**31st Annual Meeting of the DPS, October 1999**

*Session 28. Near Earth Asteroids*

Contributed Oral Parallel Session, Tuesday, October 12, 1999, 2:00-3:30pm, Sala Plenaria
[Previous] |
[Session 28] |
[Next]

## [28.04] Orbit Uncertainties, Keyholes, and Collision Probabilities

*P.W. Chodas (JPL/Caltech)*

Last year, asteroid 1997 XF11 was much in the news when
orbit calculations indicated that it might make an extremely
close approach to the Earth in 2028, and when the suggestion
was made that a collision in that year was possible. Using
observations available on March 11, 1998 (an 88-day data
arc), it could be shown that the collision probability in
2028 was essentially zero. Pre-discovery observations found
the next day only served to confirm this result. But was it
possible to rule out a collision in the years after 2028,
using only the 88-day-arc solution? New, nonlinear
techniques for analyzing orbit uncertainties were necessary
to answer this question. The resulting analysis of this
reduced-arc case showed non-negligible collision
probabilities for several subsequent years. Embedded within
the 2028 uncertainty region were many narrow "keyholes"
through which the asteroid could be perturbed onto collision
trajectories. The highest collision probability for this
hypothetical 1997 XF11 case was ~10^{-5}.

This year, two real cases with non-negligible collision
probabilities have been identified. The first of these, 1999
AN10, is remarkably similar to the case of 1997 XF11. Within
its uncertainty region, keyholes were identified which could
lead to impacts in the years 2044, 2046, or 2039. As
additional observations became available, the impact
probability rose as high as ~10^{-6}, but
pre-discovery observations were found which moved the
prediction away from these keyholes, and impact is now not
possible before 2076. The other potential impactor, 1998
OX4, has a very short arc and is currently lost; its impact
probability is ~10^{-7}. A side effect of the
increasing discovery rate for Near Earth Objects and the new
nonlinear orbit uncertainty analysis techniques will be a
growing number of asteroids with at least temporarily
non-negligible impact probabilities.

[Previous] |
[Session 28] |
[Next]