How many kinds of Sci-Fi are there?

As a genre science fiction covers a huge amount of ground. From stories of present day mad scientists to as far into the future as a writers imagination can carry us. From histories that might have been to worlds we all hope never will come to pass. The genres limits are only those of a storytellers ability to make his imagination scientifically plausible. Can every possible sub-genre be classified? Probably not, but hitting the high points can be fun and might introduce some fans to stories they haven't already read.
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