NEO News (04/04/01) Miscellaneous items

Dear Friends & Students of NEOs:

This edition of NEO News contains four short news items or follow-up
to previous stories.

David Morrison



Our Italian colleagues have started publication of a new web-based
magazine called Tumbling Stone, aimed at a broad public readership.
Andrea Milani, one of the co-founders, writes that “Tumbling Stone,
the online newsletter, is a joint initiative of NEODyS (me and my
coworkers at the University of Pisa and elsewhere) and the Spaceguard
Foundation (A. Carusi and his coworkers at CNR Roma and elsewhere).
As explained by Andrea Carusi in the first edition, “It is dedicated
to a better understanding, in the field of Near-Earth-Objects, of the
meaning and importance of announcements concerning these bodies,
their relationships with our planet, and the degree of hazard they
may represent for mankind. We feel that such an information tool is
needed for non-specialists.”

The address for Tumbling Stone is It will also be linked
directly from my impact hazard webpage.



In the March 16 edition of NEO News I reported on recent work
indicating that the lunar crater Giordano Bruno could not very well
have been formed in 1178, which is an interpretation often quoted for
the report by Gervase of Canterbury that five men witnessed an event
on the Moon that might have been an impact. I concluded with the
comment that discounting the identification of this event with the
crater Giordano Bruno undercuts the reality of the medieval report,
or at least its relevance to NEO impacts and the Taurid meteor stream.

Alan Harris of JPL wrote that “I am pleased to see yet another
“debunking” of this old Hartung chestnut”. But he notes that a
similar argument was made 8 years ago, that if a large lunar impact
had occurred in 1178 there would have been an associated meteor storm
when the ejecta reached the Earth. Harris continues: Regarding the
Giordano Bruno claim, I pointed out (Harris, JGR 98, 9145-9149, 1993)
that “our final conclusion must be, even in the extraordinarily
unlikely event that such a compact clump of ejecta were injected into
heliocentric orbit from the Moon, perturbations from the Earth would
have quickly dispersed the clump [so that] annual showers of more or
less constant intensity should be expected. . . Either by initial
dispersion in the velocity or by subsequent perturbations, an event
such as Hartung proposes [the Giordano Bruno impact] should have the
result that the sky would really light up every midsummer for a week
or two.” The bottom line is, I agree completely with Withers that
there should have been an “apocalyptic” level meteor storm in the
days following the claimed event, but furthermore it should have been
followed by a pretty spectacular meteor storm on each anniversary
since then.

Benny Peiser replied to my note in CCNet for March 19 as follows:
“Not so fast, my friend. While I agree that the Giordano Bruno crater
appears to be too large indeed to be convincingly associated with a
hypothesised lunar impact in 1178, neither Gervase’s report nor the
possibility of an observed impact on the Moon – and not even the
speculation about a link with the Taurid meteor stream have been
“debunked.” As Ed Vega rightly stresses, it’s Carl Sagan’s (and
others’) association of the eyewitness report with the Giordano Bruno
crater that has been “debunked”, not the report and its
impact-interpretation itself. BJP”

In reply to Peiser, I believe that in the absence of physical
evidence (such as a young lunar crater), the report from Gervase of
Canterbury has little weight. The report is already inconsistent
internally with an impact on the Moon, since it says that “this
phenomenon was repeated a dozen times or more, the flame assuming
various twisted shapes and then returning to normal. Then, after
these transformations, the Moon from horn to horn, along its whole
length, took on a blackish appearance”. The above does not sound to
me like the description of an impact. The story held some credibility
only because of the claimed association with crater Giordano Bruno.
Each person makes his or her own judgement about the veracity of
ancient documents, but for me the debunking of the association of
this 1178 report with any physical evidence on the Moon effectively
undercuts the entire connection of Gervase’s report with NEO impacts.

David Morrison



This question is still often raised. Old astronomy books almost
always assert that the Tunguska impactor was cometary. However, two
independent analyses published almost simultaneously in 1993 provided
strong arguments in favor of the asteroidal hypothesis. The
publications were:

Chyba, C.F., P.J. Thomas, and K.J. Zahnle: The 1908 Tunguska
explosion: atmospheric disruption of a stony asteroid. Nature
361:40-44 (1993)

Hills, J.G. and M.P. Goda: The fragmentation of small asteroids in
the atmosphere. Astronomical J. 105:1114-1144 (1993)

In both cases these teams argued that models of the rapid
deceleration and fragmentation of the impacting body in the
atmosphere showed that a comet would have disintegrated at a much
higher altitude, while only a projectile of rocky strength and
density could have penetrated to within 8 km of the surface, the
height at which the Tunguska explosion occurred.

