Duncan Steel <>

    Luigi Foschini <>

    Petr Pravec <>

    Mike Baillie <>

    Andrew Glikson <>

    Timo Niroma <>


From Duncan Steel <>

Dear Benny,

I would like to make a few comments about items contained in CCNet
dated 14 March, concerning the recent paper by Rabinowitz et al.
in Nature. Actually, my comments are not about the paper as such
(which seems a valuable and timely contribution), but rather the
reaction to it as re-circulated in CCNet.

As in most of these things, I like to play the devil's advocate
(when I said that at Torino, David Morrison said that I *am* the
devil - accepted in good humour), simply because someone has to
give an alternative viewpoint lest essential new understandings or
beliefs be missed. In all these things I am also trying to act in
good humour, and see no point in acrimonious debates which simply
estrange the researchers who should be pulling together.

To that extent I did feel that your response to the thoughts of
David Morrison, Clark Chapman and Al Harris was perhaps unwise,
in that such abrupt statements can do little except to cause the
recipients to become defensive and thus not amenable to an open

Now let me proceed. There are several lines of reasoning which
could be followed from the Rabinowitz et al. paper. In terms of
the overall aim of planetary defence (as opposed to the goal -
perhaps false - of the Spaceguard Survey as defined by DM and CC),
there are various other thoughts one could muse upon. Let me
ponder a few.

(a) David Morrison correctly cautions that the lowering of the
estimate of the number of NEAs does not imply a downgrading of the
estimate of the hazard, because the hazard estimation is based (to
a large extent) on the cratering record of the moon and Earth. But
one can jump from that to a corollary: if the present NEAs don't
explain the craters then something else is required in the overall
model. Three things come to mind (and I'm sure the reader could
invent others):

(i) The NEAs as observed/defined do not comprise all the *asteroid*
    impact risk;

(ii) The population of NEAs (and/or other impactors) has varied in time
     and we're in a lower-than-average phase;

(iii) Other impactors (small parabolic comets, say) comprise a
larger chunk of the impact hazard than believed until now. (Of
course it is also possible, as DM pointed out, that the cratering
rate in hand is out by a factor of two, and in itself that could
explain matters.)

If (ii) is correct then the hazard in this epoch is lower than
we'd thought, although that could change rapidly (e.g., a Centaur
falls into the inner solar system and fragments);

If (i) or (iii) is correct then the Spaceguard Survey is chasing
the wrong goal, and the goalposts must be moved. Various comments
could be made based on that notion. It is clear that the
Spaceguard Survey target (as in the 'find 90 percent of NEAs'
goal) is simple. It *may* be correct, but my own view has always
been that it is not only simple, but also simplistic. The reality
is perhaps more complex.

(b) Al Harris wrote in his abstract for a forthcoming conference
that: "we define a Near-Earth asteroid (NEO) as one with a
perihelion less than 1.3 AU."

This is not necessarily helpful. As DM mentioned, different
authors using different definitions clouds the issues. Many of us
have differentiated between near-Earth asteroids (NEAs), and
near-Earth objects (NEOs) as being NEAs plus comets approaching
the terrestrial orbit. This may well have been a slip on Al's
part, but in the present context the distinction could be very
important. The Spaceguard Survey is (apparently) working to a goal
of finding 90 percent of NEAs larger than 1 km. Of course we
cannot find 90 percent of all NEOs within the next century, if one
accepts the definition of NEO to include long-period comets,
because the vast majority of them will not come to perihelion
soon. But by confusing the definition one can confuse the issue.
This may connect with point (iii) in (a) above.

Turning to point (i), Al's definition does not say so, but seems
to be concerned only with asteroids with orbits which are small
(say, cis-jovian). We do know of asteroids with Halley-type orbits
and perihelia in the Mars region. I see no reason to expect that
there are no Earth-crossers awaiting discovery amongst that group.
The known asteroids of this type are large (e.g., 5335 Damocles)
and so there might be a substantial number of small ones. Although
one would expect this to be a minor component (else we would have
found a handful by now), it does not need to be a vast number to
upset the 90-percent rule which the US astronomers have adopted,
for better or worse. My point is that the allowed residual is just
10 percent. If that residual were 200 asteroids pre-Rabinowitz, it
has now shrunk to 100. If there are in reality just 50 asteroids
larger than 1 km on Earth-crossing orbits with a mean period of 50
years then only one a year would come to perihelion. What's the
chance we'd know of this population? And 50 is a large
perturbation on the allowed residual of 100; also one might expect
there to be more than 50.Those goalposts are shaking.

