  Tunguska and Chelyabinsk impact events occurred inside a geographical area of
only 3.4\% of the Earth's surface. Although two events hardly constitute a
statistically significant demonstration of a geographical pattern of impacts,
their spatial coincidence is at least tantalizing. To understand if this
concurrence reflects an underlying geographical and/or temporal pattern, we
must aim at predicting the spatio-temporal distribution of meteoroid impacts on
Earth. For this purpose we designed, implemented and tested a novel numerical
technique, the "Gravitational Ray Tracing" (GRT) designed to compute the
relative impact probability (RIP) on the surface of any planet. GRT is inspired
by the so-called ray-casting techniques used to render realistic images of
complex 3D scenes. In this paper we describe the method and the results of
testing it at the time of large impact events. Our findings suggest a
non-trivial pattern of impact probabilities at any given time on Earth.
Locations at $60-90\deg$ from the apex are more prone to impacts, especially at
midnight. Counterintuitively, sites close to apex direction have the lowest
RIP, while in the antapex RIP are slightly larger than average. We present here
preliminary maps of RIP at the time of Tunguska and Chelyabinsk events and
found no evidence of a spatial or temporal pattern, suggesting that their
coincidence was fortuitous. We apply the GRT method to compute theoretical RIP
at the location and time of 394 large fireballs. Although the predicted
spatio-temporal impact distribution matches marginally the observed events, we
successfully predict their impact speed distribution.
