 Solution

For some orbital phase of the moon to permit an impact, the comet's planetocentric hyperbolic Kepler orbit must at least cross the moon's circular orbit. At the limiting case $a_m$ is the comet's periapsis, so the radial velocity in part (iv) vanishes. ConsequentlyThis is gravitational focusing by the planet; an actual collision additionally requires the moon to occupy the crossing point at the right time.