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(meteorobs) Fw: [meteorite-list] Meteorites Don't Pop Corn


----- Original Message -----
From: Ron Baalke <baalke@zagami.jpl.nasadot gov>
To: Meteorite Mailing List <meteorite-list@meteoritecentral.com>
Sent: Friday, July 27, 2001 10:55 AM
Subject: [meteorite-list] Meteorites Don't Pop Corn


>
>
> http://science.nasadot 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...."
>
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