Along these lines one also might mention that we now know of many
Centaurs in the 50-200 km size range in the outer planetary
region. These are unstable. So, of course, would be any large
population of smaller objects in similar orbits that we have yet
to spot. Is there any reason to believe that there are no 1 km

Do not think that I am arguing against setting specific targets.
The simple '90 percent' rule is nice, for other reasons that
simplicity. Because once one gets close to 90 percent completeness
for NEAs (as per the definition of NEAs) then the residual 10
percent is overtaken, in terms of hazard posed, by the other
objects in the solar system. It may well be that the cross-over
occurs at 70 or 80 percent (residual NEAs 30 or 20 percent). But
wherever it occurs, you get to the point that to start tackling
the remaining hazard you need to develop more sophisticated search
systems (e.g. larger apertures for distant objects, or space-based
systems for Atens sunward of Earth most of the time, and so on).

(c) Now return to long-period comets. Most analyses have led to a
result showing that such comets comprise a small fraction of the
hazard, perhaps 10 percent, whilst there have been suggestions
that it is more like 40 percent based on the high impact speeds
(mean around 55 km/sec as opposed to 20 km/sec for NEAs). Of
course one needs to differentiate between the numbers of the
objects (not in absolute terms, but rather in terms of the number
passing perihelion within 1 AU per year) and the hazard they pose,
but let me leave that aside, and simply put another spin on this
(just like politicians do).

Take the 40 percent suggestion. That was correlated with 60
percent coming from the NEAs. Now the impact rate from NEAs has
been downgraded (perhaps) by a factor of two. Keeping the same
impact rates for comets, the contribution would now be 57 percent.
If you thought that the overall impact rate must be the same (to
accommodate the lunar craters) then the cometary contribution
would be 70 percent.

I do not believe those figures, but I put them is as pause for
thought. They again point towards the need for larger search
systems: the requirement to spot long-period comets when they are
beyond Saturn or even Uranus (i.e. where they are virtually
asteroidal in appearance).

(d) Clark Chapman wrote:
"The US Congress and NASA for example (and other entities) have 
adopted, as a definition of the goals of the Spaceguard Survey,
finding 90% of NEAs larger than 1 km diameter."

In fact the Council of Europe, in 1996, adopted 0.5 km as the
limit. That makes the number to be found rather larger, and the
difficulty in doing so is proportionally higher (larger number but
brightness drops as the square of the size, and that greatly
reduces the volume of the search cone for any camera). I know that
'other entities' are also looking at a lower size limit. There are
good reasons to do so.

Clark continued:
"Operationally, there is a major difference: Most people in this
field believe that, in order to multiply the detection rate by a
factor of 8 (the shortfall as previously estimated), new and
larger telescopes would have to be constructed more-or-less
immediately.  On the other hand, if Rabinowitz et al. are right,
then modest extension of current programs would meet the goal."

Is the goal the correct one? Leaving aside the debate of 1 km
versus 0.5 km NEAs, the other points I've made above indicate that
an alternative interpretation of the Rabinowitz et al. paper is
that the risk has a larger component in the outer solar system
than that assumed to date. If that is the case, then far from
being an argument for the current programmes being near-adequate,
in fact the argument is quite the contrary: that new and large
search systems are indeed required to scour the outer planetary
region. The goal as set to date is simply wrong, in my opinion,
and this new work may be interpreted to support such a view.

(e) Finally, I believe that to date we (mostly astronomers) have
been wrong to define and opportion impact risks and hazards in the
ways which we have (mea culpa). I do not think it is our job. I do
not think we have the expertise to do so. I believe it should be a
political decision. If it were left as a political decision with
the relevant experts providing governments with the necessary
information, then the appropriate programme would be carried out.
(Instead we've argued about it and not given the necessary inputs,
which has dissipated energy and effectiveness.)

