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The Earth has been getting hit by asteroids and comets for its whole
life. The planets formed from collisions of smaller objects, and even
our water may have come largely from comets. Heavy bombardments may have
continued until as recently as 4 billion years ago, making it difficult
for life to get a foothold at all.
WHAT ARE THE ODDS TODAY
are a lot more small rocks than big ones. So while Earth is constantly
being "hit" -- accumulating over 100 tons of matter ever day -- most of
this is in the form of dust or tiny sand-grain sized meteors that appear
as shooting stars. By comparison, school bus-sized asteroids may hit
every thousand years or so, medium sized (say 300 meter) asteroids might
be once every 50,000 years, and extinction level events only every
billion years. And those estimates are going down.
HOW BIG IS BIG?
In reality it could be very, very small compared to Earth. The "dino
killer" is estimated to have been between 5 and 15 kilometers across (3
to 10 miles). That's large compared to the rocks in your backyard, but
tiny compared to Earth.
To lead to a global catastrophe, an asteroid
or comet only has to be big enough to launch large amounts of dust in to
the atmosphere. That leads to the abrupt change in climate that wipes
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