[meteorite-list] THE CARANCAS PERU METEOROID IMPACT CALCULATIONS

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 7 Oct 2007 02:27:15 -0500
Message-ID: <11e401c808b3$81ed7450$b92ee146_at_ATARIENGINE>

Hi, List

Dirk's suggestion of close aerial stereo pairs is the
best suggestion in a while. It would not only permit
measurements of the crater but it would map the
ejecta blanket, the symmetry (or asymmetry) of
which is an important datum.

The impact excavated about 350+ cu. meters of soil and
rock, weighing about 500-600 tons, of which about
25 cu. meters or 40 tons is to be found in the rim.
The ejecta blanket extends up to 200 meters from
the crater and covers roughly 125,000 sq. meters. If
the impact had a low angle, the ejecta blanket would
be asymmetric, but we don't know because nobody...
etc., etc.

I noticed that the pictures of the crater Graham posted
(and which were taken soon after the event, I presume)
a number of good-sized rocks showing whitish patches
lying on the near ejecta blanket. In Mike's pictures of
the crater from the same angle, taken days later, the
ejecta blanket looks the same, but all those "white-patch"
rocks are gone. So, there were some multi-kilo stones,
6 to 8 at the least, that were collected.

The north portion of the rim is higher than the south
portion; the impactor came from the north. The slope
of the crater wall on the south is less than on the north;
this argues a steep angle of impact for the object (>60
degrees), which means that it came more or less from
the "top" of the sky.

The time of the impact was shortly before noon, the
time when the other object in the "top" of the sky was
the Sun. Now, an object can graze the top of the Earth's
atmosphere at a wide range of initial angles and end in
a downward path steeper than its encounter path. That's
pretty much the way it works. But a very steep downward
path can only result from a fairly steep angle of approach.

This would suggest that the object was approaching the
Earth from the sunward side at altitude of 60 degrees or more.
Very likely, its initial encounter velocity was high, given the
characteristics of such an orbit, if it was eccentric enough
to be a Main Belt object (which most orbit-determined
meteorites turn out to be).

In that case, the question of fragmentation or episodes of
multiple and progressive fragmentation is not as relevant as
it might be. The lateral dispersion velocity of the fragments
is very slow compared to the high speed of the object (now
the cluster) and fragments have very little time to disperse.

We have all seen fireball videos in which fragmentation
takes place. Even in prolonged flight, the separate fragments
are seen to be moving in virtually the same path at slightly
different velocities (because of their differential drag values).
In a high speed, very high angle impact, whether it's one
huge chunk or 1000 individual pieces hardly matters to the
result if they are closely, even intimately, clustered. The
crater could have been made by a very tight cluster, but
only a very tight cluster.

The size and persistence of the smoke trail suggests that
ablation was proceeding at a rapid rate, with great loss of
mass; this probably produced a high rate of deceleration.
To be seen easily, noticeably, head-turningly at noon means
it must have been very bright indeed.

We have many reports of the fireball from Desaquadero,
20 km.* NNE, on the shore of Lake Titicaca. It would be
a big help if someone could determine if there were any
sightings from locations further NNE, like Tiquina. The
absence of sightings 50 or 100 km. away would indicate
a steep descent; finding more distant reports would indicate
a shallower descent. It would help rough in the geometry
of the fall.
 [* The INGEMMET report says 20 km. from Carancas to
Desaguadero. The map says 10.8 km. The distance of
Carancas from the crater has been given as 1200 meters
up to 5000 meters.]

But I now have a reason to believe it is more likely to
have been a single (surviving) object than a fragmenter.

Rob noticed Doug and I playing one-on-one volleyball.
Our respective uniforms, his "Slow Impact" jersey and
my "Fast Impact" jersey, were provided by local merchants
trying to keep us off the street. It's a good thing it was a
one-on-one game, because everybody's on the "Slow
Impact" Team because we get More Meteorite that way.

The point is well taken that the best way to get a meteoroid
to make that difficult personal transition to a meteorite is to
slow down, sneak up on the Earth's atmosphere sidewise,
and to be as frisbee-shaped as possible. I've made that point
on the List many times before: low entry speed, low angle of
approach, and an aerodynamic shape.

