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.
Physical Science
Comments
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.
Posted by: Rod | October 28, 2009 12:26 PM
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.
Posted by: D. C. Sessions | October 28, 2009 1:44 PM
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.
Posted by: Nathan Myers | October 28, 2009 3:35 PM
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.
Posted by: idlemind | October 28, 2009 4:12 PM
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.
Posted by: ScentOfViolets | October 28, 2009 4:33 PM
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
http://www.sandia.gov/news/resources/releases/2007/asteroid.html
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.
Posted by: Gray Gaffer | October 28, 2009 6:51 PM
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):
http://neo.jpl.nasa.gov/news/news165.html
Posted by: bifyu | October 28, 2009 7:46 PM
Interesting piece.
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?
Posted by: doug l | October 28, 2009 10:22 PM
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.
Posted by: IanW | October 29, 2009 7:04 AM
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?
Posted by: MadScientist | October 29, 2009 7:56 AM
But where do the UFOs enter into the picture then?
Posted by: Sili | October 29, 2009 9:13 AM
Ethan, (and Doug at 8)
Phil Plait also says it is ram pressure, not friction that causes the heating.
http://blogs.discovermagazine.com/badastronomy/2008/12/14/meteor-propter-hoc/
Posted by: ColonelFazackerley | October 29, 2009 5:06 PM
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.
Posted by: zeke | October 29, 2009 5:34 PM
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
http://sites.google.com/site/dragonstormproject/Home
Posted by: Dennis Cox, Fresno, Ca. | January 12, 2010 4:24 PM