Episode 2 of James Cameron’s very personal look at science fiction is devoted to the topic of “Space.” And once again, the best elements are not always the clips from TV shows and movies, which don’t always seem weighed and selected with the proper historical perspective. Instead, the most fascination bits are when Cameron sits and talks, one on one, with some of his fellow filmmakers and genre visionaries. Steven Spielberg was great last week, for example, and tonight the conversationalists include, among others, Star Wars creator George Lucas.