Friday, 23 March 2018

Updated Tiangong-1 reentry predictions (updated March 23)

[post last updated March 23, 22:20 UT]
click diagram to enlarge

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 
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 
March 23          3.8 April +- 3 days
March 22            3 April +- 3 days

Currently indicated is a reentry during the first days of April. But depending on how the periodic atmospheric density variation develops, the last days of March certainly remain in the picture too.

The first set of predictions 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 is made by modelling the orbital evolution in GMAT, using the MSISE90 atmosphere and a Price-Dormand78 integrator.

The diagram above shows you how the SatAna/SatEvo prediction develops. Still some 10 days away, there is still quite an uncertainty and day-to-day shift in the estimated moment of reentry, due to variations in the atmosphere and the attitude of the space station.

There is an initially ~5.5 day periodic variability in the drag parameter B* (perhaps pointing to a slow rotation of Tiangong-1 - but see below!), as can be seen in the diagram below. Over the last few weeks this periodicity seems to slow down, to an about 6.6-6.8 day periodicity currently. I expect the reentry prediction to rock back-and-forth by a few days with a similar periodicity.

click diagram to enlarge

For the orbital data of the past two weeks I calculated area-to-mass ratio's. Using a  mass for Tiangong-1 of 8500 kg, I initially got the following variation in drag surface:

click diagram to enlarge
In an e-mail discussion with Jon Mikkel, he convinced me that the mass I used (8500 kg) 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, 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 22 appears to vary between 19 m2 and 35 m2 (for a mass of 8500 kg) or 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.

(a ~6.6-day periodicity is indicated as a red dashed sinusoid in the diagram above. It is superimposed on a much slower trend with a period of several weeks).

[important edit, 22 March 21:35 UT]
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...!".

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!

These are the kind of cases I like most. It produces advancing insight.

Tiangong-1 is nevertheless 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 of about 4 minutes. They by the way also captured amazingly detailed RADAR images of Tiangong-1, which can be seen here.

Back to the drag: as can be seen in the diagram, near March 18-19 the drag was near maximum values. It  currently looks like the drag is decreasing again, with a corresponding shift of the estimated reentry date towards a later date. But given the periodicity, we can expect that somewhere next week, the drag will likely increase again. At this moment the first days of April still seem to hold the best cards, but depending on how the discussed periodic atmospheric density variation develops, the last days of March certainly might come into the picture again too.

The problem is that both SatAna/SatEvo and GMAT do not take into account this kind of slowly varying drag. GMAT uses a constant drag surface. SatAna and SatEvo use the NDOT/2 value, which reflects the drag, for the (relatively short) data arc under consideration, and these NDOT/2 values might not well represent the feature. This means that long-term predictions by these software packages are not very accurate. We can expect that in the last 1-2 days before the reentry, the predictions become more reliable.

click diagram to enlarge

Perigee of the Tiangong-1 orbit is currently below 210 km altitude and rapidly decreasing. The average orbital altitude is currently decreasing by 2-3 km/day (see diagram above), and that value will likely increase over the coming days.

click diagrams to enlarge

This diagram shows the frequent orbital raising manoeuvres, ending late 2015, after which the station goes steadily down:

click diagrams to enlarge
The rate of decay, clearly going up:
click diagram to enlarge

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.

click map to enlarge

(note: this post has been updated, and parts added or rewritten, repeatedly. Text and figures are updated daily)

The reentry of Humanity Star

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.

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.

Friday, 9 February 2018

STARMAN (Falcon Heavy/Tesla Roadster) 2018-017A imaged in Space

click image to enlarge

The image series above shows the Falcon Heavy upper stage 2018-017A, with the Tesla Roadster of Elon Musk and STARMAN attached to it, coasting through interplanetary space towards the orbit of Mars.

At the time these images were taken, 16:39-16:50 UT on 8 February 2018, it was well beyond the moon, at a distance of 550 000 km or about 1.4 Lunar distances c.q. 0.0037 AU. The images are 30-second exposures taken by Peter Starr and me with the 0.43-m F6.8 remote robottic telescope of Dubbo Observatory in Australia

I also created an animated GIF of these images:

These 4 images are part of a lerger seies of images taken from Dubbo and from Q65 Warrumbungle, and they show a clear, slow brightness variation of +- 2 magnitudes between ~+14.6 and +16.6, with a period of perhaps approximately 4m 42s (the dataseries is not very detailed, so the real periodicty might be off from this estimate).

