You must visit every single body in all of the Kerbol system. Including Kerbol, every planet, every moon, every dwarf planet and any other celestial body in the Kerbol system. For it to qualify it must be orbiting or on the body, does not have to be inhabited, even if there is a kraken attack during your flight and you forgot to quick save it does not count, and it must be in stock KSP and no textures or mods, none at all. Edit: you can use any textures and MechJeb plus any kind of launch window alarm.