Click to view The success of Harry Potter has established that the young adult market in fiction can be insanely lucrative, as have other successful scifi series like Philip Pullman's His Dark Materials trilogy and Scott Westerfeld's Uglies series. Now traditionally adult scifi authors like Cory Doctorow and John Scalzi have released YA novels, and publishers promise more smart YA fiction is on the way. But this is hardly the first time that YA fiction in the scifi genre has flowered: in the 1950s, Scribner's did an entire Juveniles series, with over a dozen novels devoted to teen space adventure (including some of Lester Del Rey and Robert Heinlein's most beloved books). But these weren't the only cool kid scifi books of the pre-Potter era. We've got six more great, old-school YA books for you to rediscover or read for the first time.

The Rolling Stones, by Robert Heinlein. (1952)

One of Heinlein's early Juveniles, The Rolling Stones is about a zany family of Loonies (moon-dwellers) who go on a weird family trip through the inner solar system, picking up a fast-breeding, Tribble-esque "flat cat" along the way. They also visit the crazy "wild west" mining areas around the asteroid belt and get into silly adventures on Mars and Saturn. The troublemaking, anarchic family of Stones show up in some of his other novels too. The Rolling Stones is more fun than Heinlein's groundbreaking first Juvenile, Rocket Ship Galileo (which, like Harry Potter, proved there was a market for YA scifi), and has a lot of the freewheeling, libertarian-hippie flavor for which the author later became known.

A Wrinkle in Time, by Madeleine L'Engle (1962)

Meg and Charles Wallace are a brother and sister who discover late one night that their mother's strange visitors are actually extra-dimensional beings who have come to ask their help. The kids' father has been missing for a while, and it turns out his experiments with "tesseracts," FTL technologies that involve folding space, have landed him on a dangerous planet controlled by a psychic computer called IT. Traveling through a harrowing galactic and psychic landscape, the children must deal with everything from bizarre physics experiments to the nature of identity and evil. Beautiful and haunting, this is still one of the most mind-blowing novels a kid could ever read.

The White Mountains, by John Christopher (1967)

Set in a post-apocalyptic England 100 years after aliens called "Tripods" (much like those in War of the Worlds) have taken over, the novel focuses on two kids who are starting to question the values of their society. Every person over the age of 14 is fitted with a mind-controlling "cap" by the Tripods, and afterward loses all ambition or creativity. The humans' culture remains stuck in a kind of perpetual middle ages. A year away from being capped, our heroes decide to strike out on their own and look for a mysterious place in the "white mountains" where humans live uncapped.

Sweetwater, by Laurence Yep (1973)

For slightly younger readers, Sweetwater is a the tale of a kid who lives on a planet full of seamonsters and unknown threats. Rising waters threaten his small town, but even more threatening is the main character's growing relationship with a native of the planet. His own family is part of a colonizing group that has always distrusted the natives. And now our hero must unite the planet's peoples, as well as stop the eco-disaster that's consuming the town.

Star Ka'at, by Andre Norton (1976)

I've raved about this novel before, but I can't say enough good things about it. Like Sweetwater, this one is for slightly younger readers. Psychic cats called Ka'ats from another planet arrive on Earth to gather up their brethren before humans destroy themselves with atomics. Two of the Ka'ats meet up with two very realistic human kids — a nearly-homeless little girl and an orphan boy in a foster home — and befriend them. Not only does the novel deliver a nice dose of psychic kitty, which is pretty much equal to awesome in a kids' book, but it's also full of fairly grownup commentary on race relations and poverty without ever getting preachy or boring.

Alan Mendelsohn the Boy from Mars, by Daniel Pinkwater (1979)

For any nerdy kid who wishes he had psychic powers and could travel between dimensions, this weird novel by NPR commentator Daniel Pinkwater is pretty much the best there is. After Leonard moves to a new school, he's picked on by everybody until he teams up with another dorky kid named Alan — and the two of them embark on a book-fueled journey through consciousness and space. Helped along by a bookstore owner who loans them books on mind-control and interdimensional travel, the two learn to fight the mean kids at their school using their brains. Later, of course, they manage to save an entire other dimension from slavery. Wackier and leftier than a Heinlein novel, but with the same sense of anarchic fun, Alan Mendelsohn should be on any outsider kid's list of must-reads.