Tunguska Question

[KingMerv00]

Something I've always been curious about...Tunguska was caused by an asteroid/comet that exploded above the ground right? Where did 3-5 megatons of energy come from without hitting the ground?

I am not disputing the asteroid theory, I'm just ignorant of the mechanics involved.

Thanks in advance. (Hopes Phil Plait sees my question.)

 

[Timothy]

The object did not have to hit solid ground to slow down. The kinetic energy ( 1/2 m v^2 ) transferred from the object to something else -- heat, acoustic, shock, fragments going sideways versus the original path -- through interaction with the atmosphere. The amount of energy dissipated when an object goes from high velocity to zero velocity is the same whether it smashes into the ground or breaks up in the atmosphere (or partially breaks up in the atmosphere and some slower fragments hit the ground).

The actual form of the energy dissipation will be different, of course, since the dynamics of how quickly the various forms of energy interact with the environment will be different.

Many lay people do not understand this fact when they suggest "blowing up" an approaching asteroid, unaware that (to first order) a large mass impacting the earth will have identical results whether it's in one chunk or a billion particles.

 

[KingMerv00]

The object did not have to hit solid ground to slow down. The kinetic energy ( 1/2 m v^2 ) transferred from the object to something else -- heat, acoustic, shock, fragments going sideways versus the original path -- through interaction with the atmosphere. The amount of energy dissipated when an object goes from high velocity to zero velocity is the same whether it smashes into the ground or breaks up in the atmosphere (or partially breaks up in the atmosphere and some slower fragments hit the ground).

The actual form of the energy dissipation will be different, of course, since the dynamics of how quickly the various forms of energy interact with the environment will be different.

Many lay people do not understand this fact when they suggest "blowing up" an approaching asteroid, unaware that (to first order) a large mass impacting the earth will have identical results whether it's in one chunk or a billion particles.

Would it be a fair analogy to compare the asteroid hitting the atmosphere to it hitting the ocean? Tidal waves would be comparable to shock waves in the air?

 

[alfaniner]

...
Many lay people do not understand this fact when they suggest "blowing up" an approaching asteroid, unaware that (to first order) a large mass impacting the earth will have identical results whether it's in one chunk or a billion particles.

I have to admit that I'd think several smaller pieces hitting the Earth would be somewhat less devastating than one huge one, assuming they are spread out over a larger area.

 

[Timothy]

I have to admit that I'd think several smaller pieces hitting the Earth would be somewhat less devastating than one huge one, assuming they are spread out over a larger area.

It depends on how you define "devastating." You may be particularly fond of a specific patch of ground ... New York City, for example ... in which case the difference between a single rock hurtling towards Times Square will have a different result than a billion pebbles spread over the East Coast. But in the case of a significant mass, where the overall damage comes from the amount of energy imparted into the system (as opposed to specific, pinpoint damage) the result does not depend on the cohesion of the mass.

Two examples. The Chicxulub dinosaur-extinction asteroid would have had essentially the same effect whether a solid rock or a swarm of infintessimal particles of the same mass, since the destruction was caused by the energy imparted to the atmosphere.

In the Tunguska event, the damage was caused by the particulars of how and when the object exploded. If it had impacted, there would have been a large visible crater, the surrounding trees would probably have not been bowled down over as large an area, etc. The *amount* of energy would have been the same. It's just that the distribution of where it went would have changed.

 

[blutoski]

There is some suggestion that it may have been an antimatter particle. Specifically: this would explain locally elevated gamma radiation.

 

[MG1962]

The generally held opinion was the object was infact a comet - Now the comet would shed mass comming through the atmospehre, boiling off so to speak. Where the difficulty comes is all the gas trapped in pockets through out the object that begin to expand and find a way out.

The air in front of the comet is being compressed as it moves through the atmosphere, with the added element of heating, the atmosphere is being pushed in one direction, while at the same time trying to spread out as it heats up

Despite repeated attempts, no stoney or iron fragments have been recovered from the swamp centred under the event

 

[Schneibster]

I am pretty familiar with the Tunguska research, and know of no locally elevated gamma radiation. Furthermore, a google of "tunguska gamma" followed by one of "tunguska gamma radiation" turns up no such claim. Are you sure you didn't misinterpret statements about the EM radiation that would be created by the extreme heating of the atmosphere in front of an incoming massive object at Solar System speeds, which would be sufficient to create a pulse of radiation that, depending on temperature, might have a blackbody peak in the gamma ray region of the EM spectrum, with persistent gamma radiation, which would indicate the presence of radioactive material? The two are very different.

