(meteorobs) ?Errant Geminid? - Meteorite Destroys Warehouse inAukland, New Zealand?

Sun Dec 14 14:43:24 EST 2008

Hi Alister,

There are a bunch of minor details in your overall reasonable post ...

"Ballistically, without atmosphere, d = 1/2 * a * t^2, so 45,000m means 
95
seconds of fall, but one can assume it will be longer"

(As we know, meteoroids don't start from free fall, so there is 
friction, but there is the etail of initial velocity which we know is 
fast)

" While I don't have peer-reviewed numbers at my fingertips, you need 
to be looking at least at ground arrival size in the
cubic metre/yard range."

(Probably no meteoroid is hot enough to cause fires.  Yes,  know I'll 
get flack for this from some purists, but in incandescent flight the 
ablation keps them quite cold, irons being the only exception - which 
represent something like 5% of falls.  But even irons won't peak at 
more than 120 degrees for any reasonable mass > 0.  They will start 
around 100-110degress C at most from radiative heating at 1 AU.)  There 
is a very fine heated rind on the outside of the irons recovered and an 
interior basically unaffected by heat.  We can begin to know this by 
looking at the crystal structure of iron octahedrites.)

"The recent fall on the Alberta-Saskatchewan border (Nov 20-21, 2008) 
had a
13-kg (28-lb) piece (smaller than a 10-pin bowling ball) made a hole 
about
half its depth, with not the slightest evidence of any singing of the 
very
dry grass."

Of course not.  Being an ordinary chondrite it began to fall at very 
close or slightly less than 0 degrees C, due to radiative heating.  
 From there the heated visual portion in the atmosphere wicked most of 
the heat away.  It was probably still frozen.  There will be the 
atmospheric cooling arguers jumping in here.  Sure that happens too, 
but the times are only going to affect significantly the smallest 
recovered stones.  I have a friend who had a kilogram H5 stony 
meteorite land 20 meters from him.  He promptly picked it up and found 
it very cold to touch, with what he seems to recall as slight icing.  
Interestingly, the next day he went back and found another piece, but 
this was very hot due to simple radiative heat in a hot Sun.

"You would need to be talking an object that is supersonic fireball all 
the
way to ground before it can cause a fire."
Except for Sikhote Alin and Carancas Peru, we really don't have any 
other data on this.  A Supersonic fireball at ground level is an 
impending disaster for many other reasons.  Even large pieces usually 
(ehmmm Carancas) fragment and become small enough to go subsonic.

One final thing to mention usually glossed over.  The thin air at 
altitudes over 6 kilometer balances out a great deal of the frigid 
temperatures always cited, so sure there is some effect, but the heat 
transfer coefficient will be much smaller and thus they will be much 
better insulated; this coupled with the sweating effect of ablation 
which probably by "sweating material" keeps everything pretty 
intrasient temperaturewise.  We really shouldn't be unconsciously 
comparing it with sea level pressures IMO, not to mention there is 
still radiative heating up there in dark flight regardless of the 
outside temperature.  Gets back to the misconceptions of the 
'temperature of space' which is actually quite comfortable on a skewer.

Anyway, hope this helps.

PS, I can look up a three page careful analysis of the temperatures of 
meteoroids I did some time ago and posted, but I don't find it at the 
moment.

Best wishes and happy holidays
Doug

-----Original Message-----
From: Alister <aling at telus.net>
To: Global Meteor Observing Forum <meteorobs at meteorobs.org>
Sent: Sun, 14 Dec 2008 1:18 pm
Subject: Re: (meteorobs) ?Errant Geminid? - Meteorite Destroys 
Warehouse inAukland, New Zealand?

It is a common myth that meteorites are still hot enough to cause 
fires.
Only a *tiny* percentage can. While I don't have peer-reviewed numbers 
at my
fingertips, you need to be looking at least at ground arrival size in 
the
cubic metre/yard range.

The recent fall on the Alberta-Saskatchewan border (Nov 20-21, 2008) 
had a
13-kg (28-lb) piece (smaller than a 10-pin bowling ball) made a hole 
about
half its depth, with not the slightest evidence of any singing of the 
very
dry grass.

Remember that fireballs stop glowing at altitudes of roughly 50km
(30-miles), so they have 45km of falling through air that is MINUS 40 
F/C or
colder. (The space shuttle stops glowing at Mach 12, similar altitude,
160,000 feet).

Ballistically, without atmosphere, d = 1/2 * a * t^2, so 45,000m means 
95
seconds of fall, but one can assume it will be longer because it will 
be
slower. Even at 200m/s, the wind chill is almost beyond imagination. 
Even if
it started red hot, that's plenty of cooling by the time it hits the 
ground.
Remember the Peekskill fall some 10 years ago? Broke through the roof, 
hit a
couch, then bruised a person. They said it was warm, nothing more.

It also makes sense when one thinks of the Apollo return capsules - no 
huge
cloud of steam when they touched water. None of the returning Soyuz 
capsules
cause wildfires. Sure the composition/duration are different from
meteorites, but they're the same order of magnitude.

You would need to be talking an object that is supersonic fireball all 
the
way to ground before it can cause a fire.

The day after the Nov 20 meteorite, there was a report from a farmer 
whose
barn had caught fire - we all figured it was an attempt to get 
insurance
money - and of course it turned out nowhere near the fall.

Alister Ling
Edmonton, Alberta.

----- Original Message -----
From: <Desantisk at aol.com>
To: <meteorobs at meteorobs.org>
Sent: Sunday, December 14, 2008 10:22 AM
Subject: (meteorobs) ?Errant Geminid? - Meteorite Destroys Warehouse
inAukland, New Zealand?

>
> This link came across the Amateur Astronomers Association of  
Pittsburgh
> listserver:
>
> 
_http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10547987_
> 
(http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10547987)
>
>
>>From the Monday, December 15, 2008, New Zealand Herald newspaper:
> "Some people were convinced the fire was caused by what may have been 
a
> meteorite, which was seen from various parts of the upper North 
Island
> streaking
> across the sky just after 10 o';clock.?"
>
> Perhaps an errant Geminid?
>
> Kathy DeSantis
> **************Make your life easier with all your friends, email, and
> favorite sites in one place.  Try it now.
> 
(http://www.aol.com/?optin=new-dp&icid=aolcom40vanity&ncid=emlcntaolcom00
000010)
> _______________________________________________
> Mailing list meteorobs: meteorobs at meteorobs.org
> To UNSUBSCRIBE, email: owner-meteorobs at meteorobs.org
> http://lists.meteorobs.org/mailman/listinfo/meteorobs

_______________________________________________
Mailing list meteorobs: meteorobs at meteorobs.org
To UNSUBSCRIBE, email: owner-meteorobs at meteorobs.org
http://lists.meteorobs.org/mailman/listinfo/meteorobs