Mostly we've talked about individual death probabilities, global
threshold events and the like. What should be done is simply to
apply the same sorts of rules as governments already apply to
major disasters. At the UK NEO meeting at Cambridge in 1997, Nigel
Holloway showed the sorts of matrices which the UK government
applies. A one in 100,000 annual probability event is classed as
possibly tolerable if less than 100 people would die as a result;
between 100 and 10,000 people dead implies that something should
be done about it if economically feasible; greater than 10,000
deaths means *intolerable*, and measures should be taken even if
no solution is known. Now, we spend a lot of time arguing about
whether NEO impacts (sizes over 1 km) have annual probabilities of
one in 100,000 or a million , but that's irrelevant. The
intolerability limits for those might be 10,000 and 100,000
deaths, or thereabouts, but the actual deaths (say 10 million for
the UK alone) are far above no matter how one tries to wrangle
the sums.

It follows that action must be taken. That needs communicating to
public and politicians alike. Talk about when and how this
arbitrary and false goal of "90 percent completion" will be
achieved means that those involved are in disregard of basic rules
governing such matters as nuclear reactor safety: they are saying
that impacts are outside of the simple guidelines which cover
natural and man-made disasters in terms of what response is
appropriate in the face of a hazard.

Lay the 90 percent figure aside. It implies that the public is
being cheated: it pays no attention to the most likely
next-impactor to cause deaths (probably a 50-m rock), nor to
objects like Hale-Bopp and Hyakutake. If they understood this,
along with the nature of the hazard, the public would be outraged.
Rightly so.

I cannot resist noting that I write on March 15th. "Beware the
Ides of March," the soothsayer told Julius Caesar. He didn't. He
died. The comet associated with his death in fact did not appear
until mid-year (in 44 BC). But if Julius Caesar had had our
technology, he could have seen it in time.

Duncan Steel


From Luigi Foschini <>

Dear Benny,

I would like to inform you that I have just uploaded in the server the revised version of the paper:

The impact hazard from small asteroids: current problems and open

by L. Foschini


The current philosophy of impact hazard considers the danger from
small asteroids negligible. However, several facts claim for a
revision of this philosophy. In this paper, some of these facts
are discussed. It is worth noting that while the impact frequency
of Tunguska--like objects seems to be higher than previously
estimated, the atmospheric fragmentation is more efficient than
commonly thought. Indeed, data recorded from airbursts show that
small asteroids breakup at dynamical pressures lower than their
mechanical strength. This means that theoretical models are
inconsistent with observations and new models and data are
required in order to understand the phenomena..

The preprint will be available on the web as of tomorrow.



Dr. Luigi Foschini
Institute TeSRE - CNR
Via Gobetti 101, I-40129 Bologna (Italy)
Tel. +39 051.6398706 - Fax +39 051.6398724
Email: (home)
Home page:


From Petr Pravec <>

Charles F. Peterson <> wrote:  

> In terms of useful information about NEAs, it would seem that the
> velocity of the surface of the asteroid tells more than the rotational
> period.  Am I correct?
> The surface of a 100 meter asteroid with a rotational (day/night)
> period of 10 minutes is not moving as fast at its equator as the
> surface of a one kilometer asteroid with a much longer rotational
> period.  Surface velocity would tell something about origin and impact
> history, right? Day/night period seems to be an interesting but
> inconsequential artifact of the relationship of diameter to surface
> speed. 

Actually, the most important information brought by the detection
of a fast asteroid spin is the ratio between centrifugal
acceleration and gravitational acceleration on the asteroid's
surface. The superfast spins tell us that the asteroids are
rotating under tension and therefore are monolithic. (I.e., not
gravitationally bound "strengthless" bodies.)  The information
about asteroids internal structure is one of the main reasons why
to study asteroids spins. The fast day/night changes are less
important scientifically (but interesting for general public to
remember, thus they were pointed out in the press release so
widely distributed), but they are still relevant e.g. for
computation of temperature distribution on the asteroid surface.

Petr Pravec


From Mike Baillie <>


I note  Neil Bone's reply re DRAGONS IN THE SKY and how an
alternative explanation would be auroral displays. Fine for the
dragon in the sky motif but less satisfactory in explaining the
quite plain 'In the places they passed all the trees were broken'.
The latter is not a known feature of aurora but is a known feature
of the two best documented forest impacts in the last century,
namely Tunguska and Brazil.

