  It has long been assumed that the planet Jupiter acts as a giant shield,
significantly lowering the impact rate of small bodies on the Earth. However,
until recently, very little work had been carried out examining the role played
by Jupiter in determining the frequency of such collisions. In this work, the
third of a series of papers, we examine the degree to which the impact rate on
Earth resulting from the Oort cloud comets is enhanced or lessened by the
presence of a giant planet in a Jupiter-like orbit, in an attempt to more fully
understand the impact regime under which life on Earth has developed. Our
results show that the presence of a giant planet in a Jupiter-like orbit
significantly alters the impact rate of Oort cloud comets on the Earth,
decreasing the rate as the mass of the giant increases. The greatest
bombardment flus is observed when no giant planet is present.