While I did image objects in trans-Lunar orbit before, this is the first time I imaged something on an outbound true interplanetary trajectory.

The trajectory and ephemerids are available on JPL HORIZONS. An early orbit integration I made yesterday before orbit updates from telescopic observations became available, suggests 2018-017A will be close to earth again in 2073. I did not have time yet to redo the integration now telescopic observations are improving the orbit, but will do so later. So stay tuned.

[UPDATE: based on the (thanks to observations like these!) improved orbit, the 2073 close encounter that the initial orbit suggested, is no longer on the table.]

Safe travels, Starman! 

 UPDATE 2: I am quoted in this CNN article, which also features some of my imagery.

image: SpaceX

Thursday, 8 February 2018

There's a Starman Waiting in the Sky

image: SpaceX

This is the freakiest, most surrealistic image related to Space I have ever seen.

On February 6/7, as part of the Maiden Flight of the SpaceX Falcon Heavy, a Tesla Roadster with a dummy called Starman behind the wheel (and various other references to pop-culture tucked in the car) has been orbiting Earth in a 180 x 6950 km orbit for 6 hours, after which it was boosted into a heliocentric Apollo orbit with aphelion near the orbit of Mars.

Yep, that's right: a car in space! That is something beyond my wildest dreams.

Image: M. Langbroek

Above is the  ground track of the slightly under 6 hours earth orbit. Launch was at 20:45 UT (6 February) from pad 39A at Cape Canaveral; SECO 2 was over Africa 28m 52s later, boosting it into a 180x 6950 km, 29 degree inclined elliptical coasting orbit. After just under two orbital revolutions, a third and final boost sent it into heliocentric orbit. The boost was widely observed from the US West Coast (see for example this hefty 256 Mb movie shot by Derek Breit in Morgan Hill, California).

The heliocentric orbit is below. It has perihelion at 0.98 AU, aphelion at 1.67 AU and an orbital inclination of 1.05 degrees. The orbital period is 1.53 year. If this was an asteroid instead of a rocket stage and a car, we would call it an Apollo orbit.

image: M. Langbroek
The aphelion distance of the orbit is similar to the aphelion of Mars, but located near the perihelion of Mars.

The rocket stage and car will periodically come back to Earth's orbit. Near 27 January 2073, the rocket stage and car might make an actual close approach to Earth. My current orbit integration with MERCURY 6 has it passing at a nominal distance of ~0.004 AU or ~1.6 Lunar distances, and likely will be in reach of telescopes on Earth then. [EDIT 15 Feb 2018: after new orbit updates based on optical observations, the 2073 close approach is off the table]. The real distance might be more (or less) as the current orbit probably isn't very accurate (SpaceX earlier presented an orbit that was dead wrong) and the object(s), being of low area-to-mass ratio and shiny, moreover will be strongly influenced by Solar Radiation Pressure, which will perturb the orbit and is difficult to model over a 55 year timespan.

A  less close approach (nominal values in the order of 9 Lunar distances) will happen in March 2137. Close approaches to Mars will not happen over the next three centuries.

image: SpaceX

image: SpaceX
image: SpaceX

To me, this was the most exciting launch since I watched the first Shuttle launch on tv when I was a teenager. That big Falcon Heavy roaring into the sky was very impressive. Even more impressive was the synchonous return of both side boosters, landing smoothly and brotherly next to each other. The core booster alas did not fare that well, and smashed to bits in sea.

And then there were those surreal images of the Tesla orbiting earth, with "Starman" at the steering wheel. I reckon these will be iconic images for a very long time.

The whole idea of launching a car into orbit is crazy of course, and it has drawn critique from some people. I do not share that critique. This is one of the daring, crazy, whimsical things that is so characteristic of humanity, and it fits iconic moments in exploration. The World needs people who are a bit crazy, in a good way. Otherwise it would remain dull and boring, with very little progress.

Say what you want of Elon Musk, and of course this is primarily a publicity stunt (and brilliant marketing), but it appears Elon Musk is giving the human space program a real boost of the kind we haven't seen in a long time. After this stunt, I for the first time in my life get the feeling that I might really see humans walk on Mars in my lifetime. After all, if we can send a Tesla Roadster towards the orbit of Mars, we can send more. To my mind, this was absolutely awesome!

