https://www.newscientist.com/article/dn28671-needle-formation-lets-space-rubble-sneak-up-on-us/
Needle formation lets space rubble sneak up on us New Scientist December 15 2015 Look out below! Large, rocky bodies may crash to Earth more frequently than we knew - by rearranging their formation. There have been a number of showstopping meteor strikes in recent memory, like the one that exploded over Chelyabinsk, Russia, in 2013. But it was thought that unless the space rocks were at least 50 metres across, or made of very hard material, they would break up and explode as they passed through the atmosphere, never reaching the ground. But that doesn't explain why observations of Venus from the Magellan spacecraft in the 1990s showed many more craters than expected, or events like one in 2007 near Carancas in Peru (pictured above), when an impact left a crater but not the expected hard meteorites or visible fireball. Peter Schultz of Brown University in Rhode Island and his colleagues now suggest an alternative. They say rocks that enter the atmosphere as loosely bound piles of rubble, or that break up in the atmosphere, can rearrange into a needle-like formation, guided by the shock wave they cause by coming in faster than the speed of sound. Sliding through This allows the rocks to pass through the atmosphere with much less resistance, and without the fireball that is the signature of a disintegrating cloud of debris. Schultz simulated this process in experiments at the NASA Ames Vertical Gun Range by breaking up projectiles into a cloud of debris and tracking the clouds path. He presented the results at the American Geophysical Union meeting in San Francisco on 14 December. This affects how dangerous space rocks could be, Schultz says. "We didn't think we were at risk from objects less than 10 metres across actually hitting the ground." Such impacts have seldom been reported so far, but that's not actually surprising, he adds. Over human history, most rocks probably landed in the ocean or in uninhabited areas. "The population explosion has made it much more likely that people would see an impact, but that's only happened in the last few hundred years," he says."Life's just been too short." Reference: https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/80236 ______________________________________________ Visit our Facebook page https://www.facebook.com/meteoritecentral and the Archives at http://www.meteorite-list-archives.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com https://pairlist3.pair.net/mailman/listinfo/meteorite-list
