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INCOMING!, Detection and observation of Earth-approaching asteroids.
Tman
post Oct 10 2008, 07:13 AM
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Some analyses of the Meteosat images http://www.eumetsat.int/Home/Main/Media/Features/707785?l=en


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ugordan
post Oct 10 2008, 07:35 AM
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I haven't seen their ideas on whether the Meteosat images captured the streaking bolide or the fading fireball after disintegration. The fact IR and visible channel hotspot locations don't match, it's plausible the IR frame was captured a couple of seconds earlier with the "RGB" channels capturing the static fireball.
Thoughts?


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Guest_PhilCo126_*
post Oct 10 2008, 06:56 PM
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Looks like our doubts on the 2008 TC3 video were correct:

http://www.astronomy.com/asy/default.aspx?c=a&id=7468
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Guest_jumpjack_*
post Oct 11 2008, 08:49 AM
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QUOTE (PhilCo126 @ Oct 10 2008, 08:56 PM) *
Looks like our doubts on the 2008 TC3 video were correct:

http://www.astronomy.com/asy/default.aspx?c=a&id=7468

what?!?
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Tman
post Oct 11 2008, 09:15 AM
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Nice GIF of the fast gyrating http://www.skyandtelescope.com/community/s...g/30686199.html
QUOTE
...telescopic observations show that 2008 TC3 was gyrating wildly before it hit. According to Czech asteroid specialist Petr Pravec, it was "definitely a tumbler." His analysis reveals two distinct periods of 49 and 98 seconds long. One is probably due to rotation and the other to spin-axis precession, but he can't tell yet which is which. But he notes that this ranks (or ranked!) as one of the three fastest-spinning asteroids known.


Another one here http://news.astronomie.info/ai.php/headlines/200810020


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Tman
post Oct 11 2008, 04:11 PM
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That's by far a better one to trick with it - wow!

Today's APOD of a huge bolide http://antwrp.gsfc.nasa.gov/apod/ap081011.html

QUOTE
Astronomy enthusiast Howard Edin reports that he was looking in the opposite direction at the time, but saw the whole observing field light up and at first thought someone had turned on their car headlights.


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dilo
post Oct 11 2008, 05:49 PM
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Really impressive and beatiful lucky picture! ohmy.gif


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Guest_jumpjack_*
post Oct 11 2008, 06:02 PM
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QUOTE (Tman @ Oct 11 2008, 06:11 PM) *
That's by far a better one to trick with it - wow!

Today's APOD of a huge bolide http://antwrp.gsfc.nasa.gov/apod/ap081011.html

it's NOT 2008 TC3, so what?!? huh.gif
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claurel
post Oct 11 2008, 06:46 PM
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While hoping and waiting for some images of the bolide, I created another visualization of the trajectory and ground track of 2008 TC3 right before atmospheric entry. The trajectory comes from a HORIZONS integration on Oct. 6 (I don't think there have been any subsequent refinements since then.) The trajectory is marked off in minutes.

Attached Image
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Spin0
post Oct 11 2008, 07:59 PM
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QUOTE (ugordan @ Oct 10 2008, 10:35 AM) *
I haven't seen their ideas on whether the Meteosat images captured the streaking bolide or the fading fireball after disintegration. The fact IR and visible channel hotspot locations don't match, it's plausible the IR frame was captured a couple of seconds earlier with the "RGB" channels capturing the static fireball.
Thoughts?

Could it be possible that the explosion had a highspeed directional jet of hot gas and dust? The explosion would be visible in visible channels and the hot jet-burst in infrared. And as the burst is very high-speed maybe it explains the 23 km distance between the two hotspots.

Last year Sandia Labs made new computer models and simulations regarding Tunguska explosion. Their simulations showed a higly directional high-speed jet of hot gas and dust emanating from an exploding asteroid when it explodes in the atmosphere. Here's a link to their news (with cool animations!): http://www.sandia.gov/news/resources/relea...7/asteroid.html

QUOTE
Simulations show that the material of an incoming asteroid is compressed by the increasing resistance of Earth’s atmosphere. As it penetrates deeper, the more and more resistant atmospheric wall causes it to explode as an airburst that precipitates the downward flow of heated gas.



