[meteorite-list] Needle Formation Allows Small Asteroids to Impact Earth

[Ron Baalke]

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 cloud’s 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