.35 x 10.4 meter. It is hence a large and heavy object, which is why this re-entry is of concern. It is likely that parts will survive the re-entry and reach Earth surface intact.

Land masses inside the risk zone include southern Eurasia, Australia, Africa, South and Middle America and the United States. It is however most likely that the re-entry will be over an ocean.

As can be seen from the map above, my own country, the Netherlands, is well outside the risk zone.

I will follow the orbital evolution and re-entry predictions for Tiangong-1 on this blog as they evolve.

Tiangong-1 image on 18 July 2017 by Alexandre Amorim from Brazil
this is a stack of 4 separate images
(image (c) Alexandre Amorim, used with permission)

NOTE: new reentry estimates, updated daily, are consolidated in this new blog post.

Thursday, 1 March 2018

On PBS Newshour, about Open Source investigation of the North Korean missile program

In December of 2017, I was interviewed by Miles O'Brien for PBS Newshour, about Open Source investigation into the North Korean missile program.

The item aired on 28 February 2018. It is 9 minutes in duration and alternatingly features Jeffrey Lewis of the Middlebury Institute and me showing what we can learn from analysing DPRK propaganda photographs and video imagery.

(the video above starts at the start of the item).

Wednesday, 28 February 2018

Imaging FLOCK 2E 4 near decay

In the early morning of 27 February 2018, I was imaging a rocket stage from a classified launch, the NOSS 3-4 r/b, when suddenly a very fast, flashing object entered the FOV, and I followed it as it looked interesting (it was so fast that in the first instance I thought it was a meteor). It turned out to be the cubesat FLOCK 2E 4 (1998-067 JH, #41487).

FLOCK 2E 4 is a cubesat that was released from Cygnus OA-6 in May 2016. It is currently in a 247 x 260 km, 51.6 degree inclined orbit, and from the imagery it is clearly tumbling. It is coming down fast, with several km/day, as it is close to decay. An analysis with SatAna and SatEvo predicts that it will re-enter in about a week, on or near 2018 March 6-7.

The diagram below shows how the orbital altitude changed since it was released at 400 km altitude from Cygnus OA-6 in May 2016:

click diagram to enlarge

FLOCK 2E 4 was built by Planet Labs and was one of the imagers in their FLOCK constellation. It basically is a small 9 cm telescope with a camera, and delivered imagery of the Earth's surface with a resolution of a few meters.

FLOCK cubesat (image: Planet Labs)

It is a very small object, the smallest I have managed to image in Earth Orbit so far. The body measures only 34 x 10 x 10 cm, and with solar panels deployed, the maximum dimension is 34 x 30 cm. A lucky capture!

The camera used was the WATEC 902H with a Canon FD 1.8/50mm lens.

Saturday, 17 February 2018

UPDATED: TLExtract 3.5

I recently, as part of learning myself to code .NET Windows programs in Visual Basic, have started to create small, user-friendly (I hope) programs to aid satellite observers. TLExtract is a new program I have just released.

TLExtract is a program to select TLE's (satellite orbital elements) from a larger file with TLE's (for example classfd.tle or the full JSpOC TLE file), based on a custom-set condition. The resulting selection can be saved to a new TLE file.

For example, you can use it to select all objects with perigee below 2000 km from the original file. Or to select or exclude all objects containing "DEB" or "COSMOS" in the name. Or all objects with an orbital inclination larger than 45 degrees. Or all objects with a period near 1.0 rev/day (GEO). Etcetera.

Element-sets in the input file need to be 3-line elements, i.e. they need to have the line 0 with the object name.

The program runs under .NET in Windows. It accepts only one selection criterion per run, but when you want a selection to satisfy multiple criteria, you simply run another session on the output of the first session.

The program can be downloaded at my website, where you can also find other programs useful to satellite orbservers, such as IOD Entry and TLEfromProxy, as well as some general astronomical programs - for example a program to calculate Solar Longitudes, and a program to calculate the Local Sidereal Time.

UPDATE: version 3.0 features another improvement in speed, and solved a problem with hidden line carriers in the output, that messed up some software when the output file was read into them. I thank Jim King for his suggestions that led to these improvements.

UPDATE 2:  by popular request, version 3.5 includes an option to select on catalogue number.

Thursday, 15 February 2018

UNID galore! Locating Govsat-1

The evening of 13 February 2018 was very clear. I used the WATEC video camera to track objects in Low Earth Orbit in evening twilight, and later in the evening did a short session on Geostationary satellites.

Over the course of this I recorded as much as 3 initially unidentified objects ("UNID's"): objects that at the time of observation did not match with a known orbit in either the unclassified JSpOC orbit database or our amateur database of classified objects.

The first of these UNID's was the one in the footage above, which appeared while I was waiting for another satellite to pass. It didn't match anything known. The ~62-degree orbital inclination from a circular fit to the data suggested something NOSS-ey. Mike McCants later managed to identify it as probably NOSS 6 (C), (1984-012C), last observed 10 months ago.

The other two UNID's appeared in my photographic imagery from later that evening aimed at geostationary objects. The first one was a short Northwards moving trail pointing to an object in GTO that was alas only visible in two pictures. Here is one of these two images (the image also shows the classified Early Warning satellite SBIRS-GEO 2 (2013-011A), which was the target of the image):

click image to enlarge
This turned out to be the same object as a UNID imaged by Cees Bassa that same evening, and correlated by him to the object we designate as "Unknown 091017" (2009-790A). This object was first seen in 2009 and more recently "lost" for a while, as it was last seen 566 days (1.5 years) ago. This object in a 25 degree inclined GTO orbit (image below) is probably a rocket stage from a classified launch.

click to enlarge

The third UNID was an object near Geostationary altitudes, close to the commercial GEO sat Astra 3B. It is the object indicated with a yellow arrow below:

click image to enlarge

I initially somehow managed to miss this object when going through my imagery, but Cees Bassa had imaged it that same evening and urged me to look for it in my imagery, after which I found it too.

This object is most likely Govsat-1 (SES 16; 2018-013A), launched for a joint-venture of SES and the Luxemburg government by SpaceX two weeks earlier on 31 January 2018. It is "aimed exclusively at government and defence users" and its orbit is classified (although its operational slot at 21.5 E has been made public). The military of several NATO countries will use it for secure communications as part of military and humanitarian operations. The satellite was built by Orbital ATK.

The orbital slot assigned to it is at 21.5 E. On the evening of 13 February we detected it at 23.8 E, some 2 degrees to the East of this, so it probably is still slowly drifting westwards towards 21.5 E, where it will arrive somewhere in the coming few days.

The object had a noted brightness variability (in the image above it was at its brightest, while it was completely invisible in some of the other images), indicating it is spinning, probably spin-stabilization while in transfer to its orbital destination.

This object will be very interesting to follow in the feature, as it has a port that allows another still to be launched object to dock to it.