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(meteorobs) Excerpts from "CCNet, 003/2000 - 7 January 2000"




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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 7 January 2000
Date: Fri, 7 Jan 2000 12:13:38 -0500 (EST)


CCNet, 003/2000 - 7 January 2000
--------------------------------


     QUOTE OF THE DAY

     "Until Jan. 4th, 2000, the two big NEO movies of 1998 had made 
     together 903 million dollars in worldwide theatrical box office 
     sales alone (not even counting video rentals and TV rights) - an 
     amount to ponder... Armageddon is now at # 11 of the all-time 
     charts, with $ 554.4m, while Deep Impact is at # 50 with $ 348.6m, 
     according to the list at http://us.imdb.com/Charts/worldtopmovies. 
     Earlier impact movies like "Meteor" are not mentioned there, so 
     the overall sum should be close to one billion dollars."
         -- Daniel Fischer, University of Bonn

[...]

(4) CATASTROPHIC DISRUPTIONS OF MINOR PLANETS
    W. Benz*) & E. Asphaug, UNIVERSITY OF BERN

(5) LABORATORY ASTEROID COLLISIONS
    K.R. Housen & K.A. Holsapple, BOEING CO

(6) COLLISIONAL DISRUPTION OF ICE BY HIGH-VELOCITY IMPACT
    M. Arakawa, HOKKAIDO UNIVERSTY

(7) EXPERIMENTAL STUDY OF IMPACT DISRUPTION
    D.D. Durda*) & G.J. Flynn, SW RES INST

[...]

================================================

(4) CATASTROPHIC DISRUPTIONS OF MINOR PLANETS

W. Benz*) & E. Asphaug: Catastrophic disruptions revisited. ICARUS, 
1999, Vol.142, No.1, pp.5-20

*) UNIVERSITY OF BERN,INST PHYS,SIDLERSTR 5,CH-3012 BERN,SWITZERLAND

We use a smooth particle hydrodynamics method to simulate colliding 
rocky and icy bodies from centimeter scale to hundreds of kilometers in 
diameter in an effort to define self-consistently the threshold for 
catastrophic disruption. Unlike previous efforts, this analysis 
incorporates the combined effects of material strength (using a brittle 
fragmentation model) and self-gravitation, thereby providing results in 
the ''strength regime'' and the ''gravity regime,'' and in between.
In each case, the structural properties of the largest remnant 
are examined. Our main result is that gravity plays a dominant role in 
determining the outcome of collisions even involving relatively small 
targets. In the size range considered here, the enhanced role of 
gravity is not due to fracture prevention by gravitational compression, 
but rather to the difficulty of the fragments to escape their mutual 
gravitational attraction. Owing to the low efficiency of momentum 
transfer in collisions, the velocity of larger fragments tends to be 
small, and more energetic collisions are needed to disperse them. We 
find that the weakest bodies in the Solar System, as far as impact 
disruption is concerned, are about 300 m in diameter. Beyond this size, 
objects become more difficult to disperse even though they are still 
easily shattered. Thus, larger remnants of collisions involving targets 
larger than about 1 km in radius should essentially be self-gravitating 
aggregates of smaller fragments. (C) 1999 Academic Press.

================================================

(5) LABORATORY ASTEROID COLLISIONS

K.R. Housen*) & K.A. Holsapple: Scale effects in strength-dominated 
collisions of rocky asteroids. ICARUS, 1999, Vol.142, No.1, pp.21-33

*) BOEING CO,SHOCK PHYS,MS 8H-05,POB 3999,SEATTLE,WA,98124

The application of laboratory collision experimental results to the 
larger scales of asteroid impacts is complicated by the fact that the 
dynamic strength of rock typically decreases as the loading duration 
increases. Because loading times increase with the size scale of a 
collision, large bodies are effectively weaker than small ones. While 
this effect has been postulated for over a decade, it has never been 
verified in actual collision experiments. This paper summarizes 
collision tests performed under the conditions required to examine 
scale effects, i.e., increasing the size scale of the experiment while 
holding the impact velocity and impact kinetic energy per target mass 
constant. Granite targets are used, with a diameter variation of a 
factor of 18, The larger targets experienced significantly more 
collisional damage than small ones, confirming a decrease in dynamic 
strength with increasing size scale. The results are compared to a 
scaling model based on the concept that fragmentation is accomplished 
through the growth and coalescence of preexisting flaws. Measurements 
of the actual flaw-size distribution are used to validate the model. 
Field observations of flaw and fault sizes at scales to 10 km are used 
to construct a scaling model that is believed to apply to the 
shattering of a wide range of rock types. The results show that 
kilometer-sized rocky bodies may be significantly weaker than indicated 
by previous estimates, (C) 1999 Academic 
Press.

