  We assess the risk of an Earth impact for asteroid (99942) Apophis by means
of a statistical analysis accounting for the uncertainty of both the orbital
solution and the Yarkovsky effect. We select those observations with either
rigorous uncertainty information provided by the observer or a high established
accuracy. For the Yarkovsky effect we perform a Monte Carlo simulation that
fully accounts for the uncertainty in the physical characterization, especially
for the unknown spin orientation. By mapping the uncertainty information onto
the 2029 b-plane and identifying the keyholes corresponding to subsequent
impacts we assess the impact risk for future encounters. In particular, we find
an impact probability greater than 10^-6 for an impact in 2068. We analyze the
stability of the impact probability with respect to the assumptions on Apophis'
physical characterization and consider the possible effect of the early 2013
radar apparition.
