Author Topic: Meteor Strikes Chelyabinsk, Russia  (Read 136691 times)

Offline srtreadgold

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #480 on: 03/21/2013 01:21 PM »
In regards to asking whether they could have come from the same parent body at some point way back in time: "Definitely not from 2012 DA14 - the timing and direction of the Russian meteor are wrong to be from that asteroid or to have been traveling in a similar orbit. So no to the parent body as well." - from the Meteoroid Environment Office at MSFC

Offline aero

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #481 on: 03/21/2013 01:58 PM »
I don't even know how to precisely phrase this question but it has to do with the probabilities of significantly large meteoroids hitting earth.

Because two meteoroids reached earth vicinity, one strike and one near-miss on the same day, does that change the calculation of the probability of an impact on earth in the future?

If the meteoroids did have the same orgin, then the answer would clearly be, "No." But the experts see these two bodies as being totally independent, with the only thing in common being their nearness to earth on the same day. What is the probability of that happening?

A related question, "How large does a meteoroid need to be to cause a visible fireball on impacting earth's atmosphere, as apposed to the streaks of light commonly seen in meteor showers?"

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Online kevin-rf

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #482 on: 03/21/2013 02:20 PM »
To be visible at night they often say the size of a pea, to be considered a bolide (fireball greater than -3 magnitude) they often say baseball sized. Wish I could find a good internet source on meteor sizes verses intensity.

It is variable since observing distance from the meteor and meteor entry speeds  (15-70 km/s) vary greatly.
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Online pippin

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #483 on: 03/21/2013 02:28 PM »
Because two meteoroids reached earth vicinity, one strike and one near-miss on the same day, does that change the calculation of the probability of an impact on earth in the future?
Not at all. You can't calculate any probabilities based on a single event, coincidence event are no exception to that.
Theoretically, these two could even have been the only ones coming close to earth and the probability of that happening at the same day could be a one-in-100-billion-years equivalent, then that one event in 100 billion years could still have been that very day.

So no, no change. To calculate probabilities you need to count asteroids, lots of them and calculate their orbits.
Or you can count historic impacts, although that's hard on earth.
Or you can count impacts on the moon, historic and current.

Offline Robotbeat

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #484 on: 03/21/2013 04:22 PM »
It does somewhat affect one's calculation, but not in a straight-forward way...

There's always a chance your original calculation based just on analysis of asteroids (and not on impacts actually that occur) is making a large systemic error, leading either to over- or under-conservative estimation of risk. Having a few asteroids hit or make close call does effect one's interpretation of the calculation because it provides extra data (as does not having any impacts), though it is statistical in nature (but what isn't).

EDIT:To give a synthetic example: Suppose you calculated that the chance of a near-miss or collision of an object in the size range we talked about was 10^-10 per year... Clearly, if you have two objects hit or nearly hit within a short timeframe (a day or so) and appear to be completely independent events, then it is much more likely your calculation of the probability was wrong than that you just experienced an event not likely to happen within a trillion times the age of the Universe.
« Last Edit: 03/21/2013 11:07 PM by Robotbeat »
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Offline Warren Platts

Re: Meteor Strikes Chelyabinsk, Russia
« Reply #485 on: 03/21/2013 06:21 PM »
Do we know for sure that the one isn't a fragment of the other?
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Online kevin-rf

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #486 on: 03/21/2013 07:10 PM »
Considering how different the orbits are, if it is, it happened a very very long time ago and had it's orbit altered greatly. More likely they both came from different parent bodies.

Do we know the composition of DA14? If they are not the same, that would be a clincher.
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Offline Targeteer

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #487 on: 03/21/2013 10:59 PM »
For those in the US, PBS' NOVA program on 27 Mar will cover the search for fragments.
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Offline smoliarm

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #488 on: 03/22/2013 12:00 AM »
Considering how different the orbits are, if it is, it happened a very very long time ago and had it's orbit altered greatly. More likely they both came from different parent bodies.

Do we know the composition of DA14? If they are not the same, that would be a clincher.

>>Do we know the composition of DA14?
No.
We do know it is a type L asteroid, of S-class. This classification is based on spectral data and in general reflects composition, but we do not know any details.

