The Ejby fall and the sound recordings made
by Kristian Pontoppidan Larsen

by Frank W. Rasmussen
11.2.2018

The Ejby fall is so far probably mainly of scientific interest, because the bolide was monitored by several instruments and accompanied by meteorites collected on the ground.
These instrumental data have enabled detailed characterization of the trajectory and heliocentric orbit, including the exact timing, as published by Spurny et al. in 2017 (1).
A potentially valuable piece of instrumental information has so far gone rather unnoticed. It is the sound recordings (2) made by Kristian Pontoppidan Larsen in Copenhagen on the evening of the fall. Possibly because it can be difficult to determine whether the explosion-like deep distant sounds, are truly related to the Ejby fall, or simply background city/street noises – which the recording is also full of.
Sound recordings can be a valuable and rich source of information for characterizing – not only – the dark flight of especially the leading main mass meteoroid, but also for determining the position of late fragmentation events.
Since the timing of the bolide is described in details, we set on to investigate if there was a match between the expected/calculated arrival of the main mass’s sonic boom and the actual recordings made by KPL.
The number of instrumentally recorded falls has risen significantly in recent years (Approx 30). Still, very few have instrumental recordings of dark flight events. If we could substantiate KPL’ recordings, the Ejby fall could become an even more important case for confirming and/or refining methods for predicting location of meteorites on the ground.

Summary:

Ejby fall – Sonic boom sound recordings made by Kristian Pontoppidan Larsen, have been demonstrated to be in good accordance with instrumental recordings of the bolide. A dark flight simulation with perfect timed sonic boom arrival, estimated that transition to subsonic speed happened in a height of approx. 9 –10 km at 21:07:26.5 UTC. This is 9 sec after beginning of the bolide at 85.5 km of height and 3.5 sec into the dark flight, that began at 18.28 km of height. In addition, the sound of a dozen explosions were recorded approx. 8-10 seconds delayed, corresponding to fragmentation occurring at a height of 12 – 13 km during dark flight. This is in accordance with the 3 meteorites (> 1.2 kg in total) that all lacked part of the fusion crust, found within 100 meters west of Vaengedalen in Ejby.
The details of the dark flight simulation are not important and have to be adjusted/corrected by experts. But it remains that the sound recordings of sonic booms and fragmentation explosions, fits so well with bolide observations and recovery of partly non-crusted meteorites, that the probability for it to be random noise is small. No similar sound profile was present on the 15 min. long recording.

Report

As described by Spurny (1), the timing and location of the Ejby meteoroids atmospheric entry as well as velocity (14.5 km/s) is known down to approx. 36 km of height. Dark flight starts at 18.28 Km of height at an also known location. Finally we know the fall location of the 6.5 kg main mass.
Kristian Pontoppidan Larsens sound recorder was situated at a position behind the fall location of the main mass in Herlev, relative to its incoming direction. The meteoroid therefore continuously reduced its distance to the recorder, which simplify calculations. The first sonic boom to arrive will be the leading main mass passing from supersonic to subsonic speed.
At time 21:07:21.3 UTC the meteoroid is located exactly in 38 km height on its incoming trajectory still travelling @ 14.5 km/s (1).
If we imagine the scenario, that the meteoroid could continue at this speed all the way to the Herlev impact site, then the impact occurred at time 21:07:24.3. The sonic boom would be released and travel towards the sound recorder and reach it at time 21:07:50.7.
If on the other hand, we imagine that the meteoroid goes directly to a subsonic velocity as the bolide terminates at height 18.28 km – the rather slow sound will have to travel 25.4 km to the recorder, arriving at approx. 21:08:45.5.
Thus the sonic boom must be recorded within the time interval: 21:07:50.7 – 21:08:44.5; a time window of less than a minute. In fact, the first sonic boom was recorded 21:08:19.5 UTC (recorder timing (22:08:23) adjusted, see calc. section), so it isn’t possible to reject the observation on behalf of timing.

Let’s now make a more realistic simulation of the trajectory involving the deacceleration of the main mass.
There is a time difference of 58.2 sec from the meteoroid is at height 38 km to the first recorded sonic boom by KPL. This time must be shared between the meteoroid travelling to the subsonic transitionpoint and the time it takes for the sound (released as the meteoroid goes to subsonic speed), to travel to the recorder.
Assuming that all the 58.2 sec was spend on the sound travelling, it can be calculated that the meteoroid sonic boom had to originate from a position along the trajectory corresponding to height of 11.1 km.
If we allow the sound 57 sec of travel, it matches a trajectory height of 10.7 km, leaving 1.2 sec for the meteoroid to travel from 38 km of height to 10.7 km of height. Still impossible!

