CCNet DIGEST 2 July 1998

    Juan Zapata-Arauco <>

    Alan W Harris <>

    Benny J Peiser <>

    Duncan Steel <>


From Juan Zapata-Arauco <>

Dear Dr. Peiser:

I am currently doing multidisciplinary research on neo-catastrophism.
Would you be so kind to put to consideration of the network the
implications of the work by Asphaug and co-authors for the notion of a
"Earth's Protecting Shield" that many of the CCNetworkers (e.g. Kobres,
Clarke) are fond to sell as the best technology against likely future
NEO impacts?

In the otherwise excellent essay "Presidents, Experts and Asteroids"
(SCIENCE, 5 June 1998) Sir Arthur Clarke risks the prediction that
Reagan's 1983 "Star Wars" technology "will one day be regarded as a
work of political genius" and "may come to be useful in ways
unanticipated at its inception. The projected SDI armory of lasers and
interceptors could one day be used to save not only the United States,
but indeed the entire human race from the threat of comets and

On the other hand the consequences of recent "real world" simulations
of energy depositions (high-velocity impacts, nuclear explosions,
high-energy laser rays) on NEOs seem in every way far from the
simplistic assumptions we are used to trust when we think of using
SDI for a kind of "SpaceGuard" against incoming NEOs. In the commentary
to the article of Asphaugh et al: "Making and braking asteroids"
(NATURE, 4 June 1998) Alan W. Harris refers that

"most studies of these processes have modelled the offending asteroid
as a coherent solid body rather than a loose collection of debris...If
the only thing holding the body together is gravity, then one cannot
apply an impulsive change in its motion greater than the escape
velocity from the surface without disrupting the body into many pieces.
This means a kilometer-sized body can be given a change of course of
only about a meter per second. Such a small impulse would have to be
applied a fair fraction of a year before the projected time of
collision in order to accumulate a change of path of a couple of Earth
radii. The smaller the object, the smaller the impulse allowed, so THE
IMPACTS IS SERIOUSLY FLAWED; better to discover asteroids far in
advance in an orderly survey, allowing plenty of time to respond."

In which way do these observations modify the notions maintained on the
CCNet about the technologies available for a projected umbrella
defense system against NEOs? In which way do they favour a pressing
need for a survey of NEOs far deeper than the now painfully obtained?

With my best regards

Juan Zapata-Arauco


From: NATURE , vol. 393, 4 June 1998, pp. 418-419

By Alan W Harris

There is an old joke about the limitations of physics and its need
for mathematical abstraction, the punchline of which runs, “We
assume a spherical chicken...”. In trying to model the collisional
evolution of asteroids, however, theorists have been unable to
avid assuming a spherical chicken.  To create the present asteroid
belt, asteroids must have been undergoing collisions throughout
the age of the Solar System, but to make tractable analytical
calculations of what happens when one asteroid hits another, one
is forced into the unrealistic assumption that the bodies are
spherical and rigid.

Now at last we can do something better: as reported on page 437 of
this issue (1), Asphaug and his co-authors (along with several
other research groups) are progressing beyond analytical
calculations to detailed numerical models of what happens when
irregular, inhomogeneous asteroids suffer high-velocity impacts.
Of particular interest are the near-Earth asteroids -not just
'making' them, but the possibilities for 'braking’ one that might
chance to be on a collision course with the earth (the subject of
a couple of Hollywood productions, Deep Impact and Armageddon,
which either have been or will shortly be released).

Such detailed modeling has now become possible, partly because we
now know the shapes of a few asteroids from radar mapping and
spacecraft imaging, but mostly because high-speed supercomputers
and advanced hydrodynamic codes are becoming available for
non-military applications.  Although the 'peace dividend' has yet
to pay any obvious cash rebates, it appears to be yielding some
research benefits.  Over the past few years, similar computations
have yielded realistic models of the tidal breakup (2) of the
progenitor of comet Shoemaker-Levy 9 by Jupiter in 1992; models of
the impacts and plume formations (3) of those fragments as they
plunged into the atmosphere of Jupiter in 1994; similar models of
the entry into the Earth’s atmosphere of giant meteoroids such as
the Tunguska bolide (4) of 1908; and simulations of the formation
of the Moon by an enormous glancing collision between some large
body and the Earth, early in the Solar Systems (5).  A rich suite
of problems has become ripe for solution with the advent of these
powerful computing tools.

