[meteorite-list] A simple question

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Jan 27 01:05:29 2005
Message-ID: <41F88486.C5D91D89_at_bhil.com>

G?ran Axelsson wrote:

> What I'm looking for is a simple number of meteorites
> per square km (mile, parsek, ...) and year that hits
> the Earth so I could do an approximation of meteorites
> per year in Sweden. -- G?ran

Goren, List,
    I posted a study on the fall rate on the List back in Dec. 2000.
Here's
some of the conclusions.
    Many years ago, Nininger estimated that 500 meteorites ranging
from
100 grams to 10 kilograms in mass fell on land each year
(approximately
2000 for the entire Earth). His estimate was based on his field
experience.
More recently, Canada's Meteor Observation and Recovery Project
(MORP) estimated 23,930 meteorites per year as the worldwide fall
rate, using radar detection of fireballs. The Phil Bland study (1996)
put
it at 18,000 to 84,000 per year, using another methodology based on
the find rate vs. the weathering rate of the find area. My study was
based on how often meteorites hit houses, ships, cars and trucks!
(Hey,
there a fixed and measurable target area and very well distributed,
plus
people notice when a space rock hits their wheels or their dwelling!)
    It's interesting that such varying methodologies end up with the
similarity of result that they do.
    Taking the area of the Earth to be 510,000,000 sq km and the
meteorite flux to be 23,930 per yr, this yields the assumed
collisional
cross section of the earth to be 21,360 sq km per yr. This rate means
that
one meteorite per year falls on an area of 21,320 square kilometers.
The
inverse function of this value is how long we have to wait for a
meteorite to fall on a standard area, or the mean time to impact:
21,360
yr per sq km. To put this flux into perspective, if you owned a house
with a
half-acre yard, you would have to wait 10,552,000 years for a
meteorite
to fall in your front or back yard or on your roof! (On average, that
is; it could happen tomorrow.)
    However, calculating from impacts on U.S. automobiles (a fixed
target area well distributed) is 250% of expectation, again
suggesting a flux of 59,825 meteorites per year, or a terrestrial
collisional cross section of 8528 sq km per yr.
     My analysis suggests that the actual meteoritic flux is much
greater than what is currently assumed (23,930 meteorites per year).
The
data implies a better fit with a meteoritic flux of 60,000 to 100,000
meteorites per year at a minimum.
    In the case of Sweden, if you assumed every meteorite that fell
was still lying in place undisturbed since the ice went away 10,000
years ago, there would have accumulated about one meteorite per
square kilometer.
    To calculate, the number of meteorites that fall on Sweden each
year, divide the area of the nation in sq km by 8500 (corresponds
to a fall rate of about 60,000 per year for the planet). Adjust
appropriately if you believe in higher or lower fall rates.
    The same calculation would apply to any northern hemisphere
area glaciated. Equatorial deserts (NWA) have accumulated
meteorites for much longer (but the NWA area was a
grassland with forest clumps and rivers during the ice age, so
preservation was not as good as today).
    Rob Matson thinks the fall rate is even greater than I do (he said

120,000 to 150,000 per year for the planet).


Sterling K. Webb
Received on Thu 27 Jan 2005 01:04:54 AM PST