56 sec sound travel — > 10.33 km height — > 2.2 sec meteoroid travel – require 14.1 km/s average velocity!
55 sec sound — > 9.95 km — > 3.2 sec meteoroid travel – require 9.90 km/s ave. velocity.
54 sec sound — > 9.57 km — > 4.2 sec meteoroid travel – require 7.65 km/s ave. velocity.
53 sec sound — > 9.18 km — > 5.2 sec meteoroid travel – require 6.26 km/s ave. velocity.
52 sec sound — > 8.79 km — > 6.2 sec meteoroid travel – require 5.32 km/s ave. velocity.
51 sec sound — > 8.39 km — > 7.2 sec meteoroid travel – require 4.64 km/s ave. velocity.

The later scenarios are very realistic, but to determine which deacceleration is most relevant is rather complicated, as it depends on a number of factors: The atmospheric entry angel, body mass (which is altered due to ablation and fragmentation), atmospheric density (very height dependent), sound speed is influenced by several factors including wind, trajectory bending etc. Programs exists for this, but that is mainly for experts and is outside the scope of this report.
However, it should be possible to make a qualified estimate.
Reports describe a typical start of dark flight at velocities of 2 – 5 km/s for a stone meteoroid (4), (5).
Simulations showed that for the meteoroid to go to approx. 8-10 km of height in supersonic velocity, it requires a high velocity at start of dark flight (at 18.28 km height). 90% of the atmospheric mass lies below the height of 12 km (4), thus resistance becomes stronger and stronger, the lower it reaches down with high velocity.
In principle multiple solutions exists, but to achieve a rather smooth deacceleration-curve and taking other requirements into account, the following solution seems fairly realistic:
If the meteoroid travels the 22.28 km long trajectory path from height 38 km to height 18.28 km (entering dark flight) at an overall average speed of 13 km/s, then it will take 1.7 sec. If it travels the next 10.28 km path in 3.5 sec at an average speed of 2.95 km/s, it will reach the height = 9.18 km at a speed of 0.30 km/s releasing the sonic boom timely for the speed of sound to carry it to KPL’s recorder arriving at exactly 21:08:19.5 UTC.
Corresponding to 53 sec. sound travel and 5.2 sec meteoroid travel (58 km height — > 9.2 km height).

In conclusion: It is absolutely realistic that KPL recorded the leading main mass sonic boom 21:08:19.5.
Had it been recorded just 5 sec earlier – it would have been impossible. Also 5 sec later would require really deep penetration into the atmosphere (height =7.1 km) with supersonic speed, which also becomes more and more problematic. Thus the realistic time-window for the recorded sonic boom is rather narrow, increasing the probability that it was indeed the sonic boom that was recorded.
In addition, a series of explosions/booms recorded 8-10 sec after the first boom, could very well be the sound of fragmentation and fragments going subsonic. Fragments that break off during dark flight and low attitude will only be partly covered by fusion crust. This fits the 3 meteorites (> 1.2 kg in all) that landed close together in Ejby west of Vaengedalen.
One could speculate the fragmentation might have occurred from the main mass that landed in Herlev.
Unfortunately the 6.5 kg main mass scattered into hundreds of fragments upon impact with tiled surface. The biggest fragment weighing only approx. 635 grams, thus not easy to judge if it was completely crusted all way round, before impact.
In principle it should be possible to reconstruct the main mass as a 3D puzzle and determine for a fact whether or not it fragmented during dark flight. Perhaps one of the 3 meteorites (including the 522 g meteorite) from Vaengedalen in Ejby fits perfectly onto the main mass’s surface.
Still 18 sec after the first sonic boom, it is possible to hear booms, which corresponds to events in 15 km H.