An important question about the structure and evolution of
asteroids is whether they are monolithic bodies or 'rubble piles'
- that is, loose agglomerations of debris fractured by past
collisions but not quite dispersed into separate bodies.  Asphaug
et aL conclude that this is likely for many asteroids, but that it
may depend critically on the first impact suffered by an initially
monolithic body.  Once fracture zones are created, they tend to
protect the material on one side from damage due to impacts on the
other side, as the shock wave is absorbed, reflected or attenuated
by the smaller rocks in the zone. Generally speaking, these
effects tend to make the asteroid more resistant to global
disruption, and hinder the rate at which further fracturing occurs
to convert the entire mass into a 'rubble pile'.  A few large
pieces can survive.

There is observational evidence that even very small asteroids are
rubble piles, from the statistics of spin rates (6).  It seems
that no asteroids spin so fast that they would be in a state of
tension.  Among very small asteroids, the average rate of spin is
fast enough that there appears to be a barrier in the distribution
of spins at the rate corresponding to this limit, suggesting that
most asteroids indeed have no tensile strength, as would be the
case for heavily fractured bodies.  On the other hand, rubble pile
structure does not help to explain the few very slowly spinning
asteroids. 253 Mathilde, for example, imaged in detail by the NEAR
spacecraft last year, is one such very slowly spinning body, with
a rotation period of around 17 days.  The images returned were
astonishing for the number and large size of craters revealed, so
it is not plausible that Mathilde somehow avoided spin-inducing
impacts. However, the very large craters found on Mathilde do
indicate a rubble pile structure, because according to the results
of Asphaug et aL a monolithic body struck hard enough to produce
such large craters would probably be blown apart, whereas a
'sandbag' might avoid complete disruption.  The low bulk density
of Mathilde found by its gravitational perturbations on the NEAR
spacecraft also supports the idea that this is a structure with a
lot of vacuum in among the rock (7).

Perhaps the most interesting aspect of this work for the general
public is the implication for defence against asteroid impacts on
the Earth.  If an asteroid were found to be on a collision course
with the Earth, could we avoid it?  The front-running technique is
to explode a nuclear bomb some distance from the asteroid,
vaporizing a thin layer of its surface on one side, and thus
giving it a nudge.  But most studies (8) of this process have
suffered from the spherical chicken problem, modelling the
offending asteroid as a coherent solid body rather than a loose
correction of debris.  The new work may mean that deflecting an
asteroid from a collision course would be more like clearing a
landslide off the road than pushing a boulder aside. If the only
thing holding the body together is gravity, then one cannot apply
an impulsive change in its motion greater than the escape velocity
from the surface without disrupting the body into many pieces. 
This means a kilometre sized body can be given a change of course
of only about a metre per second.  Such a small impulse would have
to be applied a fair fraction of a year before the projected time
of collision in order to accumulate a change of path of a couple
of Earth radii.  The smaller the object, the smaller the impulse
allowed, so the concept of a 'Star Wars' type shield protecting
the Earth from imminent impacts is seriously flawed; better to
discover asteroids far in advance in an orderly survey, allowing
plenty of time to respond.

Alan W Harris is in the Earth and Space Sciences Division of the
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California 91109, USA.

(1) Asphaug, E., Ostro, S.J., Hudson, R.S., Scheers, D.J. & Benz, W. Nature 393, 437-440 (1998)
(2) Asphaug, E. & Benz, W. Icarus 121, 225-248 (1996)
(3) Zahnle, K., & MacLow, M. Icarus 108, 1-17 (1994)
(4) Chyba, C.F., Thomas, P.J. & Zahnle, K.J. Nature 361, 40-44 (1993)
(5) Cameron, A.G. Icarus 126, 126-137 (1997)
(6) Haris, A.W. Lunar Planet. Sci. 27, 493-494 (1996)
(7) Yeomans, D.K. et al., Science 278, 2106-2109 (1997)
(8) Ahrens, T.J. & Harris, A.W. Nature 360, 429-433 (1992)