 

[blutoski]

I am pretty familiar with the Tunguska research, and know of no locally elevated gamma radiation. Furthermore, a google of "tunguska gamma" followed by one of "tunguska gamma radiation" turns up no such claim. Are you sure you didn't misinterpret statements about the EM radiation that would be created by the extreme heating of the atmosphere in front of an incoming massive object at Solar System speeds, which would be sufficient to create a pulse of radiation that, depending on temperature, might have a blackbody peak in the gamma ray region of the EM spectrum, with persistent gamma radiation, which would indicate the presence of radioactive material? The two are very different.

I don't subscribe to the antimatter theory, myself. Gamma radiation from annhialation would be elevated very briefly as a one-time pulse. Lingering radiation could only be from decaying isotopes, and - if true - would more likely suggest a chunk of some kind of radioactive isotope in the meteor.

Like yourself, I've found few credible reports of localized radiation that were not easily explainable by fallout from upwind bomb tests.

The radiation reports are usually bound to "It was a damaged spaceship" claims, and almost all tied to a man named Zolotov, but sometimes also referring to Willard Libby's C14 tree-ring research. (Libby's findings show a jump in C14 isotope fixation in 1908, but Libby is certain this is a natural flucutation)

 

[BenBurch]

There is some suggestion that it may have been an antimatter particle. Specifically: this would explain locally elevated gamma radiation.

What gamma radiation?

 

[Schneibster]

Raise the temperature high enough, and you'll get a blackbody radiation peak in the gamma, no antimatter required. The Tunguska object was big enough and going fast enough to do this, simply by transferring kinetic energy to the atmosphere. So you get a gamma pulse from that, too.

 

[Skeptic Ginger]

This may help some:
Sandia supercomputers offer new explanation of Tunguska disaster

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.

Because of the additional energy transported toward the surface by the fireball, what scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons. The physical size of the asteroid, says Boslough, depends upon its speed and whether it is porous or nonporous, icy or waterless, and other material characteristics.

Different news account of same scientific report:
Supercomputers offer new explanation of Tunguska disaster

The new simulation — which more closely matches the widely known facts of destruction than earlier models — shows that the center of mass of an asteroid exploding above the ground is transported downward at speeds faster than sound. It takes the form of a high-temperature jet of expanding gas called a fireball.

This causes stronger blast waves and thermal radiation pulses at the surface than would be predicted by an explosion from a point-source at the height where the burst was initiated.

 

[Beth]

This may help some:
Sandia supercomputers offer new explanation of Tunguska disaster




Different news account of same scientific report:
Supercomputers offer new explanation of Tunguska disaster

Thanks. That was very interesting. Another aspect of it being caused by a smaller asteroid than previously thought is it makes that particular explanation easier to assume correct because it a more probable event.

 

[Soapy Sam]

http://www.alaska.net/~meteor/SAinfo.htm
http://www.videosift.com/video/The-Tetons-Meteor-of-1972-a-near-miss

How many strikes before you're out?

 

[Schneibster]

This may help some:
Sandia supercomputers offer new explanation of Tunguska disaster




Different news account of same scientific report:
Supercomputers offer new explanation of Tunguska disaster

Good stuff, SG. I'm going to have to go poke around that site a while and see what they've been doing. A couple years back there was a pretty good analysis on there of a large strike they had simulated- looks like they might have done some more work on the subject to tune their models a bit. Nice find.

ETA: All locked down, unfortunately. I did find a 2002 talk by a geoscientist who had modeled the Chesapeake Bay impact using code Sandia had developed.

 

[Schneibster]

http://www.alaska.net/~meteor/SAinfo.htm
http://www.videosift.com/video/The-Tetons-Meteor-of-1972-a-near-miss

How many strikes before you're out?

There aren't any dinosaur killers out there for a couple hundred years last I checked- but we're talking 100Gt+ when we talk about that. A big object, something we'd see coming for sure. Little ones, 1Gt, 10Gt? Any time now. Bound to happen. We likely won't see it coming, either- too small, and we don't watch closely enough.

Imagine what would have happened had the Eastern Med meteor come down twenty or thirty years ago. Understand that the object was 10m- 30 feet for the metric-challenged- in diameter. And it made an explosion the size of a small nuclear weapon. That's not a very big rock, and it's not anything we're going to spot any distance away. Coming at 40 km/s or so, it's going to be minutes from impact even if we do spot it before it hits, and it's lottery odds we will. What if it's nickel-iron, and doesn't detonate in the atmosphere? What if it hits a city? The only difference between that and a nuclear strike is radiation; the rest of the effects are identical, up to and including a mushroom cloud, and a thermal pulse that will ignite anything that will burn within a few miles.

