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In this post on his Bad Astronomy blog, Phil Plait describes the Tunguska event as having had a fireball which was followed by a shock wave:

A chunk of rock (or possibly ice) about 30 meters across—the size of a house—barreled in at a speed probably 50 times that of a rifle bullet. Ramming through the Earth's atmosphere, incredible forces compressed it, crumbled it, and when it reached a height of just a few kilometers above the ground, those forces won. In a matter of just a few seconds the energy of its immense speed was converted into heat, and it exploded.

[...] The fireball created a huge forest fire over hundreds of square kilometers of the Podkamennaya Tunguska River region of the Siberian forest ... but then the immense shock wave from the blast touched down. It blew the fire out and swept down those trees like a rolling pin, knocking down untold millions of them.

I guess the forest-fires-then-shock-wave makes sense as an explanation for the large number of scorched and partially scorched trees found at the site. However, I'm confused by the explanation - wouldn't the shockwave be the first thing to hit the ground? Can anyone in the know comment on the anatomy of this explosion?

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    $\begingroup$ In analogy of nuclear explosions I would venture to guess that what is meant here is that the thermal radiation from the fireball reached the ground, first. Since this happens almost at the speed of light, it will definitely outrun the shockwave. Given that the fireball was supposed to be just a few km above ground, the delay between the source and the absorbing ground was only on the order of $10\mu s$. $\endgroup$ – CuriousOne Sep 23 '15 at 11:53
  • $\begingroup$ @CuriousOne It could be (just a guess) that the nuclear explosion analogy is being wrongly applied here: my understanding is that the fireball in a nuclear blast forms pretty much instantaneously (i.e. with only $c$ delay) as X rays are absorbed by the atmosphere, which almost simultaneously becomes more opaque to them owing to chemical changes. So you have a fireball area heated up in the order of $10\mu s$ and thus the fireball would touch the ground first (nuclear blasts of this size give fireballs big enough to reach through the estimated altitude of the Tunguska bolide) ...... $\endgroup$ – Selene Routley Sep 23 '15 at 12:16
  • $\begingroup$ ..... But I'm not sure what form of EM radiation is given off by a disintegrating bollide: maybe it wouldn't be as short wavelength and as strongly absorbed by the atmosphere as nuclear blast X rays: it seems to me that knowledge about the bolide's demise would determine the answer to Emilio's question. $\endgroup$ – Selene Routley Sep 23 '15 at 12:18
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    $\begingroup$ @WetSavannaAnimalakaRodVance: The fireball in a nuclear explosion develops in a matter of microseconds (but it still takes some time after that to peak in total radiated power, I believe), so it would be much faster than the meteor fireball which should develop on the order of $r/v \approx 100m/10km/s = 10ms$. In that sense the delay of the radiation between the meteor and the ground probably won't matter, it's basically instantaneous. One can probably estimate the peak temperature with a black body model. I doubt that it's much more than a few 10,000K and it would cool very fast. $\endgroup$ – CuriousOne Sep 23 '15 at 12:21
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    $\begingroup$ Apologies if you have already seen this, but if you look at this video, (most of it is drama,) but scroll to 40 mins on and there is a simulation of the event that might be related slightly to your question youtube.com/watch?v=HXfvhJoNi90 $\endgroup$ – user81619 Sep 23 '15 at 13:50
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As illustrated HERE on Astronomy SE, since the velocity of the meteor or comet nucleus is going to be about 40 km/s (essentially the escape velocity from the sun at at 1 AU) and the velocity of Earth is about 30 km/s, the relative velocity of the two can be anywhere from 10 km/s to 70 km/s (varying by a factor of 7) depending on the direction the object is coming from.

The kinetic energy of the object is proportional to the square of the velocity, so it varies by a factor of 50! All of the kinetic energy is converted, in milliseconds, to heat in the air (which quickly become visible and UV light) - or to forming a crater if it reaches the ground. Examining the damage, something of an estimate of the energy can be made, but this could be a smaller object coming in relatively fast, or a larger object coming in relatively slower. This means the mass of the object is very poorly constrained.

The Tunguska aerial explosion is thought to be from an object coming in at a shallow angle. This matters for two reasons: objects coming in at such an angle is more likely to explode before impact, and when they do, they leave a butterfly shaped pattern of damage under them owing to the explosion happening along a path segment rather than at a point.

Light from the explosion, not the fireball itself, reaches the ground essentially instantaneously. The explosion itself takes milliseconds. The light from the plasma created by the explosion fades quickly. Most of the light happens within a small fraction of a second. This light pyrolyzes, rather than burns, the vegetation, carbonizing some of it. Only in the case of a fireball barely above the surface would thr fireball itself interact with the surface.

The shock wave, which knocks over the trees and cools the trees to the point they will not burn, reaches the ground directly under the blast travelling at the speed of sound, in 1 to 3 seconds, depending on height. Ground some distance away from the center is hit a little later, also depending on distance. Trees directly below lose branches but stay standing, similar to Genbaku Dome under the atomic bomb at Hiroshima.

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  • $\begingroup$ Nice answer, I only want to add that speed of Sound is 340 m/s and the speed of fireball is 10 000 -70 000 m/s $\endgroup$ – Jokela Nov 22 '15 at 9:27

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