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

From: Joe Jefferson (jjstrshp_at_mindspring.com)
Date: 10/22/04 

Date: Fri, 22 Oct 2004 19:20:44 GMT

Eric Stevens wrote:
>
> On Fri, 22 Oct 2004 01:50:31 GMT, Joe Jefferson
> <jjstrshp@mindspring.com> wrote:
>
> >Eric Stevens wrote:
> >>
> >> It
> >> certainly appears as though there should have been a Tunguska size
> >> impact about once every century with even more of smaller bolides.
> >
> >Okay, we'll take your figure of once a century. That's within the same
> >degree of magnitude as the figure I've seen. Going back to the NASA web
> >site, I see that the explosion flattened trees within about a 40km
> >radius, so call it 5,026 square kilometers. According to my home atlas,
> >the total surface of the Earth is 512,175,090 square kilometers. So the
> >critical missing factors are, first the average percentage of the
> >Earth's total surface that was inhabited during the time period you're
> >interested in, and second the percentage of those inhabited regions
> >about which we know enough to be able to tell whether or not they were
> >affected by an impact event. My gut feeling is that neither of these is
> >very large for most of human history, but it's not my gut that matters
> >here. What percentages were you using when you concluded that
> >statistically there should have been more significant impacts than
> >archaeologists and/or historians have believed?
>
> I think you are trying to oversimplify the problem. Lets use the
> figure 1-1 I referred to above. If we assume the smallest noticeable
> impact is a mere (?) 10 kilotons (I mean most people would notice a 10
> kiloton explosion in their neighborhood) you read from the graph that
> there should be about 8 to 10 per year.

People might notice a bolide of that size, but it wouldn't be likely to
have much of an effect on their lives. They'd see a bright flash high up
in the air, or if they happened to be looking in just the right
direction they might see a fireball cross the sky and disappear in a
bright flash. Maybe, MAYBE, something nearby might be hit be a small
piece of debris. In the modern era there might be a story about it in
the local newspaper. As you said, these events occur fairly commonly.
They do not do anything that could be considered historically significant.

> That is say 27,000 in the last
> three millennium. If you go to the say 100 kilotons you get about one
> every 3 years. Say 1000 in the last 3000 years. At the 1 megaton level
> we get an impact about once a decade. Say 300 in the last 3000 years.
> Tunguska is about once per century - say 30 in the last 3000 years.

Your numbers are too high by a factor of three, probably because the
chart you're looking at is poorly designed. Try the NASA diagram at
http://liftoff.msfc.nasa.gov/academy/space/solarsystem/meteors/ImpactHazard.html
and the trend is much clearer. For every tenfold increase in explosive
power, there is a tenfold decrease in frequency, starting with a 10 kt
blast about once a year.

> What I have done is so crude that it doesn't even amount to Simpson's
> rule but if you integrate the probability curve between the 10 kiloton
> and Tunguska range you end up with (have a guess) 60,000 impacts in
> the last 3000 years.

Integration is the wrong procedure. You can't treat these numbers as a
continuum; that's just an artifact in the charts to make the trend more
obvious. (The observed frequency of events similar in size to Tunguska
over the past century should make this very obvious.) You'll get a much
better approximation by turning the chart into a histogram with the bars
each covering a factor of ten, then simply adding together the ones big
enough to be interesting.

> Not all of them are going to do damage and it depends upon whether
> they are predominantly icy, rocky, or iron. This will determine
> whether they will end up as an air blast (Tunguska), a shower of
> stones (China 500 years ago) or a hole in the ground. But leaving that
> out, if the impacts are evenly distributed (which they will not be) it
> gives rise to one impact per 8536 sq km. In terms of circular area,
> that is one impact per 100 km diameter circle. Now, if the blast area
> covers only a 20 km radius (one quarter of the area of Tunguska), that
> means that 4% of the earth's surface has been within the blast zone of
> an impact within the last 3000 years.

You're enormously overestimating the average blast radius. For anything
less that a 1 megaton event, the blast radius at the Earth's surface is
likely to be zero; the explosion occurs so high in the atmosphere that
the blast doesn't reach the surface at all. Remember that we do have
pretty good data on damage caused by meteor impacts for at least the
past 100 years, and any estimate of past effects needs to take that into account.

> In fact the affected area is much larger than that in which trees are
> laid flat. It is likely that on average every one of those 100 km
> diameter circles will have contained an impact sufficiently close to
> the people (if any) living there to make them realise that something
> rather drastic has happened.

Okay, now I see where you're coming from. But both the frequency of
events and the average blast radius you're assuming are very much higher
than can be justified by the observed rate of damaging meteor impacts in
modern times.

> My suspicion is that most people would have so little understanding of
> what had happened that they could not describe it in terms which are
> comprehensible today. In fact, because the ideas of such disasters are
> beyond the knowledge of most of us, I strongly suspect that some of
> their stories have come down to us even today in a form which we
> presently cannot readily recognise. The question is whether or not
> some of the physical evidence is unrecognisable to us for much the
> same reason.

What do you base that suspicion on? Ancient peoples were able to
describe volcanoes as big fiery explosions coming from the ground. Why
do they think they couldn't just as easily describe a big fiery
explosion in the sky?

> By the way, talking of disasters, would you care to have a crack at
> the etymology of 'disaster'? :-)

It comes from Greek astrology by way of Latin; loosely meaning "opposed
by the stars". It can be compared with Shakespeare's "star-crossed lovers". 

-- 
Joe of Castle Jefferson
http://www.mindspring.com/~jjstrshp
Site Updated November 25th, 2001
"Defend the cause of the weak and fatherless; maintain the rights of the
poor and oppressed. Rescue the weak and needy; deliver them from the
hand of the wicked." - Psalm 82:3-4