And this isn't some nightmare that never happens; Chesapeake Bay. The Northern coast of Mexico and the Quintana Roo. Hudson Bay, maybe. They started looking from satellites for circular formations that might be impact craters, and there are so damn many of them they can't sort the wheat from the chaff. This happens ALL THE TIME, geologically speaking. And it's going to happen again, sure as death and taxes. The question is, are we ready? And the answer is, no, not by a long shot. We sure might be glad we have nuclear weapons sometime soon, you know? Because while we might not be able to do anything about a dinosaur killer, we can vaporize a 100m rock no problem. We ought to be thinking about delivery systems, because a 100m rock is going to make one hell of a big boom, and one hell of a big hole. Think about that the next time you read a story about how NASA is "wasting money."

 

[Soapy Sam]

I was thinking of Clarke's opening to "Rendezvous with Rama", where he makes the point that Tunguska size events are not that rare, historically, but that 2/3 of the Earth is ocean and until recently, most of the rest was wilderness. As human cities spread, the odds of a catastrophe start to climb.

The movie clip of the object over the Tetons is particularly arresting.
When you think of the human and economic cost of 9/11- then scale that up to a Tunguska scale event in (or above) any major city- the loss of life might be the least of it. Had the Soviet Union owned a nuclear ballistic missile programme in 1947 when Sikhote- Alin exploded near Vladivostok, the cold war might well have gone hot right there.

ETA- the Ries meteor museum in Nordlingen is well worth a visit, for anyone in Germany.
And there's the recent comet impact model of the younger Dryas cooling event, which opens a whole barrel of worms relating to climate change.

 

[Uncle Vanya]

Something I've always been curious about...Tunguska was caused by an asteroid/comet that exploded above the ground right? Where did 3-5 megatons of energy come from without hitting the ground?

I am not disputing the asteroid theory, I'm just ignorant of the mechanics involved.

Thanks in advance. (Hopes Phil Plait sees my question.)

I'm surprised no one has mentioned Nikola Tesla's doomsday machine theory yet. I think the Tesla doomsday machine is a much cooler hypothesis than boring old asteroids, so I'm going to accept that theory as true

 

[ben m]

Raise the temperature high enough, and you'll get a blackbody radiation peak in the gamma, no antimatter required. The Tunguska object was big enough and going fast enough to do this, simply by transferring kinetic energy to the atmosphere. So you get a gamma pulse from that, too.

I beg to differ. Let's very conservatively suppose that the threshhold between "x-ray" and "gamma ray" is 50 keV. (I'd call that a "hard x-ray", but YMMV.) To get the blackbody peak up to 50 keV, you have to heat the gas to 600,000,000 K. For comparison:

• Surface of the Sun: 5000 K
• Core of the Sun: 15,000,000 K
• Core of nuclear bomb: few x 10,000,000 K
• Highest temperature achieved on Earth: 2,000,000,000 K (Z-machine)

It's hard to create a gas that hot because, to oversimplify a bit, the hotter it gets the more efficiently it moves that heat elsewhere, both by radiation and by simply expanding under pressure.

So, when you're talking about warm things, you're talking about IR blackbody radiation. Talk about hot things and maybe you can think about the visible. If you think you're pushing the limits, it's worth thinking about UV blackbodies. Before you get to x-rays, though, you need to do a sanity check and see if the physics actually holds up. Gamma rays are almost out of the question.

 

[Skeptic Ginger]

There aren't any dinosaur killers out there for a couple hundred years last I checked- ...."

Except we really haven't found all the objects yet with a potential to hit us.

We're up over 5,000 near Earth objects discovered with over 700 of them being large enough for planet wide devastation. 12 or so of those large ones were only discovered in 2007, so it isn't like we've yet found the bulk of the large ones and we are still finding hundreds of smaller ones every year.

http://neo.jpl.nasa.gov/stats/

There's only one NEO listed with a slight chance of hitting Earth and that isn't until 2048. That would be NEO 2007 VK184. It was only discovered in Nov 07. But that also means a bit more orbit calculating and we may find it really isn't coming that close.

Then there is Asteroid MN4 (apophis) and the "keyhole" hypothesis. It was only discovered in 2004. For whatever reason it doesn't rate a risky as VK184, perhaps in 2029 we'll see whether it should. In that year we are supposed to see if its orbit is perturbed and that sends it into the Earth in 2036.