In 1908, a huge fireball streaked across the sky and exploded a few kilometers above the Earth's surface, downing trees for miles and miles around but leaving no impact crater on the ground. This mystery was known as the Tunguska event.
But how did this happen? The amount of energy released was estimated to be somewhere between 5 and 30 Megatons of TNT. (Comparably to a "typical" hydrogen bomb.) What could've caused this devastation?
My answer: a large meteor or small asteroid/comet could have done this easily. How? Let me explain.
When a meteor enters Earth's atmosphere, it's moving very, very quickly relative to the Earth. Meteors have a speed relative to Earth anywhere between about 40,000 and 260,000 kilometers-per-hour (11 to 72 kilometers/second), which is incredibly fast. The Earth's atmosphere works -- through friction -- to slow this meteor down, heating it up and causing it to glow.
But if there's a lot of ice and/or frozen carbon dioxide in this meteor, it's going to heat up and start to boil. If you have a solid piece of rock with a cavern of boiling water inside, it's only a matter of time before the pressure builds up enough to cause a powerful explosion.
If I assume my meteor moves at the maximum speed its allowed, 260,000 kilometers-per-hour, I can figure out how massive it needs to be to produce 10 Megatons (4 x 1016 Joules) of energy. The answer? A little over 1,000 tonnes, which means it was probably a rock a little less than ten meters on each side. Which means it was about the size of the smaller rock to the left of Haystack Rock in this picture.
And physics will take care of the rest: convert that kinetic energy into heat energy, use that heat to boil liquid inside, and -- just like it did for the mythbusters -- the increased pressure will cause the explosion we're all looking for. And the only scientific principle you need to know to make this possible? The conservation of energy.
And that's it. Plain-and-simple, how simply hurtling through the atmosphere, if you're filled with something that can boil, can cause you to explode with a tremendous amount of energy. And there's no faking that.
Wasn't there a recent expedition to the area to look in a nearby lake for some sort of magnetic anomaly? I heard of this I think last year but no more info has reached the public. The thinking was that a significantly large fragment carved a small lake near the site, and a piece may still be in the mud at the bottom of this lake.
Or no water required. A partially-consolidated mass would reach a point where drag forces caused it to break up, at which point the smaller masses would each have higher rates of energy transfer, thus breaking up even more, etc.
The result is catastrophic failure of the object's integrity -- and a very rapid transfer of all that kinetic energy to the atmosphere. Which, when you get down to it, is all that the water would do: cause the object to come apart suddenly with the resulting rapid transfer of kinetic energy to the atmosphere.
DC has got it. "Vaporizing ice" explanations, whether for exploding meteorites or comet orbital anomalies, always smack of hand-waving.
I'm with Rod, too. Supposedly the lake, five miles from the surface epicenter of the explosion, wasn't even there until after the Event.
Another reference to Mythbusters: consider what happened when they fired bullets into water. The higher-speed bullets broke into fragments and actually stopped sooner. To an object moving a hundred times faster than a bullet, hitting the earth's atmosphere must create similar forces.
Right. It's possible to have shocks moving through a contiguous body 'faster than the speed of sound'. For something metallic like an iron-nickel meteor, that's a very high speed; but not higher than several km/sec. When this happens, it's quite possible that the material will fragment from cavitation.
Note also that not all meteorites are stony. As I understand it, the Tunguska event was notable for not leaving a crater. Sure, a meteor could have done that sort of damage, but absent that unique fingerprint, there has been all sorts of wild speculation - everything from mini black holes to exploding flying saucers. In fact, it is quite possible for a meteor of the right type, some sort of chondritic form for example, to explode in midair and leave no trace.
Those interested really ought to check out the incredible and incredibly high resolution images and movies Sandia Labs made whilst modeling the Tunguska Event. See them at
They came to some interesting conclusions, especially about the dynamics of the explosion turning out to amplify the effect, so that the object need not have been as large as was previously thought. Their visualization media room is a treat in itself, for those of us who find such things irresistible. I want one.
I didn't see this reported previously, but it appears an estimated 10m asteroid detonated over Indonesia earlier this month with an energy of about 50 kilotons.
