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


>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...

Don't be sorry Bill ...i appreciate being corrected if I'm wrong,thanks!

I always thought that "carbonaceous chondrite" equates to cometary material 
(my (very basic!)error ...they are in fact usually asteroid material?) but 
the abstract you provided says "a body intermediate between chondritic 
asteroids and cometary bodies".

Would i be correct in saying that the density they quote for the recovered 
fragments is also intermediate between asteroids and cometary bodies? 
....and this (the density) is the primary criteria in classification of the 
type of parent body ? Is this then the first discovered and studied sample 
of material in the intermediate category?

Leo

>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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