Nerilka's Story: The Dragonriders of Pern, Volume 5

Rukbat, in the Sagittarian Sector, was a golden G-type star. It had five planets, two asteroid belts, and a stray planet that it had attracted and held in recent millennia. When men first settled on Rukbat's third world and called it Pern, they had taken little notice of the strange planet swinging around its adopted primary in a wildly erratic orbit. For two generations, the colonists gave the bright Red Star little thought -- until the path of the wanderer brought it close to its stepsister at perihelion. When such aspects were harmonious and not distorted by conjunctions with other planets in the system, parasitic organisms indigenous to the wandering planet sought to bridge the space gap between their home and the more temperate and hospitable planet. At these times, silver Threads dropped through Pern's skies, destroying anything they touched. The initial losses the colonists suffered were staggering. As a result, during the subsequent struggle to survive and combat the menace, Pern's tenuous contact with the mother planet was broken.

To control the incursions of the dreadful Threads -- for the Pernese had cannibalized their transport ships early on and abandoned such technological sophistication as was irrelevant to the pastoral planet -- the more resourceful men embarked on a long-term plan. The first phase involved breeding a highly specialized variety of fire-lizard, a life form indigenous to their new world. Men and women with high empathy ratings and some innate telepathic ability were trained to use and preserve the unusual animals. The dragons -- named for the mythical Terran beast they resembled -- had two valuable characteristics: They could instantaneously travel from one place to another, and after chewing a phosphine-bearing rock, they could emit a flaming gas. Because the dragons could fly, they could intercept and char the Thread in midair before it reached the surface.