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Forum » SpaceEngine » Science and Astronomy Discussions » Planetary Impacts (Stuff concerning objects in space colliding)
Planetary Impacts
WatsisnameDate: Monday, 04.03.2013, 11:06 | Message # 16
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Yeah, you two are arguing over semantics.

No asteroid/comet can hit earth's atmosphere at under the escape speed, and at that speed there is significant ablation of the surface due to heating. If you define 'intact' as zero mass loss, then it is correct that no asteroid makes it through the atmosphere intact.

If you instead define intact as making it to the surface of the Earth before breaking up into fragments or exploding, then there will be a dependency on the size, density, entry angle, and initial velocity of the object. Under this definition, events such as Sikhote-Alin, Chelyabinsk, and Tunguska are all examples of objects that did not come through intact, whereas Chicxulub and any other event that left a multi-kilometer sized crater did come through intact.

One may play with this interactive applet to get an idea of what parameters lead to what results for an impacting body.







Edited by Watsisname - Sunday, 03.03.2013, 22:17
 
WatsisnameDate: Monday, 04.03.2013, 11:06 | Message # 17
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I'll just quote myself again, this time with an even larger font. I'm tired of repeating myself, tired of having to provide additional material which is evidently not being read, and tired physically. I'm really beginning to feel HarbingerDawn's frustration here.

Quote
This process is ineffective for impactors whose mass is much greater than the mass of atmosphere they displace during entry.


Quote
Also, if the object is as you say 'in one piece technically' then that's all we originally cared about. We did not care if it would have fragmented anyway if the ground wasn't there.


Goodnight.





 
midtskogenDate: Monday, 04.03.2013, 11:06 | Message # 18
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Quote (HarbingerDawn)
[Discussion about planetary impacts moved to here]

You misspelled it...





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midtskogenDate: Monday, 04.03.2013, 11:06 | Message # 19
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Quote (HarbingerDawn)
Ok, but I clearly defined "intact" the way I was using it, so that would be intact. It doesn't matter what you call it. By your definition of "intact" even the Apollo spacecraft didn't make it back to Earth intact. Yet that's an absurd thing to say since by all reasonable criteria it did.

Neither of us is using "intact" in the literal sense, "untouched", or "visually as untouched". But I think deformed (like the melted crust of a meteorite) is stretching the meaning too far. But let's not argue over a word if what you say is that deformation is irrelevant and insignificant mass loss mean up to a few percent (e.g. 10,000 kg meteoroid in space resulting in a 9700 kg meteorite on the ground. Perhaps that's possible, but I think evidence speaks against it. It's probably hard to prove since we don't know the exact mass in space.

Quote (HarbingerDawn)
There are also objects (artificial, of course) that have survived higher speeds.

Examples?

Quote (HarbingerDawn)
you strongly implied in your earliest post on this page that a large object would survive mostly intact through the atmosphere and disintegrate on impact, and also implied that a lump of iron of a certain size would survive intact through both atmosphere and impact. Please clarify if I misunderstood.

I wrote "the main lump might reach the ground". And I was then thinking of a fall near terminal velocity, as opposed to the next sentence. "Main lump" was a vague description of something at least 50% of what flew in space. I think we have to be vague here. It's very hard to tell from a meteorite how big it was in space, how much got vaporised and how much or if it fragmented. In general, if you find a meteorite weighing a few kg, you can expect that there are many more to be found weighing a few grams as well. Or if you find a meteorite weighing a tonne, expect many weighing a few kg.

Quote (HarbingerDawn)
Craters and the objects that made many if not most of them.

I think you need to be more specific. Even if most craters were made of meteors not significantly breaking up in the atmosphere, they generally disintegrate at impact if they were big enough to create a crater.

The meteor that created the crater in the Arizona desert has been estimated to weigh 300,000 tonnes, and probably less than half vaporised. It was an iron meteor. Yet, only 25 tonnes have been found and the biggest fragment weighs 639 kg.

The Wolfe Creek Crater meteor was 50,000 tonnes of iron, and there are no fragments larger than 250 kg.

Going smaller, the iron meteorite that hit Whitecourt a thousand years ago creating a 36 m crater totally disintegrated. 3000 fragments have been found, the largest 31 kg.

The Kamil crater (45m): more than 5000 fragments found (iron), 1.7 tonnes, largest piece 83 kg.

The pattern is the same: only fragments are found in or near the craters. There's a lengthy list of craters in the Earth Impact Database.

Quote (HarbingerDawn)
But what size would be the largest meteorite we've ever examined (as in size of original object)? Have we ever examined the remains of an object 1 km wide? 10 km wide? 50 km wide?

Meteors that large simply vaporise upon impact, so what's left to study is transformed, earthly rock. I suppose some fragments could survive if falling a bit away from the impact site, but you will probably not find them since meteors larger than 1 km hits Earth only every 3-4 million years or so, so any fragments are easily lost in erosion.

The most recent impact of fairly catastrophic proportions probably is the Zhamanshin meteor. According to Russian Wikipedia it was 200-400 meters across and impacted a million years ago.

