Method of Distribution
Roughly every 250 years the Red Star, a rogue planet in the Rukbat system with an extremely eccentric orbit, would pass through the Oort Cloud located on the edge of the system. As it passed through the cloud, the planet would drag material with it, including the ovoids (Zebedees) that would later become the organism known as Thread. About eight to ten years after passing out of the Oort cloud, the thread ovoids would be close enough to drop down to the planet. Intense heat and friction of entry into Pern's atmosphere burned off the outer casing of the thread, leaving attenuated filaments of the voracious life form to fall to the planet's surface. It took roughly fifty years for Pern to escape from the trail, in which time thread fell at regular intervals. The fifty year period was known as a Pass. The remaining 200 +/- years of the Red Star's orbit, when it was not close enough to Pern to drop Thread, was known as an Interval.
Origin and Make-up
The organism known as thread is far more complex than either the settlers or the inhabitants of Pern knew, until steps were taken to finally capture and dissect the organism after the discovery of AIVAS. The organism exists on the red star in a relatively dormant state as ovoids with a tough, frozen exterior shell containing ice and oort debri. As it is a life developed in the incredibly cold climate of space, much of it contains liquid gasses such as liquid helium in particular. Its biological make-up shows that thread ovoids were originally more complex and intelligent lifeforms, perhaps even resulting in sentient beings, from the Oort cloud. However, during the journey away from the Oort cloud, the liquid helium within the organism gradually depleted, resulting in a more machine-like, non-intelligent lifeform. The possibility that thread actually could be connected to an intelligent, and thus far more dangerous species, is what leads AIVAS to aggresively pursue the developement of predatory parasites to seed the Red Star, and later the Oort cloud, with an order to fully eradicate them.
As thread enter the atmosphere, the heat from the friction of its entry would break through its hard and nearly impenetrable external casing, activating the organism inside.
Early Hypothesis Regarding Origin
When Thread first began to fall on Pern, the colonists, many of whom were still shaken from the events of the Nathi War, began to invent theories as to the origin of their new, sky-borne enemy. For example:
- Thread was a new weapon of the Nathis, designed to 'soften them up' before an eventual invasion;
- This was a defense system installed on the thought-to-be uninhabited Red Star
The colonists, under the organisation of Paul Benden and Emily Boll, managed to move virtually the entire population of Pern from the open, and geographically unsound Southern Continent to found a new hold at Fort on the Northern Continent. Though not a final solution, it allowed protection of the otherwise unprotected residents. The administration of the colony assigned AIVAS the task of coming up with a method for the eventual elimination of Thread.
In order to protect valuable farmland and forestry, squadrons of flamethrower-equipped sleds were organised to literally flame Thread out of the sky before it landed. Any Thread that made it through these protective units would be exterminated by Ground crews on the surface. As the numbers of usable power-packs declined, the sleds were eventually replaced by Dragons and their riders.
in the Ninth Pass, when the Pernese finally rediscovered Landing, AIVAS informed them that it had finally produced a solution to Thread. This involved removing the antimatter engines of the colony ships, and placing them along an enormous canyon on the Red Star's surface. In an aptly named "Operation Overkill", a number of parasites were also seeded among the Thread population in the Oort Cloud, with the intention of destroying every one of the organisms in existence.
References in Other Literature
Anne McCaffrey has, in the past, made subtle connections between different series she has worked on, connections that are often denied but still possible - one example would be the possible connection of a forming planet and crystaline lifeform discovered in the last book of the Crystal Singer series, and the possible connection of it as the source of the planet PTB on which The Powers That Be series takes place. In the series "Acorna", co-written by Anne McCaffrey, a voracious and destructive insect-like race is introduced throughout the series. They find and inhabit planets with living organisms and use it as a hatching ground for their "young", which are never clearly shown in the series itself. One member of the race, when referring to "the young", indicate very strong similarities between it and a more-sentient version of thread: they are voracious and completely devour a planet, leaving it devoid of all organism life and the planet itself an empty, dead shell. The adults of the species can eat their way through organic matter such as plants, and leave a trail of matter in their wake that rapidly hardens. The first of the Acorna series was written years after "All the Weyrs of Pern", and so it is entirely plausible that such a connection could be made, the Oort cloud being the source of the dreaded "Kheevi" from that series.