Here's Russ's comment:
Now. Sandra, I'm going to take issue with your strict delineation between sci fi and horror. I don't think they can be so easily separated. Sci fi is about the unknown, about discovery, about mystery. These can be wonderous and enlightening, or they can be terrifying. Think E.T. vs. Alien. Which makes sense, by the way, because horror is about the unknown too - something lurking out of sight, a mysterious hidden threat - that sort of thing. The genres cover the same themes, so it makes sense that the delineation between the two frequently blurs.
And here's my response:
Well, yes and no, Russ. Science fiction can evoke a sense of wonder, but that's not necessarily its primary job. It's really more about the relationship between humanity and science/technology. Without the speculative scientific element, the story would fall apart. Here's what Robert Heinlein has to say: "A handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method. To make this definition cover all science fiction (instead of "almost all") it is necessary only to strike out the word 'future.'" (See more definitions here: http://www.panix.com/~gokce/sf_defn.html)
As for horror, this is directly from the Horror Writers Association website: http://www.horror.org/horror-is.htm
...horror can deal with the mundane or the supernatural, with the fantastic or the normal. It doesn't have to be full of ghosts, ghouls, and things to go bump in the night. Its only true requirement is that it elicit an emotional reaction that includes some aspect of fear or dread. ...By this definition, the best selling book of all time, the Bible, could easily be labeled horror, for where else can you find fallen angels, demonic possessions, and an apocalypse absolutely terrifying in its majesty all in one volume?
So, yes, there can be overlap, but the two genres are trying to do two different things.