[Jack Hughett]

It looks like several mine entrances may be dug in the side of the mesa just below the pit at the top of the mesa. Could possibly be old gold mines. They built a lot of diversion dams, I'm thinking they used the diverted water to clean their ore.

[Jack Hughett]

Originally Posted by TheBigDog

It seems to me that "hitting the earth" is a relative thing for bodies of such incredible energy levels. To simplify the variables we choose to consider many of them as static. But let us think about some other variables which may make a difference to the behavior of a large body in close encounter with the earth.

Survivable Construction: we make reentry vehicles that survive in ways that we do not expect from most natural objects. It seems plausible to me that an object in space could become sedimentary in nature over years of random encounters and as a result behave in the atmosphere in ways that our models may not expect.

Spin: Look at the dynamics of pitching a baseball. Now apply those dynamics to a high energy object coming into the atmosphere. How might spin affect the behavior and path of such an object?


Food for thought.

Bill

Hello Bill,

I agree! You mentioned Survivable Construction. Considering that our atmosphere is relatively thin. I would think if an object is large enough only the surface layer of the object would burn away. I would be willing to bet (I usually don't bet but how can I be proved wrong) that the interior of the object wouldn't even change temperatures.

Spin: I haven't thought of this before, but I agree with you, it would appear that spin could play a part in the trajectory. With this in mind, the bottom of the object would be the first to make contact with the atmosphere causing a drag to be on the bottom surface. It would seem that this would put a top spin on the object driving it down. Which makes it more difficult for me to prove this theory.

Jack

[Turtle]

Originally Posted by TheBigDog

I love threads like this one.
...
Non-Impact Bounce: An object entering the atmosphere creates a huge pressure wave ahead of itself. The energy needed to create this wave is usually what overcomes the object and destroys it. Imagine that the object survives the pressure wave (Survivable Construction) but gets low enough to complicate the whole thing with a Ground Effect with the pressure wave bouncing off the surface of the planet. So the object never actually touches the earth even as it scars it. The ground effect echo slows the object, but also pushes it back up and away from the earth. The object might skip like a rock on the surface of a lake in this ground effect before finally slowing enough to make impact. Just like skipping a stone it would need a very low angle of impact.

Food for thought.

Bill

Mmmm....tastes like chicken! Not just like skipping a stone though. Here's a bit I think helps support my continuing claim of no skip, no plough, no bounce, no hop, no nothing but kaboom!

Originally Posted by COLLINS. MELOSH, MARCUS

Atmospheric entry has no significant influence on the shape, energy, or momentum of impactors with a mass that is much larger than the mass of the atmosphere displaced during penetration.
...
This equation applies for impacts into solid rock targets
where gravity is the predominant arresting influence in crater
growth, which is the case for all terrestrial impacts larger than
a couple of hundred meters in diameter...

http://www.lpl.arizona.edu/~marcus/CollinsEtAl2005.pdf

For all intents and purposes, any meteor forming a crater is a bomb, not a rock on a pond. That's all I got.

On a side note, here's the largest iron meteorite ever found: >> Hoba meteorite

[TheBigDog]

I know that we already covered the fact that the moon having no atmosphere makes a totally different beast, but I love the splash-zone around this crater, and as it is on my desktop wallpaper I often ponder exactly what the impact was like to cause it.

I call it the Butterfly Crater.

Bill

[Turtle]

Originally Posted by TheBigDog

I know that we already covered the fact that the moon having no atmosphere makes a totally different beast, but I love the splash-zone around this crater, and as it is on my desktop wallpaper I often ponder exactly what the impact was like to cause it.

I call it the Butterfly Crater.

Bill

Is it this one perhaps? Proclus (crater) - Wikipedia, the free encyclopedia

Originally Posted by wiki

The crater has a notable ray system that extends for a distance of over 600 kilometers. The rays display an asymmetry of form, with the most prominent being rays to the northwest, north-northeast, and northeast. There is an arc with no ejecta to the southwest. These features suggest an oblique impact at a low angle...

