[meteorite-list] Scientists Size Up, Classify Park Forest Meteorite

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:14 2004
Message-ID: <200404141716.KAA05299_at_zagami.jpl.nasa.gov>

Scientists Size Up, Classify Meteorite That Nearly Landed in Their Backyards
AScribe Newswire
April 14, 2004

CHICAGO -- The meteorites that punched
through roofs in Park Forest, Ill., on the evening of March 26, 2003, came
from a larger mass that weighed no less than 1,980 pounds before it hit the
atmosphere, according to scientific analyses led by the University of
Chicago's Steven Simon, who himself also happens to live in Park Forest.

Simon, a Senior Research Associate in Geophysical Sciences at the
University of Chicago, and seven co-authors will publish these and other
findings in the April issue of the journal Meteoritics and Planetary
Science. Simon holds a unique distinction among scientists: his home sits in
the middle of the strewnfield, the area from which the meteorites were
recovered.

"I don't know of any other time when a meteoriticist was in the
middle of a strewnfield," said Lawrence Grossman, Professor in Geophysical
Sciences at the University of Chicago and one of Simon's co-authors.

In fact, Simon actually saw the flash the meteorite created. He had
the drapes closed when the rock entered the sky over Illinois, but "the
whole sky lit up," he said.

Grossman, who lives in Flossmoor, not far from Park Forest, also
experienced the meteorite's arrival firsthand. He was awakened by the sound
of the meteorite entering the atmosphere that night. "I heard a detonation,"
Grossman said. "It was sharp enough to wake me up."

The team calculated the projectile's size range based on measurements
of the galactic cosmic rays that it absorbed. Measurements of a radioactive
form of cobalt provided the projectile's minimum size. "If the object is too
small the cosmic rays will just pass through and not make cobalt," Simon
explained.

Simon and Grossman classify the meteorite as an L5 chondrite, a type
of stony meteorite, one low in iron that was heated for a long period of
time inside its parent body, probably an asteroid. "It's a fairly common
type of meteorite," Simon said.

The Park Forest meteorite also showed signs that it had been highly
shocked, probably when it was part of a rock that was broken from a much
larger asteroid following a collision. The evidence for shock includes
shocked feldspar. Apollo astronauts recovered shocked specimens of the
mineral from the moon, as well, Simon said. Impact shock was common in the
early history of the solar system because of the large quantity of
interplanetary debris then in existence.

Witnesses in Michigan, Illinois, Indiana and Missouri reported seeing
the fireball that the meteorite produced as it broke up in the atmosphere,
Simon and his colleagues report. Local residents collected hundreds of
meteorite fragments totaling approximately 65 pounds from an area extending
from Crete in the south to the southern end of Olympia Fields in the north.
Located in Chicago's south suburbs, "This is the most densely populated
region to be hit by a meteorite shower in modern times," the authors write.

One meteorite narrowly missed striking a sleeping Park Forest
resident after it burst through the ceiling of a bedroom. The meteorite
sliced through some window blinds, cratered the windowsill, then bounced
across the room and broke a mirror before coming to rest.

The meteorites were recovered from a track that trends southeast to
northwest. Satellite data analyzed by Peter Brown of the University of
Western Ontario indicates that the meteorite traveled from southwest to
northeast, however.

"The meteorite broke up in the atmosphere, and the fragments
encountered strong westerly winds as they fell," the authors write. "The
smallest pieces were deflected the furthest eastward from the trajectory,
and the largest pieces, carrying more momentum, were deflected the least."

Contributing to the paper in addition to Simon and Grossman were the
University of Chicago's Robert Clayton and the late Toshiko Mayeda, Jim
Schwade of the Planetary Studies Foundation in Crystal Lake, Ill.; Paul
Sipiera of Harper College in Palatine, Ill.; John Wacker of Pacific
Northwest National Laboratory in Richland, Wash.; and Meenakshi Wadhwa of
the Field Museum of Natural History in Chicago.

Their research was supported by grants from the National Aeronautics
and Space Administration, the National Science Foundation, and the Planetary
Studies Foundation.
Received on Wed 14 Apr 2004 01:16:13 PM PDT