Re: Meteoric and Cometary impacts in historical times - Hard Evidence

From: Eric Stevens (eric.stevens_at_sum.co.nz)
Date: 10/26/04 

Date: Tue, 26 Oct 2004 21:55:33 +1300

On Mon, 25 Oct 2004 17:02:27 GMT, Joe Jefferson
<jjstrshp@mindspring.com> wrote:

>> The estimate of the number of
>> potential impactors has increased exponentially since then. Right now
>> they are arguing about whether there is even a larger number of
>> virtually indetectable dark bodies out there which will increase the
>> numbers even again. This question won't really be answered until after
>> NASA launches its orbiting infrared telescope called the Wide-field
>> Infrared Survey Explorer (WISE) in 2008.
>
>A first approximation of the answer can be had be counting the number of
>impacts large enough to do significant damage within the past 50 years -
>the time frame in which large meteors hitting anywhere in the northern
>hemisphere should have been detected by the radar networks set up to
>detect nuclear missiles.

At this point I decided to split the thread as I believe it is
necessary to explain at this point what appears to be the emerging
view on impacts. It is essential to understand this to make sense of
the statistics. I'm going to make use of the 'seminal' book on the
subject 'The Cosmic Serpent' by Victor Clube and Bill Napier. Although
this was published in 1982 all that the major advances have since done
is sharpen up the numbers. If anyone has a copy the essential chart is
on page 142.

There are five types of impactors:

1. Fireballs.

2. Meteorites.

3. Earth-crossing asteroids.

4. Main-belt asteroids.

5. Comets.

All of these originate outside the solar system and principally differ
only in their history. There is convincing evidence that they all
started off as extra-solar comets which have degraded in various
fashions for various reasons. That they are classified as they are is
dependent on their history of interaction with the sun and planets, of
which last Jupiter is the major example. With very few exceptions even
the largest asteroid is likely to be the hulk of a burned out comet.

The composition of comet nucleii is extremely variable. Some are dirty
snowballs consisting only of ices and dust. Others are aggregations of
stones with, in many occasions, the odd large boulder. Others again
consist of a few large lumps bound together by their mutual gravity.

The rate of attrition of these bodies is such that their stock must be
being topped up by one or more mechanisms. The Oort belt is a popular
explanation but first it has never been convincingly demonstrated to
exist and second, even if it exists it cannot have contained as many
bodies as is required to explain the evident impact history of the
various planets.

Clube and Napier's explanation is that the solar system's stock of
cometary bodies was last topped up when the solar system passed
through the 'Gould Belt' which is one of the spiral arms of the
galaxy. Tyheory which can be confirmed by observation and arithmetic
is that the increased stock of comets which accompanied this event
should have led and did lead to an increased rate of planetary
bombardment which since then has slowly tapered off as the stock of
comets has been exhausted.

However - from time to time a giant comet makes its way from the outer
regions into the inner regions. Such comets are made of - whatever -
and progressively disintegrate after passages past the sun and
jupiter. They break into smaller comets which fragment further in
turn. The principal mechanisms of fragmentation appears to be either
gravitation of planets or the sun, or the forces arising from the
evaporation of ices.

It is self evident that when a comet - giant or otherwise - breaks up,
all the various parts end up travelling on very slightly different
orbits, each one of which has its own orbital period. Although all of
these various bits and pieces end up coming back to much the same
point in space, they don't all arrive simultaneously. The end result
is that the original orbit is filled with a circulating stream of
rubble.

Not all of the rubble can have the same orbital history. e.g. if
Jupiter passes near the orbit it will disturb the items of those parts
which happen to be nearby but leave untouched the items on the other
side of the orbit. This means that from time to time various lumps can
be thrown out of the main stream to take up an entirely different
fresh orbit. If they are lucky they can finish up in a nice stable
orbit well clear of other planets e.g main belt asteroids.
Alternatively, they can settle down to an earth crossing orbit which
virtually guarantees that one day we will meet.

Without going into all of the details, the chance of the earth meetin
a particular type of object depends on where it is. The evidence
appears to be that the earth is emerging from a period of bombardment
arising from the break up of the giant comet Encke. The core of the
comet is still slightly active but is likely to end up as an asteroid
within the next 100 years.

