CCNet DIGEST, 7 August 1998

(1) TOUTATIS IN 1992
    Gareth V. Williams <>

    Michael Paine <>

    Linda Porter <>



(1) TOUTATIS IN 1992

From Gareth V. Williams <>

I'm afraid the article by Kunich contains at least one glaring error:

> about 40,000 megatons of TNT, or 2,000 standard-size hydrogen bombs. On
> an even larger scale, on December 8, 1992, a large asteroid named
> Toutatis missed hitting this planet by only two lunar distances.

On 1992 Dec. 8, (4179) Toutatis passed by Earth at a distance of
0.0241 AU, which is about 9 lunar distances.  On 2004 Sept. 29, it
will pass 0.0104 AU from the earth, about 4 lunar distances.


From Michael Paine <>

During 1997 I started an unofficial Spaceguard Australia website partly out of
frustration with the Australian Federal Government (which stopped
funding the NEO search in 1996). Since then I have acquired information
about the subject and decided to prepare a proposal to re-establish the
Australian program. Most of the NEO search proposals that I have come
across on the Internet appear too technical for the target audience -
that is the politicians and their "minders". I spent quite a few years
as a bureaucrat myself, learning how to say "no" in dozens of ways. I
have therefore prepared a proposal aimed more at the policitian level.
It is in draft form at

Comments and suggestion are most welcome.

On a related matter, I am seeking more information about the
hazard from tsunami generated by NEO impacts. For links I have
found to date see:

Michael Paine 7 August 1998


From Linda Porter <>

August 5, 1998: This summer's science-fiction offerings were full of
large-meteorite impacts with harrowing consequences. But the
science-fact of the summer skies promises to deliver just as beautiful
a show, with a lot less mess to clean-up. The Perseid Meteor Shower
makes its annual return to the summer skies on August 11/12, with as
many as 80 meteors per hour visible from dark-sky locations throughout
the Northern Hemisphere.

As in the popular movie "Deep Impact", the action of the Perseid
meteor shower is caused by a comet, in this case periodic comet
Swift-Tuttle. Fortunately there's no danger of Swift-Tuttle hitting the
earth. It's about 6 miles wide and a collision would be catastrophic.
Instead, the stars of this show are tiny grains of dust and debris,
most smaller than a grain of sand. They are the rubble left behind when
Swift-Tuttle occasionally visits the inner solar system.

As comets enter the inner solar system, they are warmed by the sun, and
ablated by the solar wind, which produces the familar tails that we
see, sometimes quite strikingly, as in the case of comet Hale-Bopp in
1997 (image left). This debris is left in space, and is comprised of
particles of ice, dust, and rock. When the Earth encounters these
particles on its journey around the Sun, they strike the atmosphere
with tremendous speed. Most are observed as a bright streak across the
sky that can last for several seconds, but occasionally a large
fragment will explode in a multicolored fireball. Most of the streaks
are caused by meteoroids about the size of a grain of sand, although
meteoroids are porous and much less dense than sand.

Impact Hazards?

At its peak, the Perseids produce 50 - 150 meteors per hour. Are we in
any danger from falling debris? Probably not. Most of the dramatic
streaks we see in the sky are caused by particles that incinerate
before they hit the ground. However, satellites and spacecraft can be
damaged. Meteors can poke holes in solar panels, pit surfaces, and
short out electronics. The image (left) shows a meteroid impact crater
in the the Hubble Space Telescope. It was discovered in 1994, after the
1993 Leonid meteor storm.

Most meteor experts do not expect the Perseids to pose a significant
hazard to the more than 2500 commercial, military and science
satellites in Earth orbit. The Leonids may be a different story. Once
or twice every 33 years the earth passes through a dense stream of
debris from periodic comet 55P/Tempel-Tuttle. The result is a
spectacular display of 1,000 to 200,000 meteors per hour. The next
severe Leonid meteor storm is due this November, and satellite
operators are devising stretegies to protect their hardware. Antennas,
cameras, and other delicate instruments will be be turned away from the
expected stream of particles to minimize damage.

How to View the Perseids

The Perseids are perhaps the most famous and most watched of all meteor
showers. They begin in late July and are most intense during the nights
of 11/12 and 12/13 August. Viewing conditions this year will not be
ideal because a bright, waning gibbous moon will make the dimmer
meteors difficult to see. The good news is that Perseid showers in
recent years have produced a high proportion of bright meteors. 

