Author: Joseph Lazio <jlazio@patriot.net>

The Earth is constantly pelted by bits of cosmic debris.  Most of this
simply burns up in the atmosphere (as one can attest by simply
watching meteors on a dark night).  However, if an object is big
enough it can survive passage through the atmosphere.  The damage done
by a meteorite (an object that strikes the Earth) depends upon its
initial size.

10--100 m: Objects in this size range can produce devastation similar
to that of an atomic blast (leading to them occasionally being called
"city-busters").  Effects include severe damage to or collapse of
standing buildings and the ignition of flammable materials leading to
widespread fires.  The radius over which such effects occur would vary
depending upon the size and composition of the object, but could
easily exceed 10 km.  The Tunguska event, in Siberia, of 1908 is
thought to have been from an object about 60 m in size; it led to
trees being flattened out to 20 km and trees 40 km away being damaged.

At the small end of this size range, objects about 10 m strike the
Earth about once a decade.  Fortunately, only the densest objects,
those containing iron, survive to the surface; most of the objects of
this size explode sufficiently high in the atmosphere that there are
no effects (other than maybe a loud noise) on the ground.  At the
larger end of this size range, it is estimated that the Earth is
struck several times a millennium or about 1 impact every 100--200 yr.

100 m--1 km: Objects in this size range are likely to cause severe
damage over a regional area, possibly as large as a continent (hence
the name "continent-busters").  If they strike land, they will almost
certainly produce a crater, while an ocean impact will generate large
tidal waves.  A 150 m object might produce a crater 3 km in diameter,
an ejecta blanket 10 km in diameter, and a zone of destruction
extending much farther out.  For a 1 km impactor the zone of
destruction might reasonably extend to cover countries.  The death
toll could be in the tens to hundreds of millions.  A 1 km impactor
could begin to have minor global consequences, including global
cooling caused by vast amounts of dust in the atmosphere.

Estimates from the geologic record suggest that craters are formed on
the Earth roughly once every 5000 yr.

1--10 km: Objects in this size range are likely to cause severe global
effects ("species-busters").  An impact 65 million years ago by an
object of 5--10 km in diameter is thought to have been partially or
fully responsible for the extinction of half the living species of
animals and plants at the time, including the dinosaurs.  The crater
alone from such an impact will be 10--15 times larger than the object
itself.  World-wide crop failures from dust injected into the
atmosphere could imperil civilization, and the largest-sized objects
could make the human species extinct.

The frequency with which the Earth is struck by such objects has to be
estimated from the geological and paleontological record.  At the low
end of this size range, estimates are that such impacts occur roughly
every 300 000 yr; at the upper end of the size range, impacts occur
about every 10 million years.