Much of the material in this section is drawn from the SpaceGuard
Survey report, <URL:http://ccf.arc.nasa.gov/sst/spaceguard.html>.  

A crucial point about asteroid impacts is that they are random.  Below
are various estimates of the frequency with which the Earth is struck
by objects of various sizes.  These estimates are, roughly speaking,
averages over the Earth's history.  For instance, the average time
between the impact of a 100 m diameter object is roughly 100--200 yr.
The actual time between the impacts of such objects could be shorter
than 10 yr or longer than 1000 yr.

For more information about Near-Earth Objects, those asteroids (or
minor planets) that have orbits similar to Earth's, see the following.
A list of "Potentially Hazardous Asteroids" (PHAs) is at
<URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html>.  These have
a projected closest distance to Earth of less than 0.05 AU (7.5
million km, about 1000 Earth radii).  A list of closest approaches to
the Earth by PHAs between 1999 and 2099 is available at
<URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html>.  A list
of moderately close (to within 0.2 AU) approaches to the Earth by
asteroids and comets between 1999 and 2032 is available at
<URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html>.  It is worth
emphasizing that, at the moment, *none* of the known objects presents
a serious risk of collision.