Monday, 24 December 2012

The flashing behaviour of North Korea's tumbling Kwangmyongsong 3-2 satellite

North Korea's first satellite Kwangmyongsong 3-2 (KMS 3-2) cannot be seen from the northern hemisphere at the moment (and hence cannot be observed by me currently). On the southern hemisphere, Greg Roberts (CoSatTrak) in South Africa is however successfully tracking the satellite.

He had a particular good pass on December 20th and obtained a very nice video record, tracking on the satellite with a motorized mount (note: movie has a period of black screen between opening title and start of the video record):

Greg Robert's video from S-Africa
(posted with permission)

The satellite is the object near the center of the screen, flashing about each 8.5 seconds with periods of invisibility inbetween. The moving streaks are stars (the mount is tracking the satellite as it moved along the sky): the other stationary dots in the image are hot pixels on the sensor of the video camera.

The video allows for an analysis of the flashing behaviour of the satellite. I used LiMovie to measure the satellites' brightness on the frames, resulting in the following lightcurve:

click diagram to enlarge

Visible is a clear ~8.45s periodicity with flashes of a specular character (suggesting a flat reflective surface). I have marked this with red triangles 8.45 seconds apart. In between the main flashes, a pattern of smaller secondary flashes can be discerned in a semi 8.45 second peridicity too (green triangles). They are not exactly positioned halfway between major flashes.

Assuming that each major flash is a flash caused by one of the sides of the KMS 3-2 cube-shaped body, then it completes a tumble once every ~33.8 seconds. Assuming that the less clear secondary flashes are due to a side of the cube as well, the tumbling periodicity would be half of that, i.e. 16.9 seconds.

Greg recorded the UNHA-3 r/b from the launch too. That one too is tumbling:

Greg Robert's video from S-Africa
(posted with permission)

Again, I used LiMovie to extract brightness information from each video frame. That was less successful with this video, because Greg's mount had difficulty keeping up with the fast-moving r/b for much of the record. A considerable part of the video could not be used for analysis, and I had to chop up the analysis in little non-continuous chunks:

click diagram to enlarge

What can be seen, is a flashing behaviour that starts slow and gentle and is increasing in rapidity near the end of the analysis, this being an effect of changing viewing angle.

Contrary to what some alarmist (sometimes almost hysterical) media reports have suggested, the tumbling of KMS 3-2 is by no means dangerous. David Wright over at All Things Nuclear has a very good debunking story about this all, pointing out the many misconceptions rampant in the reporting.

Monday, 17 December 2012

[UPDATED] A post-analysis of the N-Korean launch window, and N-Korean Spooks on my weblog?

On December 12, North Korea surprised the Western world by successfully bringing its first independently confirmed satellite into orbit: Kwangmyongsong 3-2, a reportedly 100 kg cubesat. For images of the satellite and an analysis of its likely components, see here. The satellite was launched with an UNHA 3 rocket from Sohae Launch Centre in Cholsan.

The successful launch came as a surprise for two reasons. First, all previous North Korean satellite launch attempts abysmally failed (even if N-Korea claimed they were a success).

Second, North Korea had indicated days before the launch that the launch was to be postponed to late December, for technical reasons. This appears to have been a deliberate disinformation ploy by N-Korea. According to South Korean press sources, it appears they also tried to play a ruse on the Americans, by pretending to dismantle the rocket when US imaging satellites were overhead, and continuing launch preparations when they were not.

Analysing the time of launch and US satellite coverage of the launch site

Well then: did the North Koreans indeed try to evade US (and Japanese) satellite surveillance?

First, they would not have been able to evade detection of the launch itself by US infra-red early warning satellites such as the DSP satellites and SBIRS in geostationary and HEO orbit. Coverage by these satellites is continuous.

But that was probably not N-Korea's goal anyway. Their goal reportedly was to try to convince analysts of imagery from US imaging satellites (optical and radar) that the rocket was not yet complete at the launch site, and not yet ready to launch for a while. The aim was apparently to throw off US predictions about the "when" of the launch until the very moment of launch itself.

Their concern hence was with US and Japanese optical and radar imaging satellites such as the KH-12 Keyholes, Lacrosses, FIA and IGS. These imaging satellites move in LEO, and coverage is not continuous- not yet at least.

