PLEASE NOTE:
*
CCNet DIGEST, 7 August 1998
---------------------------
(1) TOUTATIS IN 1992
Gareth V. Williams <graff@cfa0.harvard.edu>
(2) SPACEGUARD AUSTRALIA
Michael Paine <mpaine@tpgi.com.au>
(3) WEAK IMPACT: THE 1998 PERSEID METEOR SHOWER
Linda Porter <lindaporter@sslab.msfc.nasa.gov>
http://science.nasa.gov/newhome/headlines/ast05aug98_1.htm
(4) ABRUPT CLIMATE CHANGE & PUNCTUATED HUMAN EVOLUTION
S.H. Ambrose, UNIVERSITY OF ILLINOIS
(5) DEFORMATION ACCOMPLISHED IN SECONDS: BRECCIAS FROM THE
KENTLAND
IMPACT CRATER
M.G. Bjornerud, LAWRENCE UNIVERSITY
==================
(1) TOUTATIS IN 1992
From Gareth V. Williams <graff@cfa0.harvard.edu>
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.
=================
(2) SPACEGUARD AUSTRALIA
From Michael Paine <mpaine@tpgi.com.au>
During 1997 I started an unofficial Spaceguard Australia website
http://www1.tpgi.com.au/users/tps-seti/spacegd.html
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
http://www1.tpgi.com.au/users/tps-seti/sg_prop.html
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: http://www1.tpgi.com.au/users/tps-seti/spacegd7.html
Michael Paine 7 August 1998
======================
(3) WEAK IMPACT: THE 1998 PERSEID METEOR SHOWER
From Linda Porter <lindaporter@sslab.msfc.nasa.gov>
http://science.nasa.gov/newhome/headlines/ast05aug98_1.htm
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:
http://science.nasa.gov/newhome/headlines/ast05aug98_1.htm
===================
(4) ABRUPT CLIMATE CHANGE & PUNCTUATED HUMAN EVOLUTION
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
UNIVERSITY OF ILLINOIS, DEPT ANTHROPOL, 109 DAVENPORT HALL, 607 S
MATHEWS AVE, URBANA, IL, 61801
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.
==================
(5) DEFORMATION ACCOMPLISHED IN SECONDS: BRECCIAS FROM THE
KENTLAND
IMPACT CRATER
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
LAWRENCE UNIVERSITY, DEPT GEOL, APPLETON, WI, 54912
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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