From: L' Ermit (lhermit@hotmail.com)
Date: Wed Jan 09 2002 - 02:23:01 MST
Blunderov asked:
The last big asteroid hit, it is thought, took place about 65 million years
ago and precipitated the end of the dinosaur dynasty, birds excepted.
There have been about 4 or 5 big hits on the earth that we know of, at least
one was more catastrophic than the event which took out the dinosaurs.
I have a question for the astronomers and statisticians of CoV:aren't we
about due for another big hit sometime soon? What are the odds? How "soon"
is soon in geological time?
====
Hermit responds:
Good questions.
"RE: virus: Space Race?", Hermit, Wed 2001-04-18 03:06"
<quote>
Hermit 2] Excluding the turkey (which shares most of its DNA with the later
dinosaurs) (and other birds of course, but the turkey seems somehow
symbolic), there is not much sign of the immense wealth of life that made up
the Jurassic age. So I suspect that if you could dig up a diplodocus that it
might disagree with you. How many phenotypes do you think survived the
Quaternary boundary event? More than 10%? If so, we could always look to the
Cambrian seas and wonder whatever happened to the trilobites and
ammonites... and untold millions of others. Life on Earth came to a
catastrophic halt at least four times in the Cambrian era alone. Working
from memory, there have been Vendian, Precambrian, Cambrian, Ordovician,
Devonian, Permian, End Cretacious and Holocene (or Quaternary) mass
extinctions - in fact, the paleontological era are punctuated by the
termination of vast numbers of species. While we might like to think that we
are somehow charmed, I would argue that an actuary might be forced to
disagree. Statistics prove that lightning does strike the same places on a
fairly regular basis. Common sense makes one think of insurance for things
of value. It is only for things which have no value that insurance becomes
superfluous. I rather like to think that mankind, some of mankind anyway,
has value.
</quote>
Investigation (cores of deposition layers of off-planet sourced heavy
metals) shows that the Quaternary boundary event (c 65 Myears BCE) was
almost certainly caused by a meteorite colliding with Earth and causing a
climatic catastrophe, as was the catastrophic reduction of our ancestors to
a population of 100-10 000 at c 75 kyears BCE.
One of the best papers on the topic is "Near Earth Objects", Prof. Mark
Bailey, which can be found at http://www.arm.ac.uk/paseg/ accessed
2002-01-09. The following quotes, unless attributed elsewhere are from that
paper.
<quote>
In recent years, many studies have addressed the issue of the actuarial risk
- from several perspectives. Notable, in my view, are the contributions by
G. Canavan (Hazards book, 1994) and N. Holloway (RGO Spaceguard Meeting,
Cambridge 1997). Despite the scientific uncertainty, there is notable
agreement that the potential threat to civilization - while a very rare
event - is high in relation to hazards that Society routinely seems to care
about. The question, then, is what should be done?
</quote>
Near Earth Objects
<quote>
These objects are basically great lumps of rock, or rock-and-ice in the case
of comets. Their significance lies in their large numbers (recently
discovered) and the fact that many have orbits that, sooner or later, may
intersect that of the Earth. Their high velocities - tens of kilometres per
second, or many tens of thousands of miles per hour - and large sizes
(weighing billions of tonnes) mean that they carry vast amounts of kinetic
energy. A one-kilometre diameter asteroid, impacting at 20 km per second has
the kinetic energy equivalent of some 50 thousand Megatons of TNT.
</quote>
To put a potential impact into perspective (more in the references below
this letter and on the source site [supra]):
Object Diameter Result of Impact
50-100m 19-100Mt TNT Craters to 1km half-sphere - could destroy a megopolis
100-500m 100-10,000Mt TNT craters up to 5km half-sphere - land impact will
destroy an area the size of a state, sea impacts will cause sea scale
tsunamis
0.5 - 2km 10,000 to 500,000Mt TNT craters to 20km half-sphere, tsunami's
would be oceanic scale. Land impacts will destroy a small country -
significant global climatic effect expected
2km-10km craters 20km-100km half-sphere would have a global climatic effect
leading to global geological extinctions, the immediate death of perhaps 25%
of all humans and potentially if consequent climatic effects (Nuclear
Winter) are serious enough, all humans - although this not having happened
in some 65 million years is of lower - but far from non-existent
probability.
<quote>
Perhaps surprisingly, the best estimates of the mean collision rate of
comets and asteroids on the Earth are obtained by looking down, not up. The
cratering record now contains more than 155 known craters (by May 1999 some
authors gave the figure as 172), many of which (~70) have dimensions larger
than the canonical 10-kilometre size above which we would expect a
significant impact hazard to civilization.
The terrestrial cratering record indicates that roughly one
1-kilometre-sized object runs into the Earth every 100,000 years.
Looking upwards, there are still very large uncertainties, particularly
affecting the cometary component. Here, not only are cometary masses not
known to better than a factor of a few or so, but the proportion of
unobserved, "dark" long-period objects (LPOs) to long-period comets (LPCs)
is not known, nor are the corresponding ratios for Halley-type objects
(HTOs) to Halley-type comets (HTCs) and Jupiter-family objects (JFOs) to
Jupiter-family comets (JFCs). The result is a large uncertainty in the
predicted cometary collision rate with the Earth, that will only be resolved
by instigation of the proposed all-sky Spaceguard Survey.
