This past July, Marvel let pretty much everyone in the universe know that they were changing Thor into a female. At first blush, this just seemed like a publicity stunt to make the character relevant once again and, in turn, sell some more books.
The power of "Thor" has been transferred to aliens before (the whole Beta Ray Bill arc) and also to a frog (Throg...yes, he was actually called that), so why not a woman? But who will it be? In true Norse mythology, Thor has a sister (Thora), but as far as I know, she's never been in the comics. There's also Valkyrie in the Marvel Comics pantheon, who would be an interesting choice to take up the mantle of Thor Odinson; as would the sometimes evil Enchantress who has carried a torch for Thor for some time, but I'm unsure how she would be worthy to wield the hammer.
According to Vulture, in the months/issues leading up to this, Thor has somehow become unworthy of lifting his mystical hammer Mjolnir (and, as anyone who knows the comic mythology can tell you, Mjolnir can only be wielded by those the hammer somehow deems "worthy"), but the new, female Thor was nowhere to be seen...until now. See the preview pages below that Marvel released today for a sneak peek:
And here's the reveal/teaser page. It's kind of cool seeing the "he" on the inscription change to "she" as the unseen woman touches the hammer. One thing confuses me though...whoever this woman is says, "There must always be a Thor." Thor is his name. It's not a superhero name or a stage name or whatever; it's his birth name. To refer to him as "a Thor" seems a bit off to me.
This Wednesday, the ongoing "Thor" series gets rebooted with a new, first issue written by Jason Aaron and penciled by Russell Dauterman. Be there or be square, true believers...