There are enough pieces of evidence from history,
dendrochronology, legend, poetry and myth to suggest that there
were some really quite unpleasant happenings in the sixth century
AD involving the sky, or things in the sky, as well as cold and
plague on the ground. Both volcanoes and cometary debris have been
suggested as the possible primary agents. In my view the
bombardment case may just have an edge. Without doubt there is a
scientific case for a catastrophic event and auroral displays do
not provide a satisfactory explanation.

Mike Baillie


From Andrew Glikson <>

Dear Benny,

I thank Stephen Ashworth for his comment/question (CCNet 15.3.00)
regarding the comparison I made between a Swan Lake ballet and the
bee dance in the essay "The Violent habitat of early terrestrial
life" (CCNet 13.3.00), namely the criteria for measuring

It is said that "History is written by the winners".  There is an
inherent inclination for Homo Sapiens to take its species name too
literally, i.e. define intelligence in terms of the faculties in
which it excels, compared to other creatures.  When a pilot flies
a jet from London to New York, or an architect designs a building,
these are defined as intelligent achievements.  On the other hand,
when a Starling navigates from Greenland to the Cape of Good Hope,
or an ant colony builds the most elaborate termite nest, these are
attributed to "instinct" or to genetic codes - a clear double

The design of I.Q. tests emphasizes intuition, quick grasp and
memory. Wisdom - the product of a cumulative process over time by
trial, error and discretion, or common sense (a difficult
attribute to quantify) are rarely taken into account. Inherently,
definitions of intelligence involve subjective value judgments. 

Evolutionary time can not in itself ensure success, since species
have been known to become overspecialized, wane and extinct.  Nor
does brain mass appear to be the factor, as testified by the ever
diminishing dimensions of increasing powerful computers, or the
communal intelligence of the colonial insects.  No distinguishing
physical qualities were found in Albert Einstein's brain compared
to any other. The answers may reside in yet little understood
quantum information laws referred to, for example, by Paul Davies
(The Fifth Miracle, 1998, Penguin Press, London.  See also  
Homo Sapiens has a long way to go before it understands the
intelligence which underlies life, let alone play god, for example
through genetic engineering blind to its consequences on a
biosphere that took some 4 billion years to evolve.

Andrew Glikson
Research School of Earth Science
Australian National University
Canberra, ACT 0200


From Timo Niroma <>

Dear Benny,

While reading ancient texts and legends, many hitherto mystic
description and term has got a very acceptable and understandable
content when we take cosmic impacts into consideration. One thing,
however, has gone beyond my comprehension, and has bothered me
for 15 years. Why is there red rain, why do the lakes and rivers
turn red, why is red so dominating in these accounts?

Of course there was blood everywhere, there were many animals
including people in the area, but it is rapidly diluting and only
rarely comes as a rain.

The more obvious an explanation, too obvious, because it has to my
mind obscured the real thing, is heat. Of course bolides and
smaller meteorites heat to glow (not necessarily as red, but
anyway). Great impacts cause fires that burn huge amount of
forests and savanna. And last but not least there are surely to be
volcanic eruptions from the deep of the Earth.

But still there is something that does not match. The lava will
soon cool off in lakes and rivers. Unless the eruptive volcano is
nearby. But if it is nearby, it is too hot for people to get near
it. I speak of lakes, that remain red and cool, of rain that is
poisonous and red, but not hot. The burning forests will soon be
ashes and the smoke is not red, nor the cooled ash.

Cool, but red and poisonous? I have a suggestion. There have been
studies regarding impact to rock, resulting to coesite and
diamonds among others, needing great pressure to be born plus a
strewn field of tektites. There are studies regarding water, at
least causing huge tsunamis. But what about the atmosphere?

Frederico Gorelli from the University of Florence has now
investigated what happens to oxygen when in very great pressure
(Physical Review Letters, vol 83). Oxygen molecules in the
atmosphere have normally two atoms and as we know it is
colourless. Oxygen molecules with three atoms is called ozone, and
as we also know, it is bad on the ground, and good in the
stratosphere. But according to Gorelli and his collegues, oxygen
can consist of four (or even more) atoms under very great
pressure. But this kind of pressure in the atmosphere seems only
possible as a result of impacts.

What happens to this four-atom oxygen then? First of all this kind
of oxygen is not any more gas, but turns to liquid. So what?
Gorelli tells us that this kind of oxygen is - bright red! And I
think, poisonous also.

Timo Niroma

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