Modeling the impact physics on the computer, of course, has its
uncertainties. Therefore it is worthwhile repeating a simple physical
argument developed at the time by Kevin Zahnle of NASA Ames, and
recently recalled by Chris Chyba of the SETI Institute. Independent
of modeling details, a dense stony object (asteroid) will penetrate
deeper than a less dense icy object (comet). Also, there are many
more asteroids than comets in Earth-crossing orbits. Thus if Tunguska
were a comet, and it exploded at 8 km, then asteroids of similar size
and energy must reach the surface to produce craters. If asteroids
are more common that comets (one estimate is a factor of 10), then
there would be many more asteroid impacts in the 10-20 megaton range
than comets, and we should see their associated craters. For every
Tunguska-like airburst, we might expect ten craters the size of
Meteor Crater. But these craters are not there, thus refuting the
hypothesis that Tunguska was cometary. The alternative, that Tunguska
was asteroidal, and that only the rare, very strong iron objects make
it to the ground in the 10-20 megaton energy range, is of course
entirely consistent with the rarity of features like Meteor Crater
(which was produced by the impact of an iron object).

Chyba recently succinctly summarized this argument as follows: ” I am
impressed with Kevin Zahnle’s argument that if comets of Tunguska
size almost penetrate the atmosphere, then stony asteroids of that
size certainly will, in which case
there should be far more 1 km-size craters on Earth than in fact
there are. This seems like a very difficult argument for the
“Tunguska was a small comet” point of view to counter.”



Recent “the sky is falling” news stories have dealt with the fall
from orbit of the Mir Space Station rather than NEO impacts. The
press and public are clearly interested in the risk of falling
objects, but sometimes risks are perceived in ways that are quite
different from the hazard as calculated numerically. The following is
a very rough estimate of risks, in “order of magnitude” terms only.
It is intended to be illustrative, but certainly not precise. By risk
I mean the chance or probability that any individual will be killed
as a result of either a spacecraft atmospheric entry or the impact of
a NEO.

Let’s start with the risk of death as a result of being struck by a
piece of Mir on the assumption that the fragments could land anywhere
on Earth. Suppose 1000 large metal fragments survive to hit the
ground, and that if you are within 1 meter of the impact point you
will be killed. Thus 1000 square meters are at risk, out of a total
surface area of the Earth of about 100 trillion (10**14) square
meters (not counting the Polar Regions). This is one part in 100
billion of the Earth’s surface, and that is the risk to each
individual. Multiplying by the Earth’s population of 6 billion, we
get a chance of about 1 in 20 that one person on Earth would be

In fact, the Mir atmospheric entry was far from random. It was
steered to an impact point in the mid-Pacific Ocean. Unless you lived
in that part of the world, the risk to you was zero (not allowing for
an uncertainty in how well this controlled entry would be executed).
Since the total population of the Pacific is only a few million, the
chance that someone would be killed was less than 1 in 20,000. The
folks who sold the Russians a $200 million insurance policy were very
unlikely to have to pay off any claims.

For comparison, consider the annual risk of dying as a result of an
NEO collision with the Earth. A number of studies (e.g., the paper
that Clark Chapman and I published in Nature in 1994) have shown that
this risk is dominated by near Earth asteroids of about 2 km
diameter. There is a roughly 1 in a million chance of such an impact
each year, with estimated death of 1-2 billion people. Thus the
annual risk to each of us of from NEO impacts is about 1 in a few
million, or more than 10,000 times greater than the risk from an
uncontrolled Mir entry. (Note: This is a conservative estimate; many
would argue for a NEO-impact risk that is higher by an order of

We see from these simple calculations that the risk (per year) to
each of us from asteroid impact is thousands of times greater than
from an uncontrolled Mir entry, and millions of times greater than
from a controlled Mir dive into the Pacific. Yet no one is taking out
insurance policies to protect from cosmic impacts, and this risk
receives less news coverage than the demise of Mir. Why the
disparity? For one thing, the death of Mir was a known event that
provided a good story, while we have no specific prediction of any
NEO impact. For another, Mir was a human-built object over which we
had some control (and responsibility), while an NEO impact is
considered an “act of God”. But I suspect that the difference also
reflects the fact that very few reporters tried to make a
quantitative comparison of these risks. If they had, the results
might have surprised them!

David Morrison