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midtskogen
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13 Jan 2021 13:04

The fireball image below which I posted in the astrophotography section triggered a discussion, which rather belong here.
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This fireball has turned out to be quite interesting.  A thundering sound was heard over a large area, including the area where a recent quick clay slide destroyed a number of homes and killed 10 people, so it caused quite a bit of concern and media attention.  Norwegian seismologists also recorded the event at a nearby infrasound array, and given the camera recordings we had from our meteor network, I could quickly point out that the source was up in the air rather than in the ground.

I also soon realised that the trajectory ended pretty much right over the infrasound station.  Knowing that it would be a far shot, I asked the seismologists nevertheless to go through the seismic data as well and look for something out of the ordinary 1 to 3 minutes after the initial infrasound signal.  And guess what, precisely two minutes after the infrasound a weak, but clear, seismic signal shows up in multiple sensors in the array.  Trying various dark flight simulations, we found that the timing matches with an impact of a meteorite between ~100g and 1 kg, which would hit the ground at speeds between 55 and 70 m/s.  We know that the luminous path ended at ~20 km altitude, and the sonic transition point must have been at 16 km or below (12 - 16 km).  We also assume an pre-atmospheric mass of 5 to 10 kg (at a speed close to 15 km/s giving a peak luminosity of -8 or -9 matching the video evidence well), and the expected surviving mass would be between 0.5 and 2 kg (probably not in one piece).  So, could the seismic signal really be a meteorite impact?  To my knowledge, it would be the first time a fall is documented with video, infrasound data and seismic data.  The main concern is that the source of the seismic signal is a few km off the expected fall location, but still within the range of possibilities given the uncertainties in the data from the cameras and infrasound detectors, wind conditions and fragmentation.  We know that the seismic signal is real and very close to the sensors, and we don't have any good explanations other than that something hit the ground.  We've considered a falling tree, but it would likely give a different signal, or frost expansion in the ground, but this was the only signal that night, and frost expansion should trigger a series of signals.  And animals, not even a moose, could create a single signal like this.

Usually a fireball dropping at most a couple of kg over a forested area would not be worthwhile organising a search for.  But in this case we have a highly accurate candidate position triangulated from multiple sensors up to a few 100 meters away.  The trouble is that it's winter.  It's like looking for a wallet lost in the woods after it has snowed.  But who wouldn't give it a try anyway?  So we brought skis and gave it a try today.  It's in an easily accessible, rural area 300 masl.  We were 5 people including one of the seismologists.  Ok, it was pretty futile, but interesting nevertheless.  We found that the snow wasn't very deep, quite shallow in the densest parts of the forest, so it seems totally credible that a falling meteorite would penetrate the snow and cause a seismic signal.  Also, the fallen trees in the area were covered by old snow, so they must have fallen before the fireball event.  But there were animal tracks all over the place, as well as cavities in the snow surface caused by lumps of snow that had fallen off the trees, and there was no way we could check how deep every hole went.  So we need to try again in spring.

Images from the seismic spot, how to find a meteorite here?:
► Show Spoiler


National TV followed our little expedition giving us a couple of minutes of fame in the prime time news today, here interviewing the seismologist:
► Show Spoiler
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A-L-E-X
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13 Jan 2021 22:30

Congrats Mid!  I had thought a meteorite would be easier to spot in the snow (like meteorites that had come from the moon or mars are sometimes found in Antarctica) but I take it there's a big difference between snow in Antarctica and snow where you live.
 
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midtskogen
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13 Jan 2021 22:51

In Antarctica there's very hard wind-blown snow, or ice, and much of Antarctica is a very dry desert.  Wind and time grind snow crystals into fine powder which compacts a lot.  Also, in the summer the sun might be able to melt or sublimate the snow, in particular around dark objects like a rock.  Here we have lots of precipitation and the fluffy kind of snow, but at the same time that makes a seismic signal possible.  The TV news story.
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13 Jan 2021 22:58

Yes it is very similar here....and the infrasound equipment helps with detection also?  Thanks going to check out the story now!
 
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midtskogen
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13 Jan 2021 23:07

Since there are multiple infrasound sensors, a direction towards the source can be computed, but this is less accurate than cameras.  Perhaps more valuable is the timings (due to the speed of sound), which put some hard constraints on the distances.
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20 Jan 2021 06:33

midtskogen wrote:
In Antarctica there's very hard wind-blown snow, or ice, and much of Antarctica is a very dry desert.  Wind and time grind snow crystals into fine powder which compacts a lot.  Also, in the summer the sun might be able to melt or sublimate the snow, in particular around dark objects like a rock.  Here we have lots of precipitation and the fluffy kind of snow, but at the same time that makes a seismic signal possible.  The TV news story.