Indeed, the timing of the North Korean launch (00:49 UTC on December 12) is interesting. It coincides with the end of a one hour long interval with no coverage of the launch site by US or Japanese Low Earth Orbit imaging satellites.

By contrast, in the hours prior to and after this one-hour gap in coverage, such periods of non-coverage were much shorter (typically 10-15 minutes at best), as can be seen in the coverage analysis images below and the movie near the top of this post (movie, images and analysis made using JSatTrak).

click image to enlarge

As can be seen from the coverage analysis, this hour long interval between 23:45-00:45 UT really stands out compared to the hours before and after. The N-Koreans launched right at the end of this interval at 00:49 UT, just when the launch site was coming into reach of the FIA Radar 2.

I feel the launch right at the end of this interval is no coincidence: they picked a moment where prior to launch they would have a substantial gap in US satellite surveillance available to complete their launch preparations. The one-hour long interval seems to have provided the North Koreans enough time to remount whatever they dismounted or camouflaged as a ruse, and launch.

(some remarks on the analysis and movie above: for a few of the satellites shown, positions are not 100% certain. For example, the Keyhole USA 186 hasn't been observed for a while because of the midwinter blackout. Satellites included in the analysis are the Japanese IGS, the US Keyholes, Lacrosses and FIA [edit: plus SPOT, Worldview and Quickbird commercial imaging satellites]).

Update 17/12 12:45 UT: 
I initially forgot to include the GeoEye Worldview and Quickbird commercial imaging satellites in the analysis. These commercial sats are frequently hired by the US government for selected imaging and used by independent analysts as well.

I have now added these satellites to the analysis, and the one-hour gap coverage between 11 Dec 23:45 and 12 Dec 00:45 UT keeps standing:

click image to enlarge

Update 17/12 16:00 UT:
Also added the French SPOT satellites to the analysis. Again, the 1-hour coverage gap between 11 Dec 23:45 and 12 dec 00:45 UT keeps standing.

Korean Spooks on my weblog?

There is a bizarre twist to this all that involves this weblog. In the late morning of December 8th, four days before the launch, an IP solving to North Korea visited this weblog. It entered through web-searches that included the keywords 'tle', 'KH-12' and 'Lacrosse 5'. A screenshot of the web statistics is below:

click image to enlarge

North Koreans with access to international websites are about as rare as, well, North Korean unicorns. Only a very select handful of North Koreans -mostly direct family members of Kim Jong Un- are allowed access to the internet.

Disclaimer: I was (and am) slightly suspicious. IP's can be spoofed, and two things caught my eye. One is the OS listed, Windows Xp. N-Korea is supposed to have its own OS, 'Red Star'. But then, maybe they only use this for their own, completely internal version of the internet. Or maybe western webstatistics providers cannot properly recognize it and list it as Xp (plus it could be a knock-off of Xp, even).

Second initially suspicious detail: the 10:05 and 10:07 visits have the word "satelliet", not "satellite" in the search string. That raised some suspicion as "satelliet" is the Dutch word for "satellite". However: that could also be a simple typo (switching the last two characters - a very common kind of typo) instead of a Dutchman typing.

Assuming that this was a real N-Korean visit, then the visit is highly interesting with reference to the apparent ruse played on US satellite surveillance of N-Korea as analysed above.

For here we apparently have a North Korean, a country where the average Kim is not allowed access to the internet, looking for orbital information on US surveillance satellites on my weblog!

This moreover was someone with at least some knowledge of satellites - again, not your average North Korean Kim, but suggestive of someone from the NK space program or intelligence program. The specific keywords 'tle' (two-line elements, i.e. a set of satellite orbital elements) and 'Lacrosse 5' (a US radar imaging satellite) and 'KH-12' (US Keyhole-12/Advanced Crystal optical imaging satellites, i.e. the satellites USA 129, USA 161, USA 186 and USA 224) bear this out.

Yet this person wasn't perhaps entirely informed. He or she searched for orbital information on those US optical and radar imaging satellites that form the backbone of US space-based surveillance, but notably missing from the search queries is the most recent addition to the US radar surveillance constellation, the two FIA Radar satellites. Also missing are Japan's IGS satellites. But, maybe, after checking for the KH-12 and Lacrosse 5 they realized they should not be on my blog for this information - they should be at Mike's website for that.

