In the fantasy realm, you have Harry Potter and the Narnia series. In horror, you have Goosebumps. Mystery has The Hardy Boys and Nancy Drew. Yet science-fiction doesn't have an equivalent "entry" program for young readers. There are individual works that are brilliant -- A Wrinkle In Time and some of Ray Bradbury's shorts -- but I can't think of one children's series that could be legitimately thought of as science-fiction. Why is that?