I think they can attack the space port itself then?
Yeah, looking at the rules, if they don't already have a presence on that planet, I think it would need to be against the space port first.
Since the imperials have the spaceport AND shield generator, the spaceport cannot be attacked, but any hex adjacent to the spaceport can be attacked from another spaceport.
Yours Truly, Skymarshall Baron Von Castle Void Shield