Expert: Philip Stahl - 2/22/2009

Question
QUESTION: I was looking at two explanations as to why meteors burn as they go through the atmosphere. One says it has to do with friction as it whizzes through the atmosphere. The other says that as the meteor moves through the atmosphere, it compresses the air in front of it thereby heating the air up so much that it in turn heats the meteor to such a point that it starts burning up.
The two links with this info are:
http://science.howstuffworks.com/question308.htm
http://science.howstuffworks.com/question486.htm/printable
So which explanation is right?
Thanks for your help.

ANSWER: Hello,

The key point is that "friction" does play a role in whatever case, but the particular form of compression of air in front of the object dominates (since there are many more smaller particles incepting meteors than larger). The other explanation about the surface ablation factor is also true, since ionization of the outer layers of a particle *will* occur but this is more critical and relevant to larger objects (like bolides). For smaller ones, the air compression & heating explanation is more valid in terms of accounting for the brightness.

The other point is that (in the case of small-particle meteors) the heating effect results in a very large relative envelope of heated gas *around* the object. Thus it is that even a grain -sized object can appear extremely bright in the dark sky.

Bear in mind we are all about kinetic energy here. And that is a function of both mass and velocity, though in the form:

KE = mv^2/ 2

thus, even if a particle is extremely small, its velocity will be very large, and note that it is *squared*.

Thus it is that even a grain of sand-sized object can generate a very bright meteor.

---------- FOLLOW-UP ----------

QUESTION: Just to make sure I read you right; as an object goes through the atmosphere the dominating factor is the compression of air in front heating up the object, but at the same time friction also plays a part.

But, when I think about it, if an object is enveloped by heated gas how can the atmosphere even touch the object and cause friction?
Thanks again for your help.


ANSWER: Hello,

The very emergence of the heated gaseous envelope is caused by "friction". You have an object entering the atmosphere at some very high velocity and it will surely ionize the surrounding gas as it traverses. Its kinetic energy is converted to heat energy that causes the ionization and gives rise to the envelope. In other words, the processes are intertwined, they are only separated for the purposes of description.

In a way the very process of description or explanation renders the dynamic process somewhat artificial, since people are led to think, "ok x comes first here, now y, now z".  Thus, in explanatory terms we are conditioned to think of things in a serial manner, when the combined processes are more parallel.

In the same way, some confusion arises because the interaction of atmosphere with object VIA friction is not properly considered, or thought of as apart from the overall scenario. All I am saying is that ablation and ionization-expansion of surrounding air occurs more or less simultaneously. So, it isn't a matter of atmosphere prvented from "touching the object". The "touching" is what the friction is all about and its incepted at first "contact".

Once the object *enters* the outermost layer of atmosphere the interaction commences, and the 'game is on' so to speak in terms of friction, ablation, ionization-formation of gaseous envelope. The visible meteor we behold is the product of all of these, though to varying degrees depending on the actual size-mass of the object.



---------- FOLLOW-UP ----------

QUESTION: Forgive me for belaboring this topic, but you would still say that it holds true that the dominating factor is the compression of air in front of the object (which by virtue of being compressed heats up) and consequently heats the object itself as well as the friction does?
(I ask again because you didn't say anything about compression in this last response.)
Thank you so very much.


Answer
Hello,

I think what I meant to say - based on the *first* reply- is there are two modes, and the compression in front probably plays the larger role in terms of smaller mass objects entering at high velocities. The effect of the compression then heats the surrounding air and creates an envelope of heated and ionized gases surrounding it.

Hope this clarifies it.