But here we have a different problem. I see every sign that
this was a fast impact, to the annoyance (I'm sure) of those
who want More Meteorite or a big Jilin clone in the mudpit.
So, how do we get a fast object to the ground without it
burning up in the process?

We change its shape. We are taught (I was) to generalize
to an abstraction. Ask a physicist to model anything and
the first thing he will do is "consider the object as a sphere
of radius N." (Look at Chris Peterson's email to Mike on
10/02/07; there's a man too wise to waste time playing
volleyball with imaginary balls and an invisible net.) What
if the object ISN'T a sphere?

I've seen lots of pictures of very small asteroids and none of them
were spheres: bent peanuts, dumbbells, pancakes with dome-poles,
and something vaguely the size and shape of a stripmall-in-space,
but not one sphere. The smaller the object, the more irregular.

What if the meteoroid was roughly a cylinder 4-5 times longer
than wide? How would it fare hitting the atmosphere at 60
degrees tangent to the ground and 17,000 meters a second?
Well, it depends on its weight, almost entirely, as it turns out.
One ton just barely gets to ground at a few hundred miles per
hour and ten tons bores in at 8600 meters per second, intermediate
weights at all intermediate speeds, any speed you want. None of
them ablate away completely and none of them fragment. They
all make a crater.

What a remarkable result!

Back in February '07, when we were talking about a new and
big Holbrook find, I posted this reference which has an
analysis of that strewnfield, asserting that it was the product
of a multiple fragmentation. It uses composite scaling analysis
to model strewnfields, and in so doing the authors discover that
the original SHAPE of the meteoroid has a much stronger influence
on the descent to Earth than we realized, may in fact be the big
determining factor in what gets to ground and how fast or
slow it does it.

The link was publicly accessible then, but is now only
accessible to those with big bulgy pockets or members
of The Institutional Academic Scholars Union Local. We
must keep our arcane knowledge out of the hands of poor
people; it is our duty as a civilization, eh, what? (Sorry;
I get this way when I Google too much...)
http://www.iop.org/EJ/article/0295-5075/43/5/598/node4.html
L. Oddershede, A. Meibom, J. Bohr: Scaling analysis of meteorite
shower mass distributions. EUROPHYSICS LETTERS, 1998,
Vol.43, No.5, pp.598-604

Turns out the only way you can get the original mass of
the Sikhote-Alin object to the ground is to make it, too, a
long shape, ratio 3:1 or more. A chip off some bigger block.

The link that Mike just posted to the List:
http://home.comcast.net/~C_Shipbaugh/Impact.html
are calculations by a nanotechnologist who has obviously
never analyzed a meteorite fall before and manages to
get it amazingly right (physics is physics, you know). He
does silly things like over-estimating the volume of the crater
by a factor of two because he does not know it's conical!
Doha!

He arrives at a 5 ton TNT impact without apparently knowing
that the seismic signal was rated at 5 tons of TNT. He, too,
thinks it was a slow impact, which is why he favors 10 or
20 ton objects, but says 4.5 tons at 3000 m/sec is most likely
guess (which is the same as 1.125 ton at 6000 m/sec).

He introduces the factor of shape in the form of the "ballistic
parameter or coefficient," but then goes ahead and models it
as a SPHERE. See, all physicists think alike (well, most).

You are probably saying about now, what is this all about?
Well, remember the glory days of starting into space and how,
after envisioning spaceships all our Buck Rogers life, we were
amazed to see the first spaceship, the Mercury capsule, was
an Ice Cream Cone?

It re-entered on its butt, er, blunt, end for maximum resistance.
The re-entry end was a segment of a sphere (probably so the
physicists could model it better). And everyday dumb people
said, "Why don't they come back with the pointy end down;
wouldn't that be faster? Better yet, why isn't it all sleek and
thin like a jet plane?"

Well, we know the answer to that one, of course. Because a long
cylindrical object with an (ablated) point would bore into the ground
at tremendous speed. That's the ballistic parameter. We wanted the
Mercury capsule to SLOW DOWN. If we wanted it to make a big
crater, it would have looked like the Bell X-1 without wings.

All it takes to get any meteoroid to the ground at a high speed
is to stop imagining that God made all the billions of little rocks
in space perfect spheres to make life easy for physicists. He likes us...
But not that much.


Sterling K. Webb
Received on Sun 07 Oct 2007 03:27:15 AM PDT