UPDATE: see more in my follow-up post here with my telescopic imagery of the Falcon Heavy/Tesla Roadster in Space!

And I am quotes (and some of my imagery features) in this article on the CNN website.

Monday, 5 February 2018

The flashing behaviour of USA 81 (1992-023A)

The video footage above was shot by me on 25 July 2017 and shows the classified satellite USA 81 (1992-023A). It is flashing rapidly, at ~5 flashes per second.

USA 81 is one of two satellites (the other is USA 32; the attempted launch of a third one failed) on which little is known, but that were probably ELINT gathering satellites, probably in the Singleton/SBWASS program. It was launched from Vandenberg AFB on 25 April 1992 with a Titan 23G.

With its earlier sistership USA 32, USA 81 is renowned for the light shows it puts on. It has a very rapid and at times very conspicuous flash cycle of sharp specular flashes, like a disco-ball: the video above shows an impressive example of this. How pronounced the flashing is depends on the viewing angle: the video above catches a brief period, halfway the footage, where it becomes very pronounced.

The flashing is very regular and specular, with a main period of 0.2 seconds and a secondary period half of that, 0.1 seconds.

The diagrams below show this periodicity well. They are the result of an analysis of (part of) the video above with LiMovie and PAST - the analyzed part of the video was the second part, after the camera repositioned, when flashing was most pronounced.

The first diagram shows the observed very specular flashes (blue) and a fitted double sinusoid of 0.2 and 0.2 seconds (red, dashed). They fit very well:

click diagram to enlarge

The two periods are also well discernable in the Lomb periodogram and the fast Fourier analysis below:

click diagram to enlarge
click diagram to enlarge

The earlier sister ship of USA 81, USA 32 (1988-078A), shows the same brightness behaviour (but with a main periodocity of 1.2 seconds) with the same periodicity. This points out that the flashing is intentional.

There are two options to explain the flashing behaviour. One is that the satellite is spin-stabilized. The other is that the satellite could have a large and shiny rotating element, for example a large rotating antenna.

Friday, 26 January 2018

Meet the amateur astronomers who track secretive spy satellites for fun

Somehow I totally forgot to post this here: but after the Zuma launch and subsequent rumours of a failure made headlines, I was interviewed by Mary Beth Griggs of Popular Science magazine. It resulted in a very nice article about our amateur tracking activities, that you can read here.

Monday, 22 January 2018

TOPAZ/FIA Radar 5, the NROL-47 payload

click image to enlarge
The small streak in the center of the image above in a bright blue, partially cloudy twilight sky, is TOPAZ/FIA Radar 5 (2018-005A, 43145), the NROL-47 payload (see a previous post) imaged in evening twilight of 19 January 2018 with the sun at only 8 degrees below the horizon and the satellite at 24 degrees elevation in the northwest.

NROL-47 was launched on 12 January 2018, a week before the photo above was taken. Shortly after launch, Cees Bassa and Scott Tilley already detected the payload by radio, determining a preliminary orbit from the Doppler curves with an orbital altitude near 1057 km and inclination near 106 degrees.

The first optical observations were done in the evening of January 14th, two days after launch, by Cees Bassa in the Netherlands, after which it was picked up by a number of other observers as well (amongst others Leo Barhorst in the Netherlands, Greg Roberts in South Africa and Paul Camilleri in Australia). The latest optical observations have improved the orbit for this new satellite and show it is in a 1048 x 1057 km, 106.0 degrees inclined orbit.

The payload was making very low (11-12 degrees maximum elevation) twilight passes in the north during the first few days after launch for my locality, where I have obstruction from buildings. Passes gradually climbed higher in the sky over the week, but also deeper into twilight. Combined with dynamic weather, I started to lose hope of imaging it, but finally was succesful in the evening of 19 January. I had a 24 degree maximum elevation pass in the southwest near 17h UT, with the sun barely 8 degrees below the horizon. The pass was high enough to clear rooftop level near culmination.

The weather was again very dynamic that evening, with fields of clouds forming as soon as the sun set. Using the 1.4/85 mm lens, I took a series of images while clouds were moving in fast. Due to the very bright sky background, I was restricted to 1 second exposures at 400 ISO.

