Monday, September 24, 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.

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Blogger Arie Nouwen said...

Marco, the fireball was visible in Belgium. I have a report of someone in "het Kempense Dessel" in Belgium. See my blog plus comments on this: Is this location consistent with the trajectory of the fireball you show in your blog?

24/9/12 18:19  
Blogger SatTrackCam Leiden said...

It is certainly possible that this fireball has been seen from Belgium (indeed I have seen Belgian reports too) with this trajectory. It should have been going from low NE and then low through the north towards low NW.

24/9/12 20:02  
Blogger damian mc veigh said...

i saw these fireballs from belfast from a S.E. direction going N.W. @ the same time. i only managed to grab a desperate pic as it passed overhead, which although not a great pic ,when enlarged apears to be a lump of hot rock with vapour tails either side. find me on twitter @damianmcveigh and the picture is on a post from 21 sept.

24/9/12 20:41  
Blogger Paddyman said...

Our amateur astronomy group was having an observing meeting at Delamont country park N54.382307,W5.676842 when the fireballs appeared behind the trees on the horizon, they appeared to rise and pass almost directly overhead, perhaps just slightly to the North of our position. One of the members in Lisburn at the time managed to take this photo A few members on the forum have posted their reports on the sights and sounds from that night. The trajectory on your map matches what we seen also.

28/9/12 13:12  
Blogger Robin Brazenall said...

Hello. I witnessed this amazing sight whilst walking along a beach with a mate between Prestatyn and Rhyl North Wales at around midnight on the Saturday. Being familiar with astronomy the event seemed to last around 40 seconds or so and (as the trajectory you posted seems to confirm) was travelling due east. Conveniantly the "fireball's" path appeared underneath Merak and Dubhe in Ursa Major and beautifully underscored the constellation. Because of the incandescent brightness and colour of the central hub of the object I assumed that the object was a satellite unless hurtling rocks contain magnesium aluminium and other exotic materials..20-30 fragments followed the central orb all with their own comet-like -though briefer- tracks which appeared in "distress"..The central object about 20x size of Jupiter was like an argon arc weld light and from our perspective seemed to be heading towards Snowdonia. I did wonder for a split second whether this event would result in something dramatic like an end to this tory government or a bigger new saner beginning for our species but alas it is now probably merely a lump of rock on the bottom of the Atlantic being investigated by deep sea fish. I feel lucky to have witnessed this having seen many natural and man-made heavenly sights in my lifetime. Interesting article which may change my perspective..Robin Brazenall Stourbridge West Midlands UK

4/10/12 15:27  
Blogger Boo Long said...

Interesting to find this page. Your trajectory map fits in exactly with my observation.. From east Lancashire (about 20km west from Halifax) I saw it from the bottom of a valley so it appeared over the eastern horizon and was still visible as it disappeared over the west horizon.
I think the commentor above has his east and west confused!

4/1/13 07:18  

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