What really happened during the Tunguska Event?

[thetexan]

I'm watching a show on meteor catastrophes.

They can cause damage by hitting the Earth causing a mechanical explosion due to the energy of the mass transferring into the Earth as kinetic energy.

What I don't understand is what makes them explode above the
surface like Tunguska?

Tex

 

[Vanadium 50]

Hitting air at a zillion miles an hour is like hitting a brick wall.

 

[Drakkith]

Hitting air at a zillion miles an hour is like hitting a brick wall.

Perhaps, but that probably doesn't help the OP understand the physical mechanisms underlying the explosion.

 

[nsaspook]

Nova had a program about the 2013 meteor strike.

 

[davenn]

Nova had a program about the 2013 meteor strike

Grrrr that video is blocked from being viewed in Australia and maybe other countries HI Tex ...

The difference between an object exploding/vaporising in the atmosphere, Tunguska, or hitting the ground and producing a crater like the Arizona
Meteor Crater, is mainly composition. Lower density stony ( including ice/stone composition of comets) will break up and disintegrate easier in the atmosphere.
Where as the very solid nickel/iron ones are much more likely to punch through the atmosphere. They will/can still loose up to around up to half their mass
during their passage through the atmosphere

here's one good article
http://www.britannica.com/EBchecked...nd-meteoroid/258997/Basic-features-of-meteors

cheers
Dave

 

[OmCheeto]

I'm watching a show on meteor catastrophes.

They can cause damage by hitting the Earth causing a mechanical explosion due to the energy of the mass transferring into the Earth as kinetic energy.

What I don't understand is what makes them explode above the
surface like Tunguska?

Tex

My guess is that "explosion" is a poor word for what happens.

From wiki:

Tanguska event

Krasnoyaretz newspaper, July 13, 1908:

Kezhemskoe village. On the 17th an unusual atmospheric event was observed. At 7:43 the noise akin to a strong wind was heard. Immediately afterwards a horrific thump sounded, followed by an earthquake that literally shook the buildings, as if they were hit by a large log or a heavy rock. The first thump was followed by a second, and then a third. Then the interval between the first and the third thumps were accompanied by an unusual underground rattle, similar to a railway upon which dozens of trains are traveling at the same time. Afterwards for 5 to 6 minutes an exact likeness of artillery fire was heard: 50 to 60 salvoes in short, equal intervals, which got progressively weaker. After 1.5–2 minutes after one of the "barrages" six more thumps were heard, like cannon firing, but individual, loud and accompanied by tremors.

I would describe it more like a powerful, extended, disintegration. It sounds like the Tunguska event took 7 to 8 minutes.

The whole thing is powered by the kinetic energy of the meteor/comet. It heats up due to air friction.

Here's an interesting comparison:

https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf
...punch in the numbers for the Space Shuttle, we'd find that its total mechanical energy is E = 3.23 × 10¹² joules = 3.06 × 10⁹ Btu Let’s put this number in perspective by recognizing that heating the average house in Colorado takes only about 73.4 × 10⁷ Btu/year. So, the Shuttle has enough energy during re-entry to heat the average home in Colorado for 41 years! The Shuttle has kinetic energy due to its speed of 7700 m/s and potential energy due to its altitude. It must lose all this energy in only about one-half hour to come to a full stop on the runway (at Earth’s surface). But, remember, energy is conserved, so where does all the “lost” energy go? It converts to heat (from friction) caused by the atmosphere’s molecules striking its leading edges. This heat makes the Shuttle’s surfaces reach temperatures of up to 1477° C (2691° F). We must design the re-entry trajectory, and the vehicle, to withstand these high temperatures.

I've actually been doing a bunch of meteorite energy calculations for about a week, since I found a meteor simulator. Mostly, I've discovered that I don't know how to do math, but that's besides the point. For your problem, we just need a few numbers, and some simple algebra to see what's going on.

From the above shuttle example, we get 3.23e12 joules. From my work, I know that there are 4.2e9 joules in a ton of TNT.
Simple math: The shuttle has to dissipate the equivalent explosive energy of 775 tons of TNT, in about 30 minutes.
Or about 26 tons of TNT each minute.
Roughly half a ton per second.

The shuttles were designed to do it. Asteroids, are not.

ps. I've never seen a TNT explosion. I tried to find an appropriate video, but they were mostly annoying.

 

[D H]

My guess is that "explosion" is a poor word for what happens.

My guess is that "explosion" is an extremely good word for what happens. It's not just my guess. It's the guess of many, many scientists.

From wiki:

That was overly selective copying, Om. There were other reports that indicated it was indeed an explosion.

Eyewitness accounts are not the most reliable. This is particularly so with respect to calamitous events. People's recollection of what happened is anything but perfect, and their sense of time can get quite skewed. You are relying on accounts of very scared people to make your claim.

Moreover, you are confusing the duration of the explosion with the duration of the consequences of the explosion. The explosion created shock waves that flattened trees across 50 or more kilometers. People somewhat removed from the explosion would have first seen the explosion, then experienced the shock wave, and then heard the sound of 80 million trees being blown down. These events would not have occurred simultaneously. They would instead have been spread across a few minutes. The scariness of this easily accounts for the perceived eight minute long event.

