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(meteorobs) Meteoroid heating: Ram pressure, friction or both?

I had the pleasure to browse an excellent page today
by Dr. Phil Plait, Webmaster & principle debunker of
the revered http://badastronomy.com . In particular,
I followed a link found on Joe Rao's interesting site
"10 confounding cosmic questions", at:
  http://www.msnbc.com/news/825959.asp?pne=msn
(This is a great site, by the way, Joe! And as we'd
expect from Joe, comets, artificial satellites, and
meteor showers all figure among the "cosmic questions"!)


From there I got to Dr. Plait's "Top 5 Cosmic Myths":
  http://www.space.com/scienceastronomy/top5_myths_020903-1.html

The Good News (:>) is that TWO of Phil's top 5 myths
have to do with meteors and meteorites. The particular
one that sparked my email today is Myth # 4, entitled
"Meteors are heated by friction as they pass through
the atmosphere". (Again, this is labeled a "Myth"!)

In that page, Phil states, "In reality, it isn't
friction, but *ram pressure* that heats meteoroids.
When a gas is compressed it gets hot, like when a
bicycle pump is vigorously used to inflate a tire.
A meteoroid, moving at 33,500 mph (15 kilometers a
second) or more compresses the air in front of it
violently. The air itself gets very hot, which is
what heats the meteoroid. That's fact, not friction."

What a fine pun! :) But on to that QUESTION: I can
recall reading in Oepik's "Physics of Meteor Flight
in the Atmosphere", that the formation of a pocket
of compressed air under a meteoroid critically de-
pends on the SIZE (cross-section) of that particle:
below a certain cross-section, the particle would
in fact (per Professor O) NOT create it's own "air
shield", and so would be directly impacted by air
molecules throughout it's flight... And of course,
we can assume that particles which are JUST large
enough to form air shields, might still experience
occasional but significant direct molecule impacts.

So the question is, Which is really more important
for the average *visible* meteor: the ram pressure
Phil describes so well, or direct impact friction?

Last but not least, I noticed that Phil specially
mentions the figure "15 km/s or more", as the point
at which ram pressure will cause rapid heating. Is 
this in fact some sort of theoretical cutoff? I.e.,
for meteoroids moving at 11-14 km/s (like those of
Summer's "June Bootids"), is the physics different?


(I know, I know: three weeks before the Leonids is
not the BEST time to bring this up... But I was so
curious - and I thought some of us could use a wee
break from "Leonid fever" for a while, anyway. :>)

Clear skies and happy pondering!

Lew Gramer (GRALE), IMO, NAMN
Webmaster: http://www.meteorobs.org
Webmaster: http://www.visualdeepsky.org
Assistant: http://www.namnmeteors.org

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