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Meteorite fractures and scaling for asteroid atmospheric entry

Kathryn Bryson
Asteroid Population Characterization
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Abstract Text: 

We are attempting to understand the behavior of asteroids entering the atmosphere and quantify the impact hazard. Arguably the major difficulty faced to model the atmospheric behavior of objects entering the atmosphere is that we know very little about the internal structure of these objects and their methods of fragmentation during fall. In a study of over a thousand meteorite fragments (mostly hand-sized, some 40 or 50 cm across) in the collections of the Natural History Museums in Vienna and London, we identified six kinds of fracturing behavior. (1) Chondrites usually showed random fractures with no particular sensitivity to meteorite texture. Approximately 80% of these indicated no point of origin, while 20% show an origin. (2) Approximately 10% of the chondrites, have a distinct and strong network of fractures making an orthogonal (T-intersections) or triple intersection (Y-interesctions) structure. The Chelyabinsk meteorite has the triple intersection structure of fractures, which explains the very large number of centimeter-sized fragments that showered the Earth. (3) Fine irons with large crystal boundaries fragmented along the crystal boundaries. (4) Coarse irons fractured along kamacite grain boundaries, while other (5) fine irons fragmented randomly, c.f. chondrites. Finally, (6) CM chondrites showed that water-rich meteorites fracture around clasts. To scale the meteorite fractures to the fragmentation behavior of near-Earth asteroids, it has been suggested that the fracturing behavior follows a statistical prediction made in the 1930s, the Weibull distribution, where fractures are assumed to be randomly distributed through the target and the likelihood of encountering a fracture increases with distance. This results in a relationship: σl = σs(ns/nl)α, where σs and σl refers to stress in the small and large object and ns and nl refer to the number of cracks per unit volume of the small and large object. The value for α, the Weibull coefficient, is unclear. A relationship exists between the distributions of measured trace length and actual fracture size, where the slope of a log-log plot of trace length versus fracture density is proportional to α. Our ultimate objective is to determine a scaling factor for fracture parameters in meteorites to better understand asteroids entering the atmosphere.

"H. Kent Hills, Code 690.1/NSSDCA, Adnet Systems Inc., U.S.A...
SSERVI Identifier: 

Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."