[meteorite-list] Meteorite-list Digest, Vol 36, Issue 28

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 7 Dec 2006 21:30:26 -0600
Message-ID: <005201c71a79$350cba20$a925e146_at_ATARIENGINE>

Hi, Visual, Chris, List

    For the benefit of Listees following the question
of how slow a meteoroid can be...

    The orbital velocity for any body is maximally
the escape velocity divided by the square root of 2,
or 70.707070707...%. Can we just call that 71%?
Escape velocity is 11,263.04 meters per second. So, the
highest orbital velocity is 7964.17 meters per second.

    That's the orbital velocity at the lowest possible
orbit, skimming over the surface. The orbital velocity
gets less and less the higher the orbit, so that geo-
synchronous orbital velocity is positively pokey,
around 3000 meters per second. You have to go
faster than that just to get there, then slow down
to stay there. Crazy stuff, that gravity.

    The only orbit that can "decay" is one close
enough to the top of the atmosphere to be slowed
into re-entry. But (big but), the only way an object
from somewhere "not of this earth" can get to the
top of our atmosphere is to fall there, in the course
of which fall, it will acquire additional velocity, up
to escape velocity.

    Escape velocity is like taxes, in that there just
doesn't seem to be any way to wiggle out.

    By the time an object gets to the top of the
atmosphere, it will have acquired all of escape
velocity except that which it would (try to) pick
up in the last 50 miles.

    By even the Earth's escape velocity of 22,263 mps
is quite slow compared to the approach of most
meteoroids. Leonids are among the fastest (70,000
mps) in approach velocity (theirs and ours). Most
objects from the asteroid zone are going to intercept
Earth at twice our escape velocity or more.

    The "slow" fireball is a rarity, but the one most
likely to get something to the ground. The statistics
of meteorites (on the ground) are misleading: irons
are much rarer than their proportion on our collections.
It's just that they can withstand re-entry so much
better than rocks and that they can persist longer in
an Earth environment than mere rocks do. In re-entry,
irons are better than rocks; slow rocks are better
than fast ones; big rocks are better than little ones.
A meteorite in the hand is better than 1000 in freefall.


Sterling K. Webb
--------------------------------------------------------------------
----- Original Message -----
From: "Chris Peterson" <clp at alumni.caltech.edu>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, December 07, 2006 5:48 PM
Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28


> Objects in orbit around the Earth reenter close to Earth's escape
> velocity, which sets the lower limit for anything entering our
> atmosphere (the upper limit is set by the escape velocity of the Sun at
> the Earth- it's unlikely that anything we encounter would be faster than
> that). And for the most part, as you note, reentering objects are
> usually in flat trajectories, so they burn much longer, and are likely
> to slow down enough to stop burning before vaporizing. The Air Force has
> a group whose mission is to recover fallen junk.
>
> I'm not sure what you mean by "close to the ground"- anything you saw
> was probably more than 20 miles high, with 50 being more likely. There's
> no way to tell by eye how high a fireball actually is.
>
> Chris
>
> *****************************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
>
> ----- Original Message -----
> From: <VisualThinker7 at aol.com>
> To: <meteorite-list at meteoritecentral.com>
> Sent: Thursday, December 07, 2006 12:17 PM
> Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
>
>
>> I'm guessing that 'space junk' is slower because it was in orbit, and
>> as the
>> orbit decayed it entered the atmosphere as a shallow angle. Then, as
>> the
>> atmosphere grew thicker, it slowed gradually.
>>
>> All of the green fireballs I've seen during my years of hiking and
>> camping
>> out west were close to the ground. The much smaller and more numerous
>> ones
>> further away always appeared white.
>
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Received on Thu 07 Dec 2006 10:30:26 PM PST