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(meteorobs) Excerpts from "CCNet 105/2001 - 10 October 2001"
Only two brief items of interest to meteor researchers. But the second is a
concise and very clear statement on the most frequent confusions arising in
public accounts of "meteorite falls". Wonderful reading!
Clear skies,
Lew Gramer <owner-meteorobs@atmob.org>
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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 105/2001 - 10 October 2001
Date: Wed, 10 Oct 2001 11:35:28 +0100
CCNet 105/2001 - 10 October 2001
================================
(1) MAYBE IT WAS METEORS: WILDFIRE CLUSTER PROBED
Denver Post, 9 October 2001
(2) METEORITES & METEORWRONGS - CLUES (CCNet 13 December 1999)
Matthew Genge <M.Genge@nhm.acdot uk>
[...]
================================================================
(1) MAYBE IT WAS METEORS: WILDFIRE CLUSTER PROBED
>From Denver Post, 9 October 2001
http://wwwdot denverpost.com/Stories/0,1002,53%257E174190,00.html
By Electa Draper
Denver Post Four Corners Bureau
Tuesday, October 09, 2001 - PAGOSA SPRINGS - Firefighters are ieser a meteor
shower ignited a strange cluster of wildland fires across 4 miles in the Cat
Creek Road area the night of Sept. 25.
Up to 18 small fires erupted around 7:30 that night. The Pagosa Springs Fire
Protection District extinguished the small blazes and had them mopped up by
early the next morning. In four or five cases, area residents were the first
to douse flames since they were threatening their homes, officials said.
District officials don't suspect arson, believing instead that some sort of
natural event ignited the blazes, firefighter David Vega said. Crews used
metal detectors last week to try to find bits of any unusual elements in the
area of the burns. So far they have come up empty-handed.
Officials are asking anyone with information regarding the fires to call
970-731-4191.
Copyright 2001 The Denver Post
==========
(2) METEORITES & METEORWRONGS - CLUES (CCNet 13 December 1999)
>From Matthew Genge <M.Genge@nhm.acdot uk>
Dear Benny,
As a meteoriticist who often deals with members of the public who believe
they have found meteorites, the description of the fall from Guyra sounds
very familiar indeed. The eye-witness accounts that accompany meteorwrongs
(as opposed to meteorites) frequently have a common thread that reflect
public misconceptions of meteorite falls. The following are the most common
supporting facts given at the Natural History Museum in London as definite
proof of an
extraterrestrial origin:
(1) "It must be a meteorite because it fell from the sky". This is of course
accurate and true but is not necessarily proof of an extraterrestrial
origin, lots of things fall from the sky including birds (and occasionally
frogs and fish) and the vast majority of them are not meteorites. In a
number of cases the objects in question at the NHM were in fact tarmac, one
of which had a tyre track embedded on one side. The possibility of human
intervention (particularly when the sample has fallen through a greenhouse)
must also be considered. In one particularly impressive case (recounted by
Alex Halliday) a large block of limestone had been dropped through the roof
of a barn by a light aircraft pilot.
(2) "It was glowing when it hit the ground". Fireballs usually experience
terminal detonations at altitudes of ~20 km after which most fragments are
falling at atmospheric settling velocity and are no longer able to ionise
the atmosphere to produce a fireball phenomenon. Only larger, crater-forming
bodies may reach the Earth's surface as fireballs and therefore unless the
observation is accompanied by a description of an impact flash, shock wave
and a big
hole in the ground it is unlikely to have been a meteorite. It is worth
mentioning that it is very difficult to estimate the trajectory and distance
of a fireball from a single eye-witness account. The frequently described
observation that "It fell behind the house/tree/car" may relate to a
fireball that had simply passed over the immediate horizon. In this case it
is quite natural that people will then go to where they believe the object
fell and recover the most unusual thing they find (industrial slag/iron,
coal and in one case a concrete fence post).
(3) "The meteorite was hot when I picked it up". Heat loss by ablation
during atmospheric entry is very efficient and the fusion crusts of
meteorites are usually less than a millimetre in thickness and have cooled
and solidified in the last few seconds of luminous flight. Unless a
meteorite is large enough to experience significant shock on impact (e.g.
Canyon Diablo) it is unlikely to be hot to touch. Again the presence of a
sizeable crater might be expected for
such hot meteorites.
(4) "There was an explosion". Detonations and shock waves at the impact site
are only likely for larger bodies that either impact the ground to form a
crater or experience a terminal detonation at relatively low altitude. One
recent case, for example, in which a detonation in a farm yard in the dead
of night was accompanied with a small crater and milk churns peppered with
holes was probably not a meteorite since the event occurred in County
Antrim. An explosion that occurred in Northern Kenya was slightly more
puzzling because a 300 kg iron meteorite was recovered. The eye-witness
accounts describe a detonation, a pall of smoke and metal fragments that
were embedded in trees in a banana plantation. However, the iron meteorite
was weathered and evidently had been exposed on the Earth's surface for
sometime. Since the area is politically sensitive and has been shelled and
land-mined it may suggest the discovery of the meteorite was somewhat
serendipitous.
(5) "It smashed through my window". Except for large meteoroids that reach
the ground at a significant fraction of their original velocity, meteorites
are usually falling vertically by the time they land. Once exception was a
fragment of Barwell that bounced through a window in Leicestershire on
Christmas Eve in 1965. A report from the metropolitan police that a
meteorite had fallen through an open window in London and set fire to a
carpet under a budgie cage was however spurious. Inspection of the blackened
residue claimed to be the remnants of the meteorite revealled only millet
seeds. These are not counted amongst the most important components of
chondritic meteorites.
Meteorite falls are usually not particularly impressive events at the
locality of the fall itself. The description of the Glatton fall in 1991
near Peterborough is typical since Mr Arthur Pettifor assumed 'hooligans'
had thrown the meteorite into his garden. Only where meteorites are large
(e.g. Sikhote-Alin which probably impacted the ground at ~1 km/s) or strike
property (generally the roofs of occupied buildings or our precious cars)
are meteorite falls
particularly dramatic.
____________________
Dr Matthew J. Genge
Researcher (Meteoritics)
Department of Mineralogy, The Natural History Museum
Cromwell Road, London SW7 5BD, UK.
Tel: Int + 020 7 942 5581
Fax: Int.+ 020 7 942 5537
email: M.Genge@nhm.acdot uk
Staff internet page http://www.nhm.acdot uk/mineralogy/genge/genge.htm
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