[meteorite-list] Impact Question

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 25 Apr 2011 01:28:09 -0500
Message-ID: <8BA364CBABD545CAB0A547F867B63DBA_at_ATARIENGINE2>

Stuart, Barrett, List,

Let's get our physics straight.

The mechanisms being talked about here:
"burning from entry" and "inertia... travel[ing]
into the earth" are missing the point. These are
not "causes," but rather "effects."

All isolated, disconnected bodies have a certain
amount of energy in them. A small asteroid traveling
through space has energy that can be described
in many way depending on what other body you
reference it to.

All bodies in free motion in a field, like a gravitational
field, have a potential energy determined by their
position and their motion. Imagine you are standing
in your backyard with a nice chunk of rock in your
hand.

Motionless in your hand, held up by a force from your
muscles equal to the Earth's gravitational pull, it has
no kinetic energy at all, because it isn't moving. But it
does have potential energy. It you were to release it,
that rock would move under the invisible influence
of the mysterious gravity field that we all take for
granted because we've lived in it all our lives. It
would start to fall...

It would speed up as it fell and acquire an increasing
amount of kinetic energy, more the faster it went.
Oddly, its total energy wouldn't change. No, its potential
energy would decrease as its kinetic energy increased
because it was getting closer to the center of the
gravitational field, a position of less potential energy.

And the total energy of the rock, the sum of its potential
and kinetic energy would be constant, unchanging.
Dropping rocks is not that exciting, so let's throw that
rock straight up with all your strength, as high as you
can toss it.

It's going very fast when it leaves your hand, but as it
rises and gets further away from the center of the Earth's
force field of gravity, it goes slower and slower. Its
potential energy is increasing from its new position at
the expense of its declining kinetic energy.

Finally, all the energy you gave the rock from your
muscles will be transformed into potential emery of
position in the Earth's gravitational field and the rock's
kinetic energy will be zero -- it will be standing still,
just for a split second, at the top of your huge toss,
before...

It starts down, down, gaining speed all the way, as its
increased potential energy is turned back into kinetic
energy. Better not let it hit you... But through the entire
exchange, its total energy hasn't changed, but it has
been transformed from one kind of energy to another
and back again.

Enough about little rocks! What about BIG rocks, like
Chicxulub? Well, the story is the same. If it's going
about its business and a planet happens to get in its
way, it will come to a very rapid stop, planets being the
equivalent of a solid brick wall 8000 miles thick.

Whatever kinetic energy that big rock had, it would be
transformed into other forms of energy, which would
act on whatever matter was present and cause another
transformation of energy, etc., until the vast majority
of those energies would turn into the Lowest Common
Denominator of Energy -- Heat.

Heat is just the random motion of everything and
because of the tendency of entropy to always increase
everything tends toward the same random motion,
which we call "temperature." (Entropy? Don't ask!
I'm only allowed One Mystery per Met-List Post...)

What's odd about kinetic energy is this: it increases
with the square of the speed, like this:

               Energy = Mass x 1/2 x Velocity^2

Double the velocity; four times the energy. Triple it,
eight times the energy. Ten times the speed, 100
times the energy...

Let's say I take a tiny pellet like a bullet and throw it
at you as hard as I can, say, at 35 meters per second,
you would jump up and yell, "Ouch! What the heck
did you do that for?"

If you took the same pellet and mounted it in a
cartridge, put it in a gun and fired it back at me at
700 meters per second, I wouldn't say much, being
dead. Twenty times the speed, 400 times the energy.

Now, let's get to that asteroid. Contact velocity with
the Earth, after its initial speed has been augmented
by the pull of the Earth's gravity drawing destruction
down on itself? Let's say 35,000 meters per second
(a perfectly reasonable figure for a mildly eccentric
asteroid).

Gram for gram, that big asteroid has 1,000,000 times
more kinetic energy than the pellet I threw and 2500
times more kinetic energy than the same pellet you fired
back as a bullet.

It has more energy per gram than the amount necessary
to melt it, in fact, many times more than is needed to
melt the average rock or a chunk or iron, more energy
than is needed to vaporize rock or the chrome-nickel-steel
of a meteorite. Energy can't be destroyed, only transformed.