NASA press release (includes YouTube video link):
Is it actually friction that causes the heat? I'd read that the compressive forces were resonsible for the heating at the front of a shock wave caused by the objects hypersonic speeds. I'm not sure what the difference would be as far as heating of the object for the brief time it's exposed to its surface. It seems like the longest time I've seen for the few I've seen is no more that a few dozen seconds. How fast can heat be transmitted through nickel iron? Ice is a lousy conductor of heat, or so I thought. We need to put a camera on one of these things. No? If it's big enough does the interior actually have enough time to heat before instabilities tear it apart at high velocities or it impacts, or what else can they do? Can they "skip" through the atmosphere or get smeared-out and result in a sloppy shower of cosmic water, soot, diesel fuel and mud? What do you think of the 12.9 KYA Younger Dryas Impact theory?
Ethan - totally off topic for this blog, but on topic for your blog in general: I hope you're planning on writing something about the new gamma ray burst with a redshift of over 8 which has been discovered. I heard about it this morning on NPR.
The speeds are so high that heating by friction (+conduction) would only have an opportunity to heat the outermost layer, not the interior of the rock (and rock has far poorer thermal conductivity than iron-nickel). So I'm still unconvinced by the line about expanding heated gases in the interior. The mechanical shock as the atmosphere is encountered would be tremendous and since rocks tend to have many flaws in them (especially if they are formed by accretion of material) the rock could certainly shatter. The rock is also traveling much faster than air molecules (even many times the velocity of hydrogen) so it would be compressing a huge volume of air (with subsequent heating and later expansion) without much of that air escaping to the sides. As the rock breaks up, the expanding air (which is from our atmosphere, not the rock) can scatter the smaller rock pieces over a huge area with one loud BANG!
Of course that's just a notion without having looked at anything in any detail - I'd be happy to follow leads to other articles about what goes on.
Another thought: if the rock had much material which is gaseous at comfortable temperatures within it, wouldn't it streak off and glow beautifully while the rock was in space and relatively close to the earth (since that places it relatively close to the sun)? How far does the rock have to be from the sun in order to preserve its volatiles? Looking at our long-time neighbor (which has had billions of years to bake near the sun), what is the constituent fraction of volatiles in moon rock?
But where do the UFOs enter into the picture then?
Ethan, (and Doug at 8)
Phil Plait also says it is ram pressure, not friction that causes the heating.
Dr. Ethan is not quite right. It's the pressure difference between the front of the comet/asteroid (ram pressure) and the corresponding low pressure wake that forms at the back that causes the breakup. The object is subjected to an internal pressure gradient which exceeds its material strength, causing it to shatter into smaller pieces. Those pieces, in turn are subjected to the same forces causing further breakdown The end result is that transfer of kinetic energy into frictional heating of the air occurs very rapidly, i.e., an explosion. Comets and stony asteroids typically breakup this way.
There is no end to the impact theories related to that event. Some are good, and some not so good. And I've no doubt, you have heard them all by now.
But here's a fresh viewpoint that looks at the actual ground effects of such an event from a fluid dynamic/blast analysis point of view:
The Dragonstorm Project
Exploding flying saucers? Sounds like a couple of episodes from "The Invaders"(1967-68, a Quinn-Martin Production) that I know about.
I didn't read everyone's comments but those I did read were missing the point of the boiling water inside a solid shell. Water expands 1600 times its volume when going from liquid to gaseous states. So if it becomes super heated in an enclosed space the resulting pressure is huge, resulting in a detonation when released. That is what would cause the mutli-megaton forces.
The only problem with this explanation is that the Tunguska meteor(?) had indeed a very slow speed compared to Earth. All contemporary eyewitnesses agreed that it was clearly visible passing through the atmosphere for minutes(!) before the explosion. Meaning its speed could not be faster than that of an average jet. So where those enormous friction energy could come from?
Why not put the measurements in yards and mph so the average reader can get an idea size and speed without going to a conversion table?
Would the heating of the atmosphere ahead of the object cause dissociation of CO2 to CO + O2 in large quantity? Or some other high temperature reactions that are rare at 3000 degrees but exist at 50,000 degrees. The explosion of the atmosphere would contribute to light emission and energy absorption. The tail of the dragon would appear to move slowly through the sky. At 3000 degrees roughly 3% of CO2 breaks up into CO + O2. At 50,000 degrees who knows? But then after the object goes by the CO and O2 recombine in what would appear to resemble flue gas recombustion flame. We see the flame extinguishment behind the front and think we are seeing the object moving slowly. Am I right?