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ugordan
post Oct 11 2008, 08:02 PM
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QUOTE (Spin0 @ Oct 11 2008, 09:59 PM) *
Could it be possible that the explosion had a highspeed directional jet of hot gas and dust? The explosion would be visible in visible channels and the hot jet-burst in infrared.

Why wouldn't the explosion be visible in the infrared as well?


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DDAVIS
post Oct 11 2008, 09:58 PM
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I created another visualization of the trajectory and ground track of 2008 TC3 right before atmospheric entry.

What would the entry angle have been?

Don
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claurel
post Oct 12 2008, 04:51 AM
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QUOTE (DDAVIS @ Oct 11 2008, 02:58 PM) *
What would the entry angle have been?


Using the HORIZONS trajectory, I calculated it to be just over 19 degrees above the horizon plane within about a minute of impact.

--Chris

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Spin0
post Oct 12 2008, 11:24 AM
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QUOTE (ugordan @ Oct 11 2008, 11:02 PM) *
Why wouldn't the explosion be visible in the infrared as well?

Well... um.. a good question!
You are right - the two different hot spots couldn't have been simultaneous. It looks like the 23 km difference in distance could have been actually a difference in time. Here's how I think it happened. And I really tried to be short but bear with me.

I found a Eumetsat pdf describing the imaging system of Meteosat 8 (SEVIRI): MSG Level 1.5 Image Data Format Description (pp.12-13)

Meteosat 8 spins 100 rpm in its geostationary orbit. For imaging it has 12 spectral channels and each channel has 3 detectors, except HRV has 9. The detectors are arranged into two 'rows' in the focal point: the 'warm row' has visual channels (HRV and VIS aka VNIR) and the 'cold row' has IR channels. A mirror reflects light into each detector line by line as the satellite spins and scans the image. During image scanning all 12 channels are recording the image simultaneously, but there will be a small timing difference between each channel. Within the same row the maximum timing difference is only 432 microseconds. But between warm and cold rows it's 3.6 seconds because of the time it takes for the mirror to turn it's angle to reflect the same spot of Earth to different rows. Timing differences are compensated by readdressing the images' lines and columns accordingly. As Earth and clouds move relatively slowly this difference in timing is not a big deal.

According to Eumetsat the meteor's entry velocity was 12.8 km/s.

The Eumetsat's page has three images of the fireball: two visual HRV and NCOL, and one infrared. Their channels are accordingly HRV, VIS and IR3.9. In the visual channels the images of the fireball seem to overlap and there's no mention of a noticeable difference in distance. The difference is noticeable in the IR-image and can be measured as a distance of approx. 23 km between the IR and visual channel image.

In the satellite the detectors for HRV and VIS channels are in the 'warm row' and IR3.9 is in the 'cold row'. So one would expect for the images from the same row to have a tiny timing difference of a few hundred microseconds, and the images from different rows to have a timing difference of 3.6 seconds plus some microseconds. And it looks like this is what we see - HRV and VIS channels' images don't show a noticeable difference in distance. But the timing difference becomes notable when compared to the IR3.9 channel image from the cold row. I think that's why there's two separate fireballs and not three.

The maximum timing difference between the 'warm row' channels is 432 microseconds, at 12.8 km/s velocity it translates to 0.55 km distance. This is well within the satellite's interchannel Residual Misregistration Requirement of 1.5 km and would not be noticeable in the visual channel images. But the 3.6 s timing difference between visual and IR channels would be noticeable at the meteor's velocity.

But 3.6 seconds at 12.8 km/s would translate to 46 km distance - not 23 km as measured in the images. Could this be due to meteor's deceleration?
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ugordan
post Oct 12 2008, 11:38 AM
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Great analysis, Spin0. I think you've nailed the reason for the discrepancy. I had a hunch it had something to do with different timings in the satellite, but I didn't expect the thing to be rotating so fast and with mirrors and all!

I agree that the smaller 23 km distance likely comes from deceleration - the time is near expected impact, well into peak heating and deceleration.


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