Thursday, 29 November 2012

Video of my lecture (in Dutch) on Hunting for Asteroids and the Van der Bilt Prize ceremony, 10 November 2012

As related earlier on this blog, I was much honoured to receive the Dr J. Van der Bilt Prize of the Royal Dutch Astronomy and Meteorology Association (KNVWS) on November 10, 2012.

The post linked above already provides a narrative of that day and a couple of photographs. This current post is to point the interested reader to video of the ceremony (in Dutch), shot by my GF:

As is customary, I did a 1-hour lecture on (part of) the activities which earned me this prestigious prize. In my case, the lecture focussed on my asteroid search activities.

Video of that is below, in three parts of approximately 20 minutes each. The lecture is in Dutch. Ignore the camera repositioning near the start of part I, it becomes stable after a few minutes (when also my lecture gathers more steam):

Part I (20 minutes):

Part II (20 minutes):

Part III (15 minutes):

Friday, 23 November 2012

UNID geosat Unknown 121118 is probably the classified SIGINT Mercury 1 (94-054A)

As I reported on this blog earlier, I discovered an unidentified object in an 8.4 degree inclined geosynchronous orbit in the evening of November 18 (see images below). It was given the designation "Unknown 121118" and also observed by Greg Roberts from South Africa the next evening, showing it to be truely geosynchronous at longitude 48 E.

click images to enlarge

I initially thought it would probably be a commercial satellite that had been recently relocated. However, as USSTRATCOM has still not identified the object, we are beginning to suspect it is a classified object, and we have some idea of its identity.

Ted Molczan pointed out that the 8.44 degree inclination is similar to that of Mercury 1 (94-054A). The brightness of the object is similar too. Mercury 1 was last located over the western Atlantic near 43 W, and had not been observed for over two months. Peter Wakelin imaged its former position on November 21 and could not find it. So there is a good chance it has been moved, and is my Unknown 121118, now located at 48 E.

click map to enlarge

The Mercury (also known as 'Advanced Vortex' ) geostationary satellites are classified US military SIGINT ('eavesdropping') satellites. Two were launched during the 1990-ies: the launch of a third one failed when the rocket booster malfunctioned, destroying the satellite. Mercury 1 (94-054A) launched on 27 August 1994 was the first. Given that it now appears to have been repositioned and is station-keeping at 48 E, is appears to be still operational, 18 years after it was launched.

Why it has been repositioned over 48 E, somewhere within the last two months (and probably near the more recent part of that timespan) is a matter of speculation. Maybe it is monitoring communications on the Syria-Turkey border; maybe it is listening in on Iran; or maybe it is monitoring communications of Somalian pirates near the Horn of Africa.

In June this year I imaged Mercury 1's sister ship Mercury 2 (see here), which was of a similar brightness as 'Unknown 121118'. That object at that time seemed headed for a graveyard orbit and hasn't been observed for a while. It has a slightly larger orbital inclination than Mercury 1 / Unknown 121118.

Wednesday, 21 November 2012

PAN, other Geostationary satellites, and another UNID (this time Greg's)

As reported earlier I had a prolific observing session on Geostationary satellites in the evening of November 18th, discovering amongst others an unidentified geostationary object now temporarily designated Unknown 121118 (see here and follow-up here with imagery by Greg from S-Africa: an more on it near the end of the current post).

Below is some more imagery showing various classified and unclassified objects. All images were made using a Canon EOS 60D with a SamYang 1.4/85mm lens at ISO 1000.

Unknown 20121117 (Greg's UNID)

The November 18th imagery includes imagery of a second unidentified object, Unknown 121117 discovered by Greg Roberts (CoSaTrak) from South Africa a day earlier on the 17th (a third initially reported  'unid 'by Greg turned out to be identifiable as a known object, a Chinese CZ-3C r/b). So Greg recovered my Nov 18th UNID on the 19th, and I recovered Greg's Nov 17 UNID on the 18th: nice teamwork!

The image below shows it together with a number of nearby commercial geosats (the veil-like lighter streaks in the image are cirrus clouds, who had begone to invade an initially clear sky):

click image to enlarge

Below is one of Greg's images of the object from 17 November taken from S-Africa: in my image above taken a day later the object has drifted quite a distance more to the West.