Why can't one of these happen over Mexico City? Say 50 Megatons...
"Meaning its speed could not be faster than that of an average jet. So where those enormous friction energy could come from?"
Modern jets are streamlined and constructed from high quality alloys that are designed to withstand without damage friction through the atmosphere at their speeds.
Additionally, the space shuttle goes equally slow, yet it still requires a special heat shield. Why? In that case because it has come from orbit, not from an airport. The heatshield isnt needed going out, but IS needed coming back.
Since meteors don't have thrusters to fire to remove some of their orbital energy, every joule it has to lose coming in has to be dissipated by heating itself and the encompassing atmosphere.
Planes don't go into orbit and therefore have no orbital energy to remove.
Hi Jerry. I am not a satellite erepxt, just a fan who has benefitted greatly from your talks, website and books. Thanks so much for A Beginners Guide to DSLR Astrophotography. Absolutely terrific. Perhaps some users of Heavens Above may not realize that you can go back in time to check prior satellite passes. For your site and time the Resurs 1-4 rocket stage was passing just about where your image shows the trail, and at 9:02 PM. But, the mystery continues, since the maximum magnitude was estimated at 3.2. Was the stage rotating in a manner that just happened to cause the flare?The night before there was indeed an Iridium flare visible at your site at exactly 9:02 PM, but the location was a bit east of that indicated in your image and of course the star party did not commence until Friday.So, perhaps the mystery continues. The diagram provided by Heavens Abovefor the Resurs stage looks just about right, but given the magnitude???Maybe someone else can provide a more precise analysis.Best,Bob
Wow! It really is BIG! 1000 tones! hundred buses...!
This is all wrong. Meteors explode because their structural integrity fails under the tremendous forces of atmospheric entry. This leads to an exponential disintegration of the meteor which almost instantaneously transfers all of its enormous kinetic energy into the atmosphere in a relatively small volume. This heats the atmosphere and the vaporized particles to enormous temperatures which causes the blinding light and fantastic pressure wave.
Phil and other people with the same views are correct.
The "flying kettle theory" is not.
There are many dynamics that have not been talked about that support the steam theory. The first thing not talked about is why the steam doesn't just escape the rock as it becomes heated. Well if you are dealing with a rock that is 10m cubed, then steam would have to travel through 5m of rock, quickly.
The other issues not talked about is the the estimate of size is based upon the average water content of rock meteors. And the explosive forces that would result from super heated steam. If you don't like the steam theory, then provide the energy theory that explains the motive force of the explosion.
That would be the same pressure as any other material boiled off. Water or not.
Those who think simplistically "this is the only way it happens" are almost invariably wrong.
Those who insist those who think simplistically one way are right and those who think simplistically another way are wrong are ALWAYS wrong.
The heating comes from compression of the air in front of the meteor, but it's getting needlessly pedantic to say it ISN'T friction. You could say that the reason the air piles up in front is because it has too much internal friction to slide out of the way.
The flying tea-kettle idea is wrong. The friction-heated steam-bomb is an impossibility, and a popular myth.
First, you'd need a perfectly-sealed and very strong container. Yes, it would take time for steam to ooze out of a less than perfect container, but when a rock/iron ball is hitting atmo, it's breaking up and cracking. You'd need a container that let go all at once to get a good boom.
Second, getting heat through the container and into the ice would also take time. As is often pointed out, a meteor that makes it to the ground is often ice-cold---it just doesn't have time to heat up all through. (By the way, the energy needed for melting the ice is going to REDUCE the energy available for your explosion.)
Third, a steam explosion just isn't that big. Nobody is making steam bombs these days. Boiler explosions used to wreck steamboats, but those boilers were pretty big, designed to just barely hold pressure all over, and they, like the Mythbusters water tank, were heated slowly and carefully.
The Mythbusters tank failed around one seam, and remained largely intact. Where, in all the meteorites, are the remnants of a tank-like container? If you just thought that the tank would burn up, you are already seeing the problem with the steam-explosion idea---burning up is really what's happening.
The "exploding" meteors are simply breaking up into many small fragments, each of which friction/compresses into heat and light. Think of it as a large meteor shower happening in one place at one time, and you'll have it. Yes, if you want you can have steam helping to break up the main meteor. You can also have rubble and carbon and chondrites and comet cores, too---all will shred apart and slap the air like a shotgun fired into water.
