[meteorite-list] Eight Billion 'Dark Asteroids' May Lurk in Oort Cloud

[Ron Baalke]

http://news.sciencemag.org/space/2014/12/eight-billion-dark-asteroids-may-lurk-oort-cloud

Eight billion 'dark asteroids' may lurk in Oort cloud
By Sid Perkins
Science Magazine
4 December 2014 

Our solar system's asteroid belt, which lies between Mars and Jupiter, 
may contain a few hundred thousand objects. But much farther away, in 
regions long presumed to be the realm of comets and other icy bodies, 
there could be billions of rocky orbs circling the sun, a new study suggests. 
Researchers used computer programs to simulate the fate of objects circling 
our young sun once its planetary disk was largely cleared of gas and dust. 
Gravitational interactions with planets over the subsequent 4.5 billion 
years caused some objects to crash into the sun and others to be flung 
out of the solar system altogether. But many of the objects were cast 
into exile in the Oort cloud, a spherical haze of objects that stretches 
far beyond Neptune and a good fraction of the way toward our nearest stellar 
neighbors. (The image above depicts the Oort cloud as compared with the 
solar system and the much nearer Kuiper belt of objects.) Of those deportees, 
about 4% came from within about 375 million kilometers of the sun, rendering 
them rock- or metal-rich bodies like asteroids rather than icy orbs like 
comets, the researchers report online ahead of print in the Monthly Notices 
of the Royal Astronomical Society. Previous observations suggest that 
the Oort cloud contains about 200 billion comets, the researchers note. 
If that's correct, the new results suggest that those comets are accompanied 
by about 8 billion asteroids. If one of those objects ever fell toward 
Earth, it would be tougher to spot than a comet (being much darker) and 
more difficult to divert than the typical near-Earth asteroid (as it would 
be traveling much faster). Don't fear, though: The team estimates that 
a planet-killing collision with such an object might happen only once 
every billion years or so.