Barry Morse decided to leave the series after Season 1 had ended. An explanation of his absence was written into the first episode of Season 2, "The Metamorph", but never made it to the finished episode. Dialog was to indicate he had been in a faulty spacesuit.
In Britain, the second season was broadcast over the course of two years, with part shown in the autumn/winter of 1976-77, then a season break until August 1977, with new episodes running into the autumn. The first ITV region to finish the series was LWT (London) which broadcast the final episode ("The Dorcons") in November 1977. Some ITV regions did not conclude the series until 1978, and one region (HTV Wales) did not show the final episodes until 1984.
Moonbase Alpha's final fate was revealed in a short fan-made film shown at the 1999 convention, where original cast member Zienia Merton reprised her role as Sandra Benes and described the final abandonment of Moonbase Alpha for their new home on an unnamed planet.
Power Records adapted five episodes from the first season as audio dramas for children in the mid-70s ("Breakaway," "Death's Other Dominion," "Mission of the Darians," "Dragon's Domain," and "End of Eternity"). The first three were released on a single LP. The latter two were released on a second LP that also featured two original stories, "Return to the Beginning" (featuring the Alphans returning to Earth but during Biblical times and meeting Noah, a decidedly uncharacteristic story for the series) and "It Played So Softly on the Ear" (in which a strange musical signal lures the Alphans into investigating it). Both "Breakaway" and "Return to the Beginning" were released separately as book-and-record sets complete with comic book art - the "Breakaway" art was later adapted for the 2012 graphic novel SPACE 1999: AFTERSHOCK AND AWE. Interestingly, the audio stories, which ran anywhere from 10-15 minutes each, often compressed the plots significantly and did away with some plot points altogether (Simmonds appears only briefly in Power's version of "Breakaway," for example). They also at times added content to lighten the mood for their young audiences - the audio version of "End of Eternity," for instance, ends with a comical scene that does nothing to diminish the horror of the main plot, and the televised version feels somehow incomplete without it.
There were a number of format changes from Season 1 to Season 2. Besides changing from a more philosophical theme to a more action-oriented series, the credits showed more active scenes rather than just focused "glamour shots" of the leads, scenes of the episodes were omitted, Dr. Russel had a journal entry at the beginning of the show, and the philosophical zinger at the end was removed.
During the show's original run, the episodes were all novelized into two series of paperbacks (one for each season), although "Earthbound" and "The Taybor" were both omitted. In addition, four unused scripts (from season one) were made into their own paperback novels, the final one only being released in the U.S..
The trivia item below may give away important plot points.
The number of days since the original explosion ranges wildly between 300 and 3000+ in Helena's opening logs throughout the series. If we use Maya's joining the crew as a marker, Helena has said that their journey is in to the thousands of days before we ever meet Maya. However, after Maya's arrival, her logs continue to bounce from anywhere between 300 and 3000+ days.