Normally the best time to view meteors is after midnight, when the
earth's rotation aligns our line of sight with the direction of the
Earth's travel around the Sun. Then we're heading directly into the
stream of meteors. This year may be an exception. The gibbous moon
rises around 10:30 pm local time in mid-August brightening the sky from
then until dawn. So, the best time to look may be in the early evening
before the moon comes up.

Radio Meteors

An unusual method for observing meteors is growing in popularity among
amateur astronomers: radio echos. When a meteor burns up in the
atmosphere it leaves behind a trail of ionized gas. The ionization
rapidly dissipates, but transmissions from distant radio stations are
briefly reflected from the ionized trail back down to Earth. During an
intense meteor shower, a simple shortwave receiver can detect many
echos per minute from stations thousands of km away. For more
information see:


S.H. Ambrose: Late Pleistocene human population bottlenecks, volcanic
winter, and differentiation of modern humans. JOURNAL OF HUMAN
EVOLUTION, 1998, Vol.34, No.6, pp.623-651


The ''Weak Garden of Eden'' model for the origin and dispersal of
modern humans (Harpending et al., 1993) posits that modern humans
spread into separate regions from a restricted source, around 100 ka
(thousand years ago), then passed through population bottlenecks.
Around 50 ka, dramatic growth occurred within dispersed populations
that were genetically isolated from each other. Population growth began
earliest in Africa and later in Eurasia and is hypothesized to have
been caused by the invention and spread of a more efficient Later Stone
Age/Upper Paleolithic technology, which developed in equatorial Africa.
Climatic and geological evidence suggest an alternative hypothesis for
Late Pleistocene population bottlenecks and releases. The last glacial
period was preceded by one thousand years of the coldest temperatures
of the Later Pleistocene (similar to 71-70 ka), apparently caused by
the eruption of Toba, Sumatra. Toba was the largest known explosive
eruption of the Quaternary. Toba's volcanic winter could have decimated
most modern human populations, especially outside of isolated tropical
refugia. Release from the bottleneck could have occurred either at the
end of this hypercold phase, or 10,000 years later, at the transition
from cold oxygen isotope stage 4 to warmer stage 3. The largest
populations surviving through the bottleneck should have been found in
the largest tropical refugia, and thus in equatorial Africa. High
genetic diversity in modern Africans may thus reflect a less severe
bottleneck rather than earlier population growth. Volcanic winter may
have reduced populations to levels low enough for founder effects,
genetic draft and local adaptations to produce rapid population
differentiation. If Toba caused the bottle necks, then modem human
races may have differentiated abruptly, only 70 thousand years ago. (C)
1998 Academic Press Limited.


M.G. Bjornerud: Superimposed deformation in seconds: breccias from the
impact structure at Kentland, Indiana (USA). TECTONOPHYSICS, 1998,
Vol.290, No.3-4, pp.259-269


Breccias from the central uplift at the Kentland, Indiana impact
structure have outcrop and microscopic characteristics that give
insight into events that may occur in a carbonate-dominated sedimentary
sequence in the moments following hypervelocity impact. Three distinct
types of brecciated rock bodies - fault breccias, breccia lenses, and
breccia dikes - suggest multiple mechanisms of fragmentation. The fault
breccias occur along steeply dipping faults that coincide with
compositional discontinuities in the stratigraphic succession. The
breccia lenses and dikes are less localized in occurrence and show no
systematic spatial distribution or orientation. The fault breccias and
breccia lenses show no consistent cross-cutting relationships, but both
are transected by the breccia dikes. Textural analysis reveals
significant differences in particle size distributions for the
different breccias. The fault breccias are typically monomict, coarsest
and least uniform in grain size, and yield the highest power-law
exponent (fractal dimension) in plots of particle size vs. frequency.
The polymict dike filling is finest and most uniform in grain size, has
the lowest power-law exponent, and is locally laminated and
size-sorted. SEM images of the dike-filling breccia show that
fragmentation occurred to the scale of microns. Material within the
breccia lenses has textural characteristics intermediate between the
other two types, but the irregular morphology of these bodies suggests
a mechanism of formation different from that of either of the other
breccia categories. The breccia lenses and dikes both have sub-mm-scale
spheroidal vugs that may have been formed by carbon dioxide bubbles
released during sudden devolatilization of the carbonate country rock.
Collectively, these observations shed light on the processes that occur
during the excavation and modification phases of crater formation in
carbonate strata - heterogeneous, polyphase, multiscale deformation
accomplished over a time interval of seconds. (C) 1998 Elsevier Science
B.V. All rights reserved.

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