Date sent: Thu, 12 Jun 1997 11:58:32 -0400 (EDT)
From: Benny J Peiser <>
Subject: Are NEO studies in such good shape?
Priority: NORMAL


from: Duncan Steel <>

When one is closely involved in a particular area of study, it is
easy to imagine that others agree with you. When it comes to
studies of near-Earth objects and the hazard they pose to
humankind, one tends to imagine that it is a matter of convincing
the general public that this is a matter to take seriously. But
what about the non-general public? For example, the members of the
National Space Society (NSS) in the USA (with a branch likely near
you, too)? Surely they would see that this is an important area?

Consider the May/June issue of AD ASTRA (the magazine of the NSS).

On page 2 there is a nicely-written and supportive editorial from
Robert Zubrin, the Chairman of the NSS, in which he discusses Comet
Hale-Bopp and makes a strong argument for humankind needing to
become spacefaring so as to be able to re-direct any potential
large impactor whilst it is still far from the Earth.

On page 6 a brief mention of the NBC mini-series 'Asteroid' appears
(yuk), and it is noted that this led to increased public/media
interest in space exploration. I guess that for us it's like being
a kid again, forced to drink Cod Liver Oil: good for you but it
sure tastes bad.

Now to the meat. On pages 42-43 there is a summary report of a
survey of the members of the NSS with regard to space policy.
Remember that the respondents are space enthusiasts, and generally
pretty knowledgeable. Indeed 65.6% of NSS members are also members
of The Planetary Society, which has a fine record of promoting and
assisting research work on NEOs.

Point 5 is connected with the issues which they feel should be
stressed to the American public as reasons to support expanded
space exploration and development. Answers were given on a scale
from 1 (most important) to 6 (least important). Various things like
weather forecasting, economic development etc. score well. At the
bottom of the scale (indeed OFF the scale, I would have thought, at
level 6.36) comes 'Other', including 'Asteroid detection/deflection
... species survival ...'

Point 6 asks the respondent to decide how to split up a mythical
$100 between different space projects. The International Space
Station gets $14.38. A human mission to the Moon gets $13.43.
Right at the bottom comes 'Other' again, including 'Asteroid
detection/deflection' amongst several categories all lumped
together so as to argue over a total sum of $1.76 to be split
between them.

I don't want to be discouraging, but are we fighting a losing
battle? To take a more optimistic line, one might ask what we need
to do to boost appreciation of the need to survey the solar system
for potentially-hazardous NEOs. If the execrable mini-series
'Asteroid' did have a positive effect, should we encourage more?

Kind regards to all,

Duncan Steel


Date sent: Thu, 12 Jun 1997 10:03:21 -0400 (EDT)
From: Benny J Peiser <>
Priority: NORMAL


Some of the early members of this list will recall the live debate
between Victor Clube (Oxford) and David Hughes (Sheffield)
about potential impact hazards which was broadcast on BBC2's News
Night programme on 7 February. One of the more memorable
sound bites in this debate was David Hughes attempt to trivialize
the impact on P/Shoemaker-Levy 9 on Jupiter:

"I just don't agree with Victor at all about this. I mean, I agree
that this dear old comet [SL9] broke up by going to close to
Jupiter; but this was a very rare event and of course all that
happened there was that these bits burned up in the atmosphere. If
this comet had hit the Earth, the bits would have burned up in our

Three weeks after this encounter, a team of Japanese astronomers
published their research on Gian Domenico Cassini's observations of
Jupiter which details the evolution of of a dark spot first noted
by Cassini on 5 December 1690 (see (Tabe et al 1997 Publ Astr Soc Jap
49, L1). This and observations of several other Jupiter anomalies
made during the last three hundred years suggest that comet crashs
on Jupiter similar to that of SL9 are far from "very rare".

Now, a new calculation of Jovian comet crashs appears to support
the notion that cometary impacts on Jupiter are in fact very
frequent. In a recently published paper by American astrophysicists M
S Roulston & T J Ahrens [Impact mechanics and frequency of SL9-type
events on Jupiter. In: ICARUS, 1997, Vol.126, No.1, pp.138-147], it
is estimated that the time interval between impacts of 0.3 km
diameter comets on Jupiter is approximately 500 years, whereas the
interval between the impact of 1.6 km comets is about 6000 years.

Obviously, the question arises as to whether this seemingly
high impact frequency has any direct bearing regarding the impact
rate on Earth. I asked British astronomer David Asher
<>, who is currently working in Japan, to comment
about the calculations by Roulston & Ahrens and its relevance
regarding Earth.


from David Asher <>


The calculation of impact frequencies with Jupiter by Roulston &
Ahrens (1997 Icarus 126, 138) doesn't really have any direct
quantitative relevance to the Earth impact rate. The main point
here is that there are different dynamical principles. Many
(conventional wisdom says most) Earth impacts are from the NEA
(near-Earth asteroid) class of objects; broadly, these are on
cis-jovian orbits and cannot undergo a single approach to Jupiter
close enough to result in a significant perturbation. Relatively,
NEAs are stable on timescales during which the generally Jupiter
approaching JFCs (Jupiter family comets) undergo substantial
orbital displacement, and thus long term collision probabilities
can be calculated on the assumption of this stability, allowing
simply for the slow (secular) precession of the orbits. That
precession is largely due to Jupiter, as the most massive planet,
but is gradually accumulated rather than happening in sudden jumps.

When assessing impact rates from different sources, it makes sense
to regard those sources as the different dynamical classes (NEA,
JFC, HFC [Halley family comet], LPC [long period comet]) rather
than a comet vs asteroid source. Perfectly reasonably, it's become
the fashion to de-emphasise the clear dividing line between comets
& asteroids. How many Damocles's (i.e. asteroids) there could be
on HFC orbits, we're not sure yet. And (I'd say) one of the most
interesting & fundamental questions in current studies of the
small bodies of the solar system is the relative numbers of NEAs
deriving from the main asteroid belt vs being extinct comets.

As regards Earth impact, what (presumably) concerns us isn't the
probability, next century say, of an SL9 per se. Rather, the
chance next century of a 10^2, or 10^4, or 10^6 Mton impact
(regardless of how big SL9 actually was) and we estimate this from
(1) the orbital dynamics, i.e., how often the Earth and a potential
impacting object end up in the same place; and (2) how many
of those potential impacting objects (including size distribution)
are flying around out there. The relevance of this single
occurrence on Jupiter (SL9) seems to be of purely scientific
interest. It gives a nice reminder of ongoing important processes
(cometary splitting, planetary impacts). One good motivation for
estimating the frequency of SL9 sized events on Jupiter is for
comparision with possible previously recorded events. There seems
to be quite good evidence for an impact spot recorded by Cassini
(Tabe et al 1997 Publ Astr Soc Jap 49, L1).

I'll conclude this waffling by noting that the current conventional
practice is to talk of `collision frequencies' as essentially a
Poisson process of single events. Some people would have us believe
that there are important things we're missing. Are macroscopic
impact events really completely uncorrelated, i.e., with no
temporal pattern? And, can there be sufficient dust input
occurring over time (not from single events) to substantially
affect the terrestrial climate? That last point is a question of
quantity; obviously, given ENOUGH input there can be something
noticeable. But how much is enough, relative to how much
meteoroidal material is around?

CCCMENU CCC for 1997