Chebarkul meteorite belongs to ordinary chondrites, namely to L-class (but this has nothing to do with type L asteroids, it's just a coincidence of classification letters). Chondrite classification is based on their chemical composition, typical values and variations for most classes are well established. But it is impossible to match meteorite compositional classes with spectral types of asteroids WITHOUT a reference point, without direct sampling of asteroids.

This is why sample-return mission to asteroid has a GREAT scientific value. For cosmochemistry and planetology, it would be comparable with effect of lunar sample return.

============================
>>Do we know for sure that the one isn't a fragment of the other?
It depends how you define "for sure" :)
They have substantially different orbital planes. This makes their relation ("a fragment of the other") HIGHLY unlikely.

Online pippin

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #489 on: 03/22/2013 12:11 AM »
Clearly, if you have two objects hit or nearly hit within a short timeframe (a day or so) and appear to be completely independent events, then it is much more likely your calculation of the probability was wrong than that you just experienced an event not likely to happen within a trillion times the age of the Universe.

No, sorry, this is wrong. This would be a single event and would have zero effect on your calculation.
To be able to even take them into account mathematically you'd need at least 30 of them (as of the central limit theorem)

You can calculate the probability for such an event given your current knowledge about the number of asteroids and so on, but with just a single event, no matter how improbably it is, you don't know whether that's within the range of your assumptions or not. As I said before: even if the probability says this is only going to happen once over the entire life of the universe you don't know whether this was that one event. And no probability, how low it may be, can ever rule out a single event.

That's a common misconception about statistics: that if you know a probability and something happens you can predict what happens next. It's a common error e.g. with numbers for a lottery. People often believe, that if a certain number is drawn now, it would be very, very unlikely that it's being drawn again the next time. But in fact it's just as likely as for any other number, even one that hasn't been drawn for ages.
« Last Edit: 03/22/2013 12:29 AM by pippin »

Offline sdsds

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #490 on: 03/22/2013 03:35 AM »
That's a common misconception about statistics

I doubt Robotbeat is suffering from the misconception you describe!

We don't know much about the frequency at which Earth encounters objects the size of the one that entered over Chelyabinsk. As I read it, Robotbeat was simply suggesting some assumptions about the rate of those encounters may have underestimated the actual rate.

Robotbeat was careful not to specify why that rate may have been underestimated, he merely observed the possibility of some sort of systemic error.
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Offline Warren Platts

Re: Meteor Strikes Chelyabinsk, Russia
« Reply #491 on: 03/22/2013 01:15 PM »
I don't think we can rule out that the meteor and asteroid are not unrelated just because it's too much of a coincidence that they both happened on about the same day. As Jorge pointed out earlier, an asteroid coming up from the south can still impact the northern hemisphere. I checked out the paper that calculated the orbit of Chelyabinsk, and we certainly can't consider it conclusive since the authors of the paper do not:

Quote
Assuming that the hole in the ice sheet of Lake Cherbakul was produced by a fragment of the meteoroid is also a very important hypothesis of this work. More importantly, our conclusions relies strongly onto assume that the direction of the trajectory of the fragment responsible for the breaking of the ice sheet in the Lake, is essentially the same as the direction of the parent body. It could be not the case. After the explosion and fragmentation of the meteoroid fragments could acquire different velocities and fall affecting areas far from the region wher we expect to find.

We don't even know if the hole in the ice of Lake Cherbakul was even caused by the meteor! After all, I thought divers were unable to find any meteorite fragments there...
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Offline mlindner

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #492 on: 03/22/2013 05:30 PM »
I don't think we can rule out that the meteor and asteroid are not unrelated just because it's too much of a coincidence that they both happened on about the same day.

NO absolutely not. We back tracked the orbits of both rocks and they are on ENTIRELY different orbits with very different energies. It is straight out impossible that both rocks had any interaction probably in the last million years at the very least. Timing has nothing to do with it. If you have enough independent events occasionally the time spaces of both of them intersect.

I'm not aware of the paper you quote. Every estimate I've seen that back tracked the orbit made no reference to that suspected crater. People used seismic data and the visual video evidence. Not to mention the satellite footage.

