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(meteorobs) Excerpt from "CCNet 19/2002 - 2 February 2002"




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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet: CARBON AT THE K/T, EPH FLOPS & DEEP IMPACT ESTIMATES
Date: Sat, 2 Feb 2002 09:55:47 -0000 

CCNet 19/2002 - 2 February 2002 
------------------------------- 

[...]

(3) ESTIMATES OF EFFECTS OF THE DEEP IMPACT INTO TEMPEL 1
    Keith Holsapple <holsapple@aa.washingtondot edu> and 
    Kevin Housen <kevin.r.housen@boeing.com>

[...]

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

(3) ESTIMATES OF EFFECTS OF THE DEEP IMPACT INTO TEMPEL 1

>From Keith Holsapple <holsapple@aa.washingtondot edu> and 
     Kevin Housen <kevin.r.housen@boeing.com>

Re CCNet 23 Jan 2002: 

    "The Deep Impact mission hopes to reveal the nature of the threat 
    and how to deflect it safely. On American Independence Day 2005, 
    Deep Impact will reach its target, the six-kilometre diameter comet
    Tempel 1. The space probe will release a 350-kilogram (770 lbs)
    projectile into the heart of the comet at 10 kilometres per second
    (six miles per second). It is expected to blow a crater the size of a
    football field and 20 metres (65 feet) deep. The comet will survive
    but should reveal the nature of its interior to add to scientific
    knowledge and to guide any future plans to deflect a killer comet
    with a nuclear nudge." 

These crater estimates have been made by Peter Schultz, and have been
quoted by several recent contributions. We would like to go on record
as noting that these estimates of the effects are vastly different than
ours. As Schultz also notes in [1] referenced below, the actual result
may simply be a compression crater with little ejecta or surface
expression, or it may be very large. Schultz's estimates are based on
the large assumption. We favor a much lower value: we would predict a
crater on the order of 10 meters or less, not the 100 meters of a
football field.

These estimates differ by factors of about 1000 in crater volume, and
corresponding differences in the amount of ejecta. Why this large
discrepancy? It arises from the simple fact that we cannot perform
experiments at the size scale of interest, with 10km/s impact velocity,
on the actual (unknown) material, and at the low gravity of the surface
of Tempel. Therefore, one does the experiments that are possible, and
extrapolates to the conditions of interest. Those extrapolations use
scaling theories: theories about how the answer depends on the
parameters of the problem. Here those parameters are the impact
velocity, the impactor size, and the surface gravity. Schultz has done
experiments in low density porous materials at 1 G and various velocity,
and then applied scaling theories. Housen [2] has done experiments in a
low density material also, and at variable gravity but at a single
velocity.  

We would not extrapolate Schultz's results the same way he does. While
not going into details here, basically he has assumed that his results
should be gravity-scaled, while we believe they should use
strength-scaling. Using his approach, the crater sizes increase as a
power law (about -1/2) of gravity. When extrapolated back to the .08
cm/sec^2 gravity on the surface of the comet, he obtains his large
estimate. Our scaling approach would imply that the results are
essentially independent of surface gravity, so do not increase at low
gravity compared to the 1G experiments. (For the definitions of these
scaling regimes, one can consult, for example, Holsapple [3] listed
below.) 

At the LPSC conference in March in Houston, Housen will be presenting
his results and interpretations [2], and Schultz will be presenting his
[1]. The large uncertainties illustrate the need for missions of this
type. Here the real results will be known in 2005. It is nice to have a
debate about something where the correct answer is forthcoming.

References:
[1] Schultz, P. H. ,Anderson, J. L. B. and J. T, Heineck, Impact crater
size and evolution: Expectations for Deep Impact, Lunar and Planetary
Science XXXIII, March 2002.
[2]  Housen, K. R., Does gravity scaling apply to impacts on porous
asteroids?, Lunar and Planetary Science XXXIII, March 2002.
[3] Holsapple K. A. (1993) The scaling of impact processes in planetary
sciences. In Ann. Rev. Earth Planet. Sci. 21 , Wetherill, Albee and
Burke, eds), 333-373.

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