================================================

(6) COLLISIONAL DISRUPTION OF ICE BY HIGH-VELOCITY IMPACT

M. Arakawa: Collisional disruption of ice by high-velocity impact
ICARUS, 1999, Vol.142, No.1, pp.34-45

*) HOKKAIDO UNIVERSTY,INST LOW TEMP SCI,KITA KU,KITA 19 NISHI 
   8,SAPPORO,HOKKAIDO 060081,JAPAN

High-velocity impact among icy planetesimals is a physical phenomenon 
important to the planetary evolution process in the outer Solar System. 
In order to study this phenomenon, impact experiments on water ice were 
made by using a two-stage light gas gun installed in a cold room(-10 
degrees C) to clarify the elementary processes of collisional 
disruption and to study the reaccumulation and the escape conditions of 
the impact fragments. Cubic ice targets ranging in size from 15 to 100 
mm were impacted by a nylon projectile of 7 mg with an impact velocity 
(v(i)) from 2.3 to 4.7 km/s. The corresponding mass ratio of the 
projectile to the target (m(p)/M-t) ranged from 10(-3) to 10(-6), which 
is two orders of magnitude lower than that used in previous studies 
(Arakawa et al. 1995, Icarus 118, 341-354). As a result, we obtained 
data on elementary processes such as attenuation of the shock wave and 
fragmentation dynamics. We found that the shock pressure attenuates in 
the ice target according to the relation of P proportional to 
(L-p/r)(2), irrespective of the mass ratio between 10(-3) and 10(-
5), where L-p is the projectile size and r is a propagation distance. 
The largest fragment mass (m(l)) normalized by the original target mass 
has a good relationship to a nondimensional impact stress (P-I, NDIS) 
defined as the ratio of the antipodal pressure to the material 
strength. This relationship is described as m(l)/M(t)proportional to 
P-I(-1.7) for a wide range of impact conditions (50 m/s < v(i) < 4 km/s 
and 10(-1) < m(l)/M-t, < 10(-6)), and shows the utility of NDIS. Using 
a measured shock wave decay constant of 2, the reaccumulation and the 
escape conditions of icy bodies in high-velocity collisions were 
estimated. As a result, it was clarified that a rubble pile could be 
formed when large icy bodies (radius > 20 km) reaccumulated. On the 
other hand, when smaller icy bodies (radius < 2 km) disrupted 
catastrophically, all fragments escaped and a rubble pile was never 
formed. (C) 1999 Academic Press.

================================================

(7) EXPERIMENTAL STUDY OF IMPACT DISRUPTION

D.D. Durda*) & G.J. Flynn: Experimental study of the impact disruption 
of a porous, inhomogeneous target. ICARUS, 1999, Vol.142, No.1, 
pp.46-55

*) SW RES INST,1050 WALNUT ST,SUITE 426,BOULDER,CO,80302

Measurements of the densities of interplanetary dust particles and 
unweathered stone meteorites indicate that both have significant 
porosity on the microscopic scale, In addition, the chondritic stone 
meteorites are generally inhomogeneous, typically consisting of strong, 
millimeter-size, olivine chondrules embedded in a weaker, fine-grained 
matrix, Since target porosity is known to influence energy partitioning 
in cratering and disruption, we have begun a series of experiments to 
study the impact disruption of inhomogeneous assemblages of two 
materials of different strengths and which exhibit significant 
porosity, Experiments were performed on three similar to 300-g targets 
of porphyritic olivine basalt, consisting of millimeter-size olivine 
phenocrysts in a fine-grained vesicular matrix (simulating a stone 
meteorite). Using the NASA Ames Vertical Gun, each target was impacted 
by a 1/4-in. diameter aluminum sphere at a speed of similar to 5 km s(-
1). To avoid measuring the secondary effects of fragmentation caused by 
material impacting on the walls of the gun chamber, we monitored 
primary debris using passive detectors. We measured the size-frequency 
distribution of the small fragments using thin foils. Most foils showed 
only small depressions, sometimes containing fragments of debris, 
indicating relatively low velocity debris. One foil showed similar to 
300 puncture holes from high-speed particles, presumably a localized 
jet or cone of target or projectile ejecta, The size-frequency 
distribution was quite steep down to the similar to 10- to 20-mu m 
limit where particle size was comparable to foil thickness. Aerogel 
cells were employed to capture dust-size primary debris. Using an in 
situ chemical analysis technique, we distinguished matrix from olivine 
and determined that fragments <100 mu m in size were matrix while the 
majority of the largest fragments (> 200 mu m in size) were olivine, We 
also collected the debris from the floor of the gun chamber, The 
largest fragments (significantly bigger than individual olivine 
phenocrysts) were representative of bulk target material, In the 
millimeter-size range we found a large number of isolated olivine 
crystals, indicating the target experienced preferential failure along 
the phenocryst-matrix boundaries. All three shots showed distinct 
changes in the slopes of the mass-frequency distribution near 0.4 g, 
the size of typical olivine phenocrysts. This suggests that the 
mechanical failure of the material was affected by the presence of the 
phenocrysts. If our results are directly applicable to chondritic 
meteorites, then impact cratering and disruption of chondritic 
asteroids may overproduce olivine-rich material from chondrules in the 
millimeter-size range and olivine might be underrepresented at smaller 
sizes in the primary debris. (C) 1999 Academic Press.

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