* Astronomy

Blobrana · L

RE: Reisadalen Impact

Blobrana · L

Meteorite Hunters

Date: Jun 26 08:06 2006

RE: Reisadalen Impact

Blobrana · L

Wayne Edwards and Peter Brown from the Western Meteor Physics Group at the University of Western Ontario in Canada, have reported that they detected atmospheric shockwaves at stations in Germany and Kazakhstan. Using both airwave and seismic data, Edwards has calculated that the impact released between 0.1 and 0.5 kilotons of energy, and landed somewhere around 69.1 - 69.5N and 22.2 – 24.2E

The meteor landed in this area (69.1 - 69.5N and 22.2 – 24.2E)
(146kb, 776 x 482)

It was estimated that if the meteorite had entered Earth’s atmosphere at 18 - 20 kilometres per second, then a conservative estimate would put its mass between 2,000 - 10,000 kilograms. Most of the meteoritic mass would burn up in the atmosphere, so only about 20 to 1000 kg might actually reach the ground.

"You’ll probably get individual fragments making it to the ground. But the area there is very rugged, mostly bush and tundra, so the prospects of meteorite recovery are very low" - Peter Brown.

Blobrana · L

According to Peter Brown of the University of Western Ontario, who analysed the infrasound and seismic data, the kinetic energy of the meteoritic fireball amounted to only 300 tons of TNT.
This estimate is a lot less than the Hiroshima-like event (12,000+ tons of TNT) that were reported earlier on.
He speculated that the meteorite probably did hit the ground.

Blobrana · L

Astronomer apologises for meteorite fuss:
A professor at the Institute for Theoretical Astrophysics at the University of Oslo has issued an editorial apology for what he called "exaggerated explosive force" linked to reports of the recent meteorite strike in Norway.
The story of the meteorite impact in northern Norway made international headlines, no doubt due to the comparison with the force of the atom bomb detonated over Hiroshima.

In an editorial at Norwegian science news site forskning.no, Professor Kaare Aksnes said it was regrettable that this comparison had been made, and that it was extremely exaggerated. Aksnes also said it was regrettable that the statement had apparently emanated from the Institute.
Aksnes goes on to explain that a meteor capable of a Hiroshima-like impact would almost completely burn up as it entered Earth's atmosphere, and that the remnants would hit the earth far too slowly - though impacts of that intensity have of course occurred. He estimates the North Troms impact to have been comparable to "a powerful conventional bomb".

The original reactions to the witness reports of the meteor, also reported on forskning.no, are attributed to popular astronomer Knut Jørgen Røed Ødegaard, and were slightly guarded and very excited. Røed Ødegaard wrote the original report about the meteorite on the Institute's web site.

"There is midnight sun in the area and objects in the sky must therefore shine very strongly to be visible at all. The object is descried as a reddish ball of fire and lit up like a powerful flash. The brightness must have been exceptional," - Røed Ødegaard

Source

Blobrana · L

The suspected impact crater is a rockslide.
The landslide appeared in the Mosko valley about a month ago, after the snows had disappeared, according to sources.
It is not an uncommon event in the area.

Article in Norwegian.

Blobrana · L

Hum,
One thing we know is that there was a seismic event recorded.

If we assume that the meteor and seismic event are related (it may even explain the suspected impact `crater` in the pictures as a landslide), we know that there can't be a small seismic signal from a stone meteorite, unless it was really big.
(A large stony meteor would create a huge air burst, and it would leave a huge strewn field with thousands of fragments - see above post)
But, the witness observations rule out a such large airburst.

However, it does seem that the meteorite did break up. Therefore there is a possibility of a multiple fall. More `craters` may be found.
(It should be noted that the impact site is still unconfirmed, so it is even possible that the shockwave of the explosion might have caused a normal rock slide. The actual crater may be somewhere else.)

-- Edited by Blobrana at 18:42, 2006-06-13

Blobrana · L

Here is an example that does not fit with the observations. (calculated from the weblink at the bottom)

Distance from Impact: 100.00 km = 62.10 miles
Projectile Diameter: 1.00 m = 3.28 ft = 0.00 miles
Projectile Density: 3000 kg/m3
Impact Velocity: 17.00 km/s = 10.56 miles/s
Impact Angle: 45 degrees
Target Density: 2750 kg/m3
Target Type: Crystalline Rock

Energy before atmospheric entry: 2.27 x 1011 Joules = 0.54 x 10-4 MegaTons TNT
The average interval between impacts of this size somewhere on Earth is less than 1 month.
The projectile begins to breakup at an altitude of 53900 meters = 177000 ft
The projectile bursts into a cloud of fragments at an altitude of 49200 meters = 161000 ft
The residual velocity of the projectile fragments after the burst is 15.3 km/s = 9.51 miles/s
The energy of the airburst is 4.29 x 1010 Joules = 0.10 x 10-4 MegaTons.
No crater is formed, although large fragments may strike the surface.
The air blast at this location would not be noticed.

However if it is a 1.00 m iron meteorite...

Transient Crater Diameter: 15.5 m = 50.7 ft
Transient Crater Depth: 5.47 m = 17.9 ft
Final Crater Diameter: 19.3 m = 63.4 ft
Final Crater Depth: 4.12 m = 13.5 ft
The crater formed is a simple crater
The floor of the crater is underlain by a lens of broken rock debris (breccia) with a maximum thickness of 1.91 m = 6.27 ft.
At this impact velocity ( less than 12 km/s), little shock melting of the target occurs. At this impact velocity, little vaporization occurs; no fireball is created, therefore, there is no thermal radiation damage.
The major seismic shaking will arrive at approximately 20 seconds.
Richter Scale Magnitude: 0.3
Mercalli Scale Intensity at a distance of 100 km:
Nothing would be felt. However, seismic equipment may still detect the shaking.
The air blast will arrive at approximately 303 seconds.
Peak Overpressure: 3.86 Pa = 0 bars = 0.000548 psi
Max wind velocity: 0.0091 m/s = 0.0204 mph
Sound Intensity: 12 dB (Barely Audible)

This fits better with what the witnesses saw and heard.

http://www.lpl.arizona.edu/impacteffects/

Blobrana · L

Truls Lynne Hansen thinks that the meteorite weigh around 12 kilos, and disputes Røed Ødegaard's description, calling it an exaggeration.

Credit: OLA SOLVANG / NORDLYS
This is where last week's meteorite is believed to have hit Norway, at Reisadalen, east of Tromsø.

"Our atmosphere is peppered with small stones from outer space all the time. Most burn up and disappear, but some land here" - Truls Lynne Hansen of the Northern Lights Observatory (Nordlysobservatoriet) in Tromsø.