explosions - Why do meteors explode?

A report on the Chelyabinsk meteor event earlier this year states

Russian meteor blast injures at least 1,000 people, authorities say

My question is

• Why do meteors explode?


• Do all meteors explode?

Answer

Meteoroids come in a very large range of sizes, from specks of dust to many-kilometer-wide boulders. Explosions like that of the Chelyabinsk meteor are only found meteors that are larger than a few meters in size but smaller than a kilometer.

Though the details are argued endlessly by those who study such phenomena (it is very hard to get good data when you don't know when/where the next meteor will occur), the following qualitative description gets much of the important ideas across.

The basic idea is that the enormous entry velocity into the atmosphere (on the order of $15\ \mathrm{km/s}$) places the object under quite a lot of stress. The headwind places a very large pressure in front of it, with comparatively little pressure behind or to the sides. If the pressure builds up too much, the meteor will fragment, with pieces distributing themselves laterally. This is known as the "pancake effect."

As a result, the collection of smaller pieces has a larger front-facing surface area, causing even more stresses to build up. In very short order, a runaway fragmentation cascade disintegrates the meteor, depositing much of its kinetic energy into the air all at once.

This is discussed in [1] in relation to the Tunguska event. That paper also gives some important equations governing this process. In particular, the drag force has magnitude $$ F_\mathrm{drag} = \frac{1}{2} C_\mathrm{D} \rho_\mathrm{air} A v^2, $$ where $C_\mathrm{D} \sim 1$ is the geometric drag coefficient, $\rho_\mathrm{air}$ is the density of air, $A$ is the meteor's cross-sectional area, and $v$ is its velocity. Also, the change in mass due to ablation is $$ \dot{m}_\text{ablation} = -\frac{1}{2Q} C_\mathrm{H} \rho_\mathrm{air} A v^3, $$ where $Q$ is the heat of ablation (similar to the heat of vaporization) of the material and $C_\mathrm{H}$ is the heat transfer coefficient. Since the mass-loss rate scales as $A \sim m^{2/3}$, sublinearly with mass, smaller objects will entirely ablate faster, setting a lower limit on the size of a meteor that can undergo catastrophic fragmentation before being calmly ablated.

Meteors that are too big, on the other hand, will cross the depth of the atmosphere and crash into the ground before a pressure wave (traveling at the speed of sound in the solid) can even get from the front to the back of the object. There simply isn't time for pressure-induced fragmentation of the entire object to occur, meaning the kinetic energy isn't dissipated until the entire body slams into Earth.

[1] Chyba et al. 1993. "The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid." (link, PDF)