Added (04.03.2013, 01:25)
---------------------------------------------
Quote (Watsisname)
If you instead define intact as making it to the surface of the Earth before breaking up into fragments or exploding

Yeah, you'll be totally intact if you fall off a cliff, but not after the landing...





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Edited by midtskogen - Sunday, 03.03.2013, 22:26
 
HarbingerDawnDate: Monday, 04.03.2013, 11:06 | Message # 20
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Quote (midtskogen)
Examples?

Apollo (~11 km/s), Genesis (11.04 km/s), Stardust (12.9 km/s), Hayabusa (12.2 km/s), Galileo Probe (47.8 km/s; Jupiter).

Quote (midtskogen)
Even if most craters were made of meteors not significantly breaking up in the atmosphere, they generally disintegrate at impact if they were big enough to create a crater.

As I have very clearly and precisely specified a few times now, I am talking ONLY about surviving atmospheric passage intact. Obviously it's going to be pulverized on impact with the surface.

Quote (Watsisname)
No asteroid/comet can hit earth's atmosphere at under the escape speed, and at that speed there is significant ablation of the surface due to heating. If you define 'intact' as zero mass loss, then it is correct that no asteroid makes it through the atmosphere intact. If you instead define intact as making it to the surface of the Earth before breaking up into fragments or exploding, then there will be a dependency on the size, density, entry angle, and initial velocity of the object. Under this definition, events such as Sikhote-Alin, Chelyabinsk, and Tunguska are all examples of objects that did not come through intact, whereas Chicxulub and any other event that left a multi-kilometer sized crater did come through intact.

That is exactly what I have been trying to say this whole time!

Quote (midtskogen)
Yeah, you'll be totally intact if you fall off a cliff, but not after the landing...

But you'd be fine until you hit the ground, which - again - was my whole point.





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HarbingerDawnDate: Monday, 04.03.2013, 12:22 | Message # 21
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Quote (midtskogen)
You misspelled it...

And you posted this in the wrong thread.

Fixed now, btw.





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SpaceEngineerDate: Monday, 04.03.2013, 21:34 | Message # 22
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C/2013 A1 almost hitting Mars in 0.97:






Attachments: 3291965.jpg(112Kb) · 3594822.jpg(117Kb) · 1528514.jpg(111Kb)





 
HarbingerDawnDate: Monday, 04.03.2013, 21:40 | Message # 23
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Quote (SpaceEngineer)
C/2013 A1 almost hitting Mars in 0.97

Is that supposed to be at closest approach?





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SpaceEngineerDate: Monday, 04.03.2013, 22:48 | Message # 24
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Uncertanity in orbital elements is quite high for now, future observation will get an answer.




 
HarbingerDawnDate: Monday, 04.03.2013, 23:05 | Message # 25
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Quote (SpaceEngineer)
Uncertanity in orbital elements is quite high for now, future observation will get an answer.

What I meant was that if the screenshots were supposed to be from closest approach then you might want to try my orbital elements; if they look slightly weird it's because I fudged them a bit to make it pass Mars at the right time. Everything should be correct to well within the margin of error though.






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SpaceEngineerDate: Tuesday, 05.03.2013, 00:04 | Message # 26
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I used this orbital elements, given me by a man from a team that discovered the comet. This elementa are already disturbed by Mars, so they are not so accurate before and after approaching, but quite accurate at it. Note that minimal approacing distance is about 300,000 km due to inaccurate orbit of Mars (SE uses simple Kepler orbits, not modern precise numeric models like DE405/LE405).

Code

   Orbit
   {
    Epoch   2456950.5
    //SemiMajorAxis    -566.4013
    PericenterDist  1.40107
    Eccentricity    1.002474
    Inclination  129.05227
    AscendingNode    301.034387
    ArgOfPericenter    2.499623
    MeanAnomaly  -0.000400
   }





 
SpaceEngineerDate: Tuesday, 05.03.2013, 00:15 | Message # 27
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Made a quick video:






 
WatsisnameDate: Tuesday, 05.03.2013, 00:41 | Message # 28
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That is fantastic; love the view from the Martian surface.

Does anyone know whether (with accurate treatment of orbits) the comet passes over the day or night side of Mars? Perhaps the uncertainty is still too large to narrow it down one way or the other.





 
HarbingerDawnDate: Tuesday, 05.03.2013, 07:52 | Message # 29
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Quote (Watsisname)
Does anyone know whether (with accurate treatment of orbits) the comet passes over the day or night side of Mars?

From what I can tell so far it will be on the night side, but I think that either could be within the realm of uncertainty.





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Edited by HarbingerDawn - Tuesday, 05.03.2013, 07:54
 
midtskogenDate: Tuesday, 05.03.2013, 09:15 | Message # 30
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Excellent! I suspect that the comet might not have such clear tails already at Mars orbit compared to the coma (but comets are very unpredictable, so why not). The comet rendering looks very believable. When the view is within the coma, it gets less spectacular, which should be right.




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