 The original concept had been to place the MX missile silos on the reverse side of tall hills or mesas.Enemy warheads approach at an angle of about 25 degrees above horizontal, so if the slope of the hill was greater, the warhead would impact the hill short of the silo.However, as the Soviet ICBMs introduced their Fractional Orbital Bombardment System and high-angle "lofted" trajectories, this no longer offered any extra protection.