(image courtesy Greg Roberts, CoSatTrak S-Africa)

Unknown 121117 is a truely uncatalogued object. There is nevertheless some idea about the identity of this satellite, but I am currently not allowed to provide more information.


PAN (09-047A) and the nearby commercial geosat Paksat 1R visible in Greg's Nov 17th image are visible on my Nov 18th imagery as well. The image below basically fits to the upper image above (see the Eutelsat pair visible in both images), giving you a sense how Greg's Unknown 2012117 has moved in a day time:

click image to enlarge

I have written about PAN on this blog several times before: it is an enigmatic classified satellite that frequently relocates.

Mentor 4, Thuraya 2 and the Mentor 1r

Among the other objects imaged were the SIGINT Mentor 4 (and the nearby commercial satellite Thuraya 2), and a r/b from the Mentor 1 launch, Mentor 1r.

Mentors (the biggest geostationary satellites in existence and the biggest man-made objects in space with exception of the ISS) are relatively bright objects (typically mag. +8):

click image to enlarge

I already posted imagery of another Mentor, Mentor 5, as well as the SIGINT Vortex 6 in an earlier post.

More on my UNID, Unknown 121118

This object in an 8.5 degree inclined geosynchronous orbit (see here and here for earlier coverage) remains 'unidentified' (i.e., is not present in public orbital catalogues such as USSTRATCOM's): we are however starting to believe it could be a classified object that has recently been moved to this location from somewhere else. It is currently positioned over 48.3 E and appears stable in longitude:

click map to enlarge

Tuesday, 20 November 2012

Greg's Nov 19th imagery of my geostationary UNID (UNK121118)

click image to enlarge (image courtesy Greg Roberts)

The imagery above is posted with kind permission of Greg Roberts (CoSaTrak, South Africa). It depicts the same geosynchronous UNID that I imaged from the Netherlands on the 18th, in an image Greg shot from South Africa on the 19th.

This object does not match with anything in USSTRATCOM's current satellite catalogue, or in our classified satellite catalogue. It probably is a commercial satellite that very recently has been relocated to this spot: if so, USSTRATCOM seems not yet aware of it.We have not been able yet to identify which satellite it could be.

For my November 18 discovery imagery of this object, click here.

As the object moves in an orbit with an inclination of 8.1 degrees, it makes a small daily "up and down wobble" perpendicular to the celestial equator (many geosynchronous objects do that). This is the reason it is showing a small trail in this 40-second exposure.

(the nearby object marked EUTE 48A in the imagery is the commercial geostationary satellite Eutelsat 48A)

Monday, 19 November 2012

An Unidentified geostationary object

Yesterday evening was (initially) very clear. I took the opportunity to image the USA 144 decoy (1999-028C)  in the early evening, and then do a survey of the eastern geostationary belt between 23h and 24h LT.

The USA 144 decoy passed close to the Andromeda Galaxy (M31) and was caught together with this galaxy and a Breeze-M tank (2011-021C, from the Telstar 14R launch):

click image to enlarge

The session on geostationary satellites later in the evening was ended by incoming cirrus, but not before a fair number of objects was imaged.

A UNID geosat in Orion

Among them was a bright unidentified object in an 8-degree inclined (near-) geostationary orbit captured in several images.

The images below (overview, and then details) show it in Orion together with the Orion nebula, a Chinese r/b (Beidou CZ-3C r/b, 2010-024B), and the classified SIGINT objects Mentor 5 (USA 237, 2012-034A) and Vortex 6 (1989-035A).

click images to enlarge

While the object is unidentified at the moment, it probably is a commercial geostationary satellite that is being moved to a new slot or to a graveyard orbit: but with the tracking network of USSTRATCOM not having noted this yet.

For the moment, the object has been designated as Unknown 121118. It moves in this very approximate orbit (calculated by Mike McCants):

Unknown 121118
1 99991U 12009B   12323.84298080 0.00000000  00000-0  00000-0 0    02
2 99991   8.1179  50.0556 0005003 274.8696  85.1304  1.00270000    07

Initially, I thought it might be the same "unid" reported by Greg in the same general area the day before, but it turns out to be another object. Greg's "unid" was identified by Mike as 2010-024B, a Beidou CZ-3 r/b, actually the same object that can be seen close to USA 237 in the images above.