</quote>
As the last significant impact was at approximately 70-75 kyears BCE, we are
probably due for a "big one" (0.5-10km) sometime in the next 25kyears, and
given the last medium impact was 100 years ago [Tungustan, 1908, infra], a
medium one (50-500m) sometime soon, but the wide variance, our lack of
knowledge but recognition that the sheer number of NEOs increases
dramatically the more we look, and the increasing number of "scares" (e.g.
1997,1999 and Walter's recent "close call" report) suggests that that there
is some high probability that there may well be far more danger than we
imagined even 10 years ago.
<quote>
note that the limit of intolerability is taken as one event per 100,000
years for 10,000 fatalities; the NEO impact hazard is just as frequent, but
would involve 10,000,000 fatalities for the UK alone... Viewed from this
perspective, there is no question that the NEO hazard is significant, or, to
put it another way: if NEOs were a business, NEO plc would not be allowed to
operate!
</quote>
While Bailey (the author of most of the above) argues simply for mitigation
and a spaceguard program, as is seen in his conclusion below, I have argued
(search the archives for "Virian Vision") that the only sure way to ensure
the ongoing development of civilization is to establish viable (i.e.
self-supporting) off-Earth colonies and have shown that this approach offers
significant near-term economic, ecological and social benefits.
<quote>
The outcome of this research into the interaction of astronomically "small"
bodies with planets has potentially the most profound implications for
mankind. We live at a special epoch: for the first time in the history of
life on Earth (3.5 billion years), the facts about impacts are at least
broadly understood, and a species has developed with not only the knowledge
but also the technical capacity to mitigate part of the risk.
The discovery of the impact hazard also provides a further example of
"spin-off" from a discipline whose activity these days is often justified in
purely cultural terms. Astronomy has its roots in culture, possibly even in
religion (e.g. Hoyle 1994: The Origin of the Universe and the Origin of
Religion), but from time to time (e.g. in navigation) the discipline has
great practical value.
Whereas curiosity killed the cat, it also confers a huge Darwinian
advantage. In this case, our species' natural curiosity may be our
salvation.
Applied Astronomy, including the exploitation of natural resources in space
(many of which reside in NEOs), is likely to see major growth in the 21st
century, while solving the NEO impact hazard should also count as a
significant economic benefit, measured in units of hundreds of £M per year.
...
Conclusion
In summary, despite continuing scientific uncertainty relating to the total
number, sizes and physical structure and composition of NEOs, there is broad
agreement that a baseline level of risk due to the NEO impact hazard has
been established.
Tunguska-size objects run into the Earth about once every one hundred years;
but the more significant kilometre-sized impactors, which run into the Earth
every 100,000 years or so, cause global devastation and carry the potential
to destroy billions of lives world-wide.
Current astronomical surveys have discovered less than 10% of the globally
most dangerous objects, and a much smaller proportion of comets and smaller
bodies. There is a clear need for further investigations into all aspects of
the impact hazard, including the interdisciplinary studies relating to
impacts on time-scales of thousands of years, comparable to that of the
development of civilization. The UK is currently well placed to play a
leading role in the Spaceguard Programme, both on the European scene and
world-wide.
</quote>
Should Virians develop a "preferred political platform," I would like to see
this risk addressed, both in terms of knowledge and in terms of off-planet
populations.
Regards
Hermit
PS Thanks for the on-list support Blunderov (and Citizen Jane), it is
neither unnoticed nor unappreciated.
For frequency and effect the following references are probably still
current:
["virus: Tungustan (Siberian) Meteorite", Hermit, Wed 2001-08-15 16:52]
Note that, rather than the 1920s, the Tunguska event took place at 7:15 June
30, 1908 (Lake Baykal time zone). There is a *lot* of Russian literature on
the event.
Here are a few links to sites that contain information that seems to me to
be "good".
http://www.totse.com/en/technology/space_astronomy_nasa/tungusk2.html {Read
first - good summary}
http://www.808.com/tunguska/ {Excellent scientific site by GF Plekhanov in
English and Russian}
http://www.psi.edu/projects/siberia/siberia.html
The appropriate subject journal, which may be available on Interlibrary loan
or on fiche is "Meteorite" by Pallasite Press, NZ. The relevant website is
located at http://www.meteor.co.nz. Their November issue will contain a
report on a conference dedicated to the Tungusta event.
On their site they have a link to a table of "Environmental Damage from
Asteroid and Comet Impacts" [Michael Paine, 2001-05-23] located at
http://www4.tpg.com.au/users/tps-seti/climate.htm which tends to confirm my
memories of yields and provides many excellent references. (To put it into
perspective, the Tungusta event is estimated to have had a yield equivalent
to a 15-megaton hydrogen bomb.)
They also provide a well researched table of links to other meteorite
related sites at http://www.meteor.co.nz/links.html
PS My memory of an 80m diameter for the Arizona Meteorite was based on
modeling done (at the Sutherland Observatory) during the 1980s. I don't have
a reference handy, but there was a lot published on size, energy and crater
size during the 1985-1995 period in Sky and Telescope and more than a few
articles in Nature. An LOC search on "bolide" and "diameter" will probably
lead you to relevant citations.
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