Göran 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. -- Göran
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 ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list