Equations are derived for the critical velocity at which damage is initiated during the end-on impact of solid and granular radioisotopic fuel rods with solid and granular earth materials. The equations show that high critical impact velocity, and hence high impact resistance, occurs for impacting materials with low density, small elastic modulus, high porosity, and high crush strength. An illustrative example is given in which the critical impact velocities are calculated for the impact of solid and granular quartz rods with dry sand, water-filled sand, and a rigid body.