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Asteroid 1999 AN10 Has Remote Chance Of Colliding With Earth In 2044 or 2046

http://neo.jpl.nasa.gov/news/news018.html

New Analyses Identify a Small Possibility that Asteroid 1999 AN10
Could Collide with Earth in 2044 or 2046

New orbital analyses for the kilometer-sized asteroid 1999 AN10 have
revealed a remote chance that this object might collide with the Earth
in the year 2044 or 2046. Although this asteroid will be monitored in the
future, it is not thought to be a serious hazard to Earth at this time.
Researchers Andrea Milani,  Steven Chesley and Giovanni Valsecchi in
in Italy, as well the undersigned at JPL, have identified these new
impacting possibilities by using new observational data, and by
projecting the asteroid's motion somewhat farther into the
future than before.  New measurements of 1999 AN10, made over
the last week and a half by amateur astronomer Frank Zoltowksi
in Australia, have allowed astronomers to make significantly more 
precise orbital calculations, and the revised predictions indicate 
that the asteroid could approach the Earth particularly closely on 
August 7, 2027.  The orbital motions of the Earth and the asteroid 
do not permit a collision in 2027, but the close approach will 
certainly change the asteroid's orbital path.  During the past week, 
researchers have focused on the range of possible paths the asteroid 
could follow after 2027.

The accompanying diagram (http://neo.jpl.nasa.gov/neo/an10a.gif)
shows the uncertainty in the predicted
close approach in 2027.  The asteroid must pass through the plane
of the diagram somewhere within an extremely skinny uncertainty
ellipse, which appears simply as a line segment.  New measurements 
taken over the weekend have moved the center of the ellipse 
(the most likely point of passage) out to a distance of
about 200,000 km from the Earth, significantly farther than last
week's estimate.  The predicted point of passage may continue to
bounce around within the ellipse as new data are added, but it
cannot decrease below a minimum of about 37,000 km from the Earth's
center.

During the 2027 close approach, Earth's gravity will change the
asteroid's orbit by an amount which depends on the precise location of
the point of passage through the uncertainty ellipse.  In particular,
a range of post-2027 orbital periods are possible: a passage on the
left side of the Earth in this diagram will decrease the orbital period;
a passage on the right side will increase the period.  If the asteroid
passes through certain narrow "keyholes" in the uncertainty ellipse,
its changed orbit will bring it back for another Earth close approach
in a later year.  The 2039 impacting scenario identified by Milani
et al. last month actually required passage through two keyholes, one
in the 2027 ellipse, and one in the 2034 ellipse, which explains why it
was so unlikely (about one chance in a billion using last month's
orbital estimate).  This week's new observations have now moved the
uncertainty ellipse completely off the 2039 keyhole, which indicates
that this impacting scenario is no longer possible.

The newly identified impacting possibilities for August 6, 2044 and
August 7, 2046 each require passage through only a single keyhole in the
2027 ellipse, and the probabilities of impact for these cases are
correspondingly larger, on the order of one chance in 500,000 for 2044,
and one chance in five million for 2046.  These odds of collision are larger
than those for any other object, but they are still less than one
hundredth the chance of an undiscovered asteroid of equivalent size
striking the Earth sometime before 2044.  Asteroid 1999 AN10 therefore
does not pose a serious impact hazard at this time, but observers should
continue to monitor its motion over the next few decades, and the chance
of impact should be carefully reassessed whenever new data become available.

Paul W. Chodas
Research Scientist
Near Earth Object Program Office
Jet Propulsion Laboratory
May 26, 1999

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