A spaceport can attack a spaceport or any adjacent hex on another planet. It says it is considered adjacent to other spaceports. He wants to attack on Geminus but we (Imperials) own both the shield generator and the spaceport. The SG protects the spaceport, so they have to take it out first.
Maybe a little rule clean up needs to be done, but I can't really do it from this conference. I'm basically working from 8 am this morning to 3-5pm tomorrow solid straight.