  It has long been assumed that the planet Jupiter acts as a giant shield,
significantly lowering the impact rate of minor bodies upon the Earth, and thus
enabling the development and evolution of life in a collisional environment
which is not overly hostile. However, in the past, little work has been carried
out to examine the validity of this idea. In the second of a series of papers,
we examine the degree to which the impact risk resulting from objects on
Centaur-like orbits is affected by the presence of a giant planet, in a
continuing attempt to fully understand the impact regime under which life on
Earth has developed. The Centaurs, which occupy orbits beyond Jupiter, have
their origins in the Edgeworth-Kuiper belt that extends beyond Neptune. The
giant planets peturb the Centaurs, sending a significanr fraction into the
inner Solar System where they become visible as short-period comets. In this
work we present results which show that the presence of a giant planet can act
to significantly change the impact rate of short-period comets on the Earth,
and that a giant planet often actually increases the impact flux greatly over
that which would be expected were it not present. (Shortened version of
abstract.)