Copyright 1998, NATURE


From Benny J Peiser <>

Dear Mr Zapata-Arauco

Thank you for raising the issue of planetary defense and the possible
implications the work of Asphaug et al. may have on this subject

Before your questions can be answered satisfactory, I believe, much
more detailed information is needed that would allow a reasonable
response. Here are just some of the items which need to be taken into

First of all it should be stressed that we still don't know very much
about the  a c t u a l  material composition of asteroids and comets.
Whilst there is circumstantial evidence that  s o m e  of these object
may indeed be inhomogeneous 'piles of rubble,' it is unknown what their
overall percentage is compared to the monolithic objects among the NEO
population. Given this vague and limited data base, it would appear
unwise, if not foolish, to jump to conclusions and to reject out
of hand current models of planetary defense.

I would also like to underline that the interpretation by Alan Harris
of what nuclear devices might and might not achieve even when applied
against rubble pile asteroids is far from representing a scientific
consensus. Some researchers and workers in the field of planetary
defense are much less pessimistic (or politically inhibited) than Alan
about mankind's capability to develop an effective 'Star Wars' type of
protective shield - and in spite of its unfashionable name. After all,
the next PHO on a collision course with earth might be a hard rock
rather than a pile of rubble.

It goes without saying that I whole-heartedly support any research
to develop alternative, i.e. non-nuclear, technology forceful and
effective enough for disintegrating or (more likely) deflecting 
earth-colliding objects. However, as long as this technology is not
viable or available, we will have to content ourselves with the most
powerful means for earth defense.

There is another interesting aspect of the work by Asphaug et al. which
might have significant implications on how impact rate statistics are
currently calculated. One of the most important variables used in these
calculations are the known hyper-velocity impact craters on earth. Due
to the existence of (mostly) individual impact structures, researchers
see these single craters as evidence for monolithic impactors.
Obviously, the collision of a rubble pile asteroid with earth would
result in multiple impacts both in the atmosphere and on the surface of
the earth depending on the features and composition of such an object.
Instead of impacting in a particularl locality (and thereby limiting
the overall environmental knock-on effects), such an object would cause
multiple impacts of a Super-Tunguska kind, releasing explosive energy
over a wide area of the globe. 

Let me emphasise once again that I appreciate the critical views by
Alan Harris and yourself. I hope they will help stimulating a fruitful
and open debate about the best and most effective ways mankind can
employ to protect ourselves against future cosmic disasters.

Benny J Peiser


From Duncan Steel <>

Dear Benny,

Several recent papers (and entries in the CC Digest) have been
concerned with investigations of variations in the terrestrial influx
(of asteroids, comets, and debris derived therefrom) over geological
time, from studies of variations in the deposition rates of
extraterrestrial material identified in rock strata. Of course the
identification of an 'anomalous' iridium layer at the KT boundary by
Alvarez et al. (Science, 1980) led to the popular growth in the large
impact hypothesis for the extinction of the dinosaurs (and mass
extinctions in general), although that paper was far from the first to
contain such a suggestion (vide Napier & Clube, 1979; Urey, 1973;
Nininger, 1941; etc.).  In view of this, it might be of interest to ask
who first suggested that the terrestrial stratigraphic record might
contain extraterrestrial material deposited in past eons either by
random large impacts or through enhancements in the accretion rate of
small particles due to deviations in the interplanetary population of
comets, asteroids, meteoroids and dust.

In 1951 Fred Whipple published the second of two papers (the first was
in 1950) in which he conducted a theoretical analysis of the survival
of small (sizes below 1 mm) meteoroids in atmospheric entry, allowing
them to reach the ground intact, thus producing micrometeorites.  At
the close of his second paper he wrote the following [the words in
square brackets are mine]:

"On a purely speculative level is the possibility of detecting evidence
for past solar system catastrophes [for example, the disintegration of
a meteorite parent body/small planet]. If a minor or major planetary
disruption actually occurred in recent astronomical time [meaning the 
past 100 Myr or so], as suggested by C.A. Bauer to account for the
meteoritic helium contents, evidence for the concomitant
micro-meteorites may conceivably be found in the chalk beds of the
Cretaceous Period or in other geological formations." F.L. Whipple,
Proc. Natl. Acad. Sci. (USA), 37, 29, 1951.

Duncan Steel

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