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RE: (meteorobs) Excerpts from "CCNet 123/2001 - 22 November 2001"

> Having said all that i will now contradict myself,and site 
> the more recent 
> case of the Tagish Lake Meteorite (January 18 2000) which was also 
> attributed to a cometary fragment. Although it also exploded 
> after entering 
> the atmosphere,some fragments were recovered,well preserved 
> from the frozen 
> surface of the lake. So i guess cometary material can survive 
> intact,albeit 
> exceptionally rare!
> 

Sorry to disagree, but Tagish Lake is thought to be a D class asteroid
(though there is a SLIGHT chance it is cometary),with an asteroidal orbit -
here is the abstract from a paper presented at this month's DPS...

The Tagish Lake meteorite fell in northern British Columbia, Canada on 18
Jan 2000. It has been classified as a C2 ungrouped carbonaceous chondrite
and displays many unique properties (Brown et al. 2000). Most notable is the
probable linkage of Tagish Lake to D-class asteroids based on spectral
reflectance (Hiroi et al. 2001). We have analyzed the associated fireball
data consisting of eyewitness reports, video and photographic observations
of the associated dust cloud, seismic signals and satellite records. These
data are interpreted in the context of a new ablation model explicitly
accounting for porosity of the initial body. From these data we find that
the fireball physical behavior is intermediate between type II (associated
with carbonaceous chondrites) and type IIIa (associated with strong cometary
material) fireballs, following the classification system of Ceplecha et al.
(1998). Measured porosities of two recovered fragments measure
37+/-6%-40+/-10% with bulk densities near 1.6 +/- 0.1 gcm-3. Our modelling
suggests the initial Tagish Lake meteoroid to have had a minimum porosity of
50%. The initial fragmentation of Tagish Lake occurred under 0.3 Mpa of
dynamic pressure, at the low-strength end of the fireball spectrum (Ceplecha
1993). The initial mass of the meteoroid is estimated to be 65+/-15 tonnes.
The orbit of the Tagish Lake meteoroid suggests a nominal linkage to
asteroidal bodies, though the Tisserand value (3.6) and 1/a=0.5 does not
rule out classification as an Encke-type cometary orbit (Levison, 1996). In
terms of physical behavior, these data suggest Tagish Lake represents a body
intermediate between chondritic asteroids and cometary bodies. Inferences
from the fall of Tagish Lake thus allow a means to assess information on
physical parameters such as strength, density, composition and porosity for
a 4m D-class NEA using ground-truth data. 

Bill Cooke
CSC/ED44
Marshall Space Flight Center, AL 35812
Phone: (256) 544-9136
FAX: (256) 544-0242
Email: bill.cooke@msfc.nasadot gov

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