UPDATE 19-11-2012, 20:20 UT: Greg Roberts just mailed me that he recovered the UNID from South Africa at virtually the same position, so it is geostationary. See follow-up post here.

Belated: imaging ATV-3 late September

click images to enlarge

I am very much belated in posting the images above: they show ATV-3 imaged by me in morning twilight of 30 September 2012. The lower of the two images was my entry to ESA's ATV photo contest and won 3rd place (earning me a nice ATV t-shirt: thank you ESA!).

This was the only morning between undocking and reentry that was not clouded (well: as you can see in the images, it wasn't exactly clear either).

The observing session in question was the last one where I was accompanied by our late cat Pippi, who died a month later.

Thursday, 15 November 2012

Is it a bird, or a possible UNID satellite?

Yesterday, asteroid (2397) Lappajarvi would occult the mag. +7.8 star HIP 19971 in Taurus. I was some 106 km outside of the predicted occultation zone but inside the 1-sigma uncertainty zone, so I covered the event using my WATEC 902H video camera and SamYang 1.4/85mm lens (no occultation was seen, as expected).

However: while setting up the system and checking it out with an aim on the Pleiades, I filmed either a bird or a potential UNID satellite:

Integrated still image (click to enlarge):

I cannot 100% exclude that it is a bird, also because objects in LEO mostly would have been in shadow already at this time of the evening and sky direction (relatively low east). The resulting orbit is moreover retrograde (inclination about 127 degrees), which is very rare among satellites.

So maybe it was a bird. It does move very linear (birds usually don't) and doesn't look like other birds I had in view that evening, yet perhaps it was.

Sunday, 11 November 2012

Receiving the Dr J. van der Bilt Prize

In 1945, the Royal Dutch Astronomy and Meteorology Association (KNVWS) established the Dr. J. van der Bilt Prize, named after professional astronomer Jan van der Bilt (1876-1962) who was the  NVWS president at that time.

The prize is awarded annually to a member of the KNVWS or one of its working groups, who (as an amateur) has conducted  work of scientific value, or who has seriously contributed to the popularization of astronomy. A list of past recipients (in Dutch) is here.

Last September I got a surprise phone call from the current KNVWS president, Niek de Kort, who informed me that the Van der Bilt Prize committee had decided to award the 2012 Van der Bilt Prize to me. Someone had nominated my name, and the committee chose me out of several nominees. I consider this a great honour!

The nicely calligraphed award certificate
(click to enlarge)

The award is for my work on meteors, asteroids and artificial satellites. I have authored/co-authored several peer-reviewed publications on meteors, the result of participation in a number of scientific research expeditions during a.o. the Perseids, Leonids, and alpha Monocerotids of 1992-2002 and recently the Draconids of 2011. I do astrometry on asteroids, and discovered several new asteroids (including a NEA). And my work on artificial satellites  is probably known to the reader as it is the main subject of this blog.

Yesterday (10 November 2012), on the Astrodag ("Astro Day") of the KNVWS in Goirle, the Prize was officially handed to me by KNVWS secretary Jan de Boer (see photograph below):

The Prize comes with a nicely calligraphed certificate (see first image in this post), listing the reasons for the award and then summing it up in an alliterating one-liner.

So it reads: "awarded because of his great merit as an observer of meteors, of asteroids and of satellites".

The alliterating one-liner following this is difficult to properly translate, but roughly translates to: "Discoverer of asteroids: at home among the gravel of the solar system" (the original in Dutch alliterates: "Thuis in het gruis van het zonnestelsel"), this referring to the central line of my interest: small solar system bodies.

The two little miniatures on the certificate show a golden meteor shooting across the starry sky, and a (slightly anthropomorph) asteroid.

Just like the naming of asteroid (183294) Langbroek after me a few years ago, I consider this award to be a great honour.

Below are some more pictures of the award ceremony. As is customary, I did a 45-minute talk following the acceptance of the award, my talk being titled "Hunting Asteroids".

(the pictures of the ceremony in this post were made by Robert Haas)

(click images to enlarge)

flowers for my girlfriend as well 

Receiving congratulations and some very kind words from astronomy Prof. emeritus Hugo van Woerden, one of the award committee members

 Receiving congratulations from BWGS Satellite Workgroup chair Bram Dorreman

Starting my lecture on Asteroids

Wednesday, 31 October 2012

A sad loss for SatTrackCam - R.I.P. Pippi (our cat), 12 May 2006 - 30 Oct 2012.

