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

Sat Jan 25 19:41:54 EST 2014

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

______________________________________________