[meteorite-list] Re: Meteorites Don't Pop Corn

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:35 2004
Message-ID: <20010728022604.31615.qmail_at_web10402.mail.yahoo.com>

"In fact, says Yeomans, it's unlikely that any
substantial meteorites reached the ground."

Hey Ron,

This NASA article did a good job of debunking that
"Cornfield" story. Less said about that
"misinformation" the better. But I wonder if Donald
Yeomans knew that there were some sonic booms reported
with this fireball. Because bolide chasers have a
saying, "If a fireball results in a SONIC BOOM, then
there ARE meteorites on the ground!" I even have a
reference to back up this commonly held contention.
I've reprinted a portion of Jim Richardson's AMS
article at the end of my message, for those that would
like a follow-up.

But then, Ron, you and I know this by first-hand
experience from watching meteor showers. And many
other meteor observers have made similar observations
which has resulted in the following consensus:

1) There are many fireballs that produce a "terminal
flare", yet don't produce a "sonic boom".

2) There have been some fireballs observed that
DIDN'T produce a "terminal flare" (but then were
observed to "fade into dark-flight"), yet a sonic boom
was still heard, anyway!

3) It has been confirmed that multiple sonic booms
have followed after a solitary fragmentation event.
(Meaning, they weren't echoes. So, how does a single
explosion produce a number of booms? The fragments
produced the sonic booms, not the fragmentation event,
itself.)

4) We all have seen or heard of evidence for
"secondary fusion crust", so we all can have an
appreciation for there being time enough to produce
this crust, somewhere between the initial
fragmentation event (or even after the terminal flare)
but before reaching the retardation point, during
which time the sonic booms are produced.

In short, this means that sonic booms only follow
fireballs that had sufficiently large fragments that
survived the terminal flare/fragmentation event. And,
that the sonic booms are produced by these rapidly
decelerating fragments, but not the fragmentation
event, itself.

Leave these images of the fireball's flight path to be
used by the astronomers to determine its trajectory
back into space. If you want to find the meteorites,
"follow the booms".

Don't forget, Ron, the Perseids are in two weeks:-)
Bob V.

P.S. - here's the reference:

Richardson, J.E. (2001, June). The Gold Basin
Meteorite Strewn Field, Meteor Trails, Journal of the
American Meteor Society (No. 12) [article]:

"Meteoroid Disintegration and The Strewn Field"
"Most meteorite dropping fireballs will become visible
at an altitude of about 70-90 km, which is below the
normal band for most meteors of about 100-120 km, due
to their very slow speeds (about 11-17 km/sec). As the
meteoroid penetrates the atmosphere to more and more
dense layers of the atmosphere, it will steadily lose
kinetic energy and decelerate, while losing mass to
the ablation process at the same time. The estimates
on the amount of mass lost vary widely, depending upon
the initial assumptions made, especially with regard
to meteoroid density and structural strength. It is
also not uncommon for fireballs to exhibit several
flares in brightness over their paths, as smaller
portions break off from the primary meteoroid due to
the large differential stresses across the length of
the body as it falls.
"At about 15-20 km in altitude, most meteorite
dropping fireballs will stop emitting visible light,
as the body is decelerated down to about 2-4 km/sec.
During the remainder of the meteorite’s flight, the
object will fall as a “dark body,” continue to
decelerate until it loses all of its initial cosmic
velocity (the retardation point), and then silently
and invisibly drops to the ground at the meteorite’s
terminal velocity. During the final deceleration phase
of the fall to the retardation point, sonic booms may
be emitted which will reach the ground a few minutes
after the meteorite has landed.
"More dramatically, some fireballs will end their
visible track in a brilliant terminal flare, as the
meteoroid catastrophically disintegrates into many
smaller bodies. If this event occurs late enough in
the meteoroid’s flight, the resulting collection of
bodies will rapidly decelerate until ablation stops,
and then fall to the ground as a “meteorite shower.”
The area on the ground where this collection of
meteorites from the same parent body are found is
called a strewn field."


-----Original Message-----
From: Ron Baalke [mailto:baalke_at_zagami.jpl.nasa.gov]
Sent: Friday, July 27, 2001 10:43 AM
To: robert.verish_at_jpl.nasa.gov
Subject: Meteorites Don't Pop Corn



http://science.nasa.gov/headlines/y2001/ast27jul_1.htm

Meteorites Don't Pop Corn
NASA Science News

A fireball that dazzled Americans on July 23rd was a
piece of a comet or an
asteroid, scientists say. Contrary to reports,
however, it probably didn't
scorch any cornfields.

July 27, 2001: Every few weeks, somewhere on Earth, a
fiery light streaks
across the sky casting strange shadows and unleashing
sonic booms.
Astronomers call them fireballs or "bolides." They're
unusually bright
meteors caused by small asteroids that disintegrate in
our planet's
atmosphere. Often they explode high in the air like
kilotons of TNT --
blasting tiny meteorites far and wide.

It happens all the time, say experts, but usually no
one notices. We live on
a big planet, after all, and very little of Earth's
surface is inhabited by
people. Most debris from space falls unseen over
oceans or
sparsely-populated land areas -- or during times when
sky watchers simply
aren't paying attention.