After a first quick cursory check of the images on the camera's LCD screen I initially thought I had lost the battle against clouds and twilight. But upon a more thorough inspection on my laptop a day later, it turned out it was in the images after all, and with enough reference stars to get some decent astrometry from the images.

The payload is in a new orbital plane for TOPAZ/FIA Radar satellites. While all previous four TOPAZ/FIA Radar satellites are in a 123.0 degree inclined orbit, this new TOPAZ/FIA Radar 5 is in a 106.0 degree inclined orbit.

I had already inferred a new orbital plane for this satellite pre-launch (see a previous post), based on the launch azimuth, which deviated from that of previous TOPAZ/FIA Radar launches from Vandenberg. The new 106.0 degree orbital plane is within 2.5 degrees of my original pre-launch estimate. The orbital altitude is somewhat lower than I initially estimated.

click to enlarge

click to enlarge

Operating in two orbital planes was also the case of a previous series of radar satellites, the Lacrosse (ONYX) satellites, of which currently only Lacrosse 5 is still in orbit. These operated in two orbital planes, at 68 and 57 degrees orbital inclination.

The 123.0 degrees orbital inclination of TOPAZ/FIA Radar 1 to 4 is the retrograde equivalent of the 57 degree inclination of the Lacrosse constellation. The new 106 degree orbital inclination is however not the equivalent of a 68 degree inclination.

The current TOPAZ/FIA Radar 5 orbital altitude of 1048 x 1057 km is slightly lower that that of the previous four TOPAZ/FIA Radar satellites, which orbit at ~1100 x 1110 km. However, it is not unlikely that over the coming weeks the orbit will be further raised to a similar altitude.

Unfortunately, I am now losing visibility of the object as the higher passes occur deeper and deeper in twilight.

TOPAZ-5 is the last of the block I TOPAZ payloads. The new 106.0 degree inclined orbital plane might be the new orbital plane for the block II payloads to be launched over the coming years.

Thursday, 11 January 2018

[Updated] A potential use for satellites in Zuma-like 50-degree inclined orbits

SpaceX's launch of the Zuma satellite on 8 January was interesting, and not just because of the ongoing saga of whether it failed or not (see a previous post).  

The odd 50-degree orbital inclination is another element that made this launch interesting (see discussion in my pre-launch post here: sightings of the Falcon 9 Upper Stage over Sudan after launch later confirmed this orbital inclination).

New ideas started to form post-launch after the Falcon 9 sightings from Sudan made me realize that while it indeed was launched into a 50-degree inclined orbit, the orbital altitude (900-1000 km apogee) was higher than I initially expected, making a proposed link to USA 276 unlikely.

And then @Cosmic_Penguin posted this small message thread on Twitter, referencing this interesting publication. That struck a chord and reinforced an emerging idea about a potential role for satellites in such 50-degree inclined, ~1000 km altitude orbits.

As @Cosmic_Penguin notes, the publication specifically discusses ~50-60 degree inclined, ~1000 km altitude orbits. And it is all about Space-based Radar.

I had just been looking into the coverage of the Zuma orbit, and it lines up with content in that report.

The map below is a ground coverage map of Zuma, would it have been alive and well. One of the uses of a ~50 degree inclined ~1000 km altitude Space Based Radar satellite mentioned in the report, is for shipping surveillance.

Indeed, a satellite in a Zuma-like orbit would basically cover all Ocean surfaces, except for the high Arctic and Antarctic, which are not that interesting for the purpose discussed below (moreover, the Arctic is extensively covered by groundbased and airborne radar).

click map to enlarge

A (Radar) satellite in this kind of orbit therefore would be very useful to keep track of illicit shipping movements on the High Seas.

Think stuff like embargo-runners, e.g. embargo-breaking shipments of coal and oil to for example North Korea, illegal weapons exports from North Korea, oil exports from Syria, illicit weapons transports to the Middle East, and human trafficking as well as drugs shipments.

Ships engaged in such illegal activities sometimes turn off their transponder, making it harder to track their whereabouts once out of sight of landbased shipping radar (see also the story about one particular embargo-breaking ship here). The classified US NOSS duo ELINT satellites and similar Chinese Yaogan triplets are meant to track ships from passive radiosignal crosslocation, but when a ship displays strict radio silence, these systems will not detect them either. But Space-Based Radar will.