Pippi, 12 May 2006 - †30 October 2012

SatTrackCam Leiden (Cospar 4353) suffered a painful loss yesterday. Pippi, our cat, died from kidney failure at the age of only 6.5 (which is young for a cat).

Over the past 3 years Pippi was my furry companion during many observing sessions. She would  join me outside, sitting next to the tripod, watching carefully what I was doing (or, when it was cold, watch me from behind the window). When I occasionally moved from the courtyard to the city moat (some 50 meters distant from our house) to target geostationary satellites, she would go with me, sit next to me, frolick about a bit in the grass along the moat: and when it took too long to her liking, cross the road again and meow untill I would finish the session and walked back home with her.

She will be dearly missed and the observing sessions will definitely be a bit more lonely here. In more than one way this was a remarkable, smart and bright cat, and Suuz and I are deeply saddened by her untimely death.

Pippi watching me from behind the window during
an observing session (photo taken in April 2010):

Pippi as a kitten:

Pippi as an adult cat:

My favorite image of her:


Saturday, 6 October 2012

OT - My latest Asteroid Discovery, 2012 SM58

It has been a while, but I finally have bagged another asteroid discovery to ad to the ones I already discovered earlier.

The new discovery appeared in MPEC 2012-T11 today as 2012 SM58 (packed designation: K12S58M). It is a high-inclination main belt asteroid.

On September 21, I was using SSON's 61-cm Cassegrain in search of an object discovered by another observatory, when I noted a faint  mag. +19.5 object moving near the upper edge of the images.  It is indicated in the blink below (which shows only part of the original images):

click image to enlarge

The images were made at a high declination (+35 degrees) well away from the ecliptic. A check showed that no known object was at this position. So, a new, high inclination asteroid? At first I was not entirely sure. The object came out of the glare of a bright star, so it was a possibility that it was a reflection in the telescope's optical system. In the first of the 4 images (taken 15 minutes apart) it was difficult to see, probably because it was closer to the glare of the star. After measuring it in Astrometrica, and trying an orbit fit with FindOrb, it did however seem to fit an asteroid orbit.

I decided to wait until I had been able to image it a second time before sending in the observations to the Minor Planet Center. Bad weather on the two telescopes I use (in Arizona and California) however meant it took six days before I could image it again, on September 27th. With a Vaïsälä orbit fit to the Sept 21 observations to go by only, 6 days is already enough to create clear positional uncertainties. I was lucky though: the object was visible (near the image edges) in the new images.

While I was quite sure it was the same object, FindOrb initially had some trouble linking the 21 Sep and 27 Sep data, so I decided to send in both sets under different temporary designations: LaMa 502 and LaMa 504. The MPC sent me back a message indicating they thought it concerned the same object:   " LaMa504 (LaMa502 "

More bad weather and the entry of a full moon next again prevented new observations, untill I was able to image it again on October 4 and 5. Below is the blink of the October 5 images:

click image to enlarge

The new discovery appeared in MPEC 2012-T11 today as 2012 SM58 (packed designation: K12S58M). It is a MBIIb asteroid with an inclination of  21 degrees. Orbital elements (from the MPC):

Epoch 2012 Sept. 10.0 TT = JDT 2456180.5                MPC
M 344.68019              (2000.0)
n   0.21885053     Peri.  110.11057          T = 2456250.50125 JDT
a   2.7271204      Node   273.19478          q =     2.3235562
e   0.1479818      Incl.   21.08086
P   4.50           H   16.7           G   0.15

Assuming a typical albedo, the absolute brightness of H 16.7 suggests an object about 1.5 km large.

The orbit in our solar system:

(click images to enlarge)

Tuesday, 25 September 2012

[UPDATED] The 21 September fireball: a small Aten asteroid?

-- edited/corrected 25/9 15:25 UT. I initially made a small error in the used trajectory azimuth (not properly taking into account effects of a spherical earth). That is corrected, but the conclusions do not alter. --

In my previous post I presented clear evidence that the splendid fireball seen over NW Europe on September 21st, 2012, was a meteoric fireball. I also presented a first, very preliminary idea of its trajectory.