If it's not the exact one, the type of impact appears similar. Could be the melt on the lower portion came after the ray-maker and covered some over too.

[CraigD]

Meteor-Earth collision simulation programs are complicated, but some rough approximations, done using a few simple data and formulas, can give us an intuitive feel for the subject.

Typical rock melts at about 1500 K, and vaporizes (boils) at about 3500. Pure iron has slightly higher melting point and a lower boiling point. Typical rock has a specific heat of about

Via the formula for kinetic energy, , then, we have that a meteor with a speed of greater than about 2500 m/s that converts all of its kinetic energy to heat will be vaporized.

A typical meteor has an impact speed of about 20000 m/s. The least speed it can have is given by the formula for gravitational potential energy, , where is the radius of the Earth, the initial distance of the meteor, effectively infinity, Earth’s standard gravitational parameter, and the meteor’s mass. Via the kinetic energy formula, this calculates to about 11000 m/s.

So, we have that the minimum impact speed is about 4 times what’s necessary to vaporize the impactor. A logical question, then, is why do any meteorites remain intact? The explanation is that a smaller meteorite has a larger frontal area to mass ratio than a larger one, so experiences greater acceleration due to air resistance. Too small, and it’s vaporized by the conversion of its kinetic energy to heat due to acceleration due to air resistance, too large, and the entire meteorite will vaporize, either partially and explosively before striking the surface, or vaporizing itself and the Earth’s crust it contacts into a spreading “rock gass” fireball.

We’re far from a precise formula, but what we have is a rough prediction that only a fairly small range of initial masses of meteorites can remain intact, permitting the possibility of gouging, burrowing, skipping, or other novel behavior. From the catalog of intact meteorite finds, I suspect the largest possible intact meteorite is one with a final radius of about 2 m, and a mass of about 100000 kg, a bit larger than the largest known meteorite, the Hoba meteorite.

[Moontanman]

Originally Posted by CraigD

Meteor-Earth collision simulation programs are complicated, but some rough approximations, done using a few simple data and formulas, can give us an intuitive feel for the subject.

Typical rock melts at about 1500 K, and vaporizes (boils) at about 3500. Pure iron has slightly higher melting point and a lower boiling point. Typical rock has a specific heat of about

Via the formula for kinetic energy, , then, we have that a meteor with a speed of greater than about 2500 m/s that converts all of its kinetic energy to heat will be vaporized.

A typical meteor has an impact speed of about 20000 m/s. The least speed it can have is given by the formula for gravitational potential energy, , where is the radius of the Earth, the initial distance of the meteor, effectively infinity, Earth’s standard gravitational parameter, and the meteor’s mass. Via the kinetic energy formula, this calculates to about 11000 m/s.

So, we have that the minimum impact speed is about 4 times what’s necessary to vaporize the impactor. A logical question, then, is why do any meteorites remain intact? The explanation is that a smaller meteorite has a larger frontal area to mass ratio than a larger one, so experiences greater acceleration due to air resistance. Too small, and it’s vaporized by the conversion of its kinetic energy to heat due to acceleration due to air resistance, too large, and the entire meteorite will vaporize, either partially and explosively before striking the surface, or vaporizing itself and the Earth’s crust it contacts into a spreading “rock gass” fireball.

We’re far from a precise formula, but what we have is a rough prediction that only a fairly small range of initial masses of meteorites can remain intact, permitting the possibility of gouging, burrowing, skipping, or other novel behavior. From the catalog of intact meteorite finds, I suspect the largest possible intact meteorite is one with a final radius of about 2 m, and a mass of about 100000 kg, a bit larger than the largest known meteorite, the Hoba meteorite.

You are not saying a meteor larger than 2m will vaporise are you? I mean even if you are right about the heat of reentry at some point the meteor will be traveling to fast for the entire thing to even get hot much less vaporise. Rock meteors probably tend to explode up to a certain size and after that they would have to be to big and traveling to fast to transfer the heat of reentry significantly to the meteor. an iron nickle body would be even less likely to vaporise above a certain size. I can imagine a body say 20 miles in diameter hitting the earth at 10 to 20 miles a second the atmosphere is about 20 miles thick I predict the body would indeed impact the earth as a solid mass and vaporise as it's kinetic energy was transformed into heat as it hit the solid earth not the atmosphere. The energy released would be many orders of magnitude greater than all the nuclear devices on the planet detonated at once.

