(meteorobs) Video of Park Forest Bolide

[Robert Verish]

[I'm surprised that there was no mention of sonic
booms as a possible source for some of the "audible
detonations" attributed to fragmentation events"?]

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.

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