THIS WEEKEND, THE SCI-FI CHANNEL presents Pilot Playhouse (April 24 and 25, 7 p.m. ET both nights), a collection of first episodes from such supernatural TV series as The Invaders and Kolchak: The Nightstalker. June Lockhart hosts the festival, which opens and closes with the original, never-before-seen 1965 pilot of CBS's Lost in Space, a show in which Lockhart starred post-Lassie (1958-64). The premise and cast are essentially the same as in the eventual series, with the Robinson family (played by Guy "Zorro" Williams, Lockhart, Marta Kristen, Billy Mumy and Angela Cartwright) marooned in space when a meteor shower strikes their ship in 2001. There are some very entertaining touches. For instance, we learn from the newscaster who serves as the introductory narrator that the Robinsons' 19-year-old Judy (Kristen) "has rather heroically postponed all hopes for a career in the musical comedy field for the next two centuries at least." Cherubic Mumy, who years later would found the off-the-wall cult band Barnes & Barnes ("Party in My Pants"), shows his nascent musical talent with a campfire rendition of "Greensleeves." After seeing this pilot, CBS sent creator-director Irwin Allen (who would go on to produce such disaster films as The Poseidon Adventure and The Towering Inferno) back to the drawing board. In the show that premiered Sept. 15, 1965, the Robinsons, now joined by a genial talking robot, would be thrown off course by a stowaway saboteur, the unforgettably venal Dr. Zachary Smith, played by Jonathan Harris.