Embargoes have become an important geopolitical tool when outright war is deemed not an alternative. We currently see embargoes enforced with regard to for example Syria and North Korea. Means to enforce embargoes including detecting and stopping potential embargo violations therefore have become important. Human trafficking and drugs trafficking are growing geopolitical problems as well.

So was Zuma meant to be an (experimental, i.e. a technology demonstrator) version of such a Space Based Radar for Ocean shipping surveillance? It is an option.

What might argue against it is the extreme secrecy surrounding the launch. Very few details were made public about the Zuma payload, the Agency operating it was not disclosed, and the launch was announced very late.

For all of this, explanations can be sought, but that admittedly all is "special pleading". For example, maybe the secrecy is there because the mission involves cutting edge experimental Radar technology. Or the secrecy could simply be the result of the "secrecy cult" in some parts of the US Government going over the top. Or it could point to operation by an Agency that wants to keep this operation on the down low - e.g. the CIA. And I can think of a few more - much more outlandish, which is why I won't mention them here - potential reasons.

We have seen this kind of secrecy before with PAN (and its later sister ship CLIO), with Prowler, and more recently with USA 276. All of these were experimental satellites doing unusual things: PAN roved between, snug up to and eavesdropped on commercial geostationary satellite telephony satellites. Prowler was an experiment for covertly inspecting other geostationary satellites on-orbit. And USA 276 remains mysterious but a series of very close encounters to the International Space Station suggest it might be a technology demonstrator for observing rendez-vous manoeuvres in space.

Zuma (the more so now it might have failed) also strongly brings the infamous USA 193 satellite to mind, although there we do know that it was a satellite for the NRO, and likely an experimental radar satellite [edit: see added note 2 below].

Nevermind what Zuma really was meant to be, and who was to operate it: the message to take home is that High Seas shipping surveillance is a potential and viable role to keep in mind for any future satellite launched in a ~1000 km altitude, ~50 degree inclined orbit.

Added note 1: Cosmic Penguin pointed out to me that this was also earlier brought up in a forum post by Ed Kyle.

Added note 2, 12 January 2018:  This article suggests Zuma might be an electro-optical/SAR hybrid and a follow-on to the infamous USA 193:

"Second, the Northrop Grumman satellite may be a follow-on to another failed satellite US 193. [...] ...., a source with direct knowledge of the program told me it was a blend of radar and electro-optical and would not provide any more detail than that. A source with wide knowledge of classified space programs has told me that the Northrop Grumman-built Zuma may be the next iteration of this. Both were apparently experimental satellites, in that they were not part of a large constellation of similar satellites."

Such a spacecraft would be well suited for the purpose indicated in this blog post.

Also, Northrop-Grumman, the company that built Zuma, has actually worked on developing ideas for Space Based GMTI Radar, which again would suit well to the purpose I suggest in this blog post.

Acknowledgement: Hat Tip to @Cosmic_Penguin on Twitter for putting ideas into my brain.

Wednesday, 10 January 2018

What is NROL-47 and in what orbit will it be launched? [updated twice]

UPDATE 10 Jan 17:25 UT: The launch has been scrubbed due to high altitude winds, and moved one day to Jan 11. New start of launch window is given as 1 pm PST = 20:00 21:00 UT. This means the launch window is shifting, indicating a prefered orbital plane and launch probably right at the start of the launch window.

Update 12 Jan: The launch was again scrubbed yesterday, and is now slated for January 12, 21:00 UT . My remark about a  shifting launch window above was in error, I missed that the Maritime Broadcast Warning window opens somewhat before the actual launch window opens.

Final Update , 12 Jan: NROL-47 successfully lifted off from Vandenberg SLC-6  at 22:11 UT!

Final Update 14 Jan 2018: Amateur observers using radio have located NROL-47 in orbit. It is transmitting in the TOPAZ frequency, 2241.52 MHz. The orbit is still very preliminary but appears to point to ~1100 km orbital altitude and an orbital inclination of ~105-106 degrees. This would identify NROL-47 as a new TOPAZ, but in an orbital plane that differs from the previous four TOPAZ satellites. Due to bad weather at the observing sites of several of our active observers (I was clouded out yesterday evening myself for example), optical observations have not yet been reported.

Hot after the excitement and drama of the Zuma launch (see my previous post), a new classified launch is upcoming on Wednesday January 10, when ULA will launch NROL-47, a classified payload for the National Reconnaissance Office (NRO), on a Delta IV from Vandenberg SLC-6 in California.

