  We present a short review of the impact regime experienced by the terrestrial
planets within our own Solar system, describing the three populations of
potentially hazardous objects which move on orbits that take them through the
inner Solar system. Of these populations, the origins of two (the Near-Earth
Asteroids and the Long-Period Comets) are well understood, with members
originating in the Asteroid belt and Oort cloud, respectively. By contrast, the
source of the third population, the Short-Period Comets, is still under debate.
The proximate source of these objects is the Centaurs, a population of
dynamically unstable objects that pass perihelion between the orbits of Jupiter
and Neptune. However, a variety of different origins have been suggested for
the Centaur population. Here, we present evidence that at least a significant
fraction of the Centaur population can be sourced from the planetary Trojan
clouds, stable reservoirs of objects moving in 1:1 mean-motion resonance with
the giant planets (primarily Jupiter and Neptune). Focusing on simulations of
the Neptunian Trojan population, we show that an ongoing flux of objects should
be leaving that region to move on orbits within the Centaur population. With
conservative estimates of the flux from the Neptunian Trojan clouds, we show
that their contribution to that population could be of order ~3%, while more
realistic estimates suggest that the Neptune Trojans could even be the main
source of fresh Centaurs. We suggest that further observational work is needed
to constrain the contribution made by the Neptune Trojans to the ongoing flux
of material to the inner Solar system, and believe that future studies of the
habitability of exoplanetary systems should take care not to neglect the
contribution of resonant objects (such as planetary Trojans) to the impact flux
that could be experienced by potentially habitable worlds.
