  The potentially hazardous asteroid (101955) 1999 RQ36 has the possibility of
collision with the Earth in the latter half of the 22nd century, well beyond
the traditional 100-year time horizon for routine impact monitoring. The
probabilities accumulate to a total impact probability of approximately 10E-3,
with a pair of closely related routes to impact in 2182 comprising more than
half of the total. The analysis of impact possibilities so far in the future is
strongly dependent on the action of the Yarkovsky effect, which raises new
challenges in the careful assessment of longer term impact hazards.
  Even for asteroids with very precisely determined orbits, a future close
approach to Earth can scatter the possible trajectories to the point that the
problem becomes like that of a newly discovered asteroid with a weakly
determined orbit. If the scattering takes place late enough so that the target
plane uncertainty is dominated by Yarkovsky accelerations then the thermal
properties of the asteroid,which are typically unknown, play a major role in
the impact assessment. In contrast, if the strong planetary interaction takes
place sooner, while the Yarkovsky dispersion is still relatively small compared
to that derived from the measurements, then precise modeling of the
nongravitational acceleration may be unnecessary.

Earth and Planetary Astrophysics