Naturally, there is a difference. And this is due to the difference in conditions. In Antarctica, after all, there are constant extremely low temperatures, while in our cities there are no such tangible frosts. It seems to me that the best way to find the places where meteorites fall is to track the trajectories from near-earth orbit. Now this is problematic, but in the future the satellite imager producer, I think, will be able to offer unique technologies that will allow tracking small and large events in the life of our planet.
 
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midtskogen
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20 Jan 2021 11:00

Challenger wrote:
Source of the post It seems to me that the best way to find the places where meteorites fall is to track the trajectories from near-earth orbit. Now this is problematic, but in the future the satellite imager producer, I think, will be able to offer unique technologies that will allow tracking small and large events in the life of our planet.

Satellites can already find trajectories of fireballs, have a global view and don't need clear weather to detect fireballs, but I think ground based cameras will still offer better resolution for some time, and more easily detect smaller meteors.  That can, of course, change in the future.
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midtskogen
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26 Jan 2021 02:29

Fragments of an iron meteorite found in Sweden.  These are fragments of a meteorite that fell in Sweden 2020-11-07.  Unfortunately, any meteorites have not been found.  See the press release in Swedish.  Google translate will probably do a reasonable job.

The confirmation that these fragments indeed come from a meteorite, is good news, though it's also an indication that someone found something early and has not reported it.

I've made several post about this fireball in other forums here, since it was recorded by our camera network.  I've also written about today's news here (in Norwegian).
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27 Jan 2021 01:07

I remember this was during that outburst we mentioned....may have been a Leonid?
 
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midtskogen
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27 Jan 2021 02:28

A-L-E-X wrote:
Source of the post may have been a Leonid?

No.  Leonids don't produce 9 tonne iron chunks.  The Leonids are comet dust.  Also, wrong radiant and wrong speed.
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28 Jan 2021 14:18

yikes that's heavy!  based on that description it sounds like an errant asteroid?
 
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midtskogen
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29 Jan 2021 02:43

A-L-E-X wrote:
Source of the post yikes that's heavy!  based on that description it sounds like an errant asteroid?

Asteroid core, one could say, since it's at least iron rich.  Probably an iron meteorite, but I guess that a pallasite can't be ruled out yet, since only fragments have been found.
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23 Feb 2021 23:50

A 14 kg iron meteorite found in Sweden after the giant fireball on 7 November 2020.  This probably is the main fragment, which I've been working quite a bit on with others in Sweden and Finland calculating where it fell.  And it was found right where what we thought the most likely area was, within less than 1 km, but we didn't rule out findings up to 2-3 away.  Very satisfying.  Before anything was found we were a bit uncertain about its mass.  If rocky, as most meteorites are, the main mass could be as high as 200 kg, but if iron the mass would be less.  So it was indeed iron, and "only" 14 kg.  Yet, it never reached terminal velocity and hit the ground at about 150 m/s.  Evidently, it hit a rock, then bounced sideways creating a trench in the ground and went up in the air again and buried itself in moss 70 meters away!

This is the world's first ever iron meteorite with known orbital elements.  Formal registration will still take some time, though.  The initial mass has been calculated as 9 tonnes.

We have quite a bit of data.  Good video recordings (though from some distance, as it was cloudy where it fell).  We have infrasound recordings.  And seismic data for the rumble.  Even satellite data.  And video recordings showing the delay between the flash and the sound recorded just a few km away from where it fell, so one can actually hear the projectile sound if the meteorite itself.
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24 Feb 2021 01:35

midtskogen, that's amazing! This entire event has been fascinating to hear as it developed, and the report of this find must certainly feel very rewarding.

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Holy ***. Can you imagine being in the area when this happened?
 
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24 Feb 2021 02:06

Not really expected, but the boulder, trench and the find location line up.  It took almost two weeks from the find of the initial impact location till the actual meteorite was found 70m away.

It shows how tough iron meteorites are.  Just imagine 14 kg of iron hitting a boulder at 5-600 km/h.  It also means that other fragments, which are likely, could be buried deep.  If such a fragment hit peat, it could be gone forever several meters into the ground.

Of 9 tonnes, not much has survived.  Fragmentation likely happened very low, perhaps around 17-18 km altitude where it still had a speed at around 10 km/s (!), i.e. barely slowed down.  Even at airline cruise altitude the speed was around 2 km/s.  It would so totally have punched through a big jet had one been in its way.  And imagine the kind of air pressure it endured.  It makes much sense that this was an iron meteorite given that the luminous path ended as low as 12 km, it appears (calculating the exact effect of refraction is a bit tricky, though).  The transsonic point was probably around 7 km altitude.

We have the sound from a few km away.  That sound is so cool.  If I heard that sound just half a minute after the flash, I would take cover, but would it make any difference?  It would punch through roof and several floors.
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