Humans have colonized the planet Pern in the Rukbat star system, but have lost much of their technology and history (including their origin on Earth) due to periodic onslaughts of Thread, a mycorrhizoid spore that voraciously consumes all organic material, including humans and their crops, given the opportunity. Thread comes from the Red Star, actually another planet. The Red Star has a 250-Turn (Pernese year) elliptic orbit around Rukhbat, and when its orbit brings it close enough, Thread rains down on Pern at predictable intervals over about 50 Turns.