I've only just got the book, barely read any, but I already know the ending - the cover gives it away.
Look here, on the back cover, just starting to appear over hogwarts:
That's right, it's the NCC-1701.. D? I think it's the D, but it could be an earlier model. You probably know it better as the Enterprise, from Star Trek
That's right: The Enterprise is going to arrive at hogwarts, reveal that Voldemort and the Dementors are aliens, use some variation on the Corbomite maneouver to trick them out of hiding, then chase them back home (and generally save the day).
J K Rowling, you're a genius.
The image of the NCC-1701 A comes from Randy Asplund's site, which you should visit to see more of his work.