Based on that trajectory, I can now present some very first, very cautious conclusions about the heliocentric orbit of this meteoroid.The solutions strongly favour an identification as an Aten asteroid.

The entry azimuth of the fireball from the reconstructed preliminary trajectory is around 80 95 degrees. Based on observations by Ramon van der Hilst who observed the fireball from Bussloo, the estimated entry angle for the fireball is about 5 degrees only: a very shallow, earthgrazing angle which explains the long trajectory. (I asked Ramon to estimate the angle of the fireball with respect to the horizontal at the moment Ramon was looking roughly perpendicular to the preliminary trajectory. That angle, about 5 degrees as Ramon reports, should be close to the entry angle)

I used these values and an 18-20 km speed estimate to compute a nominal heliocentric orbit: and then played around by widely varying the values for speed, entry angle, entry azimuth around these nominal values.

The interesting point is, that for all of these, I get an Aten orbit as a result. Aten asteroids are asteroids whose perihelion lies within the orbit of the earth and who's aphelion lies only just outside the orbit of the earth. They have a semi-major axis < 1.0 AU and aphelion (just) over 1 AU.

The aphelion values I get for the approximate fireball orbit, are in the range 1.0 - 1.15 1.05 AU, the semi-major axis values are in the range 0.9 to 0.6 AU. Solutions based on higher speeds (I varied between 12 km/s and 30 km/s in my calculations) favour the slightly larger aphelion values and shorter semi-major axis.

A wide variation in entry azimuth (I tried between 60 and 110 120 degrees) and entry angle (I tried for values between 5 and 45 degrees, the latter clearly a too large value by the way) does not alter this picture much: they all result in Aten orbits.

I need to alter the trajectory direction to values significantly larger than entry from a direction of  120 degrees (well past due east) to get aphelion values that start to get well beyond 1.15 AU and semi-major axis values > 1.0 AU.

For the current very preliminary nominal trajectory solution (entry azimuth ~82 ~95 degrees, entry angle ~5 degrees) I get these values when varying the assumed entry speed of the fireball:

[editted table 15:25 UT to reflect new calculations/correction of error]

Vini    q    Q     a     e     i

12.0   0.82  1.00  0.91  0.10  6.5
15.0   0.46  1.02  0.74  0.39  15.0
18.0   0.31  1.04  0.67  0.55  20.7
20.0   0.24  1.05  0.65  0.62  24.8
25.0   0.16  1.09  0.62  0.76  37.4
27.0   0.13  1.11  0.62  0.79  43.7
30.0   0.11  1.14  0.62  0.83  54.5

Vini is the initial speed (in km/s), q the perihelion distance (in AU), Q the aphelion distance (in AU), a the semi-major axis (in AU), e the eccentricity, i the inclination.

These values should be taken with caution and only as rough indications. There are (still) large uncertainties in the trajectory and entry angle, as well as the speed of the fireball. They do show however (as well as variations on the trajectory not listed here) that an Aten-orbit is the implied solution.

The Earth encountered the meteoroid close to the meteoroid's aphelion, when it was moving almost in parallel with the Earth.

NOTE / UPDATE 26/09/2012, 19:25 UT: There is some confusion on the web regarding my analysis and the "retrograde"/ "prograde" character of this object.
The "retrograde"character is only true for an earth-centered orbit (i.e., an object orbiting the earth, such as an artificial satellite). An east-west movement in that case means it is "retrograde" (against the motion of the earth's rotation).
This is not necessarily the case for a sun-centered orbit however. An east-west moving object then can be (and is, in this case!) in a normal, "prograde" orbit (=moving in the same direction around the sun as the planets). The difference is the frame of reference: earth-centric versus sun-centric.
So beware: the "retrograde" orbit refers to what the orbit would be for an earth-orbiting satellite (which this object was not). The Aten heliocentric orbit presented here, is however prograde.

Monday, 24 September 2012

More on the 21 September 2012 fireball: why it definitely was a meteor

I should have done this analysis earlier but did not have the time available until now. What follows now is a quick and back-of-the-envelope kind of calculation, but in my (not so) humble opinion it is adequate to the question at hand.

