From the book
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.
It took generations to develop to the fullest the potential of the dragons. The second phase of the proposed defense against the deadly incursions would take even longer. For Thread, a space-traveling mycorrhizoid spore, devoured with mindless voracity all organic matter and, once grounded, burrowed and proliferated with terrifying speed. So a symbiote of the same strain was developed to counter this parasite, and the resulting grub was introduced into the soil of the Southern Continent. It was planned that the dragons would be visible protection, charring Thread while it was still skyborne and protecting the dwellings and the livestock of the colonists. The grub-symbiote would protect vegetation by devouring what Thread managed to evade the dragons' fire.
The originators of the two-stage defense did not allow for change or for hard geological fact. The Southern Continent, though seemingly more attractive than the harsher northern land, proved unstable, and the entire colony was eventually forced to seek refuge from the Threads on the continental shield rock of the north.
On the northern continent the original Fort, Fort Hold, constructed on
the eastern face of the Great West Mountain Range, was soon outgrown by
the colonists, and its capacious beasthold could not contain the growing
numbers of dragons. Another settlement was started slightly to the
north, where a great lake had formed near a cave-filled cliff. But
Ruatha Hold, too, became overcrowded within a few generations.
Damon Knight Memorial Grand Master Award(Science Fiction and Fantasy Writers of America)