[meteorite-list] R: friction or ram pressure?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 25 Jan 2014 18:41:54 -0600
Message-ID: <688372.97353.bm_at_smtp111.sbc.mail.ne1.yahoo.com>

Hi, all,

> the hot air heats the front
> surface of the meteoroid...

That's where the complexity is hiding.
The rammed air "jacket" is hotter the
deeper you get into it. The layer closest
to the meteoroid is the hottest, achieving
the plasma state and temperatures ---
30,000F to 50,000F.

Plasma does not heat anything much by
"contact." Instead, plasma at those
temperatures transfer heat by radiation.
The rammed air layers ahead are converted
to plasma by radiation until all but the
outermost layer of the "jacket" is plasma.
The technical term for this "jacket" is the
"meteor head."

On the inner side of the "meteor head," the
plasma transfers heat to the meteoroid,
by radiation. The spectrum of 30,000F plasma
runs from long radio waves to soft x-rays.
(You can Google up how to "listen" to meteors
by radio.)

It's the other end of the spectrum that
has the most powerful effect on meteoroid
rock. Soft x-rays can boil rock 10,000
times faster than your microwave can
overcook a chicken pot pie.

Areas of lower boiling point get excavated
faster, forming regmaglypts. Regmaglypts are
also caused by the irregularities in the
meteoroid's shape which affect the meteor
head's shape. Between the plasma and the
rock face there is little heat transferred
by conduction. Radiative transfer is the
predominant mechanism.

"Rock" plasma and atmospheric plasma mix
as the plasma escapes by moving from the
center of the meteoroid face back 90
degrees to the point where it can be "shed"
into the trail, combining as the plasma
cools to the point where atoms CAN combine
into meteoritic "dust." They can be seen
as areas of the trail just behind the
meteoroid described as "burning."

There is little physical contact between
the meteoroid and the plasma in our usual
sense; hot as it is, it's not very dense
by Earthly standards, just more dense than
the atmosphere at 60 miles altitude. But,
it's just 5 or 10 times hotter than the
surface of the Sun. That does the trick
very nicely. "Friction" is not really an
adequate term for the heating mechanism.

Assuming a generally spherical shape for
the meteoroid, to travel back half the
meteoroid's diameter to reach the point
where the tail detaches it must traverse
a distance of 0.78 the meteoroid's diameter,
hence, there must be a radial "plasma wind"
accelerating across the face of the meteoroid
to a velocity 57% greater than that of the
meteoroid itself. We assume this is
responsible for the flow lines.

Here's some more (technical) information
for those more fascinated. First, an
excellent general summary of radar studies
of meteor heads and trails here:
http://www.cas.uio.no/Publications/Seminar/Convergence_Dyrud.pdf

More technical treatments here:
http://www.atmos-chem-phys.net/4/817/2004/acp-4-817-2004.pdf
and here:
http://ftp.rta.nato.int/public/PubFullText/RTO/MP/RTO-MP-IST-056/MP-IST-056-
12.pdf
and then there's this tome (seriously;
it's a book!):
http://ir.lib.uwo.ca/cgi/viewcontent.cgi?article=1177&context=etd

A lot of this stuff is hotly debated. How
many tons of meteors are delivered to Earth
every year? They're fighting over that. How
many meteors a year? Fighting. And so on.

Enjoy.


Sterling K. Webb

------------------------------------------------
-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Francesco
Moser
Sent: Saturday, January 25, 2014 1:22 PM
To: Meteorite-list at meteoritecentral.com
Subject: [meteorite-list] R: friction or ram pressure?

Thanks for your kind answer!
So we can say:
meteoroids dimensions:
 <~1cm "friction"
>~1cm ram pressure

But I have still a question... how the ablation process works on the bigger
meteoroids?
The ram pressure heats the air, the hot air heats the front surface of the
meteoroid to the fusion/sublimation temperature, right?
But is the collisional process that create oriented shape, ragmaglipts, flow
line ... or what else process??

Thanks

<x>x<x>x<x>
Francesco

-----Messaggio originale-----
Da: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com] Per conto di Chris
Peterson
Inviato: sabato 25 gennaio 2014 16:24
A: meteorite-list at meteoritecentral.com
Oggetto: Re: [meteorite-list] friction or ram pressure?

Your understanding is broadly correct, although I don't think "friction"
is quite the right word to describe the heating process for particles
smaller than about a centimeter. The mechanism of heating depends on the
particle size and on the mean free path of atmospheric molecules (and
therefore on height). For centimeter scale particles, most of the heating is
the result of creating a compressed gas zone along the leading edge. For
millimeter scale particles the heating involves collisional processes. Not
surprisingly, there's an intermediate range where both processes are
operating.

Chris

*******************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 1/25/2014 7:54 AM, Francesco Moser wrote:
> Hi all!
> I have a question for you regarding falling stars and fireballs!
> Which is the physical process that heats a meteoroid during his travel
through the atmosphere?
> Is the friction with the air molecules or the ram pressure of the
> shock
wave in front of the meteoroid?
>
> I think, but maybe I'm wrong:
> for the smaller meteoroids like the one that generate the falling
> stars
the main process is the friction, instead for the bigger meteoroid like the
one that generate meteorite on the ground or for example for a Space Ship
like the Space Shuttle the main process is the ram pressure ... I'm right?
>
> Thanks a lot!!!
>
> <x>x<x>x<x>
> Francesco

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Received on Sat 25 Jan 2014 07:41:54 PM PST