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.