From Maritime Broadcast Warnings, the launch window opens at 20:30 UT and closes at 01:26 UT. [edit 1] After a one-day delay due to weather conditions, the launch is now slated to be on the 11th of January with the launch window opening 21:00 UT. The shifting launch window time indicates a launch into a preferred orbital plane, and it is likely that launch will be right at the opening of the launch window. [end of edit 1] [edit 2] This launch was scrubbed as well, and launch is now slated for 12 January 21:00 UT [end of edit 2]

The launch is in Westward direction, into retrograde orbit. This has led some space news websites to assume that the NROL-47 payload is the 5th TOPAZ (FIA Radar) satellite.

But is it? I have some doubts.

If it is TOPAZ 5, then it is clearly a deviation from the previous four launches. The launch hazard zones from published Maritime Broadcast Warnings show that the launch azimuth is different - previous TOPAZ missions all launched into azimuth 220 degrees, but NROL-47 launches into azimuth 200 degrees, a 20 degree difference.

NROL-47 Launch hazard areas (red) compared to the areas of four TOPAZ (FIA Radar) launches
click map to enlarge

This can be clearly seen on the map above, where the NROL-47 hazard zones are in red, and the hazard zones from the four TOPAZ in purple, green, light blue and dark blue. The azimuth and locations of the zones from the four TOPAZ launches are all quite similar, but those of NROL-47 stand out as different.

All the four TOPAZ satellites are in a 123.0 degree inclined retrograde orbit. The NROL-47 launch azimuth results in a retrograde orbit too, but with an orbital inclination of 108.6 degrees, not 123.0 degrees: a 14.4 degree difference.

The orbital altitude aimed for appears to be different too. The four TOPAZ satellites are in 1100 x 1110 km orbits. But the location of the Delta IV Upper Stage de-orbit zone (between South Africa and Antarctica), its shape and the opening time of the window (23:23 UT) points to the NROL-47 payload going into a 1500 km altitude orbit instead.[edit: from the first post-launch radio observations (see update in top of this post), the payload actually appears to be in a ~1100 km orbit, similar to previous TOPAZ: but indeed in a different orbital plane than the previous TOPAZ - end of edit]

estimated trajectory of NROL-47
click map to enlarge
So if this is the 5th TOPAZ launching as NROL-47 on Wednesday, then it is going into a quite different orbit compared to the previous four TOPAZ: different in orbital inclination as well as in orbital altitude.

In theory, the Delta IV rocket could do a "dogleg" and (when launching at 20:30 UT) deliver the NROL-47 payload into the 123.0 degree inclined orbit close to the orbital plane of TOPAZ 1 (FIA Radar 1). A second manoeuvre near the south polar pass could then align the RAAN and bring it exactly into the orbital plane of TOPAZ 1.

But why do that, if previous TOPAZ launches simply launched directly into the 123.0 degree inclination orbit?

So in my view, the jury is still out regarding the identity of NROL-47. It could be a 5th TOPAZ but in a quite different orbit compared to the previous four (in itself possible: the Lacrosses also occupied two different orbital inclinations). It could also be something new. If something new, it likely will be a radar satellite (like TOPAZ), given the retrograde character of the orbit. [edit - from radio observations, it appears to be a TOPAZ, but in a different orbital p;lane than the earlier TOPAZ -end of edit]

orbital constellation of TOPAZ 1, 2, 3 and 4 in 123.0 degree inclined orbits
The orbits are spaced 90 degrees in RAAN
click image to enlarge

The deliberate re-entry of the Upper Stage happens 1.5 revolutions (2h 55m) after launch.

Estimated search orbits, based on a 108.6 degree orbital inclination, are here. New elset estimates for the new launch date and time are here. South Africa will have two visible passes after launch.

An UPDATE on this post, with post-launch imagery of the payload in orbit, is here.

Tuesday, 9 January 2018

Fuel dump of Zuma's Falcon 9 Upper Stage observed by a Dutch pilot over east Africa (and rumours that Zuma failed)

image (c) Peter Horstink, used with permission
click image to enlarge

The spectacular image above was taken by Peter Horstink, the Dutch pilot of a Boeing 747-400 freighter aircraft (Martinair Holland flight MPH8371 from Amsterdam to Johannesburg), around 3:15-3:20 UT on January 8. The aircraft was flying at 35000 feet just north of Khartoum, Sudan, at that moment, which can be seen in the foreground (the yellow lights). The image above is the first one out of four images taken by Horstink.

