From: pbrown@uwovax.uwo.ca
Subject: FIREBALLS
Organization: University of Western Ont, London
Date: Fri, 16 Oct 1992 23:33:07 GMT

A couple of quick comments about the fireball discussions.
The Prairie Network of cameras was operated in the USA (Not Canada)
and has been closed for more than 10 years. It's sole recovery was that
of the Lost City Meteorite which was found in the middle of a road
about a week after being photographed by the cameras. It was operated from
the early 1960's into the mid-late 70's.
In Canada a seperate fireball camera network existed under the auspices
of the NRC. It operated from 1970 or 1971 until closing in 1985. It was
called the Meteorite Observation and Recovery Project (MORP). It's
sole photographed recovery was that of the Innisfree meteorite in 1977. 
There are *no* fireball camera networks operational in North America at the moment.
In Europe there is a network of cameras in Germany and Czechoslovakia termed
the European Network (EN) which is still operational and has been running for
about 20 years I believe.
Green color in fireballs can have several causes which have been discussed.
While Green is rather rare in normal brightness meteors (say fainter than
Venus) occuring perhaps in 1-2 percent of the population there seems to be
a higher percentage of green colored bright meteors. Green in meteors can 
be caused by the 5577 forbidden oxygen (so called auroral green line) and
was first detected in (I think) 1961 by Halliday. My understanding is that
this is usually associated with fast meteors (such as the Perseids). The
fireball in question was not going this fast; the green may have been caused
by Magnesium or a host of other emission lines from ions emitted by
the ablating meteoric parent body. An excellent thermal model of fireball
emission and developement has been designed by J. Borovica at the Ondrejov
observatory in Czechoslovakia. He has matched his model with the spectra
from several fireballs photographed in the EN and gets detailed 
elemental abundances, temperature and line intensity profiles as a 
function of height from his model. For those interested his paper will appear
in the proceedings of the Symposium "Meteoroids and Their Parent Bodies",
held in July, 1992 (to be published in about 6 months). 
   The question of the surface temperature of a meteorite after impacting
the ground has been a source of controversy for some time (see Cosmic
Debris by Burnes). Reports have varied from ice layers at the surface of
a fallen meteorite to meteorites so hot as to set hay on fire (not verified).
It would seem that as most meteorites are associated with fireballs with
steep entry angles (hence less material ablated and more liklihood of
something surviving) and hence spend little time actually being heated the
standard thinking regarding the inability of the object to heat to the core
and hence ability to dissipate any residual surface heat in its several minutes
of dark flight through the few tens  of kilometres of atmosphere before impact
is valid. But in rare cases where the initial parent body is very large and
the angle of entry is very shallow, heating may last up to 30 seconds which
is apparently sufficient to cause substantial core heating and hence the
meteorite may still be hot when it impacts. The impact itself causes little
additional heating (except in the case of very large bodies) as the stone
has reached terminal velocity long before it hits the ground.
   One request: would anyone with information on the source(s) of the 
original videotapes from the Oct 9 fireball send this information to me
via email.  Since the event was captured on tape from several stations it
is in principle possible to derive a pre-atmospheric orbit for the body and
since the associated meteorite has been recovered (and hopefully analyzed
immediately for radionuclides for cosmic ray studies!!) it can yield as
much information as if we had sent a very expensive probe to  sample some
part of the solar system! This also could mean that
the Oct 9 fireball belongs to a very exclusive club indeed: only 3 other
meteorites have photographically determined orbits.
   Please send any information regarding a) the current whereabouts of the
recovered meteorite and b) the source of the video tape(s) to:
   peter@canlon.physics.uwo.ca
   Thanks!