[Turtle]

Originally Posted by CraigD

Meteor-Earth collision simulation programs are complicated, but some rough approximations, done using a few simple data and formulas, can give us an intuitive feel for the subject.

Have you tried this one? >> http://www.lpl.arizona.edu/impacteffects/ It accompanies the report that Modest put up and that I have referenced.

Originally Posted by Cragarino

We’re far from a precise formula, but what we have is a rough prediction that only a fairly small range of initial masses of meteorites can remain intact, permitting the possibility of gouging, burrowing, skipping, or other novel behavior. From the catalog of intact meteorite finds, I suspect the largest possible intact meteorite is one with a final radius of about 2 m, and a mass of about 100000 kg, a bit larger than the largest known meteorite, the Hoba meteorite.

I don't buy skipping or gouging as you, Jack, Lauri, et al have suggested. I note there is no crater at the Hoba meteorite, and from reading that report that I keep mentioning, I gather the Hoba (and other large intact meteorites) actually slow to the point where they simply reach terminal velocity and, well...drop like a rock.

Sign me 'Still Not Convinced',


PS What 'other novel behavior' have you got in mind there?

[CraigD]

Originally Posted by Moontanman

You are not saying a meteor larger than 2m will vaporise are you?

I estimating too coarsely and with too few factors to give precise numbers, but in short, I’m saying what happens to a meteor or meteorite falls into 3 main classes, determined primarily by mass:

• Small bodies convert all of their kinetic energy into heat, transforming from a single solid body into gas and dust before hitting the ground.
• Large bodies reach the ground farily intact and cool (except for a small amount of their outer layers), having been largely unaffected by the atmosphere, where the sudden conversion of their kinetic energy into heat from a change in speed of roughly 20000 m/s to zero in a fraction of a second to (for very large, eg: 20+ km diameter) a few seconds transforms them and the Earth rock they strike into a hot gas. Some serious and well-simulated models suggest that events such as these have occurred half a dozen times or so in the past 3.8 billion years, resulting in global catastrophies in which the oceans are vaporized and the Earth’s entire surface incinerated down to hot bedrock
• ”just right” bodies, such as the Hoba meteorite, and various smaller ones, some of which make the news when they hole roofs and vehicle sheet metal, do neither of the above, and strike the ground/ocean/unfortunate dwelling/vehicle intact.

Originally Posted by Moontanman

I mean even if you are right about the heat of reentry at some point the meteor will be traveling to fast for the entire thing to even get hot much less vaporise. …

Correct. Very big meteorites essentially ignore the atmosphere. They don’t ignore the ground, however, so the ultimate effect is the same – kinetic energy converted into heat several times greater than needed to boil/vaporize the entire meteorite, with the excess vaporizing the Earth in the area of the strike.

Note that I’ve not attempted to address the many complicating details, such as the original kinetic energy that’s transformed not into heat, but into more kinetic energy in the form of “splash” material thrown high into or even outside of the atmosphere. I’ve been attempting to just get a preliminary, order-of-magnitude estimate to see if a giant, intact meteorite scenario such as the one Jack described is physically plausible – or, in the classification scheme above, if a “big three mile wide iron ball” can be a “just right” body capable of any sort of striking, skipping, gouging, or whatever.

My preliminary conclusion is that this can’t happen. Even tweaking the numbers slightly by assuming the impactor is a ball or pure tungsten, melting point 5930 K rather than the 3500 used in my estimates, a big meteorite is going to transform from solid to gas on initial impact, so rather than any bouncing like a ball, gouging like a plow, or burrowing like a bullet, it’s going to “splat” like a (superhot) paintball

Originally Posted by Turtle

Have you tried this one? >> http://www.lpl.arizona.edu/impacteffects/ It accompanies the report that Modest put up and that I have referenced.