Calculations:
Assumptions: Earths curvature neglected (steep fall); All on ground location set at height = 0 (very flat landscape). Speed of sound (altitude and temperature dependent (6)); @ 10 km height: 300 m/s. @ ground level: 335 m/s (7 deg. Celsius). Average speed from 13 km of height to ground level: 320 m/s (6). Average speed from 20 km to ground level: 308 m/s. Sound is assumed to travel in a direct line from where it originates to the recorder, regardless of clouds etc. Sound speed is also influenced by wind. Wind speed and direction change with altitude in a complex fashion not taken into account.
Concept: All locations were designated X,Y,Z coordinates in a 3D coordinate system: 0,0,0 defined as ground location “lat: 55.6767; lon: 12.3028” (1) below the bolide terminal point (X;Y;Z)= (0;0;18.28). Beginning bolide: “lat: 55.4492; lon: 11.9116” (1) –> (X,Y,Z)= (-35,37;0;85.53); KPL’s sound recorder located at (13.71;-11.12;0). Main mass fall location: (9.60;-3.29;0) (Romancevej Herlev).
Timings: According to (1), the velocity is 14.5 km/s until at least approx. a height of 36 km (Ho video). We define the height location @ 38 km, as the potential terminal point of 14.5 km/s velocity. The exact timing can be read from fig. 7 in (1) to 21:07:21.3 (UTC). From here it follows that the 38 km height location is (-10.37;0;38) and that the bolide beginning time is 21:07:17.6 (UTC) in 85.53 km height.
Correction of sound recorder timing: Local Danish time is an hour later than UTC. The Copenhagen Town hall bell can be heard on the recordings @22:00:10 and again @22:15:10. First tone designates the time.
The town hall bell (WGS84 dec. (lat,lon: 55.675582 12.570398) is situated 2.2 km from KPL’s recorder. Thus @ a sound speed of 335 m/s (7 deg. Celsius) the bell should be heard @22:00:06.5 and 22:15:06.5 on the recording. So in conclusion, the timer of the sound recorder is 1 hr. and 3.5 sec. too late relative to UTC. The town hall harbor a famous mechanical extremely precise watch (Jens Olesens) and the tower clock/bell is maintained/ adjusted weekly by the same crew.
Calc. of impact time at 14.5 km/s – imaginative case:
Distance from Height 38 km to start of dark flight =22.28 km + distance to Herlev =20.91 –> 3.0 sec. (@ 14.5 km/s). Distance from Herlev impact to recorder: 8.84 km –>26.4 sec (sound: 335 m/s). –> 29.4 sec. in total. Thus sonic boom would arrive at the time 21:07:50.7 (UTC).
Latest possible arrival of sonic booms: Distance = 25.4 km from terminal bolide point to recorder = 82.4 sec + time for meteoroid to travel from H=38 to H=18.28: Max 3 sec. (1). – Thus 21:08.46 UTC.
Calc. of sound travel time relative to trajectory: Line (trajectory) intersection to sphere (centrum at recorder location and r= sound travel distance (straight line) relative to time).(7)
In calculations of simulated dark flight, the main mass is assumed to continue on the same linear trajectory down to approx. 8-10 km of height (sonic boom), while travelling at supersonic speed. This will be associated with some error:
The sonic boom simulation resulting at 9.2 km of height, places the meteoroid at this location: WGS dec (lat, lon) 55.7065 12.3580, which seems too far west. In reality the trajectory probably started bending before this point, reducing the distance to the recorder – which in turn demands a somewhat higher and earlier position for the sonic boom.
Listing of distinct sonic booms / fragmentation explosions registered by KPL’s sound recording: Seconds after 21 hrs 08 min: 19.5: Strong; (23: very weak); 25.5: Medium; 26.5: medium; 27.5 – 29.5 > 12 medium booms; 31.5: weak; 32.5: weak; 33.5: weak; 35.5: weak; 36.5: weak; 37.5: very weak.

References:

*1
P. Spurny et al. Planetary and space science vol. 143 p. 192-198; 2017 sept.1. : Atmospheric trajectory and heliocentric orbit of the Ejby meteorite fall in Denmark in February 6, 2016.
*2
https://recordingsofnature.wordpress.com/2016/02/08/copenhagen-urban-night-sounds-meteorite-sound-bonus/
*3
http://karmaka.de/?page_id=7682
*4
https://www.amsmeteors.org/fireballs/faqf/#8 Typically at a height of 15-20 km, the ablation process stops and visible light is no longer generated. This occurs at a speed of about 2-4 km/s. From that point onwards the stone will rapidly deaccelerate further until it is falling with its terminal velocity (100-200 m/s). “stone meteoroids tend to break up between 11-27 km altitude”
*5
The Kosice main mass meteoroid entered dark flight @ 4,5 km/s and Kosice fragment of similar size as Ejby main mass entered dark flight at 3 km/s
The Kosice meteorite fall: Atmospheric trajectory, fragmentation, and orbit.
Borovicka et al.: Meteoritics & Planetary Science 1–23 (2013)
*6
https://en.wikipedia.org/wiki/Speed_of_sound
*7
Ambrsoft.com

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