If you use a planet as alternative location, think of the size of this planet. For instance, Earth has ingame a radius of 6378, so you need a number a couple thousand larger then that to see the planet below your ship. If the distance is smaller then the radius of the object it orbits, its so to speak inside the planet, so you won't be able to see it.
Simply check the radius of the object you want to put your mission around in the solar system file.