Oh, no! The YT channel I was watching these on was shut down! Luckily, I found this one on another channel.
YT description: On the ice world of Ultima Thule the Alphans encounter members of lost earth expedition who have achieved immortality- but at a price!
Anthony Terpiloff (screenplay), Elizabeth Barrows (screenplay),
Martin Landau, Barbara Bain, Barry Morse
Title: Space 1999 Season 1 Episode 5 – Death’s Other Dominion (YT link) Uploaded by Space1999FullEp.