[meteorite-list] Anatomy of a Bolide (Park Forest)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu May 27 13:58:58 2004
Message-ID: <200405271758.KAA23378_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/743xpghk.asp

Anatomy of a bolide

Last year, a brilliant fireball shattered above the Chicago suburb of Park
Forest, Illinois. Astronomers have pieced together a detailed postmortem of
the event.

By Andrew Fazekas
astronomy.com
May 26, 2004

A little over a year ago, on March 27, 2003, a spectacular fireball lit up
northern Illinois and sent chunks of space rock crashing into the streets
and roofs of a quiet Chicago suburb of Park Forest. Incredibly, the kamikaze
dive of this extraterrestrial visitor was caught - completely by accident -
on police car and security video cameras, providing scientists an amazing
opportunity to study in detail the last fiery moments of a meteor's life.

The Park Forest fireball represents the largest bolide to streak across a
densely populated area in modern history. "With only two such events
occurring on land anytime during a year around the world, this was unique by
all standards," says Wayne Edwards, study team member and Ph.D. candidate
from the University of Western Ontario. Many different visual and acoustic
instruments around the region observed the object's descent. Astronomers
have used these measurements to determine the Park Forest meteorite's
pre-fall orbit. This is only the eighth time astronomers have been able to
calculate a reliable orbit for a meteorite.

By synchronizing eyewitness videos, audio, and seismic measurements on the
ground with infrared satellite imagery, Edwards and his team have been able
to determine the fireball's original spin rate, trajectory, and velocity
accurately. Before entering Earth's atmosphere, the Park Forest object
weighed 12.5 tons (11 metric tons), was traveling at 12 miles (20
kilometers) per second, and packed the equivalent energy of nearly 500 tons
of TNT.

Audible detonations occurred as the meteor punched through the atmosphere
and broke into smaller fragments. Analysis of audio and seismic recordings
clearly matched three distinctly visible fragmentation events. The main
fragmentation event, visible on a police-car video, occurred at a height of
44 miles (70 km) and was quickly followed by two smaller detonations at 22
and 16 miles (36 and 26 km). "Incredibly, we can trace the meteor in the
video back to the very earliest time in its entry into our atmosphere," says
Edwards.

Based on a computer model of the meteorite's fragmentation rate, the team
believes the bolide actually shattered early in its entry into our
atmosphere and simply traveled as a collection of smaller pieces. Most of
these eventually disintegrated, leaving behind only the 40 pounds (18
kilograms) of meteorites recovered. Before its fatal plunge to Earth,
the object followed the orbit of a typical Apollo-type, Earth-crossing
asteroid.

With enough data from different meteor falls, a clearer picture may emerge
as to the sources of these meteorites and what type of asteroid they come
from. "With enough orbits and enough samples from the ground, we can start
to determine what the distribution of this material is in the solar system,"
explains Edwards. Going beyond Park Forest, the Ontario team plans to
establish a system of cameras specifically designed to catch fireballs and
determine their orbits.

The team presented its findings at the 2004 Joint Assembly of the American
and Canadian Geophysical Union in Montreal, Canada, last week.
Received on Thu 27 May 2004 01:58:47 PM PDT