# How complicated is the heating of meteors?

When any gas, including our atmosphere, is compressed, it's temperature rises. When a meteor is falling through the atmosphere, it is compressing the air below it. Because this makes the air hotter than the meteor, heat moves from the air to the meteor.

Also, some amount of oxidation (burning) might also take place.

What I would like to know, is it really that simple?

Alternatively, what else causes meteors to heat up?

I've heard people say "air friction," which sounds to me like nonsense, but I am not a rocket scientist, so what do I know? If "air friction" is real I want to know how it works!

Also, I'm fairly certain that heating of air due to turbulence will only affect the air which the meteor has already moved through... not the meteor itself. If I'm wrong, I want to know!

• Air friction certainly does exist in any viscous fluid and will do some heating even in an incompressible flow where the heating due to compression does not apply. The magnitude may be small but it is no nonsense. The dissipation of turbulence to heat happens mainly in the wake but the viscosity also (or mainly) acts in the thin boundary layers at the surface and it is the final term in the kinetic energy balance equation for incompressible flow. May 1 '20 at 12:24

Your analysis is correct. Compression of the air is the cause of heating.

You call this "simple" but the compression depends on the ability of the air to flow around the meteoroid, and as the modelling the hypersonic flow is far from trivial.

"Air fiction" is the resistance to motion felt by an object moving relative to the air. That includes the reaction to the force that is compressing the air in front of the meteoroid, but also results from work done in moving the air out the way, and is dependent on shape, surface effects, turbulence etc.

The visible meteor is the trail of plasma produced in the air by the meteoroid. The actual piece of rock is too small and dim to be seen. The meteor is glowing air.

Physically, the air in front of the meteor is experiencing adiabatic compression. The jump values of density and temperature before and after the adiabatic shock are given by the Rankine-Hugoniot conditions.
The analysis of those conditions reveal that the density jump is limited to a finite value, but the temperature jump before and after the shock is proportional to the square of the shock (i.e. the meteors) mach number $$\Delta T\sim M^2$$ (source: Mihalas&Mihalas, Foundations of Radiation Hydrodynamics, Section 104, "Steady shocks"), hence in principle unlimited.

This gives rise to the enormous temperatures experienced by meteors. Of course there are multiple complications to this picture, due to ionization, turbulence, boundary layer formation etc. but this is the starting point for a physically correct reasoning.

The myth about air friction somehow became common knowledge, but essentially the viscosity of air doesn't play that much of a role. While the value of viscosity is important for the heat transport via conduction, this process is however negligible, as radiative and turbulent energy transport dominate in the domain around the meteor.

Finally, the friction or viscosity does play an important theoretical role: In general, the pre-and post-shock values of hydrodynamic values given by the Rankine-Hugoniot conditions can have different entropies. Regions of different entropies must be connected by some dissipative mechanism, which can be given by an enormously thin viscous layer in the shock structure. However this hardly merits the statement that "air friction heats meteors".

• One could maybe argue that friction is just sloppy terminology for drag (air doing work on the rock), and that compressing the air in front of you = supersonic wave drag. Although it's usually not understood that way, so maybe that's just an explanation of the chain of misunderstandings that got to this point. It's not wrong to call it air resistance, and then it doesn't take much for someone to think air friction is a synonym for air resistance, based on common knowledge of subsonic drag. Apr 30 '20 at 8:26
• Is it possible for a meteor or other object to cause any disturbance in air in front of it without heating that air enough that the speed of sound matches the surface-perpendicular speed of the object? If not, I think that would offer a nice explanation for why supersonic travel generates so much heat. Apr 30 '20 at 22:40
• @supercat: I don't get your question. In front of the shock, i.e. in the atmospheric rest-frame, by definition of a supersonic shock no hydrodynamic disturbance can arrive before the shock. For high Mach-numbers the radiative efficiency of the shock can be significant however, leading to a radiative precursor, as photons can travel faster than the local sound speed. And the nice explanation for why supersonic travel generates so much heat is written in my answer. Apr 30 '20 at 22:57
• @AtmosphericPrisonEscape: The speed of sound in the air between the object and the outside edge of the shock wave would need to be at least equal to the speed of the shock wave front, right? Your cited explanation quotes some formulas, but doesn't make intuitively clear why they hold. Saying that the air has to be hot enough for the shock wave front to move with the object seems much more intuitive, at least to me. Apr 30 '20 at 23:21
• @supercat: But that's wrong. Isothermal shocks exist, and those have the same sound speed pre and post-shock. Just saying something doesn't make it true. You have to dig into those 'some formulas' to extract intuition from them, not vice versa. But I can appreciate your idea. May 1 '20 at 1:16