What I don't understand is what makes them explode above the surface like Tunguska?

The Tunguska meteor most likely was a rubble pile asteroid, a collection of boulders, rocks, pebbles, and dust weakly held together by gravity that hit the top of the atmosphere at over ten kilometers per second. Not much happened when it first hit the top of the atmosphere. The atmosphere gets exponentially more dense as altitude decreases. The asteroid would have been shaking and losing pieces at an exponentially increasing rate, with the exponential time scale on the order of seconds to tens of seconds.

An even more rapid exponential process kicked in when it got to an altitude of ten kilometers or so. Rubble piles are not one single solid chunk of rock. They instead comprise a very large number of little pieces of material held together weakly by the asteroid's gravity. They can be broken up other forces. That's exactly what happened, and it happened very quickly.

The atmospheric drag force on an object is proportional to the product of the atmospheric dynamic pressure, the object's cross section area to the wind, and and its kinetic energy. The onset of the breakup of the asteroid made the area increased drastically. Some of this increased energy release went back into the asteroid, which acted to increase the rate at which the asteroid was disintegrating. This is a exponentially increasing positive feedback loop. While psychological positive feedback is a thing to receive, positive feedback in physics never results in anything good. Another name for exponentially increasing positive feedback with a sub-second exponential factor is "explosion". In this case, the Tunguska event released the equivalent of about 1,000 Hiroshima bombs in a few seconds' time. Another name for that is a massive explosion.

 

[Ophiolite]

This is a very technical look at the issue, but it is current and worth a look.

Grigoryan, S.S. et al "Physical Mechanism of Chelyabinsk Superbolide Explosion" Solar System Research, 2013, Vol. 47, No. 4, pp. 268–274

Abstract
The work presents modern ideas on the physical mechanism of explosion of large meteoroids (superbolides) in the Earth’s atmosphere at the end of their trajectories. As a result of our work, the values of following parameters were obtained: the altitude of the beginning of the aerodynamic destruction of a meteoroid like the Chelyabinsk superbolide; the altitude of a relatively very thin layer, characterized by sharp aerodynamic deceleration of a fragmenting and laterally expanding space object, accompanied by an impulse transformation of kinetic energy into thermal energy with plasma generation which results in intense electromagnetic radiation and an explosive shock wave; and, the initial temperature of such a plasma.

 

[OmCheeto]

My guess is that "explosion" is an extremely good word for what happens. It's not just my guess. It's the guess of many, many scientists.

At my first science forum, back in the late 90's, they would annotate such posts of mine with; (this is nonsense...moderator)
How on Earth, my brain left out the fact that 2000 km^2 of forrest were flattened, is beyond me.

That was overly selective copying, Om. There were other reports that indicated it was indeed an explosion.

Eyewitness accounts are not the most reliable. This is particularly so with respect to calamitous events. People's recollection of what happened is anything but perfect, and their sense of time can get quite skewed. You are relying on accounts of very scared people to make your claim.
...

I think my motive in the selection was to get as distant an eyewitness account as possible. People close to the epicenter seemed to be, metaphorically speaking, "inside the car" during the crash.

As part of my penance, for sharing such nonsense, I've spent the last 3 days gathering data, and skimming multi-hundred pages papers, in an attempt to make sense, of what you obviously understand.

About the only thing I concluded, was that the Tunguska event, is very peculiar, and worthy of the ≈1000 papers written about it.

The Tunguska Impact--100 Years Later
"A century later some still debate the cause and come up with different scenarios that could have caused the explosion," said Yeomans. "But the generally agreed upon theory is that on the morning of June 30, 1908, a large space rock, about 120 feet across, entered the atmosphere of Siberia and then detonated in the sky."

It is estimated the asteroid entered Earth's atmosphere traveling at a speed of about 33,500 miles per hour. During its quick plunge, the 220-million-pound space rock heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs.

"44,500 °F", is a lot.

One of the papers I looked at:

Blast Wave
Los Alamos, 1947
Authors: Hans Bethe, Klaus Fuchs, Joseph Hirschfelder, John Magee, Rudolph Peierls, John von Neumann
304 pages

Ih this report the general phenomena connected with a blast wave in
air will be discussed. The particular features of the blast wave produced
by a nuclear explosion will be emphasized, but many of the developments in
this volume will apply generally to blast waves produced by any type of explosion.

Lots of incomprehensible maths, and the chapter titles gave me an indication of how complex (ie. above my head) this problem was.

Reflection of Blast Wave, Altitude Effect, etc.
High Temperature Opacity of Air
The Equation of State of Air Below 25,000°K​

The most peculiar thing, was that when I plugged in the numbers from the NASA article above, into Purdue's "Impact: Earth!" simulator, the resulting numbers, don't match what happened.

I suspect, we'll be discussing the Тунгусский Феномен, for quite awhile.