Why did the Chelyabinsk Meteor Explode?

[Zorwell]

Why did the meteor explode? What specifically caused the fireball (the abrupt increase in brightness) for example? That is, what is the physics behind such a fireball versus one that just ablates and burns down to nothing? Meteors are made up of materials we do not normally consider explosive, so what produces the energy we see in such a fireball? What's the mechanism? Did the material of the meteor vaporize and the constituents burn?

 

[Simon Bridge]

Lots of things you don't normally think of as explosive can explode - water for example can explode into steam when heated sufficiently fast. If it is a sealed container at the time, the explosion can be quite dramatic.

So - pretty much, the meteor may have contained some volatile substances which were critically heated or exposed during flight. It may just be the kinetic energy getting dumped in the atmosphere as it broke apart.
http://www.universetoday.com/100025/airburst-explained-nasa-addresses-the-russian-meteor-explosion/

Look up "asteroid airburst" - they are quite common.

 

[Zorwell]

Lots of things you don't normally think of as explosive can explode - water for example can explode into steam when heated sufficiently fast. If it is a sealed container at the time, the explosion can be quite dramatic.

So - pretty much, the meteor may have contained some volatile substances which were critically heated or exposed during flight. It may just be the kinetic energy getting dumped in the atmosphere as it broke apart.
http://www.universetoday.com/100025/airburst-explained-nasa-addresses-the-russian-meteor-explosion/

Look up "asteroid airburst" - they are quite common.

Sure, even a water heater can explode, but such an explosion doesn't produce light. My question is, what is the physical process by which the kinetic energy of the meteor is converted into light? Is it rapid combustion of atomized (or very small particles) of the material of the meteor? So the fireball is produced by the rapid expansion of the material as it vaporizes and undergoes rapid combustion? Alternatively, imagine if a large block of ice were shot through the atmosphere at sufficient speed it would "explode" but the "explosion" would be just a breaking up of the ice and its vaporization but that largely mechanical process (i.e., no change in chemical composition) wouldn't produce any light.

 

[Simon Bridge]

OK - let's recap - taking your first post one point at a time:

Why did the meteor explode? What specifically caused the fireball (the abrupt increase in brightness) for example?

Specifically? We don't know. It may be that someone had a spectrometer pointed in the right direction at the time - still waiting for data. But, generally:

That is, what is the physics behind such a fireball versus one that just ablates and burns down to nothing?

A fireball comes with a much more rapid energy release - though a bright light does not have to come from a high temperature or, even, burning.

Meteors are made up of materials we do not normally consider explosive,

Many materials that are not usually considered explosive may still explode. Meteors may contain substances that are explosive.

so what produces the energy we see in such a fireball?

Kinetic energy - probably. The meteor may also contain volatile materials that get released as it burns up.

What's the mechanism?

The air around the meteor heats up as the meteor slows down. If it slows suddenly (say - it breaks apart, increasing surface area and thus drag) then you get a sudden energy dump which has to go somewhere.

Did the material of the meteor vaporize and the constituents burn?

Some bits may have become incandescent, so would the air.

Small meteors are not usually hot by the time they hit.
But this was a biggish one.

 

[Zorwell]

Thanks Simon, I really appreciate the blow-by-blow clarification. Essentially the light is produced by the heating of the material of the material and the air around it and the rapid increase in the size and brightness of the fireball is due to the increase in heat produced by rapid slowing in the atmosphere possibly caused by the meteor breaking up thus dramatically increasing it's surface area/drag.

I think I really wanted to ask a slightly different question which is, in general, when an asteroid of sufficient size explodes in the atmosphere, as this one did, what would we expect that the mechanism causing the explosion would be? I'm guessing that your general answer would be essentially a very large, abrupt release of heat due to the meteor fracturing increasing drag.

There have been many discussions about the effect of a large meteor striking the earth, but we have seen, in this example, that the primary effects of some meteors isn't at all in their impact with the surface but the explosive size of the shock wave they produce in the atmosphere and possibly, with a larger meteor exploding closer to the ground, the heat that they produce.

 

[Astronuc]

I suspect the explosion came from an abrupt phase change - liquid to gas.

The fireball grew as it got into the lower (denser) atmosphere and got hotter, which increased the amount of liquid to vapor.

 

[Simon Bridge]

I suspect the explosion came from an abrupt phase change - liquid to gas.

... of something. I was being cautious including several possible mechanisms.
Meteors can be quite complex can't they?

There have been many discussions about the effect of a large meteor striking the earth, but we have seen, in this example, that the primary effects of some meteors isn't at all in their impact with the surface but the explosive size of the shock wave they produce in the atmosphere and possibly, with a larger meteor exploding closer to the ground, the heat that they produce.

Ineed - small fast objects can also be devastating. The article I gave you in post #2 expands on this theme (a bit) and has some good leads for further reading.

Also see:
https://www.physicsforums.com/showthread.php?t=672021
... right at the bottom.

 

[Frank Minsky]

Just a hint. The meteor could contain some oxidizable material, e. g. iron. Then the sequence heating - cracking - pulverizing - burning could result in an effect similar to BLU-82 explosion.

 

[Frank Minsky]

Or, alternatively, even sufficiently hot inert bulk object may heat adjacent air fast enough to produce shock wave when it catastrophically breaks into many small particles with very big surface area.

 

[jfgobin]

If you dump enough energy in a solid in a very short time, you can vaporize it, and depending on the temperature, it can also ignite.

As an example: you wouldn't think that a coin and a copper coil can explode, would you? However, look at this video: MAKE Coin Shrinking in slow motion.

Yup, the copper coil vaporized and the vaport caught fire, creating a small fireball.

The same applies for asteroids, except that the energy is provided by the friction of air: the asteroids loose kinetic energy due to the friction, energy which is converted into heat.