[meteorite-list] Meteorites Don't Pop Corn

Ron Baalke baalke at zagami.jpl.nasa.gov
Fri Jul 27 13:55:22 EDT 2001


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

"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

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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