E.17.3 I heard that an asteroid was going to hit the Earth?!

These such questions typically occur after a news report of a future
close encounter between the Earth and an asteroid.  To date, all such
reports have resulted from (1) Astronomers did not yet know well
enough the orbit of a newly-discovered asteroid to say with any
certainty that it would not hit the Earth; (2) Reporters not checking
their stories or misunderstanding what they were told; or (3) both.

Objects that can potentially come close to the Earth are referred to
as Near-Earth Objects (NEOs).  The International Astronomical Union
maintains lists of such objects. About 100 asteroids are classified as
"Potentially Hazardous Asteroids" (PHAs), at
<URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html>; they all
have a projected closest distance to Earth of less than 0.05 AU (7.5
million km).  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>.  At the
moment, NONE of these encounters is thought to pose a serious risk.

The "potential hazard" of PHAs lies in their orbits and the
perturbations on those orbits from the planets and the Moon currently
not being known with sufficient accuracy to completely exclude the
possibility of a collision, but, generally, labeling these asteroids
as PHAs is erring on the side of extreme caution.  It is not worth
losing any sleep over them.