  We have performed a simulation of a next generation sky survey's (Pan-STARRS
1) efficiency for detecting Earth-impacting asteroids. The steady-state
sky-plane distribution of the impactors long before impact is concentrated
towards small solar elongations (Chesley and Spahr, 2004) but we find that
there is interesting and potentially exploitable behavior in the sky-plane
distribution in the months leading up to impact. The next generation surveys
will find most of the dangerous impactors (>140m diameter) during their
decade-long survey missions though there is the potential to miss difficult
objects with long synodic periods appearing in the direction of the Sun, as
well as objects with long orbital periods that spend much of their time far
from the Sun and Earth. A space-based platform that can observe close to the
Sun may be needed to identify many of the potential impactors that spend much
of their time interior to the Earth's orbit. The next generation surveys have a
good chance of imaging a bolide like 2008TC3 before it enters the atmosphere
but the difficulty will lie in obtaining enough images in advance of impact to
allow an accurate pre-impact orbit to be computed.
