Cover Characteristics: Book Covers featuring Spaceships/UFOs

Technically the topic this week is UFOs, but really whether something is a UFO or not is relative, right? I mean, to the aliens in the UFO, it's actually just a spaceship. Or it could be humans flying around in a ship, or something that's not even a ship, but if someone happens to see it and doesn't know what it is, voila! It becomes a UFO. Yes? This was all just a really long way of saying I'm...