It takes a little over a joule to melt a gram of rock; that's
the kinetic energy of that gram traveling at the sedate
velocity of a mere 2100 m/s. A good-sized, high-speed
impactor would turn to plasma with close to 100%
efficiency.

And since mass and velocity are the ONLY things that
matter, it makes no difference at all whether a truly big
impactor is made of rock or of iron or of pure water ice
or high-density styrofoam composite or tightly bundled
goose feathers --- ENERGY is the ONLY thing that will
determine the outcome, if the energy is above a certain
rather modest threshold.

We use the same units of measurement to describe
big impacts as we do nuclear weapons, because both
consist of the same thing: a super-hot ball of plasma
on the surface of the planet. All other defining
characteristics of the event have vanished with a
single second or two.

Plasma ball, a certain temperature, a certain energy --
that's the whole story, because that's all there is left.
The mechanisms that caused it, nuclear reactions or
kinetic energy, don't matter anymore. Only the
result... which is the same.

Impacts are, of course more energetic than nuclear
firecrackers. Chicxulub specs out at 100,000,000
megatons. The largest thermonuclear weapons, the
"super-city-busters, like the US B53 9 megaton device
(no longer in service) are of the range of 5-10 megatons.
"Ordinary" city-busters, like we would have used on
Moscow or they on Washington, were 1.0 to 1.5
megatons, like the B83 1.2 megaton device, the
largest US thermonuclear device in service today.

You would have to build a pyramid of 80,000,000
B83 city-busters and light them up all in the same
instant to create a Chicxulub. I doubt we ever had
more than a few thousand B83's and the entire
nuclear arsenal of the world at its peak wouldn't
have totaled 50,000 megatons, or less than 0.05%
of a Chicxulub. Kinetic energy is a more dreadful
and potent power than the clever trick of nuclear
reactions.

Which is why I find it so strange that some geologists
dismiss the life-extincting potential of major impacts.
Me? I think we've been dam lucky.


Sterling K. Webb
-------------------------------------------------------------------------------
----- Original Message -----
From: "Barrett" <BarrettWF at comcast.net>
To: "'Stuart McDaniel'" <actionshooting at carolina.rr.com>; "'meteorite
list'" <meteorite-list at meteoritecentral.com>
Sent: Sunday, April 24, 2011 9:20 PM
Subject: Re: [meteorite-list] Impact Question


>A very LARGE impact such as the one you are talking about, the "fire"
>is
> super heated gases. The incoming meteor is traveling apprx 14,000 to
> 40,000
> miles per hour, thus superheating everything, and a large meteor
> doesn't
> burn up on entry but travels to the earth. Upon impact and the
> resulting
> sudden stop, the inertia of it travels into the earth, creating heat,
> and
> lot's of it. The super heated gases travel outwards catching anything
> flammable on fire. Then you have the super heated debris that is
> ejected
> from the impact site that is also thrown out much like hot lava balls.
> The
> gases behind the meteor is also super heated to a state of super hot
> plasma
> generating more heat.
>
>
> -----Original Message-----
> From: meteorite-list-bounces at meteoritecentral.com
> [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of
> Stuart
> McDaniel
> Sent: Sunday, April 24, 2011 7:39 PM
> To: meteorite list
> Subject: [meteorite-list] Impact Question
>
> I am watching "Earth: The Making of a Planet" on Nat Geo right now
> and they
>
> are talking about Chixalub impact and I was wondering, probably
> something
> simple,............................but,
>
> If an asteroid the sixe of Chixalub hits the Earth where does the
> "fire"
> from impact come from?? Is it because it was still burning from entry.
> Because I don't understand how 2 object crashing together can create a
> flaming impact and "burn" up everything.
>
>
>
> Stuart McDaniel
> Lawndale, NC
> Secr.,
> Cleve. Co. Astronomical Society
> IMCA #9052
> Member - KCA, KBCA, CDUSA
Received on Mon 25 Apr 2011 02:28:09 AM PDT