  Motivated by the recent discovery of interstellar objects passing through the
solar system, and by recent developments in dynamical simulations, this paper
reconsiders the likelihood for life bearing rocks to be transferred from one
planetary system to another. The astronomical aspects of this lithopanspermia
process can now be estimated, including the cross sections for rock capture,
the velocity distributions of rocky ejecta, the survival times for captured
objects, and the dynamics of the solar system in both its birth cluster and in
the field. The remaining uncertainties are primarily biological, i.e., the
probability of life developing on a planet, the time required for such an
event, and the efficiency with which life becomes seeded in a new environment.
Using current estimates for the input quantities, we find that the transfer
rates are enhanced in the birth cluster, but the resulting odds for success are
too low for panspermia to be a likely occurrence. In contrast, the expected
inventory of alien rocks in the solar system is predicted to be substantial
(where the vast majority of such bodies are not biologically active and do not
interact with Earth).