From the satellite image you can get quite precise direction and from the angle in the videos you can get entry angle. There also several images showing the decent path from a high altitude aircraft which allows a better estimation of angle.
« Last Edit: 03/22/2013 05:35 PM by mlindner »
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Offline Warren Platts

Re: Meteor Strikes Chelyabinsk, Russia
« Reply #493 on: 03/22/2013 06:13 PM »
Sorry, here's the link:

http://arxiv.org/pdf/1302.5377v1.pdf

These guys didn't use satellite data. Perhaps you have a link to the study you're referring to?
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Offline mlindner

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #494 on: 03/22/2013 08:44 PM »
Sorry, here's the link:

http://arxiv.org/pdf/1302.5377v1.pdf

These guys didn't use satellite data. Perhaps you have a link to the study you're referring to?

Was that really released 6 days after the impact? I'm surprised they got enough data together and ran the analyses and wrote the paper in that short amount of time.

Edit: Couldn't find the article I saw, but here is an updated paper from those same authors.

http://arxiv.org/pdf/1303.1796v1.pdf
« Last Edit: 03/22/2013 08:57 PM by mlindner »
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Offline mlindner

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #495 on: 03/22/2013 09:01 PM »
I think they have also more or less confirmed that the hole in the lake is indeed from A meteorite. Unless of course, there was yet _another_ meteorite that also struck that lake in the same time period. They found meteoric fragments all around the hole.
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Offline smoliarm

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #496 on: 03/23/2013 07:55 PM »
I think they have also more or less confirmed that the hole in the lake is indeed from A meteorite. Unless of course, there was yet _another_ meteorite that also struck that lake in the same time period. They found meteoric fragments all around the hole.

It is 100% confirmed that the hole in Chebarkul ice was made by the fragment of the same meteorite - there are 50+ small chips collected from the surrounding ice. They are identical in composition with the rest of material of this meteorite (collected in other places).

From Warren Platts:
Quote
After all, I thought divers were unable to find any meteorite fragments there...

They did not find meteorite merely because they DID NOT look for it - believe it or not :)
This was a military emergency response team, they were checking for something radioactive or otherwise harmful - by the book. As for meteorite, they did not have a first clue how to identify it, that's why they did not find it.

Offline smoliarm

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #497 on: 03/23/2013 08:09 PM »
That's a common misconception about statistics

I doubt Robotbeat is suffering from the misconception you describe!


I can confirm that pippin is right on both accounts:
1. That quote from Robotbeat does have statement which is incorrect.
2. And yes, this is a common misconception.

There is nothing strange about it, this bit of probability theory is contra-intuitive, at least quarter of my students made same or similar mistake.

Offline Robotbeat

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #498 on: 03/23/2013 08:44 PM »
If you experience an incredibly, incredibly unlikely event (according to some model), you might be just experiencing  a statistical aberration, but more likely your model is incorrect (or, of course, selection bias or something...).

Note that I have no reason to think that our estimates of impact probability are wrong by several orders of magnitude (or even off at all), but if your models WERE off by several orders of magnitude then yeah, it doesn't take a large sample size for you to consider maybe your model is incorrect.
« Last Edit: 03/23/2013 08:45 PM by Robotbeat »
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Offline Robotbeat

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Re: Meteor Strikes Chelyabinsk, Russia
« Reply #499 on: 03/23/2013 08:53 PM »
Clearly, if you have two objects hit or nearly hit within a short timeframe (a day or so) and appear to be completely independent events, then it is much more likely your calculation of the probability was wrong than that you just experienced an event not likely to happen within a trillion times the age of the Universe.

No, sorry, this is wrong. This would be a single event and would have zero effect on your calculation....
Not true. It depends on the details. For instance, if I measure the resistance of a component once, even though I have a sample size of one, my confidence is pretty high (given I have a calibrated meter, etc) that the resistance is such-and-such.

Or for instance (to give a cliche car analogy), suppose I go around a corner (driving carefully, not too fast) in a car with the assumption that the road is non-slippery but I lose traction, my wheels slip, and I almost crash. What kind of person would think that this single event should have zero effect on my calculation on whether the road is slippery or not? And no, the answer is not "a scientist."

A small sample size of one or two CAN, under certain circumstances, provide useful information or cast doubt on one's model.

In these circumstances of a close pass and a collision, they don't effect current models because the models already say that close-passes are relatively common and large hits can happen between once a decade and once a century.
« Last Edit: 03/23/2013 08:55 PM by Robotbeat »
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