Last Monday was different, however. On July 23rd
hundreds of thousands of
people were looking when, unexpected, a fireball
appeared over the US east
coast. It was 6:15 p.m. local time. The Sun hadn't
set, but onlookers had no
trouble seeing the fireball in broad daylight.
Witnesses from Canada to
Virginia agreed that the colorful fireball was
brighter than a Full Moon,
and some saw a smoky trail lingering long after it had
passed.

"Contrary to some reports this was not a meteor
shower," says Donald
Yeomans, manager of NASA's Near Earth Object program
at the Jet Propulsion
Laboratory. Meteor showers happen when Earth passes
through the debris
trails of comets and countless thousands of cosmic
dust specks burn up in
Earth's atmosphere. At the heart of Monday's fireball,
however, was a
solitary object -- perhaps a small asteroid or a piece
of a comet.

Hundreds of eyewitness reports collected by the
American Meteor Society
establish that the fireball was moving on an east-west
trajectory that
carried it directly over the state of Pennsylvania.
"It was traveling
perhaps 15 km/s (34,000 mph) or faster when it
exploded in the atmosphere
with the force of about 3 kilotons of TNT," says Bill
Cooke, a member of the
Space Environments team at the Marshall Space Flight
Center. If this was a
rocky asteroid, then it probably measured between 1
and 2 meters across and
weighed 30 or so metric tons.

"Asteroids that size enter Earth's atmosphere every
month or so," says
Yeomans.

"The pressure wave from the airburst shattered some
windows in towns west of
Williamsport," Cooke continued. "Breaking glass
requires an overpressure of
about 5 millibars (0.5 kPa), which means that those
homes were within 100 km
of the explosion."

No one knows if any sizable fragments of the object
survived the blast. But
if they did, the meteorites probably landed in the
wooded, hilly terrain
west of Williamsport -- perhaps in one of the many
state parks of that area.

Says Bob Young of the State Museum of Pennsylvania:
"One of our planetarium
staff was told that the little northern Pennsylvania
town of Trout Run was
destroyed by the meteor! The witness was about 100
miles away when she heard
the tale from her hairdresser." Other reports credit
the fireball for
scorching a cornfield in Lycoming County, PA, and
littering the countryside
with burnt rocks.

In fact, says Yeomans, it's unlikely that any
substantial meteorites reached
the ground. Atmospheric friction would have reduced
most of the fragments to
dust. Even if fragments did survive, he added, they
wouldn't burn cornfields
because --despite their fiery appearance in the sky--
freshly-fallen
meteorites are not hot.

Objects from space that enter Earth's atmosphere are
-- like space itself --
very cold and they remain so even as they blaze a
hot-looking trail toward
the ground. "The outer layers are warmed by
atmospheric friction, and little
bits flake away as they descend," explains Yeomans.
This is called ablation
and it's a wonderful way to remove heat. (Some
commercial heat shields use
ablation to keep spacecraft cool when they re-enter
Earth's atmosphere.)
"Rocky asteroids are poor conductors of heat," Yeomans
continued. "Their
central regions remain cool even as the hot outer
layers are ablated away."

Asteroids move faster than the speed of sound in
Earth's atmosphere. As a
result, the air pressure ahead of a fireball can
substantially exceed the
air pressure behind it. "The difference can be so
great that it actually
crushes the object," says Cooke. "This is probably
what triggered the
airburst over Pennsylvania."

Small fragments from such explosions lose much of
their kinetic energy as
they heat the atmosphere via friction. They quickly
decelerate and become
sub-sonic. Dusty debris from airbursts (and ablation)
can linger in the
atmosphere for weeks or months, carried around the
globe by winds. Walnut-
to baseball-sized fragments might hit the ground right
away at a few hundred
kilometers per hour.

"Small rocky meteorites found immediately after
landing will not be hot to
the touch," says Yeomans. They will not scorch the
ground or start fires. On
the other hand, notes Cooke, "if we got hit by
something large enough to
leave a crater, the fragments might be very hot
indeed." A stony meteorite
larger than 50 meters might be able to punch through
the atmosphere and do
such damage -- but that's far larger than the object
that flew over
Pennsylvania.

No one knows what kind of space debris caused the July
23rd fireball. It
might have been a small piece of an icy comet, in
which case it's unlikely
that anything larger than dust grains survived. It
might also have been a
rocky asteroid -- the most likely candidate -- or
perhaps a nickel-iron
meteorite. "Iron objects are more likely to survive a
descent to Earth,"
says Yeomans, "but they are rare."

It's possible that fragments will never be found,
notes Cooke. "We still
don't have a precise trajectory for this object," he
explains. "And so much
of the targeted area (in central Pennsylvania) is
heavily forested --
searching for debris will be like looking for a needle
in a haystack."

Or should that be a needle in a cornfield?

"I suppose it's possible that some ablative fragments
fell into that field,"
says Cooke, "but it is strange that only a small area
was affected. I doubt
it's a good candidate impact site."

"I wouldn't start looking there either," agrees
Yeomans. "That scorched
cornfield story sounds a little too corny for me...."



__________________________________________________
Do You Yahoo!?
Make international calls for as low as $.04/minute with Yahoo! Messenger
http://phonecard.yahoo.com/
Received on Fri 27 Jul 2001 10:26:03 PM PDT