First of all, putting a nuclear explosive device is space is prohibited by several treaties, including the Nuclear Test Ban Treaty of 1963 (the one that killed Project Orion) and quite likely the Outer Space Treaty of 1967.

Second, it seems rather unlikely that a high-speed space probe sent on an outbound mission could come back at the Earth. Orbital mechanics just doesn't work that way. Off hand, about the only scenario that I can think of where something even remotely like that could happen would be if the probe did a sling-shot maneuver around the target planet, and lost enough velocity to fall back towards the Sun. It'd have to hit the Earth on it's way by. Rather unlikely.