The only thing that will NOT blast apart in the air is a solid iron body. Even if it has a bottle of water sealed inside it.
A little more:
If you watch the video of the Russian meteor, you will not see any point where it blows up. There is never an explosion. It builds up, and tapers off---not smoothly, but never abruptly.
The shock wave hitting the ground may seem like an explosion, but so does the shockwave of a supersonic jet plane, and that isn't blowing up. A boom isn't always from an explosion.
As for the energy of a steam explosion ... well, where does the heat come from to make the steam? It comes from friction/compressive heating, which could just as well be melting rock. If you want a steam explosion, you've got to lose energy somewhere else---you could, if you wanted, imagine a water-cooled heat-shield that would get most of the meteor to the ground un-melted, and that would work better than your steam explosion.
If there is a steam explosion, where is the bright light coming from? How does a steam explosion make a fireball? And, once the steam explosion occurs, what happens to all the solid bits of the meteor as they spread out, still moving earthward at blazing speed. Heck, what happens to the steam cloud as it expands, still moving along at meteoric velocity?
Finally, imagine that the meteorite was broken up into gravel and dust while still in space. Have it held together by gravity, static charges or even by ice, or just fly the bits in a bunch through improbability. Now, run that into the atmosphere at miles per second, and imagine what you'd get. You'd get an entire meteor shower all to once.
Now imagine firing a shotgun into water. Any meteor that is not a ball of iron is going to come apart, and the bits will come apart, and the little bits of it will "burn" all at once.
There are meteoroids and asteroids out there that are rubble piles, and there are old comet nuclei that are flimsier than that. Any of them will burn up in the air, just like the Russian meteor did.
An iron ball with water sealed inside is very improbable, and would not heat up enough to make a steam explosion in the time involved. (Heating up just enough to make an explosion, without melting through is wildly improbable.) And, if it did explode, the result would be to spread out the meteor so that it would stop moving through the air, which would cause a greater release of energy than the explosion possibly could have.
I'm not saying that water/ice in a meteor is impossible, I'm saying that what would heat it is the kinetic energy of the meteor, and the effect of a steam explosion would cause the kinetic energy to be shed into the air even more abruptly. The steam explosion would be lost in the confusion.
Little bits of water popping off would add to the spreading of a meteor, yeah. But one gigantic steam explosion just isn't going to happen, and certainly isn't needed, and wasn't seen.
Here's a challenge for you: Design and describe how you would make a steam-exploding fake meteor. We'll drop it in from the next space-flight. Tell me your metal thickness, with rates of heat transfer, and at least say whether the water will be liquid or solid, and discuss heat transfer and phase change energy. Then give me a number on pressure buildup before shell failure.
Then, after that, describe how natural forces could form something similar. Then show evidence that supports such a form existing----other than "exploding" meteors.
Then describe, in detail, exactly what you think would happen if a rubble-pile asteroid hit the atmosphere, and contrast your description with the Russian meteor.
it clearly shows an object was fired at the "meteor".
Sure, David Huddleston. A dashcam through a streaky windshield, with flares of light all over, shows a couple of blots of light that you can imagine look like "an object was fired at the “meteor”". But it doesn't show the object approaching, or any change after it hit, just a few flares on the extreme closeup. There was no explosion, no change in the meteor.
And, David Huddleston, what are you suggesting was happening?What was the meteor, then? Who shot at it? Why'd they shoot at it? What did they shoot at it with? How did they know where to shoot? Why have they kept quiet? Why'd they do something so obvious, but try to hide it?
Believers gonna believe.
@ David Huddleston
BWA HA HA! That's awesome, thanks for the link.
I really love how out of all the obvious artifacts moving as the position of the meteor changes with respect to the viewer, they single out that ONE as "clearly" being a real object.
Reminds me of a conspiracy theorist who says NASA is hiding the existence of a Jupiter-sized comet, and space aliens zapping the sun with 'lasers', because individual frames of SDO footage have either a big blob or little line artifacts, respectively.
Obviously, he was wearing ray-bans, hence the neuralizer didn't work on him...
Awesome discussion! Thanks guys!!
I figured there was a little more sophistication to the explanation of an exploding Meteoroid. I googled around to find out why a meteor explodes in the first place and came across this thread. We obviously have some experts here.
