He kind of became the de facto lead of Mission: Impossible late in its first season, once they dialed back Steven Hill's role -- even though Landau was technically only a "special guest star" the whole season. They offered Landau the series lead when Hill left, but he didn't want to be tied down to a show for more than a year at a time, so we got Peter Graves as a result.
I have recollection that it was their work on M:I that lead to Landau and Bain being cast in Space:1999 though by the the time it went into production both would have been off the first show for several years.
Have some other recollections. I can remember it airing in Australia during the 1970s but can't recall when exactly but I would have been very young the time (born in 71) so at the time I didn't comprehend that the moon was no longer in Earth orbit and it was only many years later that I saw Breakaway when it was editted into a 2 part tv movie.
I watched The Last Sunset late one night at my Grandparents. The tv was in the rumpus room where I was sleeping so I quietly turned it on and watched.
and when it aired on a Sunday night at 7:30 I went to tell my dad that it was on but he wasn't interested. Said the episode from the previous week was the biggest load of rubbish he'd ever seen. been 40 odd years of tv since so he's probably seen worse by now.