CompuServe Messages

Hi Doug, The answer to your question regarding the size of the meteorite that might be responsible for cratering of the scale seen at Bavaria …is…not…trivial. And, I can't pretend to be up on the crater-formation kinetics of meteorite impacts. As I've limited resources at arm's reach to answer your question, I should just answer " I dunno." (This is a bit longwinded and it's all borrowed, so here it goes…) So as to have some idea of the phenomenon of crater formation, I'd gone back to O. Richard Norton's neat and very readable book ROCKS FROM SPACE and the Sikhote Alin fall of 1947. The largest crater, one of over 100 craters, was 87 feet diameter by 20 feet deep. No more than 154 lbs of remnants were collected in that crater. A crater half that size yielded 232 fragments totaling 391 lbs. However, a two ton meteorite was recovered in '51 out of a crater with a diameter of only 11.5 feet! More than 25 tons have been recovered from that fall, and it's felt that another 50 tons may still be buried out there. From Philip Bagnall's THE METEORITE & TEKTITE Collector's Handbook, there's a cursory discussion about crater impact dynamics. Paraphrasing some of the contents: Most meteorites enter the atmosphere usually at less than 40 km/sec, the slowest perhaps 12 km/s. Compare that to Shoemaker-Levy 6's approx 20 km/sec before it impacted Jupiter. For meterites from within the solar system, the max velocity (when we hit a meteorite head-on) is felt to be the sum of the Earth's orbital velocity and that of a meteoroid, or about 72 km/sec. The atmosphere decelerates the meteorite depending on mass and density, the velocity, and the angle of inclination. Frictional heating starts an ablation process that may result in significant mass loss, depending on the integrity of the meteoritic material. eventually, most meteoroids lose their orbital velocity and attain a retardation speed, at which point, ablation and ionization stops and most meteorites just darken and free-fall without further acceleration. High velocity meteorites are slowed more intensely than low velocity meteorites. The typical reduction in velocity can be from the 40-70 km/sec to as slow as 100 m/sec. Many large meteorites are destroyed high up in the atmosphere. Between '75 and '92, U.S. military satellites detected 136 high atmospheric explosions, averaging 8/year. Three of theses blasts were equivalent to a 20 kiloton nuclear blast, several from meteorites several yards in length. Those were likely stony or stony-iron meteorites. If a solid iron meteorite is of sufficient mass and velocity, it'd never reach it's retardation point and impact the ground, causing cratering. If the velocity of the meteorite is less than 500 m/s, then a percussion crater of relatively small diameter may form, often with an intact meteorite within.. If the velocity were beyond 500 m/s then the diameter rapidly increases. At 3-4 km/sec, pressure propagation into the ground and into the impacting meteorite itself may result in an explosion due to violen decompression with high velocity ejection of schrapnel as in the Sikhote Alin, or for bigger irons, vaporized and molten material jetting into ejecta blankets, as might be observed on the Moon with Tycho and Copernicus craters, where there was absolutely no atmospheric retardation.. The mile diameter Barringer crater in Arizona, 600 feet deep, is felt to've been caused by an iron meteorite about 63000 tons and about the 80 feet in diameter travelling at about 17 km/s at 30 degrees incline. So,…I still dunno! Joseph.