In the mean time the earth is encountering an ever diminishing stream
of rubble in the original orbit of the comet. We know the main bulk of
these as the Taurid meteor stream. We are lucky now in that we no
longer intersect the main Encke orbit and all we see is the fringe
fireworks. It is likely that 3000 to 5000 years ago the bombardments
werevery much worse and gave rise to the descriptions of Hesiod and
Lucretius which I have quoted in another article. It is probable that
they will become much worse again in about another 1500 years.

The point is that it does not appear to be safe to extrapolate back
into pre-history on the basis of the last 1000 years. To do so is like
denying ice ages on the basis of the last 8000 years (or to deny the
Roman warm period on the basis of the last 800 years). The rate of
bombardment was very much higher in the bronze age than it is now.

The most relevant events from a historical and archaeological point of
view are those described in the following quotatoin from the 'The
Cosmic Serpent':

Begin quote (spelling by Textbridge) :
+++++++++++++++++++++++++++++++++++++++++++++==
There is one further, and very significant, class of missile.
Occasionally a meteor is so bright that it lights up the landscape.
Most probably this will belong to the class of objects known as
fireballs. A large body of data on these objects has been accumulated
through research programs such as the Prairie, Canadian and European
networks, using all-sky cameras to record meteoric events. Operation
of these networks began around 1964 with a search area of about 2
million square km. From their atmospheric deceleration and
fragmentation it is possible to deduce the physical properties of the
fireballs, which normally have end heights of 50—60 km. It turns out
that these objects are very fragile and of low density, indicative of
cometary material. Some have had high entry velocities and appear to
have come from beyond Jupiter or even to have been in retrograde
orbits before collision. The largest Prairie Network fireball recorded
had a mass of about 3.5 tons; the Sumava fireball recorded in December
1974 by the European Network had a mass of about 200 tons. Mass for
mass, the arrival rate of fireballs is about a hundred times as great
as that of meteorites (Figure 17). They are much too abundant to be a
low-mass extension ofthe active comets in the Earth's neighbourhood.
The Earth is thus encountering a considerable population of
interplanetary boulders, quite distinct from stony or iron bodies, and
which cannot be directly related to the telescopic asteroids or
comets. This population is dominant amongst the smaller projectiles
entering the atmosphere, and the question arises: what is the
frequency of larger masses amongst them? For somewhere in the range of
1 ton (from the larger Prairie Network fireballs) and 10 billion tons
(from the telescopic missiles) there must occur a turnover in their
mass distribution, as otherwise one would predict far more
kilometre-sized bodies in circum-terrestrial space than are observed.
Once again, information in the ~gap' is provided by a single
exceptional collision.

The Tunguska object may have had a mass of500,000-l ,000,000 tons
(diameter about 100 metres) and an impact speed of about 30 km/sec.
These figures have been reconstructed from the topography of the
flattened and burned forest, and from the accounts of dozens of
eyewitnesses, most of whom were hundreds of kilometres from the impact
site. The occurrence of this 40-100 megaton event as recently as 1908
is consistent neither with its being a small asteroid nor with its
being a small active comet. Extrapolation of the known fireball mass
distribution indicates that Tunguska-like land impacts should occur as
often as once in fifty years, and this is entirely consistent with
observation of one such event in the twentieth century. Indeed
fireballs intermediate between Tunguska and the Prairie Network
objects have been recorded. The ~umava fireball of 1974 has been
mentioned. One over Holland in 1958, with an end height of 45 km.
appeared almost as bright as the Sun. The Montana fireball of August
1972 grazed the Earth's atmosphere and escaped into space again. The
frequencies of these occasional events are consistent with an
extension of the Prairie Network fireballs into the Tunguska range,
and so the mass distribution appears to hold at least as far as
million-ton objects. But somewhere between 0.1 km and 1 km diameter
there must be a cut-off in these large numbers of fireballs. The
corresponding uncertainty in impact energies is very large— somewhere
between 100 and 100,000 megatons.

++++++++++++++++++++++++++++++++++++++++++++++
End quote.

Note that based on hard observational data "Extrapolation of the known
fireball mass distribution indicates that Tunguska-like land impacts
should occur as often as once in fifty years ... "

So, I ask again, where are the records of these events in history?

Eric Stevens

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