I have. It’s very cool. I’m gratified that its “The minimum impact velocity on Earth is 11 km/s” hint matches my rough estimate.

It’s a complicated program, though, with some 25 pages of empirical table-rich text just summarizing it in plain English. Even with its source code in hand, I imagine it would take days to understand, hence my attempt to take a simpler look at the physics in terms of energy only.

Originally Posted by Turtle

I don't buy skipping or gouging as you, Jack, Lauri, et al have suggested.

I don’t buy it, either.

I can imagine a “glancing impact” scenario where a large (100+ km diameter) body struck the earth and ejected nearly its original mass into space. I can, with an exercise of imagination, imagine a very weird large body that behaved essentially artificially, jetting steam, using aerodynamics, etc. to make a sort of “sort landing” involving lots of skipping and gouging, but find it hard to imagine such an object existing other than as the result of advanced engineering. While the “how could you make it?” question is an interesting one, it’s not relevant, I think, to a thread about naturally occurring meteorites.

What I don’t want to do with any of my “estimating the plausible” posting is discourage anyone from getting outside and looking for geological evidence of meteorite impacts. I’m actually contemplating a sort of “science dream vacation” involving a phoned ahead visit to the NY State Museum in Albany, then some rough hiking and informed sightseeing on Panther Mountain, to get some hands-on experience with the more visible clues of an impact crater, such as creekbed fracture patterns, actually look like. Right now, if someone were to drag me off into the woods and show me what they thought was an undiscovered crater site, I wouldn’t have much of an idea what to actually look for, or tell anyone to look for.

Being married, though, it’s a tough sell of a vacation plan. “Let’s go to upstate New York and climb mountains to look at cracks in creekbeds” comes out a lot less attractive than “let’s go to the beach and drink shaved ice wine coolers”.

[Turtle]

Originally Posted by CraigD

I have.[tried the impact calculator] It’s very cool. I’m gratified that its “The minimum impact velocity on Earth is 11 km/s” hint matches my rough estimate.

I noticed your 'hit' too, and took a smidge of gratification myself.

It’s a complicated program, though, with some 25 pages of empirical table-rich text just summarizing it in plain English.
...
What I don’t want to do with any of my “estimating the plausible” posting is discourage anyone from getting outside and looking for geological evidence of meteorite impacts.

The program itself is quite simple to use I think. The user enters only 6 parameters (distance from impact, meteor density, diameter, speed, angle of entry, & target material) and the program describes the result. I agree on your description of the report, but what I don't want to do, is discourage anyone from reading complicated material. I don't concern myself with not understanding everything in such cases, rather I just plunge in.

Originally Posted by Craigologist

I’m actually contemplating a sort of “science dream vacation” involving a phoned ahead visit to the NY State Museum in Albany, then some rough hiking and informed sightseeing on Panther Mountain, to get some hands-on experience with the more visible clues of an impact crater, such as creekbed fracture patterns, actually look like. Right now, if someone were to drag me off into the woods and show me what they thought was an undiscovered crater site, I wouldn’t have much of an idea what to actually look for, or tell anyone to look for.

Being married, though, it’s a tough sell of a vacation plan. “Let’s go to upstate New York and climb mountains to look at cracks in creekbeds” comes out a lot less attractive than “let’s go to the beach and drink shaved ice wine coolers”.

Cracked creek beds & shaved-ice wine coolers? I'm all for field work, and boy would I like to put my boots on the ground down in Chihuahua.

On the matter of other impact scenarios, one not mentioned yet is the impactor so large that it causes anti-podal focusing and rips up Earth on the opposite side from the strike. (search for Dr. Mark Boslow, Sandia Labs impact physicist) I think I cover that in this thread: >>http://hypography.com/forums/astrono...r-planets.html

Here's a bit on finding impact sites by amateurs: >> Astroseti.org : How to discover asteroid impacts

A bit on ocean impact clues: >> http://www.nytimes.com/2006/11/14/sc...in&oref=slogin