[meteorite-list] Archive: (meteorobs) Meteoroid heating: Ram pressure, friction or both?

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:13 2004
Message-ID: <20021101050450.24170.qmail_at_web80310.mail.yahoo.com>

The following FWD messages are for archival purposes,
and as a reference for the FAQ web page:

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Date: Wed, 30 Oct 2002 19:25:43 -0500
From: "Lew Gramer" <lew_at_upstream.net>
Subject: (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

The archive and Web site for our list is at
http://www.meteorobs.org
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Date: Wed, 30 Oct 2002 23:53:45 -0700
From: "Jim and Jan Gamble" <gamble_at_peoplepc.com>
Subject: Re: (meteorobs)
Meteoroid heating: Ram pressure, friction or both?

 May we entertain the theory that many meteors
illuminate our skies in much the same fashion as good
old lightning. Think about it, calculations as to how
much visible energy can be released due to heating of
a spec of dust no bigger than that of the head of a
straight pin frequently don't match up with how much
light is actually emitted. Can heat alone really cause
a rock to burn brighter than our sun. Electrical
discharges are among the brightest visible energy
sources in the universe.
  This certainly is not to say heat is not involved,
but is it heat ALONE?
Regards,
Jim Gamble
El Paso, Tx Station-Sandia Meteor Detection Network
All Sky Camera System
31.47N 106.18W
gamble_at_peoplepc.com
http://www.geocities.com/desert_lights


The archive and Web site for our list is at
http://www.meteorobs.org
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Received on Fri 01 Nov 2002 12:04:50 AM PST