The only question left unanswered for me is how much of the shock wave was caused by the exploding meteoroid and how much was caused by the "sonic boom" from exceeding the speed of sound as it entered the atmosphere. Seems to me that those would be two distinct shock waves with the sonic boom occurring first and being much smaller (because it is too high up in the atmosphere?).
Anyone care to explain the two shock wave theory?
...also, seems to me that a huge gap in the discussion here is the rate at which a meteoroid breaks apart. All at once and you have the "explosion" effect. But who's to say a meteoroid has to break apart all at once? Why can't a meteoroid break apart erratically with one chunk breaking off a few seconds before another chunk and some of the leftover piece (or pieces) remaining intact as they hit the earth?
In the latter scenario, I don't see how we would have any sort of explosion at all...or at least not on the scale of atomic weaponry like this Russian rock.
Is there something unique about meteoroids that makes them disintegrate "all at once" despite the amalgam composite of material they are made from?
Obviously, the Flying Teakettle is incorrect, for all the other reasons stated here. And it did not just "burn up"; why would there have been a pronounced shock wave at that location - the object would have been almost gone by then? No - it's as stated in various other posts - the friction and compressive forces are immense, getting bigger and bigger as the atmospheric density increases. When the object finally deteriorates catastrophically (through a process that no one is quite sure of - let's be honest), all that incredible kinetic energy is suddenly no longer there (no falling object to create it); so it instantaneously gets transferred to the disintegrating object, making it an incredibly violent exploding object. Combined with the compressed air rushing back to fill the void (like what makes thunder in a thunderstorm), the resulting shock waves are enormous. We can't be sure how much of it is due to what, but those are the forces at work. Fascinating!
How does a meteor explode?
Has considered internal vibration?
We all know how suitable sound pressure waves in air can generate internal ringing that can shatter solid crystalline glass.
I am thinking along lines of form drag, wakes, Karman vortices, turbulent buffet (like airplane wing at stall).
Even a solid iron bell could shatter if it experienced a suitable play of form-drag-induced vibration. This could be a contributory mechanism for shattering, or at least spalling off of surface layers. Not sure how mythbusters would test this one.
"All at once and you have the “explosion” effect. But who’s to say a meteoroid has to break apart all at once?"
The one that produced the Arizona crater for example, didn't explode.
The Tunguska one did.
The best theory is that the latter was mostly ice and frozen volatiles. And that would have been a bit like a flying teakettle. Once fractured, the surface area and turbulence/friction that produces would increase dramatically and the energy required to catastrophically fail the entire lot reduced massively at the same time.
This mythbusters "themite vs ice" footage might explain what had happened.
As mythbusters used thermite as heat source,
Tunguska and recently meteorite at Russia was rapidly heated because friction with Earth atmosphere.
The experiment suggest probably meteor doesn't need any rocky shell in order frozen water/Carbon dioxide to explode mid air.
Russia seems to have bad luck with exploding meteors. I wonder if the temperature difference with cold Russia and the blazing hot meteor could also make it explode?
The main reason for the explosion of a meteorite is called adiabatic compression. The meteorite is so fast that there is no time for the meteorite-air system to exchange heat to the atmosphere making the temperature to go up very fast too. The difference in temperature from one side to the other provokes stresses that make the meteor heterogeneous materials to crack. That's it. No friction, no difference in mechanical pressures, no kinetic energy, etc. Adios amigos.
Ethan, did you absorb anything in thermodynamics? How can an object piercing the earths atmosphere at 72 KPS possibly heat-up that quickly to cause an internal reservoir of anything to explode. A path normal to the surface would encounter the earths atmosphere of any significant density 1 second before impact. A ROCK will not heat up from just above absolute zero to your required temperatures that rapidly. Approaching at a tangential path would last a bit longer due to drag, but no way long enough to achieve your theoretical temps. So, it was a malfunctioning nuclear reactor in a space vehicle that went critical. PERIOD!
I don't see in the discussion above any analysis of the rotational disassembly of a non-symmetric object when subjected to the very high dynamic pressure of entry into the earth's atmosphere. Depending on the specifics of the object the torques experienced may be sufficient to begin spinning the object. From there the question of when it comes apart is a matter of material strength, drag-related torque, inertia, and time. The time is short but the forces are high. The rotational velocity required to exceed the strength of gravitationally bound accretions is small, and once it starts to come apart it does so quickly, resulting in an "explosion". A good finite element analysis of this hypotheses would add something to the discussion.