The spectacular green-blue "spiral" on the image is, given time and location and character, almost certainly the Falcon 9 Upper Stage from the launch of the classified Zuma satellite that day (see my earlier post here), depressurizing and venting fuel at the end of its de-orbit burn. Some 15-25 minutes later, it would re-enter in its designated re-entry zone in the southern Indian Ocean (see map below).

Horstink described his observation as follows (translated from his Dutch e-mail report):

"It started with a greenish light in the top of my front window. At first I thought it was a reflection from some lightsource behind me, but it turned out not to be. At about 218 UTC [this later turned out to be a mistake and must read 3:18 UTC: the aircraft passed Khartoum near 3:25 UTC - ML] with a  very clear sky and with Khartoum in our sight, a point of light (like a star but somewhat bigger) moved from above us to South of us. It moved slower than a usual satellite but clearly did move. I estimate we saw it for 2-3 minutes. The waning moon crescent at that time was almost right above us. The object was surrounded by a greenish glow in the shape of spiral arms, like a spiral galaxy. Two of them, which didn't seem to move much. The total  size of the phenomena was about three times the diameter of the moon."
(note that when measured from the photographs using the star background as a reference, the actual diameter of the spiral cloud is about 11 degrees. The cloud is at ~8 degrees elevation over the horizon, near azimuth 155 degrees. The two bright stars to the right of the cloud are alpha and beta Centauri ).

The map below gives my estimated trajectory for the Falcon 9 Upper Stage, with apogee at ~900 km. It fits the area of the sighting, the launch hazard zone direction and the de-orbit zone position (from Maritime Broadcast Warnings) in the Indian Ocean. The Falcon 9 Upper Stage should have re-entered into the atmosphere between 3:30-3:45 UT, about 30 minutes after the  window from the Maritime Broadcast Warning opened at 3:00 UT.

click map to enlarge

The sighting points to a somewhat higher orbital altitude for Zuma than I had anticipated before the launch: with hindsight, I had too much of an idΓ©e-fixe that the orbital altitude would be similar to that of USA 276. The Falcon 9 sighting over East Africa suggests an altitude over double as high, in the order of 900-1000 km rather than my original 400 km estimate.

The sighting does confirm the 50-degree orbital inclination of the orbit. A new estimated elset based on this revision of the orbital altitude is here.

The map below shows the (very) approximate position of the aircraft at the time of Peter Horstink's observation relative to the Falcon 9 trajectory (times in UT, January 8th 2018). The aircraft was flying on a heading of 170 degrees, and Horstink gives his position as "between waypoint Alpox and Khartoum VOR" which corresponds to about 16.38 N, 32.35 E. The Falcon 9 Upper Stage was coming down at an altitude in the range of 200-400 km at that time. Approximate positions for the Falcon 9 Upper Stage are indicated in 2-minute intervals:

click map to enlarge

Horstink made the image above and below plus a few more with a handheld camera, from the cockpit of the aircraft.

image (c) Peter Horstink, used with permission
click image to enlarge
Horstink's observation is not the only observation. Very similar photographs showing a spiral in the sky have been coming from the ground in Sudan, e.g. in this tweet:

image from the ground, from Sudan (author unknown)

On rumours that Zuma failed

The sightings from Sudan near 3:15-3:20 UT are significant, as in the late afternoon of the 8th, rumours appeared on Twitter of a Zuma launch failure. These rumours then were picked up by some news outlets, e.g. here and here.

I have no idea about the veracity of these rumours, and so far SpaceX has said the mission was "nominal" (indicating no problems with the Falcon 9), while Northrop-Grumman and the US military haven't given comments (they never do about classified mission status). They could very well just be rumours, perhaps born out of a misunderstanding of events in the launch seen from the ground by lay observers.

For the moment, unless the US Government comes with some statement, I think it is wise we should treat it as "just rumours", and not necessarily true.

The sighting of the Falcon 9 Upper stage venting 2 hours 15 minutes (1.5 orbit) after launch, bear significantly on the discussion, as it seems to confirm the remarks by SpaceX that the mission was nominal. Of course, for SpaceX the mission ends at orbit insertion.