It concerns, of course, the splendid slow fireball seen widely over NW Europe near 21:55 UT on 21 September 2012. I posted on it before, focussing on saying "no" to the suggestion that this could have concerned a satellite reentry. In the post that now follows, I further strengthen the conclusion that it was not a satellite reentry, but a genuine meteoric fireball.

The map above gives a quick (and not particularly accurate) back-of-the-envelope reconstruction of the fireball trajectory. It is based on trajectory descriptions from Bussloo in the Netherlands and Dublin in Ireland: by taking reported altitudes (with respect to stars) and general directions of reported start and endpoints, and an assumed altitude of 50 km, the trajectory above is what approximately results. (update 19:10 UT, 24 Sep: an updated version of the map is at the bottom of this post).

The resulting trajectory is some 1000-1200 km long. In what now follows, I have taken 1100 km as the distance travelled by this fireball.

Observers near the western and eastern ends of the trajectory would probably not see the complete trajectory. Observers approximately mid-way, in mid-Britain, would potentially see most if not all of the trajectory (from experience I know you can see bright fireballs from distances of 500 km).

Observers report durations between 20-60 seconds: most video's on the web suggest a 40+ seconds duration.

It would take a reentering satellite travelling at 8 km/s (the orbital speed at decay altitudes) about 138 seconds or roughly 2.25 minutes to travel this distance. While the reported fireball durations are long, none of the reports nor videos comes even remotely close to that value.

A meteoric fireball travelling at the lowest speed possible for such an object, 11.8 km/s, would take 93 seconds to travel that distance. This is still longer than almost all of the reports suggest, but clearly getting closer.

If we take an estimated duration of 60 seconds, the 1100 km trajectory length results in a speed of  approximately 18 km/s.

18 km/s is a very reasonable speed for a slow, asteroidal origin fireball.

(it is, let me repeat, also way too fast for a satellite reentry).

Meteorite dropping fireballs typically have speeds between 11.8 and 27 km/s. A speed near 18 km/s sits squarely in the middle of that speed interval.

(update: diagram added 14:45 UT, 24 Sep)
(click diagram to enlarge)

The 60 seconds probably represents the upper boundary value for the duration of the fireball. If we take a shorter duration of 40 seconds, the speed already increases to 27.5 km/s.

This quick back-of-the-envelope reconstruction therefore shows that this must have been a meteoric fireball, quite likely of asteroidal origin, and we definitely can exclude a satellite reentry.

The fragmentation described and filmed is not unusual for meteorite dropping fireballs (see the video's of the Peekskill meteorite fall in my previous post). The object probably entered the atmosphere under a very shallow angle, which together with the slow speed explains the unusually long duration of the event.

Meteors of this kind are rare, but they have been seen before. Think of the Peekskill meteorite fall, but also the famous 1972 daylight fireball over the Grand Tetons (that had a duration of over 100 seconds) and the Cyrilid Meteor Procession from 1913 (that lasted minutes).

Note: a previous post gives a number of other lines of evidence which likewise suggest this fireball was not man-made space debris.

UPDATE: a further update is given in a new post: a very cautious orbital solution suggests an Aten orbit.

Note 2: on how I made this quick and (emphasis) rough trajectory reconstruction. I took observations that contain clear sky locations: e.g. a sighting from Dublin stating it went "through the pan of the Big Dipper"; the description from Bussloo observatory in the Netherlands; and later adding a.o. a photo from Halifax, UK, showing it just above the tail of Ursa Major. These descriptions can be turned into directions and elevations. Next, I drew lines from these sighting points towards the indicated directions, marking distances roughly corresponding to 30, 50 and 80 km altitude as indicated by the observed elevation [ distance = altitude / tan(elevation) ]. Near the start of the trajectory I marked 50 and 80 km, for Britain and Ireland I marked 30 and 50 km. These points then provide you with a rough trajectory.
From Dublin the object passed through North towards west. From Bussloo the object started NE (azimuth 60 degrees): these are important points of information too as it shows that the object started at least as far east as the Dutch-German border (and more likely over Sleswig-Holstein in N-Germany) and had its endpoint at least as far west as the northern part of Ireland.

Above: Updated map version, 24 Sep 19:10 GMT , also showing the principle of how it was reconstructed for three sighting locations. With thanks to Ramon van der Hilst for providing more detailed information on sky trajectory as seen from Bussloo (NL) on request.