At any rate, it shows that at least the Upper stage achieved orbit (so it was definitely not a launch failure where the rocket failed to achieve orbit), and it makes sense that the payload then did as well.

So if something went wrong, if at all (a big "if" - I am skeptical), then there are three options left:

(1)  Zuma was inserted into orbit, but it is in the wrong orbit (too high, too low); 

(2)  Zuma was inserted into orbit, but is "dead", i.e. non-responsive;

(3)  Zuma achieved orbit with the Upper Stage, but failed to detach from the Upper Stage, and next de-orbitted with the Upper Stage near 3:30-3:45 UT.

JSpOC ("NORAD") did enter an object from this launch into its master catalogue on January 9th, as object nr. 43098, COSPAR 2018-001A, name USA 280. They designated it "PAYLOAD" (and the USA 280 designation would point to this as well). As usual for classified missions, they do not give details on the orbit.

screenshot showing the JSpOC master catalogue entry for a "PAYLOAD" named USA 280 associated with the launch

This suggests something achieved orbit long enough (i.e. over more than one orbit) to be detected and added to the catalogue.

While this does not necessarily mean the object is still in orbit (and it could in theory reference the Falcon 9 Upper Stage, with the "PAYLOAD" designation then in error), it does fuel my skepticism towards the truth of the rumours.

If Zuma is on-orbit but did fail, the situation becomes reminiscent of the USA 193 saga - an experimental satellite launched in December 2006 that failed after orbit insertion, and a year later was shot out of the sky with an SM3 missile, which has become infamous as "Operation Burnt Frost".

With regard to the observed fuel dump/depressurization: this is normal for most launches and does not necessarily indicate something's wrong.

Rocket stages always carry excess fuel, as you don't want the engine to cut out prematurely by running out of fuel. So it always has a sufficient fuel margin. Once its work is done, this excess fuel is often vented, also known as "depressurization".

[update] An earlier example of such a spiral resulting from a Falcon 9 venting fuel after launch into LEO, is this one from a SpaceX Falcon 9 test launch of a DRAGON in 2010. So this event over Sudan is not unusual. [end of update]

Depressurization and fuel venting avoids the risk of the rocket stage blowing up, for example as a result of static electricity building up in the rocket stage. You do not want your rocket stage to blow up, as it creates an uncontrolable swarm of debris and includes the risk that particles are ejected into orbits where they do not decay quickly,  adding to the space debris risk.

The spiral pattern results when the rocket stage is spinning, perhaps as result of the fuel vent.

At the moment, Zuma is not visible from the Northern hemisphere because all passes are in daylight or earth shadow. This will change 1-2 weeks from now, depending on the exact orbital altitude. The sighting from Sudan does confirm the orbital plane the object should be in (that is: unless it did a manoeuvre into another orbital plane after separation from the Falcon - but I doubt that). So we have to wait now untill a new object is observed in this orbital plane.

The hunt is on!

UPDATE: some news sources are now claiming sources within the US military and US Government confirm the failure, saying the second stage of the Falcon 9 "failed" and stage and satellite crashed into sea.

This does not tally with the observations over Sudan, which show the Upper Stage did reach orbit. So my skepticism remains. If there is some truth to it nevertheless, it could point to option (3) above and subsequent misinterpretation in the press.

UPDATE 2: the adapter mating ZUMA to the Falcon 9 Upper Stage was not made by SpaceX, but by Northrop-Grumman itself (which is somewhat unusual). So if ZUMA did not separate from the Falcon 9 (and did a dive into the Indian Ocean with it), the blame is not on SpaceX but on Northrop-Grumman. In that case, the SpaceX declaration that the Falcon 9 performed "nominal" is correct, even if Zuma did not separate from it.

UPDATE 3 (17 jan 2018): It turns out that a ~52 degree inclined, ~660 km altitude orbit also fits the constraints of the de-orbit area and being over East Africa at the right time. So we are adding that option to the search efforts. I did a partial plane scan of the 50-degree orbital plane two days ago.

click map to enlarge

Ackowledgement: I thank Peter Horstink for his report, for providing additional information on request, and for the permission to feature his images on this blog. I thank Govert Schilling for bringing me into contact with Peter Horstink. The photographs with this post are (c) Peter Horstink.